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Mercouri Kanatzidis ( Greek : Μερκούριος Κανατζίδης ; born 1957) is a Charles E. and Emma H. Morrison Professor of chemistry and professor of materials science and engineering at Northwestern University [ 1 ] and Senior Scientist at Argonne National Laboratory . [ 2 ]
Kanatzidis was listed as one of the most cited researchers in Materials Science and Engineering in 2016 based on Elsevier Scopus data. [ 3 ] He has published over 1,655 manuscripts ( h-index =181 Google h-index =210] [ 4 ] ) and has over 60 patents. Kanatzidis has mentored over 90 Ph.D. students and nearly 130 postdoctoral fellows. More than 90 of these alumni hold academic positions worldwide.
Kanatzidis was born in Thessaloniki , Greece. He received his B.S. degree from Aristotle University in 1979 and his Ph.D. from the University of Iowa in 1984 [ 1 ] (with Dimitri Coucouvanis). He spent two years at the University of Iowa from 1980 to 1982 and then moved to the University of Michigan when Coucouvanis moved there in 1982.
He was a postdoctoral research fellow at the University of Michigan (1985) and Northwestern University (1986–1987) where he worked with Professor Tobin J. Marks on conductive polymers and intercalation compounds .
He became assistant professor at Michigan State University in 1987. He was promoted to full Professor in 1994. He moved to Northwestern University in 2006. [ 5 ]
Kanatzidis developed synthesis methodologies to synthesizing new chalcogenide materials and intermetallics . One of his notable contributions is the panoramic synthesis [ 6 ] [ 7 ] method, which enables the design and discovery of novel materials. He is also credited with developing flux synthesis techniques that allow for reactions to occur at lower temperatures than conventional methods, leading to the formation of unique structures and compositions.
In addition to these contributions, Kanatzidis's research has resulted in the discovery of metal sulfide ion-exchangers, which have practical applications in the remediation of heavy metals in industrial waste water. These findings demonstrate his ability to not only generate new materials but also to identify and apply them in real-world settings.
Kanatzidis is also credited with defining the concept of nanostructuring in the thermoelectric field. By developing new approaches to controlling the structure and composition of thermoelectric materials at the nanoscale, he has contributed to the advancement of this field and the creation of high-performance materials with unique properties. These methods for achieving "nanostructuring" and all-scale architecturing of thermoelectric semiconductors , resulted in the creation of high-performance materials with unprecedented ZT figures of merit [ 8 ] [ 9 ] (ZT~2.5). [ 10 ] These materials feature coherently embedded nanodots, such as those found in PbTe (a phenomenon known as endotaxy), which significantly reduce thermal conductivity by over 70%, while maintaining high electrical conductivity . This unique combination of properties allows for the attainment of very high ZT values exceeding 2.5 [ 11 ] in nanostructured thermoelectric materials.
Kanatzidis, along with fellow researcher Professor Robert P.H. Chang at Northwestern, developed a Novel solar cell technology that utilizes tin instead of lead perovskite. [ 12 ] [ 13 ] In their groundbreaking study, they published the first solid-state solar cell device incorporating a film of CsSnI3 perovskitein a solid-state dye-sensitized Gratzel cell , which achieved an efficiency of approximately 10%. Kanatzidis was also the first to demonstrate the functionality of CH3NH3SnI3-based solar cells, and he discovered the anomalous bandgap dependence between lead and tin-based solid solutions APb1-xSnxI3 (A=Cs, CH3NH3, formamidinium). This discovery revealed that bandgaps as low as 1.1 eV are achievable, which is useful in the development of tandem solar cells. [ 14 ]
In 2016, Kanatzidis and Mohite demonstrated that 2D iodide perovskites form films with vertical slab orientation, and showed >12% efficiency in a solar cell with far better stability than corresponding 3D MAPbI3-based solar cells. [ 15 ] . Since then, 2D iodide perovskites have become widely used in mixtures of 2D/3D perovskites for solar cells, exhibiting both high stability and efficiency.
In 2013 he reported the x-ray detecting properties of the perovskite CsPbBr 3 semiconductor [ 16 ] with potential applications in gamma-ray spectroscopy having better than 1.4% energy resolution. [ 17 ] [ 18 ]
Kanatzidis has proposed ideas and concepts for predictive synthesis to new materials including "infinitely adaptive" homologous superseries and the panoramic synthesis strategy where with a single experiment all phases in the course of a given reaction can be detected. This offers a panoramic view of all the phases present, and could help unravel the mechanisms of how new materials form. [ 19 ]
Kanatzidis is credited with inventing a new category of materials known as chalcogels . These unique inorganic compounds exhibit aerogel properties. Chalcogels have a sponge-like structure that enables them to effectively absorb heavy-metal atoms from polluted water. Due to their high surface area-to-volume ratio, even small pieces of chalcogels can purify thousands of liters of water. Chalcogels have demonstrated the ability to reduce mercury, lead, and cadmium concentrations to parts per trillion (ppt) levels as well as radionuclides. [ 20 ] Biomimetic chalcogels containing bioinorganic Fe 4 S 4 have been reported to photochemically convert N 2 to NH 3 . [ 21 ] The International Mineralogical Association named a new mineral, Kanatzidisite , belonging to the sulfosalt class with a composition of [BiSbS3][Te2]. [ 22 ] | https://en.wikipedia.org/wiki/Mercouri_Kanatzidis |
Mercury(I) chloride is the chemical compound with the formula Hg 2 Cl 2 . Also known as the mineral calomel [ 4 ] (a rare mineral) or mercurous chloride , this dense white or yellowish-white, odorless solid is the principal example of a mercury (I) compound. It is a component of reference electrodes in electrochemistry . [ 5 ] [ 6 ]
The name calomel is thought to come from the Greek καλός "beautiful", and μέλας "black"; or καλός and μέλι "honey" from its sweet taste. [ 4 ] The "black" name (somewhat surprising for a white compound) is probably due to its characteristic disproportionation reaction with ammonia , which gives a spectacular black coloration due to the finely dispersed metallic mercury formed. It is also referred to as the mineral horn quicksilver or horn mercury . [ 4 ]
Calomel was taken internally and used as a laxative, [ 4 ] for example to treat George III in 1801, and disinfectant, as well as in the treatment of syphilis, until the early 20th century. Until fairly recently, [ when? ] it was also used as a horticultural fungicide, most notably as a root dip to help prevent the occurrence of clubroot amongst crops of the family Brassicaceae . [ 7 ]
Mercury became a popular remedy for a variety of physical and mental ailments during the age of " heroic medicine ". It was prescribed by doctors in America throughout the 18th century, and during the revolution, to make patients regurgitate and release their body from "impurities". Benjamin Rush was a well-known advocate of mercury in medicine and used calomel to treat sufferers of yellow fever during its outbreak in Philadelphia in 1793. Calomel was given to patients as a purgative or cathartic until they began to salivate and was often administered to patients in such great quantities that their hair and teeth fell out. [ 8 ]
Yellow fever was also treated with calomel. [ 9 ]
Lewis and Clark brought calomel on their expedition. Researchers used that same mercury, found deep in latrine pits, to retrace the locations of their respective locations and campsites. [ 10 ]
Mercury is unique among the group 12 metals for its ability to form the M–M bond so readily. Hg 2 Cl 2 is a linear molecule. The mineral calomel crystallizes in the tetragonal system, with space group I4/m 2/m 2/m. The unit cell of the crystal structure is shown below:
The Hg–Hg bond length of 253 pm (Hg–Hg in the metal is 300 pm) and the Hg–Cl bond length in the linear Hg 2 Cl 2 unit is 243 pm. [ 11 ] The overall coordination of each Hg atom is octahedral as, in addition to the two nearest neighbours, there are four other Cl atoms at 321 pm. Longer mercury polycations exist.
Mercurous chloride forms by the reaction of elemental mercury and mercuric chloride:
It can be prepared via metathesis reaction involving aqueous mercury(I) nitrate using various chloride sources including NaCl or HCl.
Ammonia causes Hg 2 Cl 2 to disproportionate :
Mercurous chloride is employed extensively in electrochemistry , taking advantage of the ease of its oxidation and reduction reactions. The calomel electrode is a reference electrode , especially in older publications. Over the past 50 years, it has been superseded by the silver/silver chloride (Ag/AgCl) electrode. Although the mercury electrodes have been widely abandoned due to the dangerous nature of mercury , many chemists believe they are still more accurate and are not dangerous as long as they are handled properly. The differences in experimental potentials vary little from literature values. Other electrodes can vary by 70 to 100 millivolts. [ citation needed ]
Mercurous chloride decomposes into mercury(II) chloride and elemental mercury upon exposure to UV light.
The formation of Hg can be used to calculate the number of photons in the light beam, by the technique of actinometry .
By utilizing a light reaction in the presence of mercury(II) chloride and ammonium oxalate , mercury(I) chloride, ammonium chloride and carbon dioxide are produced.
This particular reaction was discovered by J. M. Eder (hence the name Eder reaction ) in 1880 and reinvestigated by W. E. Rosevaere in 1929. [ 12 ]
Mercury(I) bromide , Hg 2 Br 2 , is light yellow, whereas mercury(I) iodide , Hg 2 I 2 , is greenish in colour. Both are poorly soluble. Mercury(I) fluoride is unstable in the absence of a strong acid.
Mercurous chloride is toxic , although due to its low solubility in water it is generally less dangerous than its mercuric chloride counterpart. It was used in medicine as a diuretic and purgative (laxative) in the United States from the late 1700s through the 1860s. Calomel was also a common ingredient in teething powders in Britain up until 1954, causing widespread mercury poisoning in the form of pink disease , which at the time had a mortality rate of 1 in 10. [ 13 ] These medicinal uses were later discontinued when the compound's toxicity was discovered.
It has also found uses in cosmetics as soaps and skin lightening creams, but these preparations are now illegal to manufacture or import in many countries including the US, Canada, Japan and the European Union. [ 14 ] A study of workers involved in the production of these preparations showed that the sodium salt of 2,3-dimercapto-1-propanesulfonic acid (DMPS) was effective in lowering the body burden of mercury and in decreasing the urinary mercury concentration to normal levels. [ 15 ] | https://en.wikipedia.org/wiki/Mercury(I)_chloride |
Mercury(I) oxide , also known as mercurous oxide , is an inorganic metal oxide with the chemical formula Hg 2 O.
It is a brown/black powder, insoluble in water but soluble in nitric acid . With hydrochloric acid , it reacts to form calomel, Hg 2 Cl 2 . [ 4 ] Mercury(I) oxide is toxic but without taste or smell. It is chemically unstable and converts to mercury(II) oxide and mercury metal.
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Mercury(I)_oxide |
Mercury(I) sulfide or mercurous sulfide is a hypothetical chemical compound of mercury and sulfur , with chemical formula Hg 2 S . Its existence has been disputed; it may be stable below 0 °C or in suitable environments, but is unstable at room temperature, decomposing into metallic mercury and mercury(II) sulfide (mercuric sulfide, cinnabar). [ 1 ] [ 2 ]
This compound was described in the 19th century by Berzelius as a black precipitate obtained by passing hydrogen sulfide H 2 S through solutions of mercury(I) salts. [ 3 ] [ 4 ] [ 5 ]
As of 1825, the London Pharmacopoeia listed a compound called "Ethiops-mineral" or Hydrargyri Sulphuretum Nigrum ("black sulfide of mercury"), a black powder that was obtained by combining solid sulfur and mercury at room temperature. This preparation did not leave the characteristic stain of metallic mercury when rubbed onto gold . When a large amount of Ethiops-mineral was vigorously ground, however, it formed mercury and cinnabar with evolution of smoke and heat. [ 6 ]
However, the existence of mercurous sulfide was disputed in 1816 by French pharmacist N. Guibourt . In his thesis he claimed that the precipitate obtained in such a manner was nothing more than an intimate mixture of mercury(II) sulfide HgS (mercuric sulfide, cinnabar) and metallic mercury Hg 2 , which could be separated by heating or grinding. (Guibourt also denied the reality of mercurous oxide Hg 2 O , for the same reason.) [ 7 ] [ 8 ] [ 6 ] [ 9 ]
Reviewing Guibourt's article in 1825, British chemist W. T. Brande disputed his conclusions. He observed that the proportions of mercury and sulfur in the precipitate are stoichometric for the formula Hg 2 S ; and that nitrogen triiodide , silver fulminate , and mercury fulminate were accepted compounds, even though they were decomposed by slight friction. He claimed that the black precipitate did not show any sign of metallic mercury or cinnabar (although it was easily decomposed into them). He also noted that hot nitric acid does not attack cinnabar, whereas it quickly turns precipitated "mercurous sulfide" to mercuric nitrate without leaving any residue. [ 6 ]
In 1894, Italian chemists Antony and Sestini claimed to have determined that mercurous sulfide was stable at –10 °C, but disproportionated into Hg 2 and HgS when heated to 0 °C. [ 2 ] [ 10 ]
According to W. T. Brande, mercurous sulfide is easily decomposed by trituration, exposure to sunlight, or heating to 300 °F. It reacts with hot nitric acid yielding mercuric nitrate. Boiling with potassium carbonate ("potassa", potash) removes part of the sulfur leaving pure cinnabar as residue. [ 6 ]
The structural formula is supposed to contain two mercury atoms bound to each other, as in the real compound mercury(I) chloride (calomel), Hg 2 Cl 2 . The latter is an ionic compound with the dimercury(I) cation , Hg 2+ 2 or + Hg–Hg + , and chloride anions Cl − .
However, like cinnabar, Hg 2 S may be a covalent polymer [–S–Hg–Hg–] n rather than an ionic compound. Many stable polymeric mercury compounds with the bonding system E-Hg-Hg-E (E = N, P, As, Sb, O, S, Se, and Sn) have been described since 1958. [ 11 ] [ 12 ]
One may also note that the stable compound Hg 4 BiS 2 Cl 5 , recently synthesized, was found to consist of two-dimensional polymeric cations [–(S–)–Hg–Hg–(S–Hg–)–Hg– ] 2 n + n balanced by one-dimensional polymeric anions [– Cl–(BiCl 4 ) – ] 2 n − n . In the cations, the sulfur atoms are tricoordinated, and the mercury atoms are dicoordinated. In each unit, two of the mercury atoms form S–Hg–S bridges, while the other two form an S–Hg–Hg–S bridge. [ 13 ] [ 14 ]
New insights that might lead to the successful synthesis of Hg 2 S has been coming since 1958 through the work of Klaus Brodersen and others. The reaction between dimercury(I) salts and Lewis bases in polar solvents normally destroys the Hg–Hg bond. The successful preparation of S–Hg–Hg–S compounds can be achieved with nonpolar solvents, weak Lewis bases, and NH acidic nitrogen compounds. [ 11 ]
In the 19th and early 20th centuries, several preparation routes for Hg 2 S have been described, but their reliability is questionable. According to W. T. Brande (1825), mercurous sulfide can be reliably obtained by passing H 2 S through a very dilute solution of mercurous chloride (calomel) or nitrate , and carefully filtering the black precipitate. [ 6 ]
According to 19th-century pharmacopoeia, the preparation Ethiops-mineral, claimed to be mercurous sulfide, was prepared by gentle grinding of equal parts of mercury and sulfur, until the mercury globules were no longer visible. [ 6 ]
According to Scherer , Hg 2 S could be obtained by reaction of mercurous nitrate HgNO 3 and sodium thiosulfate Na 2 S 2 O 3 . [ 15 ] However, a review of the procedure by J. T. Norton in 1900 cast doubts on the claim. [ 15 ]
A report from 1903 by American chemist Charles Baskerville claims that sulfuric acid left over metallic mercury in a closed bottle for over 5 years developed a crust over the metal that was found to be mercurous sulfide. [ 5 ] | https://en.wikipedia.org/wiki/Mercury(I)_sulfide |
Mercury(II) chloride ( mercury bichloride [ citation needed ] , mercury dichloride , mercuric chloride ), historically also sulema or corrosive sublimate , [ 2 ] is the inorganic chemical compound of mercury and chlorine with the formula HgCl 2 , used as a laboratory reagent . It is a white crystalline solid and a molecular compound that is very toxic to humans. Once used as a first line treatment for syphilis , it has been replaced by the more effective and less toxic procaine penicillin since at least 1948.
Mercuric chloride is obtained by the action of chlorine on mercury or on mercury(I) chloride . It can also be produced by the addition of hydrochloric acid to a hot, concentrated solution of mercury(I) compounds such as the nitrate : [ 2 ]
Heating a mixture of solid mercury(II) sulfate and sodium chloride also affords volatile HgCl 2 , which can be separated by sublimation . [ 2 ]
Mercuric chloride is not a salt composed of discrete ions, but it is made of linear triatomic molecules, hence its tendency to sublime . In the crystal, each mercury atom is bonded to two chloride ligands with Hg–Cl distance of 2.38 Å ; six other chlorides are more distant at 3.38 Å. [ 3 ]
Its solubility in water increases from 6% at 20 °C (68 °F) to 36% at 100 °C (212 °F).
The main application of mercuric chloride is as a catalyst for the conversion of acetylene to vinyl chloride , the precursor to polyvinyl chloride :
For this application, the mercuric chloride is supported on carbon in concentrations of about 5 weight percent. This technology has been eclipsed by the thermal cracking of 1,2-dichloroethane . Other significant applications of mercuric chloride include its use as a depolarizer in batteries and as a reagent in organic synthesis and analytical chemistry (see below). [ 4 ] It is being used in plant tissue culture for surface sterilisation of explants such as leaf or stem nodes.
Mercuric chloride is occasionally used to form an amalgam with metals, such as aluminium . [ 5 ] Upon treatment with an aqueous solution of mercuric chloride, aluminium strips quickly become covered by a thin layer of the amalgam. Normally, aluminium is protected by a thin layer of oxide, thus making it inert. Amalgamated aluminium exhibits a variety of reactions not observed for aluminium itself. For example, amalgamated aluminum reacts with water generating Al(OH) 3 and hydrogen gas. Halocarbons react with amalgamated aluminium in the Barbier reaction . These alkylaluminium compounds are nucleophilic and can be used in a similar fashion to the Grignard reagent. Amalgamated aluminium is also used as a reducing agent in organic synthesis. Zinc is also commonly amalgamated using mercuric chloride.
Mercuric chloride is used to remove dithiane groups attached to a carbonyl in an umpolung reaction. This reaction exploits the high affinity of Hg 2+ for anionic sulfur ligands.
Mercuric chloride may be used as a stabilising agent for chemicals and analytical samples. Care must be taken to ensure that detected mercuric chloride does not eclipse the signals of other components in the sample, such as is possible in gas chromatography . [ 6 ]
Around 900, the authors of the Arabic writings attributed to Jabir ibn Hayyan (Latin: Geber) and the Persian physician and alchemist Abu Bakr al-Razi (Latin: Rhazes) were experimenting with sal ammoniac (ammonium chloride), which when it was distilled together with vitriol (hydrated sulfates of various metals) produced hydrogen chloride . [ 7 ] It is possible that in one of his experiments, al-Razi stumbled upon a primitive method to produce hydrochloric acid . [ 8 ] However, it appears that in most of these early experiments with chloride salts , the gaseous products were discarded, and hydrogen chloride may have been produced many times before it was discovered that it can be put to chemical use. [ 9 ]
One of the first such uses of hydrogen chloride was in the synthesis of mercury(II) chloride (corrosive sublimate), whose production from the heating of mercury either with alum and ammonium chloride or with vitriol and sodium chloride was first described in the De aluminibus et salibus ("On Alums and Salts"). [ 10 ] This eleventh- or twelfth-century Arabic alchemical text is anonymous in most manuscripts, though some manuscripts attribute it to Hermes Trismegistus , and a few falsely attribute it to Abu Bakr al-Razi. [ 11 ] It was translated into Hebrew and two times into Latin , with one Latin translation by Gerard of Cremona (1144–1187) . [ 12 ]
In the process described in the De aluminibus et salibus , hydrochloric acid started to form, but it immediately reacted with the mercury to produce mercury(II) chloride. Thirteenth-century Latin alchemists , for whom the De aluminibus et salibus was one of the main reference works, were fascinated by the chlorinating properties of mercury(II) chloride, and they eventually discovered that when the metals are eliminated from the process of heating vitriols, alums, and salts, strong mineral acids can directly be distilled. [ 13 ]
Mercury(II) chloride was used as a photographic intensifier to produce positive pictures in the collodion process of the 1800s. When applied to a negative, the mercury(II) chloride whitens and thickens the image, thereby increasing the opacity of the shadows and creating the illusion of a positive image. [ 14 ]
For the preservation of anthropological and biological specimens during the late 19th and early 20th centuries, objects were dipped in or were painted with a "mercuric solution". This was done to prevent the specimens' destruction by moths, mites and mold. Objects in drawers were protected by scattering crystalline mercuric chloride over them. [ 15 ] It finds minor use in tanning, and wood was preserved by kyanizing (soaking in mercuric chloride). [ 16 ] Mercuric chloride was one of the three chemicals used for railroad tie wood treatment between 1830 and 1856 in Europe and the United States. Limited railroad ties were treated in the United States until there were concerns over lumber shortages in the 1890s. [ 17 ] The process was generally abandoned because mercuric chloride was water-soluble and not effective for the long term, as well as being highly poisonous. Furthermore, alternative treatment processes, such as copper sulfate , zinc chloride , and ultimately creosote ; were found to be less toxic. Limited kyanizing was used for some railroad ties in the 1890s and early 1900s. [ 18 ]
Mercuric chloride was a common over-the-counter disinfectant in the early twentieth century, recommended for everything from fighting measles germs [ 19 ] to protecting fur coats [ 20 ] and exterminating red ants. [ 21 ] A New York physician, Carlin Philips, wrote in 1913 that "it is one of our most popular and effective household antiseptics", but so corrosive and poisonous that it should only be available by prescription. [ 22 ] A group of physicians in Chicago made the same demand later the same month. The product frequently caused accidental poisonings and was used as a suicide method. [ 23 ]
It was used to disinfect wounds by Arab physicians in the Middle Ages . [ 24 ] It continued to be used by Arab physicians into the twentieth century, until modern medicine deemed it unsafe for use.
Syphilis was frequently treated with mercuric chloride before the advent of antibiotics . It was inhaled, ingested, injected, and applied topically. Both mercuric-chloride treatment for syphilis and poisoning during the course of treatment were so common that the latter's symptoms were often confused with those of syphilis. This use of "salts of white mercury" is referred to in the English -language folk song " The Unfortunate Rake ". [ 25 ]
Yaws was treated with mercuric chloride (labeled as Corrosive Sublimate) before the advent of antibiotics . It was applied topically to alleviate ulcerative symptoms. Evidence of this is found in Jack London's book The Cruise of the Snark in the chapter entitled "The Amateur M.D."
Between 1901 and 1904 the US Marine Hospital Service quarantined and engaged in an extensive disinfection program of San Francisco's Chinatown in response to an epidemic of bubonic plague . This program forced the closure of over 14,000 rooms and the eviction of thousands of Chinese residents whose dwellings were rendered toxic and uninhabitable from the disinfection program. Long-term mercury pollution is still a concern for construction workers in Chinatown to this day. [ 26 ]
Mercury dichloride is a highly toxic compound, [ 37 ] both acutely and as a cumulative poison. Its toxicity is due not just to its mercury content but also to its corrosive properties, which can cause serious internal damage, including ulcers to the stomach, mouth, and throat, and corrosive damage to the intestines. Mercuric chloride also tends to accumulate in the kidneys, causing severe corrosive damage which can lead to acute kidney failure . However, mercuric chloride, like all inorganic mercury salts, does not cross the blood–brain barrier as readily as organic mercury, although it is known to be a cumulative poison.
Common side effects of acute mercuric chloride poisoning include burning sensations in the mouth and throat, stomach pain, abdominal discomfort, lethargy, vomiting of blood, corrosive bronchitis, severe irritation to the gastrointestinal tract, and kidney failure. Chronic exposure can lead to symptoms more common with mercury poisoning, such as insomnia, delayed reflexes, excessive salivation, bleeding gums, fatigue, tremors, and dental problems.
Acute exposure to large amounts of mercuric chloride can cause death in as little as 24 hours, usually due to acute kidney failure or damage to the gastrointestinal tract. In other cases, victims of acute exposure have taken up to two weeks to die. [ 38 ] | https://en.wikipedia.org/wiki/Mercury(II)_chloride |
Mercury(II) fulminate , or Hg(CNO) 2 , is a primary explosive . It is highly sensitive to friction , heat and shock and is mainly used as a trigger for other explosives in percussion caps and detonators . Mercury(II) cyanate, though its chemical formula is identical, has a different atomic arrangement, making the cyanate and fulminate anionic isomers .
First used as a priming composition in small copper caps beginning in the 1820s, mercury fulminate quickly replaced flints as a means to ignite black powder charges in muzzle-loading firearms . Later, during the late 19th century and most of the 20th century, mercury fulminate became widely used in primers for self-contained rifle and pistol ammunition ; it was the only practical detonator for firing projectiles until the early 20th century. [ 1 ] Mercury fulminate has the distinct advantage over potassium chlorate of being non-corrosive, but it is known to weaken with time, by decomposing into its constituent elements. The reduced mercury which results forms amalgams with cartridge brass, weakening it, as well. Today, mercury fulminate has been replaced in primers by more efficient chemical substances. These are non-corrosive, less toxic, and more stable over time; they include lead azide , lead styphnate , and tetrazene derivatives. In addition, none of these compounds requires mercury for manufacture, supplies of which can be unreliable in wartime.
Mercury(II) fulminate is prepared by dissolving mercury in nitric acid and adding ethanol to the solution. Edward Charles Howard is credited with first preparing it in 1800. [ 2 ] [ 1 ] However, Johann Kunckel had discovered the compound more than a century before in the 17th century . [ 3 ] The crystal structure of this compound was determined only in 2007. [ 4 ]
Silver fulminate can be prepared in a similar way, but this salt is even more unstable than mercury fulminate; it can explode even under water and is impossible to accumulate in large amounts because it detonates under its own weight. [ 5 ]
The thermal decomposition of mercury(II) fulminate can begin at temperatures as low as 100 °C, though it proceeds at a much higher rate with increasing temperature. [ 6 ]
A possible reaction for the decomposition of mercury(II) fulminate yields carbon dioxide gas, nitrogen gas, and a combination of relatively stable mercury salts. | https://en.wikipedia.org/wiki/Mercury(II)_fulminate |
Soluble in excess KI( Potassium iodide ) forming soluble complex K 2 [HgI 4 ]( Potassium tetraiodomercurate(II) ) also known as Nessler's reagent
Mercury(II) iodide is a chemical compound with the molecular formula Hg I 2 . It is typically produced synthetically but can also be found in nature as the extremely rare mineral coccinite . Unlike the related mercury(II) chloride it is hardly soluble in water (<100 ppm).
Mercury(II) iodide is produced by adding an aqueous solution of potassium iodide to an aqueous solution of mercury(II) chloride with stirring; the precipitate is filtered off, washed and dried at 70 °C.
Mercury(II) iodide displays thermochromism ; when heated above 126 °C (400 K) it undergoes a phase transition , from the red alpha crystalline form to a pale yellow beta form. As the sample cools, it gradually reacquires its original colour. It has often been used for thermochromism demonstrations. [ 2 ] A third form, which is orange, is also known; this can be formed by recrystallisation and is also metastable , eventually converting back to the red alpha form. [ 3 ] The various forms can exist in a diverse range of crystal structures and as a result mercury(II) iodide possesses a surprisingly complex phase diagram . [ 4 ]
Mercury(II) iodide is used for preparation of Nessler's reagent , used for detection of presence of ammonia .
Mercury(II) iodide is a semiconductor material , used in some x-ray and gamma ray detection and imaging devices operating at room temperatures. [ 5 ]
In veterinary medicine , mercury(II) iodide is used in blister ointments in exostoses , bursal enlargement, etc. [ citation needed ]
It can appear as a precipitate in many reactions. | https://en.wikipedia.org/wiki/Mercury(II)_iodide |
Mercury(II) nitrate is an inorganic compound with the chemical formula Hg ( N O 3 ) 2 . It is the mercury (II) salt of nitric acid HNO 3 . It contains mercury(II) cations Hg 2+ and nitrate anions NO − 3 , and water of crystallization H 2 O in the case of a hydrous salt. Mercury(II) nitrate forms hydrates Hg(NO 3 ) 2 · x H 2 O . Anhydrous and hydrous salts are colorless or white soluble crystalline solids that are occasionally used as a reagents . Mercury(II) nitrate is made by treating mercury with hot concentrated nitric acid. Neither anhydrous nor monohydrate has been confirmed by X-ray crystallography . [ 1 ] The anhydrous material is more widely used. [ clarification needed ]
Mercury(II) nitrate is used as an oxidizing agent in organic synthesis , as a nitrification agent, as an analytical reagent in laboratories, in the manufacture of felt , and in the manufacture of mercury fulminate . [ 2 ] An alternative qualitative Zeisel test can be done with the use of mercury(II) nitrate instead of silver nitrate, leading to the formation of scarlet red mercury(II) iodide . [ 3 ]
Mercury compounds are highly toxic. The use of this compound by hatters and the subsequent mercury poisoning of said hatters is a common theory of where the phrase " mad as a hatter " came from. | https://en.wikipedia.org/wiki/Mercury(II)_nitrate |
Mercury(II) oxide , also called mercuric oxide or simply mercury oxide , is the inorganic compound with the formula Hg O . It has a red or orange color. Mercury(II) oxide is a solid at room temperature and pressure. The mineral form montroydite is very rarely found.
An experiment for the preparation of mercuric oxide was first described by 11th century Arab-Spanish alchemist, Maslama al-Majriti , in Rutbat al-hakim. [ 6 ] It was historically called red precipitate (as opposed to white precepitate being the mercuric amidochloride ).
In 1774, Joseph Priestley discovered that oxygen was released by heating mercuric oxide, although he did not identify the gas as oxygen (rather, Priestley called it " dephlogisticated air," as that was the paradigm that he was working under at the time). [ 7 ]
The red form of HgO can be made by heating Hg in oxygen at roughly 350 °C, or by pyrolysis of Hg(NO 3 ) 2 . [ 8 ] The yellow form can be obtained by precipitation of aqueous Hg 2+ with alkali. [ 8 ] The difference in color is due to particle size; both forms have the same structure consisting of near linear O-Hg-O units linked in zigzag chains with an Hg-O-Hg angle of 108°. [ 8 ]
HgO is soluble in many conventional strong acids through protonation of the anion. [ 9 ] The exceptions include acids which form insoluble mercury(II) salts, like mercury(II) iodide in the case of hydroiodic acid . Dissolution is also possible through complexation of the cation; e.g. cyanide ligands form stable water soluble mercury(II) complexes.
Under atmospheric pressure mercuric oxide has two crystalline forms: one is called montroydite ( orthorhombic , 2/m 2/m 2/m, Pnma), and the second is analogous to the sulfide mineral cinnabar ( hexagonal ,
hP6, P3221); both are characterized by Hg-O chains. [ 10 ] At pressures above 10 GPa both structures convert to a tetragonal form. [ 1 ]
Mercury oxide is sometimes used in the production of mercury as it decomposes quite easily. When it decomposes, oxygen gas is generated. [ citation needed ]
It is also used as a material for cathodes in mercury batteries . [ 11 ]
Mercury oxide is a highly toxic substance which can be absorbed into the body by inhalation of its aerosol, through the skin and by ingestion. The substance is irritating to the eyes, the skin and the respiratory tract and may have effects on the kidneys, resulting in kidney impairment. In the food chain important to humans, bioaccumulation takes place, specifically in aquatic organisms. The substance is banned as a pesticide in the EU . [ 12 ]
Evaporation at 20 °C is negligible. HgO decomposes on exposure to light or on heating above 500 °C. Heating produces highly toxic mercury fumes and oxygen, which increases the fire hazard. Mercury(II) oxide reacts violently with reducing agents, chlorine, hydrogen peroxide, magnesium (when heated), disulfur dichloride and hydrogen trisulfide. Shock-sensitive compounds are formed with metals and elements such as sulfur and phosphorus. [ 13 ] | https://en.wikipedia.org/wiki/Mercury(II)_oxide |
Mercury(IV) fluoride , HgF 4 , is a purported compound, the first to be reported with mercury in the +4 oxidation state . Mercury, like the other group 12 elements ( cadmium and zinc ), has an s 2 d 10 electron configuration and generally only forms bonds involving its 6s orbital. This means that the highest oxidation state mercury normally attains is +2, and for this reason it is sometimes considered a post-transition metal instead of a transition metal . HgF 4 was first reported from experiments in 2007, but its existence remains disputed; experiments conducted in 2008 could not replicate the compound. [ 1 ] [ 2 ]
Speculation about higher oxidation states for mercury had existed since the 1970s, and theoretical calculations in the 1990s predicted that it should be stable in the gas phase, with a square-planar geometry consistent with a formal d 8 configuration. However, experimental proof remained elusive until 2007, when HgF 4 was first prepared using solid neon and argon for matrix isolation at a temperature of 4 K . The compound was detected using infrared spectroscopy . [ 3 ] [ 4 ]
However, the compound's synthesis has not been replicated in other labs, and more recent theoretical studies cast doubt on the possible existence of mercury(IV) (and copernicium(IV)) fluoride. Dirac-Hartree-Fock computations including both relativistic effects and electron correlation suggest that an HgF 4 compound would be unbound by about 2 eV (and CnF 4 by 14 eV). [ 5 ]
Theoretical studies suggest that mercury is unique among the natural elements of group 12 in forming a tetrafluoride , and attribute this observation to relativistic effects . According to calculations, the tetrafluorides of the "less relativistic" elements cadmium and zinc are unstable and eliminate a fluorine molecule, F 2 , to form the metal difluoride complex. [ citation needed ] On the other hand, the tetrafluoride of the "more relativistic" synthetic element 112, copernicium , is predicted to be more stable. [ 6 ] [ failed verification ]
Subsequent density functional theory and coupled cluster calculations indicated that bonding in HgF 4 (if it really exists) involves d orbitals. This has led to the suggestion that mercury should be considered a transition metal (the group 12 metals are sometimes excluded from the transition metals because they do not oxidize beyond +2). [ 7 ] Chemical historian William B. Jensen has argued that the compound alone is insufficient to reclassify the metal, because HgF 4 represents at best a non-equilibrium transient state . [ 8 ]
HgF 4 is produced by the reaction of elemental mercury with fluorine :
HgF 4 is only stable in matrix isolation at 4 K (−269 °C); upon heating, or if the HgF 4 molecules touch each other, it decomposes to mercury(II) fluoride and fluorine:
HgF 4 is a diamagnetic , square planar molecule. The mercury atom has a formal 6s 2 5d 8 6p 6 electron configuration, and as such obeys the octet rule but not the 18-electron rule . HgF 4 is isoelectronic with the tetrafluoroaurate anion, AuF − 4 , and is valence isoelectronic with the tetrachloroaurate ( AuCl − 4 ), tetrabromoaurate ( AuBr − 4 ), and tetrachloroplatinate ( PtCl 2− 4 ) anions. | https://en.wikipedia.org/wiki/Mercury(IV)_fluoride |
Mercury is a freeware developed by the Cambridge Crystallographic Data Centre , originally designed as a crystal structure visualization tool. Mercury helps three dimensional visualization of crystal structure and assists in drawing and analysis of crystal packing and intermolecular interactions . [ 1 ] Current version Mercury can read " cif ", " .mol ", ".mol2", " .pdb ", ".res", ".sd" and ".xyz" types of files. Mercury has its own file format with filename extension ".mryx". [ 2 ]
The Cambridge Crystallographic Data Centre (CCDC) developed and launched two programs, named ConQuest and Mercury [ 3 ] that run under Windows and various types of Unix , including Linux. ConQuest as a search interface to the Cambridge Structural Database (CSD) , with Fortran code that performs a large variety of tasks, such as two dimensional and three-dimensional substructure searching. Mercury introduced as a crystal structure visualizer having the facilities for exploring the intermolecular contacts. The mercury program entirely written in object oriented C++ . The C++ Qt library is used for building the GUI and OpenGL for three-dimensional graphics rendering. The primary objective of the first generation Mercury is to provide the three dimensional viewing of crystal structures with .MOL2, .PDB, .CIF, .MOL file formats. [ 4 ] The first version have approximately 2800 users signed on to the Mercury e-mail announcement list. [ 5 ] Mercury 2.0 launched in 2008, with additional tools to interpret and compare packing trends in crystal structures. Mercury version released in 2015 and later provides an additional functionality to generate 3D print . [ 6 ] The current Version 4.0 of Mercury developed its visual interface up to a greater extent by comparing with its old versions. [ 2 ]
Mercury is available as a free download software and full version Mercury with more advanced features available with a CSD licence, advanced features are disabled in the absence of such a licence. Cambridge Crystallographic Data Centre (CCDC) provides CSD licence to academic institutions. [ 2 ] | https://en.wikipedia.org/wiki/Mercury_(crystallography) |
The mercury beating heart is an electrochemical redox reaction between the elements mercury , iron and chromium. The reaction causes a blob of mercury in water to oscillate.
The observeable reaction demonstrates an effect of a non-homogeneous electrical double layer . [ 1 ] [ 2 ] It is often used as a classroom demonstration.
In the experiment a droplet of mercury is placed in a watch glass , immersed in an electrolyte such as sulfuric acid which contains an oxidizing agent such as hydrogen peroxide , potassium permanganate , or potassium dichromate. The tip of an iron nail is positioned almost touching the mercury. If the position of the nail tip is just right, the mercury blob begins to oscillate, changing shape.
In one variation the mechanism is thought to be as follows:
The dichromate oxidizes the mercury, forming a layer of mercury oxide. In the process the dichromate is reduced to the chromium(III) ion. The oxidized layer on the mercury reduces the surface tension of the blob and the blob flattens out coming in contact with the iron nail. Then the mercury sulfate oxidizes the iron to the iron(II) ion, and in the process is reduced back to metallic mercury. Once there is no oxide coating left on the mercury blob, the surface tension increases and the blob rounds up and loses contact with the nail, ready to start the process over again.
The net reaction is that the dichromate oxidizes the iron. This favorable reaction drives the mercury oxidations/reductions and the oscillations in shape. When the dichromate is fully reduced, the reaction stops.
There may be other mechanisms involved, however. Lin et al. appear to report that the oscillations occur without the presence of the oxidizing agent, though the mercury does not appear to get an oxidizing layer on it and the oscillations are much weaker.
An electrical double layer forms between the surface of the mercury droplet and the electrolyte solution. At rest this layer is uniform. When the iron tip is introduced, a redox reaction starts in which iron is oxidized to the ferric ion and the oxidizing reagent is spent (e.g. when hydrogen peroxide together with hydronium ions is reduced to water). Because the oxidation only takes place in the vicinity of the tip, and the reduction process covers the whole droplet surface, the surface tension is no longer homogeneous; this results in the observed oscillations. [ 3 ]
Although this reaction is mediated by changes in surface tension, it is very similar in mechanism to other chemical oscillators such as the Belousov–Zhabotinsky reaction , which has several intermediate redox reactions driven by the oxidation of malate by bromine.
The mercury beating heart was first observed in the year 1800 by Alessandro Volta and William Henry . The chemical phenomenon in the form best known today was first described by German chemist Friedlieb Ferdinand Runge , the discoverer of caffeine . [ 4 ] | https://en.wikipedia.org/wiki/Mercury_beating_heart |
The presence of mercury in corn syrup was a health controversy that arose due to several studies that found that mercury residues in high-fructose corn syrups (HFCS) used in food products. [ 1 ] [ 2 ] [ 3 ] This was significant due to the toxic nature of mercury and its association with learning disabilities and heart disease. [ 4 ] [ 5 ] [ 6 ]
History of analyses
Three separate studies between 2009-2010 found mercury in high fructose corn syrup or food products containing high fructose corn syrup. [ 1 ] [ 2 ] [ 3 ] The first major study was led by United States Food and Drug Administration (FDA) whistleblower Renee Dufault , who began her research while serving as an Environmental Health Officer (EHO) at the FDA in 2004. [ 1 ] [ 7 ] [ 8 ] [ 9 ] Dufault left the agency to publish her findings, which were made public a year after she left the FDA. [ 4 ] [ 10 ] [ 11 ] High fructose corn syrup (HFCS) samples were collected by an FDA field investigator without warning from three separate corn refiners during the week of February 17-24, 2005. [ 1 ] Of the twenty samples analyzed, mercury residues were found in nine and the results of the study were published in the peer reviewed journal Environmental Health in 2009. [ 1 ]
In a follow-up study led by David Wallinga at the Institute for Agriculture and Trade Policy (IATP) , fifty-five foods with HFCS listed as the first or second ingredient, were analyzed for mercury. [ 2 ] Of the fifty-five products, mercury was detected in seventeen samples and the results were published in an institutional report in January 2009. [ 12 ] [ 2 ] [ 13 ]
The third study was led by Karen Rideout at the National Collaborating Centre for Environmental Health in Canada in 2010. [ 3 ] Rideout’s team collected nine Canadian national brand syrup products containing HFCS (known as glucose-fructose) as the first or second ingredient from major chain grocery stores in Vancouver. [ 3 ] All of the samples collected by Rideout’s team were analyzed for mercury and concentrations ranged from 0.220 -1.92 ug/l. [ 3 ] The results were peer-reviewed and published as a comment on the article published by Dufault and her collaborators in Environmental Health in 2010, [ 3 ] a year after the Corn Refiners Association had claimed that there were no quantifiable levels of mercury in high-fructose corn syrup manufactured in US and Canada production facilities. [ 14 ]
The presence of mercury in HFCS has been attributed to the use of mercury-grade caustic soda and mercury-grade hydrogen chloride in the corn syrup manufacturing process. [ 1 ] [ 2 ] [ 15 ] Both chemicals are found to contain mercury residues when derived from the mercury-cell chlor-alkali chemical manufacturing process. [ 16 ] Another source of the mercury residue in HFCS, however, is the routine application of mercuric chloride (0.01 M) on the corn during the starch extraction process. [ 17 ] | https://en.wikipedia.org/wiki/Mercury_in_corn_syrup |
Porosimetry is an analytical technique used to determine various quantifiable aspects of a material's porous structure, such as pore diameter , total pore volume , surface area , and bulk and absolute densities .
The technique involves the intrusion of a non-wetting liquid (often mercury ) at high pressure into a material through the use of a porosimeter . The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid's surface tension .
A force balance equation known as Washburn's equation for the above material having cylindrical pores is given as: [ 1 ]
Since the technique is usually performed within a vacuum , the initial gas pressure is zero. The contact angle of mercury with most solids is between 135° and 142°, so an average of 140° can be taken without much error. The surface tension of mercury at 20 °C under vacuum is 480 mN / m . With the various substitutions, the equation becomes:
As pressure increases, so does the cumulative pore volume. From the cumulative pore volume, one can find the pressure and pore diameter where 50% of the total volume has been added to give the median pore diameter.
This article about materials science is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Mercury_intrusion_porosimetry |
Mercury nano-trap water filtration is a method of decontaminating water of mercury. Mercury is one of the most notorious metal pollutants present in food, water, air and soil, but the process of eliminating it is limited. [ 1 ] Heavy metals such as mercury are formed on the Earth's crust and made into solutions with ground water through certain natural processing and pH changes occurring in the soil. [ 2 ] There are traditional methods that are used to extract mercury from the natural water sources and industrial waste water, such as chemical precipitation , amalgamation , reverse osmosis , membrane filtration and photochemical methods. [ 3 ] [ 4 ] [ 5 ] However, these methods are expensive, time-consuming, and inefficient, hence the need for a nanofiltration technology that overcomes all of these issues. Nanofiltration technology is very efficient in removal of mercury species due to its characteristics of having high surface area-to-volume and the fact that it is easily chemically functionalized. [ 6 ] Additionally, Brownian motion of nanomaterials allows them to scan large volume of solvent in short times. There are many copolymer nanoparticles (NPs) that can be used as scavengers to eliminate mercury species via redox reactions such as selenium NPs, manganese dioxide nanowhiskers, carbon nanotube−silverNP composites, silver NPs, silver NP-decorated silicaspheres, gold NP-based materials. Among these adsorbents, citrate-capped gold NP-based materials have been used intensively to capture mercury species from nature water.
High-resolution transmission electron microscopy (HRTEM) is used to obtain the HRTEM images (FEI Tecnai G2 F20 S-Twin working at 200 kV) and double beam UV-visible spectrophotometer is used for the extinction, or removal, of the NPs gold spectra. [ 7 ] A combination of scanning electron microscope and an energy dispersive X-ray detector (EDX) for obtaining EDX spectra while sub micrometer particle size analyzer and Delsa nano zeta potential measure the potential of gold NPs. Inductively coupled plasma-mass spectrometry (ICP-MS) measures the quantification of mercury species in gold NPs while the linear range of Hg2+ should be between 2.5 and 50 nM. The powder X-ray diffraction is measured using a diffractometer with Cu Kα radiation. A superconducting quantum interference device measures the magnetometry while the Fourier-transformed infrared spectroscopy spectrum is measured by a Nicolet 6700 FT-IR spectrometer.
The application of an external magnetic field helps reduce the Fe3O4 NP compositions and can be used to remove the citrate capped gold NPs or Tween 20- Au NPs. The quantity of the Hg2+ is determined by ICP-MS. Elimination efficiency is calculated as:
The equilibrium absorption capacity was determined by:
where qe is the equilibrium absorption capacity, Ce is the concentration of Hg2+, V is the volume of the solution and W is the weight of Tween 20-Au NPs. Other metals in place of mercury can be used to determine the reusability of the nanofiltration method. A simple nanofiltration process is illustrated in Figure 2.
When purifying mercury from sea water, a decrease in the volume to surface area ratio leads to a decline in the efficiency of the elimination.7 Cost efficiency of the nanotrap process is indicated in the fact that the materials can be reused to recycle more water. Tween 20-Au NPs are rapid, efficient and selective in capturing Hg2+ in high salt water concentration when the gold NPs catalyze the citrate ion induced reduction of Hg2+ to Hg0. [ 8 ] ICP-MS is used to quantify the Hg2+ concentration to determine the level of elimination efficiency. Figure 3 shows an illustration process of the nano trap filtration process.
The use of nanomaterials in removing the mercury from water is advantageous because of the high surface area to volume ratio and the fact that they are easily chemically functionalized. Nanomaterials capture five times more mercury than the maximum mercury captured predicted through the use of previous mercury filtration systems. [ 9 ] Compared to other NP-based methods for Hg2+ removal from high salt matrix, Tween 20-Au NPs is a rapid and selective technique with high efficiency. [ 10 ] For purification of drinking water, it is advisable to use nano-tablets with the filters because they are easily accessible and cost efficient. [ 11 ] [ 12 ] | https://en.wikipedia.org/wiki/Mercury_nano-trap_water_filtration |
The mercury probe is an electrical probing device to make rapid, non-destructive contact to a sample for electrical characterization. Its primary application is semiconductor measurements where otherwise time-consuming metallizations or photolithographic processing are required to make contact to a sample. These processing steps usually take hours and have to be avoided where possible to reduce device processing times.
The mercury probe applies mercury contacts of well-defined areas to a flat sample. The nature of the mercury-sample contacts and the instrumentation connected to the mercury probe define the application. If the mercury-sample contact is ohmic (non-rectifying) then current-voltage instrumentation can be used to measure resistance , leakage currents, or current-voltage characteristics. Resistance can be measured on bulk samples or on thin films. The thin films can be composed of any material that does not react with mercury. Metals, semiconductors, oxides, and chemical coatings have all been measured successfully. [ 1 ]
The mercury probe is a versatile tool for investigation of parameters of conducting, insulating and semiconductor materials.
One of the first successful mercury probe applications was the characterization of epitaxial layers grown on silicon . [ 2 ] It is critical to device performance to monitor the doping level and thickness of an epitaxial layer. Prior to the mercury probe, a sample had to undergo a metallization process, which could take hours. A mercury probe connected to capacitance-voltage doping profile instrumentation could measure an epitaxial layer as soon as it came out of the epitaxial reactor. The mercury probe formed a Schottky barrier of well-defined area that could be measured as easily as a conventional metallized contact.
Another mercury probe application popular for it speed is oxide characterization. [ 3 ] The mercury probe forms a gate contact and enables measurement of the capacitance-voltage or current-voltage parameters of the mercury-oxide-semiconductor structure. Using this device, material parameters such as permittivity , doping, oxide charge, and dielectric strength may be evaluated. The contact area of a mercury droplet resting on a semiconductor can be modified by electrowetting , [ 4 ] meaning that accurate parameter extraction may need to take this effect into account.
A mercury probe with concentric dot and ring contacts as well as a back contact extends mercury probe applications to silicon on insulator (SOI) structures, where a pseudo-MOSFET device is formed. [ 5 ] This Hg-FET can be used to study mobility, interface trap density, and transconductance .
The same mercury-sample structures can be measured with capacitance-voltage instrumentation to monitor permittivity and thickness of dielectric materials. These measurements are a convenient gauge for development of novel dielectrics of both low-k and high-k types.
If the mercury-sample contact is rectifying then a diode has formed and offers other measurement possibilities. Current-voltage measurements of the diode can reveal properties of the semiconductor such as breakdown voltage and lifetime. Capacitance-voltage measurements allow computation of the semiconductor doping level and uniformity. These measurements are successfully made on many materials including SiC , GaAs , GaN , InP , CdS , and InSb . | https://en.wikipedia.org/wiki/Mercury_probe |
Mercury silvering or fire gilding is a silvering technique for applying a thin layer of precious metal such as silver or gold (mercury gilding ) to a base metal object. The process was invented during the Middle Ages and is documented in Vannoccio Biringuccio's 1540 book De la pirotechnia . [ 1 ] An amalgam of mercury and the precious metal is prepared and applied to the object which is then heated, sometimes in oil , vaporizing most of the mercury. The technique is dangerous since mercury is highly toxic, especially in its vapor phase. Mercury silvering can be detected through a variety of methods.
The technique was also used in Asia, for example tokin plating in Edo-period Japan. [ 2 ]
This industry -related article is a stub . You can help Wikipedia by expanding it .
This metalworking article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Mercury_silvering |
Planetary symbols are used in astrology and traditionally in astronomy to represent a classical planet (which includes the Sun and the Moon) or one of the modern planets. The classical symbols were also used in alchemy for the seven metals known to the ancients , which were associated with the planets , and in calendars for the seven days of the week associated with the seven planets. The original symbols date to Greco-Roman astronomy ; their modern forms developed in the 16th century, and additional symbols would be created later for newly discovered planets.
The seven classical planets, their symbols, days and most commonly associated planetary metals are:
The International Astronomical Union (IAU) discourages the use of these symbols in modern journal articles, and their style manual proposes one- and two-letter abbreviations for the names of the planets for cases where planetary symbols might be used, such as in the headings of tables. [ 1 ] The modern planets with their traditional symbols and IAU abbreviations are:
The symbols of Venus and Mars are also used to represent female and male in biology following a convention introduced by Carl Linnaeus in the 1750s.
The origins of the planetary symbols can be found in the attributes given to classical deities. The Roman planisphere of Bianchini (2nd century, currently in the Louvre , inv. Ma 540) [ 2 ] shows the seven planets represented by portraits of the seven corresponding gods, each a bust with a halo and an iconic object or dress, as follows: Mercury has a caduceus and a winged cap; Venus has a necklace and a shining mirror; Mars has a war-helmet and a spear; Jupiter has a laurel crown and a staff; Saturn has a conical headdress and a scythe; the Sun has rays emanating from his head; and the Moon has a crescent atop her head.
The written symbols for Mercury, Venus, Jupiter, and Saturn have been traced to forms found in late Greek papyri. [ 3 ] [ b ]
Early forms are also found in medieval Byzantine codices which preserve horoscopes. [ 4 ]
A diagram in the astronomical compendium by Johannes Kamateros (12th century) closely resembles the 11th-century forms shown above, with the Sun represented by a circle with a single ray, Jupiter by the letter zeta (the initial of Zeus , Jupiter's counterpart in Greek mythology), Mars by a round shield in front of a diagonal spear, and the remaining classical planets by symbols resembling the modern ones, though without the crosses seen in modern versions of Mercury, Venus, Jupiter and Saturn. [ citation needed ] These crosses first appear in the late 15th or early 16th century. According to Maunder, the addition of crosses appears to be "an attempt to give a savour of Christianity to the symbols of the old pagan gods." [ 5 ] The modern forms of the classical planetary symbols are found in a woodcut of the seven planets in a Latin translation of Abu Ma'shar al-Balkhi 's De Magnis Coniunctionibus printed at Venice in 1506, represented as the corresponding gods riding chariots. [ 6 ]
Earth is not one of the classical planets, as "planets" by definition were "wandering stars" as seen from Earth's surface.
Earth's status as planet is a consequence of heliocentrism in the 16th century.
Nonetheless, there is a pre-heliocentric symbol for the world, now used as a planetary symbol for the Earth. This is a circle crossed by two lines, horizontal and vertical, representing the world divided by four rivers into the four quarters of the world (often translated as the four "corners" of the world): . A variant, now obsolete, had only the horizontal line: . [ 7 ]
A medieval European symbol for the world – the globus cruciger , (the globe surmounted by a Christian cross ) – is also used as a planetary symbol; it resembles an inverted symbol for Venus.
The planetary symbols for Earth are encoded in Unicode at U+1F728 🜨 ALCHEMICAL SYMBOL FOR VERDIGRIS and U+2641 ♁ EARTH .
The crescent shape has been used to represent the Moon since antiquity. In classical antiquity, it is worn by lunar deities ( Selene/Luna , Artemis/Diana , Men , etc.) either on the head or behind the shoulders, with its horns pointing upward.
The representation of the moon as a simple crescent with the horns pointing to the side (as a heraldic crescent increscent or crescent decrescent ) is attested from late Classical times.
The same symbol can be used in a different context not for the Moon itself but for a lunar phase , as part of a sequence of four symbols
for "new moon" (U+1F311 🌑︎), "waxing" (U+263D ☽︎), "full moon" (U+1F315 🌕︎) and "waning" (U+263E ☾︎).
The symbol ☿ for Mercury is a caduceus (a staff intertwined with two serpents), a symbol associated with Mercury / Hermes throughout antiquity. Some time after the 11th century, a cross was added to the bottom of the staff to make it seem more Christian. [ 3 ]
The ☿ symbol has also been used to indicate intersex , transgender , or non-binary gender . [ 8 ] A related usage is for the 'worker' or 'neuter' sex among social insects that is neither male nor (due to its lack of reproductive capacity) fully female, such as worker bees . [ 9 ] It was also once the designated symbol for hermaphroditic or 'perfect' flowers , [ 10 ] but botanists now use ⚥ for these. [ 11 ]
Its Unicode codepoint is U+263F ☿ MERCURY .
The Venus symbol , ♀, consists of a circle with a small cross below it.
It has been interpreted as a depiction of the hand-mirror of the goddess, which may also explain Venus's association with the planetary metal copper, as mirrors in antiquity were made of polished copper, [ 12 ] [ d ] though this is not certain. [ 3 ] In the Greek Oxyrhynchus Papyri 235 , the symbols for Venus and Mercury did not have the cross on the bottom stem, [ 3 ] and Venus appears without the cross (⚲) in Johannes Kamateros (12th century). [ citation needed ]
In botany and biology , the symbol for Venus is used to represent the female sex , alongside the symbol for Mars representing the male sex, [ 13 ] following a convention introduced by Linnaeus in the 1750s. [ 10 ] [ e ] Arising from the biological convention, the symbol also came to be used in sociological contexts to represent women or femininity . This gendered association of Venus and Mars has been used to pair them heteronormatively , describing women and men stereotypically as being so different that they can be understood as coming from different planets, an understanding popularized in 1992 by the book titled Men Are from Mars, Women Are from Venus . [ 14 ] [ 15 ]
Unicode encodes the symbol as U+2640 ♀ FEMALE SIGN , in the Miscellaneous Symbols block. [ f ]
The modern astronomical symbol for the Sun, the circumpunct ( U+2609 ☉ SUN ), was first used in the Renaissance . It possibly represents Apollo's golden shield with a boss ; it is unknown if it traces descent from the nearly identical Egyptian hieroglyph for the Sun.
Bianchini's planisphere , produced in the 2nd century, shows a circlet with rays radiating from it. [ 5 ] [ 2 ] In late Classical times, the Sun is attested as a circle with a single ray. A diagram in Johannes Kamateros' 12th century Compendium of Astrology shows the same symbol. [ 18 ] This older symbol is encoded by Unicode as U+1F71A 🜚 ALCHEMICAL SYMBOL FOR GOLD in the Alchemical Symbols block. Both symbols have been used alchemically for gold, as have more elaborate symbols showing a disk with multiple rays or even a face.
The Mars symbol , ♂, is a depiction of a circle with an arrow emerging from it, pointing at an angle to the upper right in Europe and to the upper left in India. [ 19 ] [ 20 ] It is also the old and obsolete symbol for iron in alchemy. In zoology and botany, it is used to represent the male sex (alongside the astrological symbol for Venus representing the female sex), [ 13 ] following a convention introduced by Linnaeus in the 1750s. [ 10 ]
The symbol dates from at latest the 11th century, at which time it was an arrow across or through a circle, thought to represent the shield and spear of the god Mars; in the medieval form, for example in the 12th-century Compendium of Astrology by Johannes Kamateros, the spear is drawn across the shield. [ 18 ] The Greek Oxyrhynchus Papyri show a different symbol, [ 3 ] perhaps simply a spear. [ 2 ]
Its Unicode codepoint is U+2642 ♂ MALE SIGN ( ♂ ).
The symbol for Jupiter , ♃, was originally a Greek zeta, Ζ , with a stroke indicating that it is an abbreviation (for Zeus , the Greek equivalent of Roman Jupiter).
Its Unicode codepoint is U+2643 ♃ JUPITER .
Salmasius and earlier attestations show that the symbol for Saturn, ♄, derives from the initial letters ( Kappa , rho ) of its ancient Greek name Κρόνος ( Kronos ), with a stroke to indicate an abbreviation . [ 10 ] By the time of Kamateros (12th century), the symbol had been reduced to a shape similar to a lower-case letter eta η, with the abbreviation stroke surviving (if at all) in the curl on the bottom-right end.
Its Unicode codepoint is U+2644 ♄ SATURN .
The symbols for Uranus were created shortly after its discovery in 1781. One symbol, ⛢, invented by J. G. Köhler and refined by Bode , was intended to represent the newly discovered metal platinum ; since platinum, commonly called white gold, was found by chemists mixed with iron, the symbol for platinum combines the alchemical symbols for iron , ♂, and gold , ☉. [ 21 ] [ 22 ] Gold and iron are the planetary metals for the Sun and Mars, and so share their symbols. Several orientations were suggested, but an upright arrow is now universal.
Another symbol, , was suggested by Lalande in 1784. In a letter to Herschel , Lalande described it as "a globe surmounted by the first letter of your name". [ 23 ] The platinum symbol tends to be used by astronomers, and the monogram by astrologers. [ 24 ]
For use in computer systems, the symbols are encoded U+26E2 ⛢ ASTRONOMICAL SYMBOL FOR URANUS and U+2645 ♅ URANUS .
Several symbols were proposed for Neptune to accompany the suggested names for the planet. Claiming the right to name his discovery, Urbain Le Verrier originally proposed to name the planet for the Roman god Neptune [ 25 ] and the symbol of a trident , [ 26 ] while falsely stating that this had been officially approved by the French Bureau des Longitudes . [ 25 ] In October, he sought to name the planet Leverrier , after himself, and he had loyal support in this from the observatory director, François Arago , [ 27 ] who in turn proposed a new symbol for the planet, . [ 28 ] However, this suggestion met with resistance outside France, [ 27 ] and French almanacs quickly reintroduced the name Herschel for Uranus , after that planet's discoverer Sir William Herschel , and Leverrier for the new planet, [ 29 ] though it was used by anglophone institutions. [ 30 ] Professor James Pillans of the University of Edinburgh defended the name Janus for the new planet, and proposed a key for its symbol. [ 26 ] Meanwhile, Struve presented the name Neptune on December 29, 1846, to the Saint Petersburg Academy of Sciences . [ 31 ] In August 1847, the Bureau des Longitudes announced its decision to follow prevailing astronomical practice and adopt the choice of Neptune , with Arago refraining from participating in this decision. [ 32 ] The planetary symbol was Neptune's trident , with the handle stylized either as a crossed , following Mercury, Venus, Jupiter, Saturn, and the asteroids, or as an orb , following the symbols for Uranus, Earth, and Mars. [ 7 ] The crossed variant is the more common today.
For use in computer systems, the symbols are encoded as U+2646 ♆ NEPTUNE and U+2BC9 ⯉ NEPTUNE FORM TWO .
Pluto was almost universally considered a planet from its discovery in 1930 until its re-classification as a dwarf planet (planetoid) by the IAU in 2006. Planetary geologists [ 33 ] and astrologers continue to treat it as a planet. The original planetary symbol for Pluto was , a monogram of the letters P and L. Astrologers generally use a bident with an orb. NASA has used the bident symbol since Pluto's reclassification. These symbols are encoded as U+2647 ♇ PLUTO and U+2BD3 ⯓ PLUTO FORM TWO .
In the 19th century, planetary symbols for the major asteroids were also in use, including 1 Ceres (a reaper's sickle , encoded U+26B3 ⚳ CERES ), 2 Pallas (a lance, U+26B4 ⚴ PALLAS ) and 3 Juno (a sceptre, encoded U+26B5 ⚵ JUNO ).
Encke (1850) used symbols for 5 Astraea , 6 Hebe , 7 Iris , 8 Flora and 9 Metis in the Berliner Astronomisches Jahrbuch . [ 34 ]
In the late 20th century, astrologers abbreviated the symbol for 4 Vesta (the sacred fire of Vesta , encoded U+26B6 ⚶ VESTA ), [ 35 ] and introduced new symbols for 5 Astraea ( , a stylised % sign, shift-5 on QWERTY keyboards for asteroid 5), 10 Hygiea encoded U+2BDA ⯚ HYGIEA ) [ 36 ] and for 2060 Chiron , discovered in 1977 (a key, U+26B7 ⚷ CHIRON ). [ 35 ] Chiron's symbol was adapted as additional centaurs were discovered; symbols for 5145 Pholus and 7066 Nessus have been encoded in Unicode. [ 36 ] The abbreviated Vesta symbol is now universal, and the astrological symbol for Pluto has been used astronomically for Pluto as a dwarf planet. [ 37 ]
In the early 21st century, symbols for the trans-Neptunian dwarf planets have been given Unicode codepoints , particularly Eris (the hand of Eris , ⯰, but also ⯱), Sedna , Haumea , Makemake , Gonggong , Quaoar and Orcus which are in Unicode. All (except Eris, for which the hand of Eris is a traditional Discordian symbol) were devised by Denis Moskowitz, a software engineer in Massachusetts. [ 37 ] [ 38 ]
Other symbols have also been invented by Moskowitz, for some smaller TNOs as well as many planetary moons. (Charon in particular coincidentally matches a symbol already existing in Unicode as an astrological Pluto.) However, these have not been broadly adopted. [ 37 ] [ 39 ]
From 1845 to 1855, many symbols were created for newly discovered asteroids. But by 1851, the spate of discoveries had led to a general abandonment of these symbols in favour of numbering all asteroids instead. [ 41 ] | https://en.wikipedia.org/wiki/Mercury_symbol |
Meredith Gwynne Evans , FRS (2 December 1904 – 25 December 1952) was a British physical chemist who made important contributions to the theory of chemical reaction rates and reaction mechanisms . Together with Henry Eyring and Michael Polanyi , Meredith Gwynne Evans is one of the founders of the transition state theory .
Meredith Gwynne Evans was born in Atherton , a suburb of Manchester , on 2 December 1904 and the son of Frederick George Evans, an elementary schoolmaster from Pembrokeshire, Wales , and his wife, Margaretta Eleanora Williams. He was the eldest son in a family of three sons and one daughter.
Evans attended the elementary school at which his father was Headmaster, won a County Scholarship to Leigh Grammar School, and was educated at the University of Manchester . [ 1 ]
One of MG's brothers, AG Evans, worked with him and later took the chair of chemistry at University College Cardiff , beating off the challenge of E. A. Moelwyn-Hughes.
Evans was appointed Assistant Lecturer at the University of Manchester from 1929 and elected to membership of the Manchester Literary and Philosophical Society on 29.10.1929. He remained in that post until 1939, when he became Professor of Physical Chemistry at the University of Leeds . One of Evans' students was George Porter , who later noted (in a conversation with Sir John Meurig Thomas ) that Evans was the most brilliant chemist he had ever met. (George Porter went on to win the Nobel Prize for Chemistry in 1967.) Evans returned to the University of Manchester in 1949. He was elected a Fellow of the Royal Society in 1947. [ 1 ]
His earliest research was on problems of adsorption of gases on chabazite and other zeolites. [ 2 ] Later on, he started working on theoretical chemistry and its link with quantum mechanics . In this research field, he worked with Douglas Hartree and Lawrence Bragg to apply quantum mechanics to chemical problems.
In 1933, M. G. Evans was awarded a Rockefeller Scholarship and went to Princeton University to work with Hugh Taylor and Henry Eyring , amongst others. Back in Manchester, Evans became one of Michael Polanyi 's principal coworkers on the development of the transition state theory . In 1935, with only one month difference, both Henry Eyring in Princeton, [ 3 ] and Michael Polanyi and Meredith Gwynne Evans in Manchester [ 4 ] published the founding papers on transition state theory, formulating what is now known as the " Eyring equation " which opened up a new era in the study of chemical kinetics .
Evans died on 25 December 1952 in Manchester . | https://en.wikipedia.org/wiki/Meredith_Gwynne_Evans |
The Meredith effect is a phenomenon whereby the aerodynamic drag produced by a cooling radiator may be offset by careful design of the cooling duct such that useful thrust is produced by the expansion of the hot air in the duct. The effect was discovered in the 1930s and became more important as the speeds of piston-engined aircraft increased over the next decade.
The Meredith effect occurs when air flowing through a duct is heated by a heat-exchanger or radiator containing a hot working fluid. Typically the fluid is a coolant carrying waste heat from an internal combustion engine. [ 2 ]
The duct must be travelling at a significant speed with respect to the air for the effect to occur. Air flowing into the duct meets drag resistance from the radiator surface and is compressed due to the ram air effect . As the air flows through the radiator it is heated , raising its temperature slightly and increasing its volume. The hot, pressurised air then exits through the exhaust duct which is shaped to be convergent, i.e. to narrow towards the rear. This accelerates the air backwards and the reaction of this acceleration against the installation provides a small forward thrust. [ 3 ] The air expands and decreases temperature as it passes along the duct, before emerging to join the external air flow. Thus, the three processes of an open Brayton cycle are achieved: compression, heat addition at constant pressure, and expansion. The thrust obtainable depends upon the pressure ratio between the inside and outside of the duct and the temperature of the coolant. [ 2 ] The higher boiling point of ethylene glycol compared to water allows the air to attain a higher temperature increasing the specific thrust.
If the generated thrust is less than the aerodynamic drag of the ducting and radiator, then the arrangement serves to reduce the net aerodynamic drag of the radiator installation. If the generated thrust exceeds the aerodynamic drag of the installation, then the entire assemblage contributes a net forward thrust to the vehicle.
The Meredith effect inspired the early American work on the aero-thermodynamic duct or ramjet , due to the similarity of their principles of operation. [ 2 ] In more recent times the phenomenon has been utilised in racing cars by mounting the engine cooling radiators in tunnels. [ 4 ]
F. W. Meredith was a British engineer working at the Royal Aircraft Establishment (RAE), Farnborough . Reflecting on the principles of liquid cooling, he realized that what was conventionally regarded as waste heat, to be transferred to the atmosphere by a coolant in a radiator, need not be lost. The heat adds energy to the airflow and, with careful design, this may be used to generate thrust. The work was published in 1936. [ 3 ]
The phenomenon became known as the "Meredith effect" and was quickly adopted by the designers of prototype fighter aircraft then under development, including the Supermarine Spitfire and Hawker Hurricane whose Rolls-Royce PV-12 engine, later named the Merlin , was cooled by ethylene glycol. An early example of a Meredith effect radiator was incorporated in the design of the Spitfire for the first flight of the prototype on 5 March 1936. [ 5 ]
Many engineers did not understand the operating principles of the effect. A common mistake was the idea that the air-cooled radial engine would benefit most, because its fins ran hotter than the radiator of a liquid-cooled engine, with the mistake persisting even as late as 1949. [ 2 ] | https://en.wikipedia.org/wiki/Meredith_effect |
CORDIC ( coordinate rotation digital computer ), Volder's algorithm , Digit-by-digit method , Circular CORDIC ( Jack E. Volder ), [ 1 ] [ 2 ] Linear CORDIC , Hyperbolic CORDIC (John Stephen Walther), [ 3 ] [ 4 ] and Generalized Hyperbolic CORDIC ( GH CORDIC ) (Yuanyong Luo et al.), [ 5 ] [ 6 ] is a simple and efficient algorithm to calculate trigonometric functions , hyperbolic functions , square roots , multiplications , divisions , and exponentials and logarithms with arbitrary base, typically converging with one digit (or bit) per iteration. CORDIC is therefore also an example of digit-by-digit algorithms . CORDIC and closely related methods known as pseudo-multiplication and pseudo-division or factor combining are commonly used when no hardware multiplier is available (e.g. in simple microcontrollers and field-programmable gate arrays or FPGAs), as the only operations they require are additions , subtractions , bitshift and lookup tables . As such, they all belong to the class of shift-and-add algorithms . In computer science, CORDIC is often used to implement floating-point arithmetic when the target platform lacks hardware multiply for cost or space reasons.
Similar mathematical techniques were published by Henry Briggs as early as 1624 [ 7 ] [ 8 ] and Robert Flower in 1771, [ 9 ] but CORDIC is better optimized for low-complexity finite-state CPUs.
CORDIC was conceived in 1956 [ 10 ] [ 11 ] by Jack E. Volder at the aeroelectronics department of Convair out of necessity to replace the analog resolver in the B-58 bomber 's navigation computer with a more accurate and faster real-time digital solution. [ 11 ] Therefore, CORDIC is sometimes referred to as a digital resolver . [ 12 ] [ 13 ]
In his research Volder was inspired by a formula in the 1946 edition of the CRC Handbook of Chemistry and Physics : [ 11 ]
where φ {\displaystyle \varphi } is such that tan ( φ ) = 2 − n {\displaystyle \tan(\varphi )=2^{-n}} , and K n := 1 + 2 − 2 n {\displaystyle K_{n}:={\sqrt {1+2^{-2n}}}} .
His research led to an internal technical report proposing the CORDIC algorithm to solve sine and cosine functions and a prototypical computer implementing it. [ 10 ] [ 11 ] The report also discussed the possibility to compute hyperbolic coordinate rotation , logarithms and exponential functions with modified CORDIC algorithms. [ 10 ] [ 11 ] Utilizing CORDIC for multiplication and division was also conceived at this time. [ 11 ] Based on the CORDIC principle, Dan H. Daggett, a colleague of Volder at Convair, developed conversion algorithms between binary and binary-coded decimal (BCD). [ 11 ] [ 14 ]
In 1958, Convair finally started to build a demonstration system to solve radar fix –taking problems named CORDIC I , completed in 1960 without Volder, who had left the company already. [ 1 ] [ 11 ] More universal CORDIC II models A (stationary) and B (airborne) were built and tested by Daggett and Harry Schuss in 1962. [ 11 ] [ 15 ]
Volder's CORDIC algorithm was first described in public in 1959, [ 1 ] [ 2 ] [ 11 ] [ 13 ] [ 16 ] which caused it to be incorporated into navigation computers by companies including Martin-Orlando , Computer Control , Litton , Kearfott , Lear-Siegler , Sperry , Raytheon , and Collins Radio . [ 11 ]
Volder teamed up with Malcolm McMillan to build Athena , a fixed-point desktop calculator utilizing his binary CORDIC algorithm. [ 17 ] The design was introduced to Hewlett-Packard in June 1965, but not accepted. [ 17 ] Still, McMillan introduced David S. Cochran (HP) to Volder's algorithm and when Cochran later met Volder he referred him to a similar approach John E. Meggitt (IBM [ 18 ] ) had proposed as pseudo-multiplication and pseudo-division in 1961. [ 18 ] [ 19 ] Meggitt's method also suggested the use of base 10 [ 18 ] rather than base 2 , as used by Volder's CORDIC so far. These efforts led to the ROMable logic implementation of a decimal CORDIC prototype machine inside of Hewlett-Packard in 1966, [ 20 ] [ 19 ] built by and conceptually derived from Thomas E. Osborne 's prototypical Green Machine , a four-function, floating-point desktop calculator he had completed in DTL logic [ 17 ] in December 1964. [ 21 ] This project resulted in the public demonstration of Hewlett-Packard's first desktop calculator with scientific functions, the HP 9100A in March 1968, with series production starting later that year. [ 17 ] [ 21 ] [ 22 ] [ 23 ]
When Wang Laboratories found that the HP 9100A used an approach similar to the factor combining method in their earlier LOCI-1 [ 24 ] (September 1964) and LOCI-2 (January 1965) [ 25 ] [ 26 ] Logarithmic Computing Instrument desktop calculators, [ 27 ] they unsuccessfully accused Hewlett-Packard of infringement of one of An Wang 's patents in 1968. [ 19 ] [ 28 ] [ 29 ] [ 30 ]
John Stephen Walther at Hewlett-Packard generalized the algorithm into the Unified CORDIC algorithm in 1971, allowing it to calculate hyperbolic functions , natural exponentials , natural logarithms , multiplications , divisions , and square roots . [ 31 ] [ 3 ] [ 4 ] [ 32 ] The CORDIC subroutines for trigonometric and hyperbolic functions could share most of their code. [ 28 ] This development resulted in the first scientific handheld calculator , the HP-35 in 1972. [ 28 ] [ 33 ] [ 34 ] [ 35 ] [ 36 ] [ 37 ] Based on hyperbolic CORDIC, Yuanyong Luo et al. further proposed a Generalized Hyperbolic CORDIC (GH CORDIC) to directly compute logarithms and exponentials with an arbitrary fixed base in 2019. [ 5 ] [ 6 ] [ 38 ] [ 39 ] [ 40 ] Theoretically, Hyperbolic CORDIC is a special case of GH CORDIC. [ 5 ]
Originally, CORDIC was implemented only using the binary numeral system and despite Meggitt suggesting the use of the decimal system for his pseudo-multiplication approach, decimal CORDIC continued to remain mostly unheard of for several more years, so that Hermann Schmid and Anthony Bogacki still suggested it as a novelty as late as 1973 [ 16 ] [ 13 ] [ 41 ] [ 42 ] [ 43 ] and it was found only later that Hewlett-Packard had implemented it in 1966 already. [ 11 ] [ 13 ] [ 20 ] [ 28 ]
Decimal CORDIC became widely used in pocket calculators , [ 13 ] most of which operate in binary-coded decimal (BCD) rather than binary. This change in the input and output format did not alter CORDIC's core calculation algorithms. CORDIC is particularly well-suited for handheld calculators, in which low cost – and thus low chip gate count – is much more important than speed.
CORDIC has been implemented in the ARM-based STM32G4 , Intel 8087 , [ 43 ] [ 44 ] [ 45 ] [ 46 ] [ 47 ] 80287 , [ 47 ] [ 48 ] 80387 [ 47 ] [ 48 ] up to the 80486 [ 43 ] coprocessor series as well as in the Motorola 68881 [ 43 ] [ 44 ] and 68882 for some kinds of floating-point instructions, mainly as a way to reduce the gate counts (and complexity) of the FPU sub-system.
CORDIC uses simple shift-add operations for several computing tasks such as the calculation of trigonometric, hyperbolic and logarithmic functions, real and complex multiplications, division, square-root calculation, solution of linear systems, eigenvalue estimation, singular value decomposition , QR factorization and many others. As a consequence, CORDIC has been used for applications in diverse areas such as signal and image processing , communication systems , robotics and 3D graphics apart from general scientific and technical computation. [ 49 ] [ 50 ]
The algorithm was used in the navigational system of the Apollo program 's Lunar Roving Vehicle to compute bearing and range, or distance from the Lunar module . [ 51 ] [ 52 ] CORDIC was used to implement the Intel 8087 math coprocessor in 1980, avoiding the need to implement hardware multiplication. [ 53 ]
CORDIC is generally faster than other approaches when a hardware multiplier is not available (e.g., a microcontroller), or when the number of gates required to implement the functions it supports should be minimized (e.g., in an FPGA or ASIC ).
In fact, CORDIC is a standard drop-in IP in FPGA development applications such as Vivado for Xilinx, while a power series implementation is not due to the specificity of such an IP, i.e. CORDIC can compute many different functions (general purpose) while a hardware multiplier configured to execute power series implementations can only compute the function it was designed for.
On the other hand, when a hardware multiplier is available ( e.g. , in a DSP microprocessor), table-lookup methods and power series are generally faster than CORDIC. In recent years, the CORDIC algorithm has been used extensively for various biomedical applications, especially in FPGA implementations. [ citation needed ]
The STM32G4 , STM32U5 and STM32H5 series and certain STM32H7 series of MCUs implement a CORDIC module to accelerate computations in various mixed signal applications such as graphics for human-machine interface and field oriented control of motors. While not as fast as a power series approximation, CORDIC is indeed faster than interpolating table based implementations such as the ones provided by the ARM CMSIS and C standard libraries. [ 54 ] Though the results may be slightly less accurate as the CORDIC modules provided only achieve 20 bits of precision in the result. For example, most of the performance difference compared to the ARM implementation is due to the overhead of the interpolation algorithm, which achieves full floating point precision (24 bits) and can likely achieve relative error to that precision. [ 55 ] Another benefit is that the CORDIC module is a coprocessor and can be run in parallel with other CPU tasks.
The issue with using Taylor series is that while they do provide small absolute error, they do not exhibit well behaved relative error. [ 56 ] Other means of polynomial approximation, such as minimax optimization, may be used to control both kinds of error.
Many older systems with integer-only CPUs have implemented CORDIC to varying extents as part of their IEEE floating-point libraries. As most modern general-purpose CPUs have floating-point registers with common operations such as add, subtract, multiply, divide, sine, cosine, square root, log 10 , natural log, the need to implement CORDIC in them with software is nearly non-existent. Only microcontroller or special safety and time-constrained software applications would need to consider using CORDIC.
CORDIC can be used to calculate a number of different functions. This explanation shows how to use CORDIC in rotation mode to calculate the sine and cosine of an angle, assuming that the desired angle is given in radians and represented in a fixed-point format. To determine the sine or cosine for an angle β {\displaystyle \beta } , the y or x coordinate of a point on the unit circle corresponding to the desired angle must be found. Using CORDIC, one would start with the vector v 0 {\displaystyle v_{0}} :
In the first iteration, this vector is rotated 45° counterclockwise to get the vector v 1 {\displaystyle v_{1}} . Successive iterations rotate the vector in one or the other direction by size-decreasing steps, until the desired angle has been achieved. Each step angle is γ i = arctan ( 2 − i ) {\displaystyle \gamma _{i}=\arctan {(2^{-i})}} for i = 0 , 1 , 2 , … {\displaystyle i=0,1,2,\dots } .
More formally, every iteration calculates a rotation, which is performed by multiplying the vector v i {\displaystyle v_{i}} with the rotation matrix R i {\displaystyle R_{i}} :
The rotation matrix is given by
Using the trigonometric identity :
the cosine factor can be taken out to give:
The expression for the rotated vector v i + 1 = R i v i {\displaystyle v_{i+1}=R_{i}v_{i}} then becomes:
where x i {\displaystyle x_{i}} and y i {\displaystyle y_{i}} are the components of v i {\displaystyle v_{i}} . Setting the angle γ i {\displaystyle \gamma _{i}} for each iteration such that tan ( γ i ) = ± 2 − i {\displaystyle \tan(\gamma _{i})=\pm 2^{-i}} still yields a series that converges to every possible output value. The multiplication with the tangent can therefore be replaced by a division by a power of two, which is efficiently done in digital computer hardware using a bit shift . The expression then becomes:
and σ i {\displaystyle \sigma _{i}} is used to determine the direction of the rotation: if the angle γ i {\displaystyle \gamma _{i}} is positive, then σ i {\displaystyle \sigma _{i}} is +1, otherwise it is −1.
The following trigonometric identity can be used to replace the cosine:
giving this multiplier for each iteration:
The K i {\displaystyle K_{i}} factors can then be taken out of the iterative process and applied all at once afterwards with a scaling factor K ( n ) {\displaystyle K(n)} :
which is calculated in advance and stored in a table or as a single constant, if the number of iterations is fixed. This correction could also be made in advance, by scaling v 0 {\displaystyle v_{0}} and hence saving a multiplication. Additionally, it can be noted that [ 43 ]
to allow further reduction of the algorithm's complexity. Some applications may avoid correcting for K {\displaystyle K} altogether, resulting in a processing gain A {\displaystyle A} : [ 57 ]
After a sufficient number of iterations, the vector's angle will be close to the wanted angle β {\displaystyle \beta } . For most ordinary purposes, 40 iterations ( n = 40) are sufficient to obtain the correct result to the 10th decimal place.
The only task left is to determine whether the rotation should be clockwise or counterclockwise at each iteration (choosing the value of σ {\displaystyle \sigma } ). This is done by keeping track of how much the angle was rotated at each iteration and subtracting that from the wanted angle; then in order to get closer to the wanted angle β {\displaystyle \beta } , if β n + 1 {\displaystyle \beta _{n+1}} is positive, the rotation is clockwise, otherwise it is negative and the rotation is counterclockwise:
The values of γ n {\displaystyle \gamma _{n}} must also be precomputed and stored. For small angles it can be approximated with arctan ( γ n ) ≈ γ n {\displaystyle \arctan(\gamma _{n})\approx \gamma _{n}} to reduce the table size.
As can be seen in the illustration above, the sine of the angle β {\displaystyle \beta } is the y coordinate of the final vector v n , {\displaystyle v_{n},} while the x coordinate is the cosine value.
The rotation-mode algorithm described above can rotate any vector (not only a unit vector aligned along the x axis) by an angle between −90° and +90°. Decisions on the direction of the rotation depend on β i {\displaystyle \beta _{i}} being positive or negative.
The vectoring-mode of operation requires a slight modification of the algorithm. It starts with a vector whose x coordinate is positive whereas the y coordinate is arbitrary. Successive rotations have the goal of rotating the vector to the x axis (and therefore reducing the y coordinate to zero). At each step, the value of y determines the direction of the rotation. The final value of β i {\displaystyle \beta _{i}} contains the total angle of rotation. The final value of x will be the magnitude of the original vector scaled by K . So, an obvious use of the vectoring mode is the transformation from rectangular to polar coordinates.
In Java the Math class has a scalb(double x,int scale) method to perform such a shift, [ 58 ] C has the ldexp function, [ 59 ] and the x86 class of processors have the fscale floating point operation. [ 60 ]
The number of logic gates for the implementation of a CORDIC is roughly comparable to the number required for a multiplier as both require combinations of shifts and additions. The choice for a multiplier-based or CORDIC-based implementation will depend on the context. The multiplication of two complex numbers represented by their real and imaginary components (rectangular coordinates), for example, requires 4 multiplications, but could be realized by a single CORDIC operating on complex numbers represented by their polar coordinates, especially if the magnitude of the numbers is not relevant (multiplying a complex vector with a vector on the unit circle actually amounts to a rotation). CORDICs are often used in circuits for telecommunications such as digital down converters .
In two of the publications by Vladimir Baykov, [ 61 ] [ 62 ] it was proposed to use the double iterations method for the implementation of the functions: arcsine, arccosine, natural logarithm, exponential function, as well as for the calculation of the hyperbolic functions. Double iterations method consists in the fact that unlike the classical CORDIC method, where the iteration step value changes every time, i.e. on each iteration, in the double iteration method, the iteration step value is repeated twice and changes only through one iteration. Hence the designation for the degree indicator for double iterations appeared: i = 0 , 0 , 1 , 1 , 2 , 2 … {\displaystyle i=0,0,1,1,2,2\dots } . Whereas with ordinary iterations: i = 0 , 1 , 2 … {\displaystyle i=0,1,2\dots } . The double iteration method guarantees the convergence of the method throughout the valid range of argument changes.
The generalization of the CORDIC convergence problems for the arbitrary positional number system with radix R {\displaystyle R} showed [ 63 ] that for the functions sine, cosine, arctangent, it is enough to perform R − 1 {\displaystyle R-1} iterations for each value of i (i = 0 or 1 to n, where n is the number of digits), i.e. for each digit of the result. For the natural logarithm, exponential, hyperbolic sine, cosine and arctangent, R {\displaystyle R} iterations should be performed for each value i {\displaystyle i} . For the functions arcsine and arccosine, two R − 1 {\displaystyle R-1} iterations should be performed for each number digit, i.e. for each value of i {\displaystyle i} . [ 63 ]
For inverse hyperbolic sine and arcosine functions, the number of iterations will be 2 R {\displaystyle 2R} for each i {\displaystyle i} , that is, for each result digit.
CORDIC is part of the class of "shift-and-add" algorithms , as are the logarithm and exponential algorithms derived from Henry Briggs' work. Another shift-and-add algorithm which can be used for computing many elementary functions is the BKM algorithm , which is a generalization of the logarithm and exponential algorithms to the complex plane. For instance, BKM can be used to compute the sine and cosine of a real angle x {\displaystyle x} (in radians) by computing the exponential of 0 + i x {\displaystyle 0+ix} , which is cis ( x ) = cos ( x ) + i sin ( x ) {\displaystyle \operatorname {cis} (x)=\cos(x)+i\sin(x)} . The BKM algorithm is slightly more complex than CORDIC, but has the advantage that it does not need a scaling factor ( K ). | https://en.wikipedia.org/wiki/Merged_CORDIC |
In astronomy , the meridian is the great circle passing through the celestial poles , as well as the zenith and nadir of an observer's location. Consequently, it contains also the north and south points on the horizon , and it is perpendicular to the celestial equator and horizon. Meridians, celestial and geographical, are determined by the pencil of planes passing through the Earth's rotation axis . For a location not on this axis, there is a unique meridian plane in this axial-pencil through that location. The intersection of this plane with Earth's surface defines two geographical meridians (either one east and one west of the prime meridian , or else the prime meridian itself and its anti-meridian), and the intersection of the plane with the celestial sphere is the celestial meridian for that location and time.
There are several ways to divide the meridian into semicircles . In one approach, the observer's upper meridian extends from a celestial pole and passes through the zenith to contact the opposite pole, while the lower meridian passes through the nadir to contact both poles at the opposite ends. In another approach known as the horizontal coordinate system , the meridian is divided into the local meridian , the semicircle that contains the observer's zenith and the north and south points of their horizon, [ 1 ] [ 2 ] and the opposite semicircle, which contains the nadir and the north and south points of their horizon.
On any given (sidereal) day/night, a celestial object will appear to drift across , or transit, the observer's upper meridian as Earth rotates, since the meridian is fixed to the local horizon. At culmination , the object contacts the upper meridian and reaches its highest point in the sky. An object's right ascension and the local sidereal time can be used to determine the time of its culmination (see hour angle ).
The term meridian comes from the Latin meridies , which means both "midday" and "south", as the celestial equator appears to tilt southward from the Northern Hemisphere . | https://en.wikipedia.org/wiki/Meridian_(astronomy) |
Meristics is an area of zoology and botany which relates to counting quantitative features of animals and plants, such as the number of fins or scales in fish . A meristic (countable trait) can be used to describe a particular species , or used to identify an unknown species. Meristic traits are often described in a shorthand notation called a meristic formula .
Meristic characters are the countable structures occurring in series (e.g. myomeres , vertebrae , fin rays ). These characters are among the characters most commonly used for differentiation of species and populations . In the salmonids , scale counts have been most widely used for the differentiation of populations within species. In rainbow and steelhead trout the most notable differences among populations occur in counts of scales. Meristic comparison is used in phenetic and cladistic analysis.
A meristic study is often a difficult task. For example, counting the features of a fish is not as easy as it may appear. Many meristic analyses are performed on dead fish that have been preserved in alcohol. Meristic traits are less easily observed on living fish, though it is possible. On very small fish, a microscope may be required.
Ichthyologists follow a basic set of rules when performing a meristic analysis, to remove as much ambiguity as possible. The specific practice, however, may vary depending on the type of fish. The methodology for counting meristic traits should be described by the specialist who performs the analysis.
A meristic formula is a shorthand method of describing the way the bones (rays) of a bony fish 's fins are arranged. It is comparable to the floral formula for flowers.
Spine counts are given in Roman numerals, e.g. XI-XIV. Ray counts are given in Arabic numerals, e.g. 11–14. [ 1 ]
The meristic formula of the dusky spinefoot ( Siganus luridus ) is: D, XIV+10; A, VII+8-9; P, 16–17; V, I+3+I; GR, 18-22
This means the fish has 14 spiny rays (bones) in the first part of its dorsal fin (D), followed by 10 soft rays. A is the anal fin, P represents the pectoral fins (near the gills and eyes), V represents the ventral or pelvic fins , and C is the caudal fin or tail (not indicated in this example). GR means gill raker count (see below).
The number of bones in the backbone is a feature which can also be used to classify fish species. Usually all the vertebrae are counted. Vertebral counts may be split into abdominal (those associated with the body cavity) and caudal (tail) vertebrae. If there are sutures in the urostyle , components are counted, otherwise the urostyle is usually counted as one vertebra. [ 2 ]
The number of gill rakers on the first gill arch can also be used to identify a fish species. Rakers are counted for the upper and lower limbs of the gill arch, and a raker at the joint of the upper and lower limbs is counted as of the lower. Counts for the upper and lower limbs are separated by a + sign and ranges are bracketed, e.g., GR: 3 + (4-5). [ 3 ] | https://en.wikipedia.org/wiki/Meristics |
Mermaid's glove is a common name referring to two different organisms: | https://en.wikipedia.org/wiki/Mermaid's_glove |
In quantum field theory and statistical mechanics , the Hohenberg–Mermin–Wagner theorem or Mermin–Wagner theorem (also known as Mermin–Wagner–Berezinskii theorem or Mermin–Wagner–Coleman theorem ) states that continuous symmetries cannot be spontaneously broken at finite temperature in systems with sufficiently short-range interactions in dimensions d ≤ 2 . Intuitively, this theorem implies that long-range fluctuations can be created with little energy cost, and since they increase the entropy, they are favored.
This preference is because if such a spontaneous symmetry breaking occurred, then the corresponding Goldstone bosons , being massless, would have an infrared divergent correlation function .
The absence of spontaneous symmetry breaking in d ≤ 2 dimensional infinite systems was rigorously proved by David Mermin and Herbert Wagner (1966), citing a more general unpublished proof by Pierre Hohenberg (published later in 1967) in statistical mechanics. [ 1 ] It was independently discovered by Vadim Berezinskii in the Soviet Union in 1970. [ 2 ] [ 3 ] It was also reformulated later by Sidney Coleman ( 1973 ) for quantum field theory . The theorem does not apply to discrete symmetries that can be seen in the two-dimensional Ising model .
Consider the free scalar field φ of mass m in two Euclidean dimensions. Its propagator is:
For small m , G is a solution to Laplace's equation with a point source:
This is because the propagator is the reciprocal of ∇ 2 in k space. To use Gauss's law , define the electric field analog to be E = ∇ G . The divergence of the electric field is zero. In two dimensions, using a large Gaussian ring:
So that the function G has a logarithmic divergence both at small and large r .
The interpretation of the divergence is that the field fluctuations cannot stay centered around a mean. If you start at a point where the field has the value 1, the divergence tells you that as you travel far away, the field is arbitrarily far from the starting value. This makes a two dimensional massless scalar field slightly tricky to define mathematically. If you define the field by a Monte Carlo simulation, it doesn't stay put, it slides to infinitely large values with time.
This happens in one dimension too, when the field is a one dimensional scalar field, a random walk in time. A random walk also moves arbitrarily far from its starting point, so that a one-dimensional or two-dimensional scalar does not have a well defined average value.
If the field is an angle, θ , as it is in the Mexican hat model where the complex field A = Re iθ has an expectation value but is free to slide in the θ direction, the angle θ will be random at large distances. This is the Mermin–Wagner theorem: there is no spontaneous breaking of a continuous symmetry in two dimensions.
While the Mermin–Wagner theorem prevents any spontaneous symmetry breaking on a global scale, ordering transitions of Kosterlitz–Thouless–type may be allowed. This is the case for the XY model where the continuous (internal) O (2) symmetry on a spatial lattice of dimension d ≤ 2 , i.e. the (spin-)field's expectation value, remains zero for any finite temperature ( quantum phase transitions remain unaffected). However, the theorem does not prevent the existence of a phase transition in the sense of a diverging correlation length ξ . To this end, the model has two phases: a conventional disordered phase at high temperature with dominating exponential decay of the correlation function G ( r ) ∼ exp ( − r / ξ ) {\displaystyle G(r)\sim \exp(-r/\xi )} for r / ξ ≫ 1 {\displaystyle r/\xi \gg 1} , and a low-temperature phase with quasi-long-range order where G ( r ) decays according to some power law for "sufficiently large", but finite distance r ( a ≪ r ≪ ξ with a the lattice spacing ).
We will present an intuitive way [ 4 ] to understand the mechanism that prevents symmetry breaking in low dimensions, through an application to the Heisenberg model , that is a system of n -component spins S i of unit length | S i | = 1 , located at the sites of a d -dimensional square lattice, with nearest neighbour coupling J . Its Hamiltonian is
The name of this model comes from its rotational symmetry. Consider the low temperature behavior of this system and assume that there exists a spontaneously broken symmetry, that is a phase where all spins point in the same direction, e.g. along the x -axis. Then the O ( n ) rotational symmetry of the system is spontaneously broken, or rather reduced to the O ( n − 1) symmetry under rotations around this direction. We can parametrize the field in terms of independent fluctuations { σ α : α = 1 , … , n − 1 } {\displaystyle \{\sigma _{\alpha }:\alpha =1,\dots ,n-1\}} around this direction as follows:
with | σ α | ≪ 1 , and Taylor expand the resulting Hamiltonian. We have
whence
Ignoring the irrelevant constant term H 0 = − JNd and passing to the continuum limit , given that we are interested in the low temperature phase where long-wavelength fluctuations dominate, we get
The field fluctuations σ α are called spin waves and can be recognized as Goldstone bosons. Indeed, they are n -1 in number and they have zero mass since there is no mass term in the Hamiltonian.
To find if this hypothetical phase really exists we have to check if our assumption is self-consistent, that is if the expectation value of the magnetization , calculated in this framework, is finite as assumed. To this end we need to calculate the first order correction to the magnetization due to the fluctuations. This is the procedure followed in the derivation of the well-known Ginzburg criterion .
The model is Gaussian to first order and so the momentum space correlation function is proportional to k −2 . Thus the real space two-point correlation function for each of these modes is
where a is the lattice spacing. The average magnetization is
and the first order correction can now easily be calculated:
The integral above is proportional to
and so it is finite for d > 2 , but appears to be divergent for d ≤ 2 (logarithmically for d = 2 ).
This divergence signifies that fluctuations σ α are large so that the expansion in the parameter | σ α | ≪ 1 performed above is not self-consistent. One can naturally expect then that beyond that approximation, the average magnetization is zero.
We thus conclude that for d ≤ 2 our assumption that there exists a phase of spontaneous magnetization is incorrect for all T > 0 , because the fluctuations are strong enough to destroy the spontaneous symmetry breaking. This is a general result:
The result can also be extended to other geometries, such as Heisenberg films with an arbitrary number of layers, as well as to other lattice systems (Hubbard model, s-f model). [ 5 ]
Much stronger results than absence of magnetization can actually be proved, and the setting can be substantially more general. In particular [ citation needed ] :
In this general setting, Mermin–Wagner theorem admits the following strong form (stated here in an informal way):
When the assumption that the Lie group be compact is dropped, a similar result holds, but with the conclusion that infinite-volume Gibbs states do not exist.
Finally, there are other important applications of these ideas and methods, most notably to the proof that there cannot be non-translation invariant Gibbs states in 2-dimensional systems. A typical such example would be the absence of crystalline states in a system of hard disks (with possibly additional attractive interactions).
It has been proved however that interactions of hard-core type can lead in general to violations of Mermin–Wagner theorem.
In 1930, Felix Bloch argued that, by diagonalizing the Slater determinant for fermions, magnetism in 2D should not exist. [ 6 ] Some easy arguments, which are summarized below, were given by Rudolf Peierls based on entropic and energetic considerations. [ 7 ] Lev Landau also did some work on symmetry breaking in two dimensions. [ 8 ]
One reason for the lack of global symmetry breaking is, that one can easily excite long wavelength fluctuations which destroy perfect order. "Easily excited" means, that the energy for those fluctuations tend to zero for large enough systems. Let's consider a magnetic model (e.g. the XY-model in one dimension). It is a chain of magnetic moments of length L {\displaystyle L} . We consider harmonic approximation, where the forces (torque) between neighbouring moments increase linearly with the angle of twisting γ i {\displaystyle \gamma _{i}} . This implies, that the energy due to twisting increases quadratically E i ∝ γ i 2 {\displaystyle E_{i}\propto \gamma _{i}^{2}} . The total energy is the sum of all twisted pairs of magnetic moments E g e s ∝ ∑ i γ i 2 {\displaystyle E_{ges}\propto \sum _{i}\gamma _{i}^{2}} . If one considers the excited mode with the lowest energy in one dimension (see figure), then the moments on the chain of length L {\displaystyle L} are tilted by π {\displaystyle \pi } along the chain. The relative angle between neighbouring moments is the same for all pairs of moments in this mode and equals γ i = π / N {\displaystyle \gamma _{i}=\pi /N} , if the chain consists of N {\displaystyle N} magnetic moments. It follows that the total energy of this lowest mode is E g e s ∝ N ⋅ γ i 2 = N π 2 N 2 ∝ L π 2 L 2 {\displaystyle E_{ges}\propto N\cdot \gamma _{i}^{2}=N{\frac {\pi ^{2}}{N^{2}}}\propto L{\frac {\pi ^{2}}{L^{2}}}} . It decreases with increasing system size ∝ 1 / L {\displaystyle \propto 1/L} and tends to zero in the thermodynamic limit L → ∞ {\displaystyle L\to \infty } , N → ∞ {\displaystyle N\to \infty } , L / N = const. {\displaystyle L/N={\text{const.}}} For arbitrary large systems follows, that the lowest modes do not cost any energy and will be thermally excited. Simultaneously, the long range order is destroyed on the chain. In two dimensions (or in a plane) the number of magnetic moments is proportional to the area of the plain N ∝ L 2 {\displaystyle N\propto L^{2}} . The energy for the lowest excited mode is then E g e s ∝ N 2 ⋅ γ i 2 ∝ L 2 π 2 L 2 {\displaystyle E_{ges}\propto N^{2}\cdot \gamma _{i}^{2}\propto L^{2}{\frac {\pi ^{2}}{L^{2}}}} , which tends to a constant in the thermodynamic limit. Thus the modes will be excited at sufficiently large temperatures. In three dimensions, the number of magnetic moments is proportional to the volume V = L 3 {\displaystyle V=L^{3}} and the energy of the lowest mode is E g e s ∝ N 3 ⋅ γ i 2 ∝ L 3 π 2 L 2 {\displaystyle E_{ges}\propto N^{3}\cdot \gamma _{i}^{2}\propto L^{3}{\frac {\pi ^{2}}{L^{2}}}} . It diverges with system size and will thus not be excited for large enough systems. Long range order is not affected by this mode and global symmetry breaking is allowed.
An entropic argument against perfect long range order in crystals with D < 3 {\displaystyle D<3} is as follows (see figure): consider a chain of atoms/particles with an average particle distance of ⟨ a ⟩ {\displaystyle \langle a\rangle } . Thermal fluctuations between particle 0 {\displaystyle 0} and particle 1 {\displaystyle 1} will lead to fluctuations of the average particle distance of the order of ξ 0 , 1 {\displaystyle \xi _{0,1}} , thus the distance is given by a = ⟨ a ⟩ ± ξ 0 , 1 {\displaystyle a=\langle a\rangle \pm \xi _{0,1}} . The fluctuations between particle − 1 {\displaystyle -1} and 0 {\displaystyle 0} will be of the same size: | ξ − 1 , 0 | = | ξ 0 , 1 | {\displaystyle |\xi _{-1,0}|=|\xi _{0,1}|} . We assume that the thermal fluctuations are statistically independent (which is evident if we consider only nearest neighbour interaction) and the fluctuations between − 1 {\displaystyle -1} and particle + 1 {\displaystyle +1} (with double the distance) has to be summed statistically independent (or incoherent): ξ − 1 , 1 = 2 ⋅ ξ 0 , 1 {\displaystyle \xi _{-1,1}={\sqrt {2}}\cdot \xi _{0,1}} . For particles N-times the average distance, the fluctuations will increase with the square root ξ 0 , N = N ⋅ ξ 0 , 1 {\displaystyle \xi _{0,N}={\sqrt {N}}\cdot \xi _{0,1}} if neighbouring fluctuations are summed independently. Although the average distance ⟨ a ⟩ {\displaystyle \langle a\rangle } is well defined, the deviations from a perfect periodic chain increase with the square root of the system size. In three dimensions, one has to walk along three linearly independent directions to cover the whole space; in a cubic crystal, this is effectively along the space diagonal, to get from particle 0 {\displaystyle 0} to particle 3 {\displaystyle 3} . As one can easily see in the figure, there are six different possibilities to do this. This implies, that the fluctuations on the six different pathways cannot be statistically independent, since they pass the same particles at position 0 {\displaystyle 0} and 3 {\displaystyle 3} . Now, the fluctuations of the six different ways have to be summed in a coherent way and will be of the order of ξ {\displaystyle \xi } – independent of the size of the cube. The fluctuations stay finite and lattice sites are well defined. For the case of two dimensions, Herbert Wagner and David Mermin have proved rigorously, that fluctuations distances increase logarithmically with systems size ξ ∝ ln ( L ) {\displaystyle \xi \propto \ln(L)} . This is frequently called the logarithmic divergence of displacements.
The image shows a (quasi-) two-dimensional crystal of colloidal particles. These are micrometre-sized particles dispersed in water and sedimented on a flat interface, thus they can perform Brownian motions only within a plane. The sixfold crystalline order is easy to detect on a local scale, since the logarithmic increase of displacements is rather slow. The deviations from the (red) lattice axis are easy to detect, too, here shown as green arrows. The deviations are basically given by the elastic lattice vibrations (acoustic phonons). A direct experimental proof of Hohenberg–Mermin–Wagner fluctuations would be, if the displacements increase logarithmic with the distance of a locally fitted coordinate frame (blue). This logarithmic divergence goes along with an algebraic (slow) decay of positional correlations. The spatial order of a 2D crystal is called quasi-long-range (see also such hexatic phase for the phase behaviour of 2D ensembles).
Interestingly, significant signatures of Hohenberg–Mermin–Wagner fluctuations have not been found in crystals but in disordered amorphous systems. [ 9 ] [ 10 ] [ 11 ]
This work did not investigate the logarithmic displacements of lattice sites (which are difficult to quantify for a finite system size), but the magnitude of the mean squared displacement of the particles as function of time. This way, the displacements are not analysed in space but in the time domain. The theoretical background is given by D. Cassi, as well as F. Merkl and H. Wagner. [ 12 ] [ 13 ] This work analyses the recurrence probability of random walks and spontaneous symmetry breaking in various dimensions. The finite recurrence probability of a random walk in one and two dimension shows a dualism to the lack of perfect long-range order in one and two dimensions, while the vanishing recurrence probability of a random walk in 3D is dual to existence of perfect long-range order and the possibility of symmetry breaking.
Real magnets usually do not have a continuous symmetry, since the spin-orbit coupling of the electrons imposes an anisotropy. For atomic systems like graphene, one can show that monolayers of cosmological (or at least continental) size are necessary to measure a significant size of the amplitudes of fluctuations. [ 14 ] A recent discussion about the Hohenberg–Mermin–Wagner theorems and its limitations in the thermodynamic limit is given by Bertrand Halperin . [ 15 ] More recently, it was shown that the most severe physical limitation are finite-size effects in 2D, because the suppression due to infrared fluctuations is only logarithmic in the size: The sample would have to be larger than the observable universe for a 2D superconducting transition to be suppressed below ~100 K. [ 16 ] For magnetism, there is a similar behaviour where the sample size must approach the size of the universe to have a Curie temperature T c in the mK range. [ 17 ] However, because disorder and interlayer coupling compete with finite-size effects at restoring order, it cannot be said a priori which of them is responsible for the observation of magnetic ordering in a given 2D sample. [ 18 ]
The discrepancy between the Hohenberg–Mermin–Wagner theorem (ruling out long range order in 2D) and the first computer simulations (Alder & Wainwright), which indicated crystallization in 2D, motivated J. Michael Kosterlitz and David J. Thouless to work on topological phase transitions in 2D. This work was awarded with the 2016 Nobel Prize in Physics (together with Duncan Haldane ). | https://en.wikipedia.org/wiki/Mermin–Wagner_theorem |
A merodiploid is a partially diploid bacterium, which has its own chromosome complement and a chromosome fragment introduced by conjugation , transformation or transduction . [ 1 ] It can also be defined as an essentially haploid organism that carries a second copy of a part of its genome. The term is derived from the Greek, meros = part, and was originally used to describe both unstable partial diploidy, such as that which occurs briefly in recipients after mating with an Hfr strain (1), and the stable state, exemplified by F-prime strains (see Hfr'S And F-Primes). Over time the usage has tended to confine the term to descriptions of stable genetic states. Merodiploidy refers to the partial duplication of chromosomes in a haploid organism. [ 2 ]
This bacteria -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Merodiploid |
Meromyosin is a part of myosin ( mero meaning "part of"). With regards to human anatomy myosin and actin constitute the basic functional unit of a muscle fiber, called sarcomere , playing a role in muscle contraction .
Biochemically viewed meromyosin form subunits of the actin -associated motor protein , myosin , as commonly obtained by trypsin proteolysis (protein breakdown). [ 1 ] Following this proteolysis, two types of meromyosin are formed: heavy meromyosin (HMM) and light meromyosin (LMM). [ 2 ]
Light meromyosin has a long, straight portion in the “tail” region. Heavy meromyosin (HMM) is a protein chain terminating in a globular head portion /cross bridge. [ 3 ] HMM consists of two subunits, Heavy Meromyosin Subunit 1 and 2 (HMMS-1 and HMMS-2). The majority of myosin activity is concentrated in HMMS-1. HMMS-1 has an actin binding site and ATP binding site (myosin ATPase) that determines the rate of muscle contraction when muscle is stretched.
Light and heavy meromyosin are subunits of myosin filaments (thick myofilaments).
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Meromyosin |
Meropenem , sold under the brand name Merrem among others, is an intravenous carbapenem antibiotic used to treat a variety of bacterial infections . [ 3 ] Some of these include meningitis , intra-abdominal infection , pneumonia , sepsis , and anthrax . [ 3 ]
Common side effects include nausea, diarrhea, constipation, headache, rash, and pain at the site of injection. [ 3 ] Serious side effects include Clostridioides difficile infection , seizures , and allergic reactions including anaphylaxis . [ 3 ] Those who are allergic to other β-lactam antibiotics are more likely to be allergic to meropenem as well. [ 3 ] Use in pregnancy appears to be safe. [ 3 ] It is in the carbapenem family of medications. [ 3 ] Meropenem usually results in bacterial death through blocking their ability to make a cell wall . [ 3 ] It is resistant to breakdown by many kinds of β-lactamase enzymes, produced by bacteria to protect themselves from antibiotics. [ 4 ] [ 5 ] [ 6 ]
Meropenem was patented in 1983. [ 7 ] It was approved for medical use in the United States in 1996. [ 3 ] It is on the World Health Organization's List of Essential Medicines . [ 8 ] [ 9 ] The World Health Organization classifies meropenem as critically important for human medicine. [ 10 ]
The spectrum of action includes many Gram-positive and Gram-negative bacteria (including Pseudomonas ) and anaerobic bacteria. The overall spectrum is similar to that of imipenem, although meropenem is more active against Enterobacteriaceae and less active against Gram-positive bacteria. Meropenem is effective against bacteria producing extended-spectrum β-lactamases but may be more susceptible to hydrolysis by metallo-β-lactamases produced by bacteria. [ 11 ] β-lactamases are enzymes that bacteria produce to hydrolyze β-lactam antibiotics, breaking the β-lactam ring and rendering these antibiotics ineffective. This mechanism helps bacteria resist the effects of antibiotics like penicillins, cephalosporins, and carbapenems, making treatment more challenging. [ 12 ] [ 13 ] [ 14 ] While β-lactam ring in meropenem is more accessible to water molecules than in the other β-lactam antibiotics, that facilitates the hydrolysis process and faster degradation of meropenem's antibacterial properties in aqueous solutions, it is more resistant to degradation by β-lactamase enzymes produced by bacteria than the other β-lactam antibiotics. [ 15 ] [ 4 ]
Meropenem is frequently given in the treatment of febrile neutropenia . This condition frequently occurs in patients with hematological malignancies and cancer patients receiving anticancer drugs that suppress bone marrow formation. It is approved for complicated skin and skin structure infections, complicated intra-abdominal infections and bacterial meningitis . [ 4 ] [ 16 ] [ 17 ] [ 18 ]
Meropenem is effective in treating bacterial pneumonia, including hospital-acquired pneumonia. [ 19 ]
In 2017, the U.S. Food and Drug Administration (FDA) granted approval for the combination of meropenem and vaborbactam to treat adults with complicated urinary tract infections . [ 20 ]
Meropenem is administered intravenously as an aqueous solution. Meropenem is stored in vials as white crystalline powder (containing meropenem as the trihydrate blended with anhydrous sodium carbonate). [ 21 ] [ 22 ] [ 23 ] For intravenous administration, if pure meropenem powder is used (rather than the powder blended with sodium carbonate), meropenem is dissolved in 5% monobasic potassium phosphate solution, since meropenem is soluble in 5% monobasic potassium phosphate solution and only sparingly soluble in water [ 22 ] ( 5.63 mg/mL ). [ 24 ] [ 25 ] [ 26 ] For intravenous bolus administration, injection vials (that contain meropenem blended with sodium carbonate) are reconstituted with sterile water for injection. [ 21 ] [ 22 ] [ 24 ]
Reconstituted (dissolved) meropenem degrades over time. [ 27 ] [ 28 ] [ 29 ] [ 30 ] The degradation may be associated with color change of the solution, typical for a hydrolysis of the amide bond of the β-lactam ring as seen with most β-lactam antibiotics, [ 31 ] while particularly for meropenem the color is changing from colorless or pale yellow to vivid yellowish. [ 32 ] Upon reconstitution, the meropenem infusion solution, prepared with 0.9% sodium chloride, exhibits both chemical and physical stability for a duration of 3 hours at a temperature up to 25 °C. If refrigerated ( 2–8 °C), the stability extends to 24 hours. However, when the product is reconstituted in a 5% dextrose solution, it is used immediately to ensure its efficacy. [ 27 ] The degradation of meropenem in a water-based solution is affected by factors such as pH, temperature, initial concentration, and the specific type of infusion solution used. [ 32 ] Meropenem solutions should not be frozen. [ 33 ] [ 34 ]
There is a bit of a paradox with meropenem that the amide bond in the β-lactam ring of meropenem makes it resistant to many β-lactamases (penicillinases), which are enzymes produced by bacteria that can break down penicillin and related antibiotics such as meropenem. [ 35 ] [ 36 ] This resistance is due to the stability of the β-lactam ring in meropenem, which is less susceptible to hydrolysis by these enzymes. [ 37 ] However, meropenem is not stable in the presence of water. [ 38 ] [ 39 ] It can undergo hydrolysis in aqueous solutions, which can reduce its effectiveness. [ 40 ] This means that while meropenem is designed to resist bacterial enzymes, it can still be broken down by water, which is a bit ironic. [ 41 ] That's why meropenem requires frequent or prolonged slow administration to supply new drug to the bloodstream to replace what was hydrolyzed by the water component of blood. [ 42 ] [ 43 ]
Meropenem is administered every 8 hours. [ 24 ]
Dosing must be adjusted for altered kidney function and for haemofiltration . [ 44 ]
Studies describe application of meropenem therapeutic drug monitoring (measurements of drug levels in the bloodstream at specific intervals) for optimal application. [ 45 ] [ 46 ]
As with other β-lactams antibiotics, the effectiveness of treatment depends on the amount of time during the dosing interval that the meropenem concentration is above the minimum inhibitory concentration for the bacteria causing the infection. [ 47 ] For β-lactams, including meropenem, prolonged intravenous administration is associated with lower mortality compared to bolus intravenous infusion, especially in severe infections or those caused by less sensitive bacteria, such as Pseudomonas aeruginosa . [ 47 ] [ 48 ]
Meropenem exhibits poor permeability across the gut and low oral bioavailability because of its hydrophilic properties, which inhibit its passive diffusion across the intestinal epithelium. [ 49 ] The challenges related to research of oral delivery of meropenem are related to high susceptibility of meropenem to degradation through hydrolysis of the amide bond in the β-lactam ring, even at relatively low temperatures and humidity. [ 49 ] This instability can result in the loss of meropenem's antibacterial activity. Besides that, meropenem is unstable in the acidic environment of the stomach, leading to extensive degradation and loss of the drug after oral administration. [ 49 ] In addition, intestinal efflux (secretory) transport can pump the drug back into the gut: efflux transporters, particularly P-glycoprotein (P-gp), present in the gastrointestinal tract can actively pump meropenem back into the gut lumen, limiting its absorption and reducing oral bioavailability; in the attempts of oral administration bacteria can develop resistance to meropenem by enhancing the active efflux of the antibiotic through efflux transporters, such as the MexAB-OprM tripartite efflux system in Pseudomonas aeruginosa. [ 49 ] That's why meropenem is administered intravenously. [ 49 ] [ 50 ]
There is insufficient data regarding the administration of meropenem during breastfeeding. However, it has been observed that, in general, the concentration of this β-lactam antibiotic in breast milk is relatively low, therefore, β-lactam antibiotics are not anticipated to induce detrimental effects in infants who are breastfed. Nonetheless, there have been sporadic reports of disturbances in the gastrointestinal flora of the infant, manifesting as diarrhea or oral candidiasis (thrush), associated with the use of β-lactam antibiotics, however, these potential side effects have not been thoroughly investigated specifically in the context of meropenem use, therefore, the safety profile of meropenem in breastfeeding mothers and their infants is unknown. [ 51 ]
Although meropenem is not approved for intramuscular or subcutaneous routes of administration in humans, there were studies that evaluated the drug bioavailability in cats and reported bioavailability of 99.69% for intramuscular route and 96.52 % for subcutaneous route of administration; these studies also compared elimination half-lives for intravenous, intramuscular or subcutaneous routes of administration in cats and reported duration of 1.35, 2.10 and 2.26 hours, respectively. [ 52 ] There was also a small study on local tolerance of meropenem intramuscular administration in humans, and it was reported as generally good. [ 52 ] [ 53 ] [ 54 ]
Among antibiotic drugs, meropenem is relatively safe. [ 4 ] [ 46 ] The most common adverse effects are diarrhea (4.8%), nausea and vomiting (3.6%), injection-site inflammation (2.4%), headache (2.3%), rash (1.9%) and thrombophlebitis (0.9%). [ 55 ] Many of these adverse effects were observed in severely ill individuals already taking many medications including vancomycin . [ 56 ] [ 57 ] Meropenem has a reduced potential for seizures in comparison with imipenem . Several cases of severe hypokalemia have been reported. [ 58 ] [ 59 ]
Meropenem rapidly reduces serum concentrations of valproic acid . As a result, people who use valproic acid for epilepsy are at increased risk of seizures during treatment with meropenem. In situations where the use of meropenem cannot be avoided, prescription of an additional anticonvulsant should be considered. [ 60 ]
Meropenem is bactericidal except against Listeria monocytogenes , where it is bacteriostatic . It inhibits bacterial cell wall synthesis like other β-lactam antibiotics. In contrast to other β-lactams, it is highly resistant to degradation by β-lactamases or cephalosporinases. In general, resistance arises due to mutations in penicillin-binding proteins , production of metallo-β-lactamases, or resistance to diffusion across the bacterial outer membrane. [ 55 ] Unlike imipenem , it is stable to dehydropeptidase -1, so can be given without cilastatin . [ 61 ]
In 2016, a synthetic peptide-conjugated PMO (PPMO) was found to inhibit the expression of New Delhi metallo-beta-lactamase 1 , an enzyme that many drug-resistant bacteria use to destroy carbapenems. [ 62 ] [ 63 ]
Meropenem has a low protein binding rate of approximately 2%, in contrast to ertapenem, which is about 90%. This pharmacokinetic difference may impact clinical outcomes, particularly in hypoalbuminemic patients. [ 64 ] Observational studies have shown that, in this population, treatment with meropenem is associated with a significantly lower 30-day mortality rate compared to ertapenem, with an approximately fourfold reduction in risk. [ 65 ]
Nebulized meropenem (inhaled route) is researched, but is not approved, for prevention of bronchiectasis exacerbation. [ 66 ] | https://en.wikipedia.org/wiki/Meropenem |
Merox is an acronym for mercaptan oxidation . It is a proprietary catalytic chemical process developed by UOP used in oil refineries and natural gas processing plants to remove mercaptans from LPG , propane , butanes , light naphthas , kerosene , and jet fuel by converting them to liquid hydrocarbon disulfides .
The Merox process requires an alkaline environment which, in some process versions, is provided by an aqueous solution of sodium hydroxide (NaOH), a strong base , commonly referred to as caustic . In other versions of the process, the alkalinity is provided by ammonia , which is a weak base.
The catalyst in some versions of the process is a water-soluble liquid. In other versions, the catalyst is impregnated into charcoal granules.
Processes within oil refineries or natural gas processing plants that remove mercaptans and/or hydrogen sulfide (H 2 S) are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide. The liquid hydrocarbon disulfides may remain in the sweetened products. These may be used as part of the refinery or natural gas processing plant fuel, or they may be processed further.
When dealing with kerosene, the Merox process is usually more economical than using a catalytic hydrodesulfurization process for much the same purpose. It is rarely (if ever) required to reduce the sulphur content of a straight-run kerosene to meet the 3000 ppm sulphur specification of jet fuel , because very few crude oils have a kerosene cut with a higher content of sulphur than this limit.
UOP has developed many versions of the Merox process for various applications:
In all of the above Merox versions, the overall oxidation reaction that takes place in converting mercaptans to disulfides is:
The most common mercaptans removed are:
In some of the above Merox process versions, the catalyst is a liquid. In others, the catalyst is in the form of impregnated charcoal granules.
Process flow diagrams and descriptions of the two conventional versions of the Merox process are presented in the following sections.
The conventional Merox process for extraction and removal of mercaptans from liquefied petroleum gases (LPG), such as propane, butanes and, mixtures of propane and butanes, can also be used to extract and remove mercaptans from light naphthas. [ 1 ] It is a two-step process. In the first step, the feedstock LPG or light naphtha is contacted in the trayed extractor vessel with an aqueous caustic solution containing UOP's proprietary liquid catalyst. The caustic solution reacts with mercaptans and extracts them. The reaction that takes place in the extractor is:
In the above reaction, RSH is a mercaptan and R signifies an organic group such as a methyl, ethyl, propyl or other groups. For example, the ethyl mercaptan ( ethanethiol ) has the formula C 2 H 5 SH.
The second step is referred to as regeneration and it involves heating and oxidizing of the caustic solution leaving the extractor. The oxidations results in converting the extracted mercaptans to organic disulfides (RSSR) which are liquids that are water-insoluble and are then separated and decanted from the aqueous caustic solution. The reaction that takes place in the regeneration step is:
After decantation of the disulfides, the regenerated "lean" caustic solution is recirculated back to the top of the extractor to continue extracting mercaptans.
The net overall Merox reaction covering the extraction and the regeneration step may be expressed as:
The feedstock entering the extractor must be free of any H 2 S. Otherwise, any H 2 S entering the extractor would react with the circulating caustic solution and interfere with the Merox reactions. Therefore, the feedstock is first "prewashed" by flowing through a batch of aqueous caustic to remove any H 2 S. The reaction that takes place in the prewash vessel is:
The batch of caustic solution in the prewash vessel is periodically discarded as " spent caustic " and replaced by fresh caustic as needed.
The flow diagram below depicts the equipment and the flow paths involved in the process. [ 1 ] The LPG (or light naphtha) feedstock enters the prewash vessel and flows upward through a batch of caustic which removes any H 2 S that may be present in the feedstock. The coalescer at the top of the prewash vessel prevents caustic from being entrained and carried out of the vessel.
The feedstock then enters the mercaptan extractor and flows upward through the contact trays where the LPG intimately contacts the downflowing Merox caustic that extracts the mercaptans from the LPG. The sweetened LPG exits the tower and flows through: a caustic settler vessel to remove any entrained caustic, a water wash vessel to further remove any residual entrained caustic and a vessel containing a bed of rock salt to remove any entrained water. The dry sweetened LPG exits the Merox unit.
The caustic solution leaving the bottom of the mercaptan extractor ("rich" Merox caustic) flows through a control valve which maintains the extractor pressure needed to keep the LPG liquified. It is then injected with UOP's proprietary liquid catalyst (on an as needed basis), flows through a steam-heated heat exchanger and is injected with compressed air before entering the oxidizer vessel where the extracted mercaptans are converted to disulfides. The oxidizer vessel has a packed bed to keep the aqueous caustic and the water-insoluble disulfide well contacted and well mixed.
The caustic-disulfide mixture then flows into the separator vessel where it is allowed to form a lower layer of "lean" Merox caustic and an upper layer of disulfides. The vertical section of the separator is for the disengagement and venting of excess air and includes a Raschig ring section to prevent entrainment of any disulfides in the vented air. The disulfides are withdrawn from the separator and routed to fuel storage or to a hydrotreater
unit. The regenerated lean Merox caustic is then pumped back to the top of the extractor for reuse.
The conventional Merox process for the removal of mercaptans (i.e., sweetening) of jet fuel or kerosene is a one-step process. [ 2 ] The mercaptan oxidation reaction takes place in an alkaline environment as the feedstock jet fuel or kerosene, mixed with compressed air, flows through a fixed bed of catalyst in a reactor vessel. The catalyst consists of charcoal granules that have been impregnated with UOP's proprietary catalyst. The oxidation reaction that takes place is:
As is the case with the conventional Merox process for treating LPG, the jet fuel or kerosene sweetening process also requires that the feedstock be prewashed to remove any H 2 S that would interfere with the sweetening. The reaction that takes place in the batch caustic prewash vessel is:
The Merox reactor is a vertical vessel containing a bed of charcoal granules that have been impregnated with the UOP catalyst. The charcoal granules may be impregnated with the catalyst in situ or they may be purchased from UOP as pre-impregnated with the catalyst. An alkaline environment is provided by caustic being pumped into reactor on an intermittent, as needed basis.
The jet fuel or kerosene feedstock from the top of the caustic prewash vessel is injected with compressed air and enters the top of the Merox reactor vessel along with any injected caustic. The mercaptan oxidation reaction takes place as the feedstock percolates downward over the catalyst. The reactor effluent flows through a caustic settler vessel where it forms a bottom layer of aqueous caustic solution and an upper layer of water-insoluble sweetened product.
The caustic solution remains in the caustic settler so that the vessel contains a reservoir for the supply of caustic that is intermittently pumped into the reactor to maintain the alkaline environment.
The sweetened product from the caustic settler vessel flows through a water wash vessel to remove any entrained caustic as well as any other unwanted water-soluble substances, followed by flowing through a salt bed vessel to remove any entrained water and finally through a clay filter vessel. The clay filter removes any oil-soluble substances, non-metallic compounds (especially copper) and particulate matter, which might prevent meeting jet fuel product specifications.
The pressure maintained in the reactor is injected air will completely dissolve in the feedstock at the operating temperature . | https://en.wikipedia.org/wiki/Merox |
Merrilactone A is one of the four sesquiterpenes that were newly discovered from the fruit of Illicium merrillianum in 2000. [ 1 ] Members of the genus Illicium include Chinese star anise , widely used as a spice for flavouring food and beverages, and also poisonous plants such as Japanese star anise . [ 2 ] Chemical studies of Illicium have developed rapidly over the last 20 years, and merrilactone A has been shown to have neurotrophic activity in fetal rat cortical neuron cultures. [ 3 ] This has led researchers to believe that Merrilactone A may hold therapeutic potential in the treatment of neuro-degenerative diseases such as Alzheimer's disease and Parkinson's disease . [ 4 ]
Merrilactone A occurs naturally in Illicium merrillianum , a plant indigenous to southern China and Myanmar . [ 3 ] The genus Illicium belongs to the family Illiciaceae and is an evergreen shrub or tree. Approximately 40 species are disjunctively distributed in eastern North America , Mexico , the West Indies , and eastern Asia . The highest concentration of species is in northern Myanmar and southern China , where nearly 35 species have been described. The fruits of the Illicium species are distinctive star-shaped follicles that have a characteristic refreshing flavour. The fruits of Illicium merrillianum also have an aromatic odor, bland taste and cause numbness of the tongue when chewed. [ 2 ]
The only economically important product from this genus is the fruit of Illicium verum , or Chinese star anise , which is widely used as a spice for flavouring food and beverages. In contrast, the fruit of Japanese star anise , Illicium anisatum , have been known to be very toxic for several centuries.
Merrilactone A was found to exhibit a significant neurotrophic activity, such as greatly promoting neurite outgrowth in the primary cultures of fetal rat cortical neurons at concentrations from 10 to 0.1 μmol/L. It was also found that this compound had a property of neuroprotection at same concentration. [ 1 ]
This compound is originated from mevalonic acid pathway which produce dimethylallyl diphosphate (DMAPP) from acetyl-CoA . Three DMAPPs, or one DMAPP and two isopentenyl diphosphate (IPP), are made into a farnesyl diphosphate (FPP), which is the fundamental precursor of sesquiterpene , and then sesquiterpene cyclise enzymes cause it a cyclization . The synthesis up to here is well known, though process after the first cyclization was unclear. Anislactone -type sesquiterpenes which merrilactone A belongs to has been thought to be biosynthesized from majucin since they have γ-lactone . [ 5 ] However, this pathway has trouble to elucidate some configurations and other feature they have. The biosynthesis shown in the figure was newly proposed and solved these problems.
The point of this pathway is that seco-prezizaane , anislactone and tashironin group which are all found in Illicium are all derived from the same intermediate 6 . This is expected to be able to give all characteristic sesquiterpenes in Illicium plants a reasonable explanation of biosynthesis.
Since the discovery of merrilactone A, many methods of total synthesis have been proposed, of which four produce racemic material, [ 6 ] [ 7 ] [ 8 ] [ 9 ] and two produce the natural enantiomer . [ 10 ] [ 11 ] | https://en.wikipedia.org/wiki/Merrilactone_A |
The Merrill–Crowe Process is a separation technique for removing gold from the solution obtained by the cyanide leaching of gold ores. It is an improvement of the MacArthur-Forrest process , where an additional vacuum is managed to remove air in the solution (invention of Crowe), and zinc dust is used instead of zinc shavings (improvement of Merrill ). [ 1 ]
The solution is separated from the ore by methods such as filtration (e.g. vertical leaf type clarifier filters) and counter current decantation (CCD). Afterwards a very clear solution is achieved by using pre-coated filters applying diatomaceous earth . Oxygen is then removed by passing the solution through a vacuum de-aeration column. Zinc dust is added to the clarified, de-aerated solution which precipitates the gold; zinc having a higher affinity for the cyanide ion than gold. Other precious metals, silver , and base metals, like copper , will also precipitate, if present. [ 2 ] [ 3 ]
Automated membrane filtration offers a cost savings alternative to CCD. Both applications are compared in detail by K. McGrew, 2016.
The gold precipitate (mixed with zinc dust) is then filtered out of the solution, and the zinc dust and gold are mixed with sulfuric acid to dissolve the zinc. The solution is filtered, and the remaining solids are smelted to a gold dore bar. These bars are sent to a refinery to remove the copper and silver, the specific process used depending upon the impurities in the gold. [ 4 ]
The basic process was discovered and patented by Charles Washington Merrill around 1900, and later refined by Thomas Bennett Crowe, working for the Merrill Company. [ 5 ] | https://en.wikipedia.org/wiki/Merrill–Crowe_process |
Merrimack Pharmaceuticals, Inc. is a pharmaceutical company based in Cambridge, Massachusetts , United States . They specialize in developing drugs for the treatment of cancer .
Merrimack's first FDA -approved drug was approved in 2015; Onivyde , a liposome encapsulated version of irinotecan is used for treating pancreatic adenocarcinoma . It was approved for use in the European Union the following year. [ 1 ] [ 2 ]
Merrimack was founded by a group of scientists from MIT and Harvard University in 2000. [ 3 ]
In 2016, Merrimack had 426 full-time employees, 103 of which had an MD or PhD. [ 3 ]
In October 2016, CEO Robert Mulroy resigned and the company announced they would be laying off 20% of its employees. [ 4 ] In January 2017, interim CEO Gary Crocker resigned and the board of directors appointed Richard Peters to be president and CEO. Peters previously worked at Sanofi and was a faculty member at Harvard University. [ 5 ]
In January 2017, French pharmaceutical company Ipsen announced they would be purchasing Onivyde from Merrimack for approximately $1 billion. [ 6 ] Following the close of the deal with Ipsen , Merrimack reduced its headcount by about 80%. By May of 2019, Merrimack planned to lay off its entire staff, including the leadership team. [ 7 ]
On November 13, 2018, the statistical programming director Songjiang Wang, received "six months in prison and one year supervised released " after a guilty verdict was handed down to Wang from a United States District Judge in July 2018 for securities fraud and conspiracy to commit securities fraud. [ 8 ] Also on December 20, 2019, the United States Securities and Exchange Commission charged Wang with Insider trading . [ 9 ]
On May 10, 2024, Merrimack announced that the stockholders at a Special Meeting held that day approved the adoption of a Plan of Dissolution. The Board of Directors declared a liquidating cash dividend in the amount of $15.10 per share, expected to be paid on or about May 17, 2024. Merrimack’s Common Stock would continue to trade on NASDAQ through May 17, 2024 and thereafter delist from NASDAQ on May 20, 2024. [ 10 ]
Merrimack has four drugs in clinical development. | https://en.wikipedia.org/wiki/Merrimack_Pharmaceuticals |
The Merritt engine is a design conceived by Dan Merritt, an engineer at Coventry University . Rather than being entirely new, it is a development of the standard petrol engine. The engine is intended to provide "diesel levels of efficiency through a lean-burn strategy similar to that of direct injection". Merritt proposes that fuel/air mixing is not done in the cylinder , but takes place beforehand in a special chamber designed to promote swirl . [ 1 ]
The Merritt technique is also known as MUSIC – for Merritt Unthrottled Spark Ignition Combustion. [ 2 ] Brian Knibb, an engineer in Derby, UK, says it is relatively straightforward to modify existing engines to run in this way. “To produce MUSIC engines, a factory would simply need to change the cylinder head fitted to engines, leaving the cylinders and the rest unchanged.” [ 3 ] | https://en.wikipedia.org/wiki/Merritt_engine |
Merry L. Lindsey is an American cardiac physiologist. In 2022 she was named the Dean of the School of Graduate Studies at Meharry Medical College. In 2019 she was named the Stokes-Shackleford Professor and Chair of the University of Nebraska Medical Center Department of Cellular and Integrative Physiology and the director of the Center for Heart and Vascular Research. In 2021, Lindsey was appointed editor-in-chief of the American Journal of Physiology. Heart and Circulatory Physiology.
Lindsey was born Stuart, Florida in 1970 [ 1 ] [ 2 ] and raised in South Florida , [ 3 ] where she attended South Fork High School. [ 4 ] Following high school, Lindsey earned her undergraduate degree in biology from Boston University and her PhD in cardiovascular sciences from Baylor College of Medicine . [ 3 ]
Upon completing her PhD, Lindsey worked at the Medical University of South Carolina as an assistant professor before joining the faculty at the University of Texas Health Science Center . In 2019, she left the Mississippi Center for Heart Research to accept an appointment as the Stokes-Shackleford Professor and Chair of the Department of Cellular and Integrative Physiology at the University of Nebraska Medical Center . [ 3 ] Upon joining the department, Lindsey also became the founding director of the Center for Heart and Vascular Research. [ 5 ] She joined Meharry Medical College as the dean of the School of Graduate Studies and Research. [ 6 ]
In 2021, Lindsey was appointed editor-in-chief of the American Journal of Physiology. Heart and Circulatory Physiology , a journal published by the American Physiological Society . [ 7 ] She received the Vincenzo Panagia Distinguished Lecture Award from the Institute of Cardiovascular Sciences at St-Boniface Hospital Research in 2021, [ 8 ] and the Distinguished Investigator Award from the British Society for Cardiovascular Research in 2022. [ 9 ] | https://en.wikipedia.org/wiki/Merry_Lindsey |
In mathematics , a Mersenne prime is a prime number that is one less than a power of two . That is, it is a prime number of the form M n = 2 n − 1 for some integer n . They are named after Marin Mersenne , a French Minim friar , who studied them in the early 17th century. If n is a composite number then so is 2 n − 1 . Therefore, an equivalent definition of the Mersenne primes is that they are the prime numbers of the form M p = 2 p − 1 for some prime p .
The exponents n which give Mersenne primes are 2, 3, 5, 7, 13, 17, 19, 31, ... (sequence A000043 in the OEIS ) and the resulting Mersenne primes are 3 , 7 , 31 , 127 , 8191, 131071, 524287, 2147483647 , ... (sequence A000668 in the OEIS ).
Numbers of the form M n = 2 n − 1 without the primality requirement may be called Mersenne numbers . Sometimes, however, Mersenne numbers are defined to have the additional requirement that n should be prime.
The smallest composite Mersenne number with prime exponent n is 2 11 − 1 = 2047 = 23 × 89 .
Mersenne primes were studied in antiquity because of their close connection to perfect numbers : the Euclid–Euler theorem asserts a one-to-one correspondence between even perfect numbers and Mersenne primes. Many of the largest known primes are Mersenne primes because Mersenne numbers are easier to check for primality.
As of 2025 [ref] , 52 Mersenne primes are known. The largest known prime number , 2 136,279,841 − 1 , is a Mersenne prime. [ 1 ] [ 2 ] Since 1997, all newly found Mersenne primes have been discovered by the Great Internet Mersenne Prime Search , a distributed computing project. In December 2020, a major milestone in the project was passed after all exponents below 100 million were checked at least once. [ 3 ]
Many fundamental questions about Mersenne primes remain unresolved. It is not even known whether the set of Mersenne primes is finite or infinite.
The Lenstra–Pomerance–Wagstaff conjecture claims that there are infinitely many Mersenne primes and predicts their order of growth and frequency: For every number n , there should on average be about e γ ⋅ log 2 ( 10 ) ≈ 5.92 {\displaystyle e^{\gamma }\cdot \log _{2}(10)\approx 5.92} primes p with n decimal digits for which M p {\displaystyle M_{p}} is prime. Here, γ is the Euler–Mascheroni constant .
It is also not known whether infinitely many Mersenne numbers with prime exponents are composite, although this would follow from widely believed conjectures about prime numbers, for example, the infinitude of Sophie Germain primes congruent to 3 ( mod 4 ). For these primes p , 2 p + 1 (which is also prime) will divide M p , for example, 23 | M 11 , 47 | M 23 , 167 | M 83 , 263 | M 131 , 359 | M 179 , 383 | M 191 , 479 | M 239 , and 503 | M 251 (sequence A002515 in the OEIS ). For these primes p , 2 p + 1 is congruent to 7 mod 8, so 2 is a quadratic residue mod 2 p + 1 , and the multiplicative order of 2 mod 2 p + 1 must divide ( 2 p + 1 ) − 1 2 = p {\textstyle {\frac {(2p+1)-1}{2}}=p} . Since p is a prime, it must be p or 1. However, it cannot be 1 since Φ 1 ( 2 ) = 1 {\displaystyle \Phi _{1}(2)=1} and 1 has no prime factors , so it must be p . Hence, 2 p + 1 divides Φ p ( 2 ) = 2 p − 1 {\displaystyle \Phi _{p}(2)=2^{p}-1} and 2 p − 1 = M p {\displaystyle 2^{p}-1=M_{p}} cannot be prime.
The first four Mersenne primes are M 2 = 3 , M 3 = 7 , M 5 = 31 and M 7 = 127 and because the first Mersenne prime starts at M 2 , all Mersenne primes are congruent to 3 (mod 4). Other than M 0 = 0 and M 1 = 1 , all other Mersenne numbers are also congruent to 3 (mod 4). Consequently, in the prime factorization of a Mersenne number ( ≥ M 2 ) there must be at least one prime factor congruent to 3 (mod 4).
A basic theorem about Mersenne numbers states that if M p is prime, then the exponent p must also be prime. This follows from the identity 2 a b − 1 = ( 2 a − 1 ) ⋅ ( 1 + 2 a + 2 2 a + 2 3 a + ⋯ + 2 ( b − 1 ) a ) = ( 2 b − 1 ) ⋅ ( 1 + 2 b + 2 2 b + 2 3 b + ⋯ + 2 ( a − 1 ) b ) . {\displaystyle {\begin{aligned}2^{ab}-1&=(2^{a}-1)\cdot \left(1+2^{a}+2^{2a}+2^{3a}+\cdots +2^{(b-1)a}\right)\\&=(2^{b}-1)\cdot \left(1+2^{b}+2^{2b}+2^{3b}+\cdots +2^{(a-1)b}\right).\end{aligned}}} This rules out primality for Mersenne numbers with a composite exponent, such as M 4 = 2 4 − 1 = 15 = 3 × 5 = (2 2 − 1) × (1 + 2 2 ) .
Though the above examples might suggest that M p is prime for all primes p , this is not the case, and the smallest counterexample is the Mersenne number
The evidence at hand suggests that a randomly selected Mersenne number is much more likely to be prime than an arbitrary randomly selected odd integer of similar size. [ 4 ] Nonetheless, prime values of M p appear to grow increasingly sparse as p increases. For example, eight of the first 11 primes p give rise to a Mersenne prime M p (the correct terms on Mersenne's original list), while M p is prime for only 43 of the first two million prime numbers (up to 32,452,843).
Since Mersenne numbers grow very rapidly, the search for Mersenne primes is a difficult task, even though there is a simple efficient test to determine whether a given Mersenne number is prime: the Lucas–Lehmer primality test (LLT), which makes it much easier to test the primality of Mersenne numbers than that of most other numbers of the same size. The search for the largest known prime has somewhat of a cult following . [ citation needed ] Consequently, a large amount of computer power has been expended searching for new Mersenne primes, much of which is now done using distributed computing .
Arithmetic modulo a Mersenne number is particularly efficient on a binary computer , making them popular choices when a prime modulus is desired, such as the Park–Miller random number generator . To find a primitive polynomial of Mersenne number order requires knowing the factorization of that number, so Mersenne primes allow one to find primitive polynomials of very high order. Such primitive trinomials are used in pseudorandom number generators with very large periods such as the Mersenne twister , generalized shift register and Lagged Fibonacci generators .
Mersenne primes M p are closely connected to perfect numbers . In the 4th century BC, Euclid proved that if 2 p − 1 is prime, then 2 p − 1 (2 p − 1 ) is a perfect number. In the 18th century, Leonhard Euler proved that, conversely, all even perfect numbers have this form. [ 5 ] This is known as the Euclid–Euler theorem . It is unknown whether there are any odd perfect numbers .
Mersenne primes take their name from the 17th-century French scholar Marin Mersenne , who compiled what was supposed to be a list of Mersenne primes with exponents up to 257. The exponents listed by Mersenne in 1644 were as follows:
His list replicated the known primes of his time with exponents up to 19. His next entry, 31, was correct, but the list then became largely incorrect, as Mersenne mistakenly included M 67 and M 257 (which are composite) and omitted M 61 , M 89 , and M 107 (which are prime). Mersenne gave little indication of how he came up with his list. [ 6 ]
Édouard Lucas proved in 1876 that M 127 is indeed prime, as Mersenne claimed. This was the largest known prime number for 75 years until 1951, when Aimé Ferrier found a larger prime, ( 2 148 + 1 ) / 17 {\displaystyle (2^{148}+1)/17} , using a desk calculating machine. [ 7 ] : page 22 M 61 was determined to be prime in 1883 by Ivan Mikheevich Pervushin , though Mersenne claimed it was composite, and for this reason it is sometimes called Pervushin's number [ citation needed ] . This was the second-largest known prime number, and it remained so until 1911. Lucas had shown another error in Mersenne's list in 1876 by demonstrating that M 67 was composite without finding a factor. No factor was found until a famous talk by Frank Nelson Cole in 1903. [ 8 ] Without speaking a word, he went to a blackboard and raised 2 to the 67th power, then subtracted one, resulting in the number 147,573,952,589,676,412,927 . On the other side of the board, he multiplied 193,707,721 × 761,838,257,287 and got the same number, then returned to his seat (to applause) without speaking. [ 9 ] He later said that the result had taken him "three years of Sundays" to find. [ 10 ] A correct list of all Mersenne primes in this number range was completed and rigorously verified only about three centuries after Mersenne published his list.
Fast algorithms for finding Mersenne primes are available, and as of October 2024 [update] , the seven largest known prime numbers are Mersenne primes.
The first four Mersenne primes M 2 = 3 , M 3 = 7 , M 5 = 31 and M 7 = 127 were known in antiquity. The fifth, M 13 = 8191 , was discovered anonymously before 1461; the next two ( M 17 and M 19 ) were found by Pietro Cataldi in 1588. After nearly two centuries, M 31 was verified to be prime by Leonhard Euler in 1772. The next (in historical, not numerical order) was M 127 , found by Édouard Lucas in 1876, then M 61 by Ivan Mikheevich Pervushin in 1883. Two more ( M 89 and M 107 ) were found early in the 20th century, by R. E. Powers in 1911 and 1914, respectively.
The most efficient method presently known for testing the primality of Mersenne numbers is the Lucas–Lehmer primality test . Specifically, it can be shown that for prime p > 2 , M p = 2 p − 1 is prime if and only if M p divides S p − 2 , where S 0 = 4 and S k = ( S k − 1 ) 2 − 2 for k > 0 .
During the era of manual calculation, all previously untested exponents up to and including 257 were tested with the Lucas–Lehmer test and found to be composite. A notable contribution was made by retired Yale physics professor Horace Scudder Uhler, who did the calculations for exponents 157, 167, 193, 199, 227, and 229. [ 11 ] Unfortunately for those investigators, the interval they were testing contains the largest known relative gap between Mersenne primes: the next Mersenne prime exponent, 521, would turn out to be more than four times as large as the previous record of 127.
The search for Mersenne primes was revolutionized by the introduction of the electronic digital computer. Alan Turing searched for them on the Manchester Mark 1 in 1949, [ 12 ] but the first successful identification of a Mersenne prime, M 521 , by this means was achieved at 10:00 pm on January 30, 1952, using the U.S. National Bureau of Standards Western Automatic Computer (SWAC) at the Institute for Numerical Analysis at the University of California, Los Angeles (UCLA), under the direction of D. H. Lehmer , with a computer search program written and run by Prof. R. M. Robinson . It was the first Mersenne prime to be identified in thirty-eight years; the next one, M 607 , was found by the computer a little less than two hours later. Three more — M 1279 , M 2203 , and M 2281 — were found by the same program in the next several months. M 4,423 was the first prime discovered with more than 1000 digits, M 44,497 was the first with more than 10,000, and M 6,972,593 was the first with more than a million. In general, the number of digits in the decimal representation of M n equals ⌊ n × log 10 2⌋ + 1 , where ⌊ x ⌋ denotes the floor function (or equivalently ⌊log 10 M n ⌋ + 1 ).
In September 2008, mathematicians at UCLA participating in the Great Internet Mersenne Prime Search (GIMPS) won part of a $100,000 prize from the Electronic Frontier Foundation for their discovery of a very nearly 13-million-digit Mersenne prime. The prize, finally confirmed in October 2009, is for the first known prime with at least 10 million digits. The prime was found on a Dell OptiPlex 745 on August 23, 2008. This was the eighth Mersenne prime discovered at UCLA. [ 13 ]
On April 12, 2009, a GIMPS server log reported that a 47th Mersenne prime had possibly been found. The find was first noticed on June 4, 2009, and verified a week later. The prime is 2 42,643,801 − 1 . Although it is chronologically the 47th Mersenne prime to be discovered, it is smaller than the largest known at the time, which was the 45th to be discovered.
On January 25, 2013, Curtis Cooper , a mathematician at the University of Central Missouri , discovered a 48th Mersenne prime, 2 57,885,161 − 1 (a number with 17,425,170 digits), as a result of a search executed by a GIMPS server network. [ 14 ]
On January 19, 2016, Cooper published his discovery of a 49th Mersenne prime, 2 74,207,281 − 1 (a number with 22,338,618 digits), as a result of a search executed by a GIMPS server network. [ 15 ] [ 16 ] [ 17 ] This was the fourth Mersenne prime discovered by Cooper and his team in the past ten years.
On September 2, 2016, the Great Internet Mersenne Prime Search finished verifying all tests below M 37,156,667 , thus officially confirming its position as the 45th Mersenne prime. [ 18 ]
On January 3, 2018, it was announced that Jonathan Pace, a 51-year-old electrical engineer living in Germantown, Tennessee , had found a 50th Mersenne prime, 2 77,232,917 − 1 (a number with 23,249,425 digits), as a result of a search executed by a GIMPS server network. [ 19 ] The discovery was made by a computer in the offices of a church in the same town. [ 20 ] [ 21 ]
On December 21, 2018, it was announced that The Great Internet Mersenne Prime Search (GIMPS) discovered a new prime number, 2 82,589,933 − 1 , having 24,862,048 digits. A computer volunteered by Patrick Laroche from Ocala, Florida made the find on December 7, 2018. [ 22 ]
In late 2020, GIMPS began using a new technique to rule out potential Mersenne primes called the Probable prime (PRP) test, based on development from Robert Gerbicz in 2017, and a simple way to verify tests developed by Krzysztof Pietrzak in 2018. Due to the low error rate and ease of proof, this nearly halved the computing time to rule out potential primes over the Lucas-Lehmer test (as two users would no longer have to perform the same test to confirm the other's result), although exponents passing the PRP test still require one to confirm their primality. [ 23 ]
On October 12, 2024, a user named Luke Durant from San Jose, California, found the current largest known Mersenne prime, 2 136,279,841 − 1 , having 41,024,320 digits. This marks the first Mersenne prime with an exponent surpassing 8 digits. This was announced on October 21, 2024. [ 24 ]
Mersenne numbers are 0, 1, 3, 7, 15, 31, 63, ... (sequence A000225 in the OEIS ).
As of 2024 [update] , the 52 known Mersenne primes are 2 p − 1 for the following p :
Since they are prime numbers, Mersenne primes are divisible only by 1 and themselves. However, not all Mersenne numbers are Mersenne primes. Mersenne numbers are very good test cases for the special number field sieve algorithm, so often the largest number factorized with this algorithm has been a Mersenne number. As of June 2019 [update] , 2 1,193 − 1 is the record-holder, [ 27 ] having been factored with a variant of the special number field sieve that allows the factorization of several numbers at once. See integer factorization records for links to more information. The special number field sieve can factorize numbers with more than one large factor. If a number has only one very large factor then other algorithms can factorize larger numbers by first finding small factors and then running a primality test on the cofactor. As of September 2022 [update] , the largest completely factored number (with probable prime factors allowed) is 2 12,720,787 − 1 = 1,119,429,257 × 175,573,124,547,437,977 × 8,480,999,878,421,106,991 × q , where q is a 3,829,294-digit probable prime. It was discovered by a GIMPS participant with nickname "Funky Waddle". [ 28 ] [ 29 ] As of September 2022 [update] , the Mersenne number M 1277 is the smallest composite Mersenne number with no known factors; it has no prime factors below 2 68 , [ 30 ] and is very unlikely to have any factors below 10 65 (~2 216 ). [ 31 ]
The table below shows factorizations for the first 20 composite Mersenne numbers where the exponent p is a prime number.((sequence A244453 in the OEIS ))
The number of factors for the first 500 Mersenne numbers can be found at (sequence A046800 in the OEIS ).
In the mathematical problem Tower of Hanoi , solving a puzzle with an n -disc tower requires M n steps, assuming no mistakes are made. [ 32 ] The number of rice grains on the whole chessboard in the wheat and chessboard problem is M 64 . [ 33 ]
The asteroid with minor planet number 8191 is named 8191 Mersenne after Marin Mersenne, because 8191 is a Mersenne prime. [ 34 ]
In geometry , an integer right triangle that is primitive and has its even leg a power of 2 ( ≥ 4 ) generates a unique right triangle such that its inradius is always a Mersenne number. For example, if the even leg is 2 n + 1 then because it is primitive it constrains the odd leg to be 4 n − 1 , the hypotenuse to be 4 n + 1 and its inradius to be 2 n − 1 . [ 35 ]
A Mersenne–Fermat number is defined as 2 p r − 1 / 2 p r − 1 − 1 with p prime, r natural number, and can be written as MF( p , r ) . When r = 1 , it is a Mersenne number. When p = 2 , it is a Fermat number . The only known Mersenne–Fermat primes with r > 1 are
In fact, MF( p , r ) = Φ p r (2) , where Φ is the cyclotomic polynomial .
The simplest generalized Mersenne primes are prime numbers of the form f (2 n ) , where f ( x ) is a low-degree polynomial with small integer coefficients . [ 37 ] An example is 2 64 − 2 32 + 1 , in this case, n = 32 , and f ( x ) = x 2 − x + 1 ; another example is 2 192 − 2 64 − 1 , in this case, n = 64 , and f ( x ) = x 3 − x − 1 .
It is also natural to try to generalize primes of the form 2 n − 1 to primes of the form b n − 1 (for b ≠ 2 and n > 1 ). However (see also theorems above ), b n − 1 is always divisible by b − 1 , so unless the latter is a unit , the former is not a prime. This can be remedied by allowing b to be an algebraic integer instead of an integer:
In the ring of integers (on real numbers ), if b − 1 is a unit , then b is either 2 or 0. But 2 n − 1 are the usual Mersenne primes, and the formula 0 n − 1 does not lead to anything interesting (since it is always −1 for all n > 0 ). Thus, we can regard a ring of "integers" on complex numbers instead of real numbers , like Gaussian integers and Eisenstein integers .
If we regard the ring of Gaussian integers , we get the case b = 1 + i and b = 1 − i , and can ask ( WLOG ) for which n the number (1 + i ) n − 1 is a Gaussian prime which will then be called a Gaussian Mersenne prime . [ 38 ]
(1 + i ) n − 1 is a Gaussian prime for the following n :
Like the sequence of exponents for usual Mersenne primes, this sequence contains only (rational) prime numbers.
As for all Gaussian primes, the norms (that is, squares of absolute values) of these numbers are rational primes:
One may encounter cases where such a Mersenne prime is also an Eisenstein prime , being of the form b = 1 + ω and b = 1 − ω . In these cases, such numbers are called Eisenstein Mersenne primes .
(1 + ω ) n − 1 is an Eisenstein prime for the following n :
The norms (that is, squares of absolute values) of these Eisenstein primes are rational primes:
The other way to deal with the fact that b n − 1 is always divisible by b − 1 , it is to simply take out this factor and ask which values of n make
be prime. (The integer b can be either positive or negative.) If, for example, we take b = 10 , we get n values of:
These primes are called repunit primes. Another example is when we take b = −12 , we get n values of:
It is a conjecture that for every integer b which is not a perfect power , there are infinitely many values of n such that b n − 1 / b − 1 is prime. (When b is a perfect power, it can be shown that there is at most one n value such that b n − 1 / b − 1 is prime)
Least n such that b n − 1 / b − 1 is prime are (starting with b = 2 , 0 if no such n exists)
For negative bases b , they are (starting with b = −2 , 0 if no such n exists)
Least base b such that b prime( n ) − 1 / b − 1 is prime are
For negative bases b , they are
Another generalized Mersenne number is
with a , b any coprime integers, a > 1 and − a < b < a . (Since a n − b n is always divisible by a − b , the division is necessary for there to be any chance of finding prime numbers.) [ a ] We can ask which n makes this number prime. It can be shown that such n must be primes themselves or equal to 4, and n can be 4 if and only if a + b = 1 and a 2 + b 2 is prime. [ b ] It is a conjecture that for any pair ( a , b ) such that a and b are not both perfect r th powers for any r and −4 ab is not a perfect fourth power , there are infinitely many values of n such that a n − b n / a − b is prime. [ c ] However, this has not been proved for any single value of ( a , b ) .
* Note: if b < 0 and n is even, then the numbers n are not included in the corresponding OEIS sequence.
When a = b + 1 , it is ( b + 1) n − b n , a difference of two consecutive perfect n th powers, and if a n − b n is prime, then a must be b + 1 , because it is divisible by a − b .
Least n such that ( b + 1) n − b n is prime are
Least b such that ( b + 1) prime( n ) − b prime( n ) is prime are | https://en.wikipedia.org/wiki/Mersenne_prime |
In analytic number theory , Mertens' theorems are three 1874 results related to the density of prime numbers proved by Franz Mertens . [ 1 ]
In the following, let p ≤ n {\displaystyle p\leq n} mean all primes not exceeding n .
Mertens' first theorem is that
does not exceed 2 in absolute value for any n ≥ 2 {\displaystyle n\geq 2} . ( A083343 )
Mertens' second theorem is
where M is the Meissel–Mertens constant ( A077761 ). More precisely, Mertens [ 1 ] proves that the expression under the limit does not in absolute value exceed
for any n ≥ 2 {\displaystyle n\geq 2} .
The main step in the proof of Mertens' second theorem is
where the last equality needs ∑ p k ≤ n log p = O ( n ) {\displaystyle \sum _{p^{k}\leq n}\log p=O(n)} which follows from ∑ p ∈ ( n , 2 n ] log p ≤ log ( 2 n n ) = O ( n ) {\displaystyle \sum _{p\in (n,2n]}\log p\leq \log {2n \choose n}=O(n)} .
Thus, we have proved that
Since the sum over prime powers with k ≥ 2 {\displaystyle k\geq 2} converges, this implies
A partial summation yields
In a paper [ 2 ] on the growth rate of the sum-of-divisors function published in 1983, Guy Robin proved that in Mertens' 2nd theorem the difference
changes sign infinitely often, and that in Mertens' 3rd theorem the difference
changes sign infinitely often. Robin's results are analogous to Littlewood 's famous theorem that the difference π( x ) − li( x ) changes sign infinitely often. No analog of the Skewes number (an upper bound on the first natural number x for which π( x ) > li( x )) is known in the case of Mertens' 2nd and 3rd theorems.
Regarding this asymptotic formula Mertens refers in his paper to "two curious formula of Legendre", [ 1 ] the first one being Mertens' second theorem's prototype (and the second one being Mertens' third theorem's prototype: see the very first lines of the paper). He recalls that it is contained in Legendre's third edition of his "Théorie des nombres" (1830; it is in fact already mentioned in the second edition, 1808), and also that a more elaborate version was proved by Chebyshev in 1851. [ 3 ] Note that, already in 1737, Euler knew the asymptotic behaviour of this sum.
Mertens diplomatically describes his proof as more precise and rigorous. In reality none of the previous proofs are acceptable by modern standards: Euler's computations involve the infinity (and the hyperbolic logarithm of infinity, and the logarithm of the logarithm of infinity!); Legendre's argument is heuristic; and Chebyshev's proof, although perfectly sound, makes use of the Legendre-Gauss conjecture, which was not proved until 1896 and became better known as the prime number theorem .
Mertens' proof does not appeal to any unproved hypothesis (in 1874), and only to elementary real analysis. It comes 22 years before the first proof of the prime number theorem which, by contrast, relies on a careful analysis of the behavior of the Riemann zeta function as a function of a complex variable.
Mertens' proof is in that respect remarkable. Indeed, with modern notation it yields
whereas the prime number theorem (in its simplest form, without error estimate), can be shown to imply [ 4 ]
In 1909 Edmund Landau , by using the best version of the prime number theorem then at his disposition, proved [ 5 ] that
holds; in particular the error term is smaller than 1 / ( log x ) k {\displaystyle 1/(\log x)^{k}} for any fixed integer k . A simple summation by parts exploiting the strongest form known of the prime number theorem improves this to
for some c > 0 {\displaystyle c>0} .
Similarly a partial summation shows that ∑ p ≤ x log p p = log x + C + o ( 1 ) {\displaystyle \sum _{p\leq x}{\frac {\log p}{p}}=\log x+C+o(1)} is implied by the PNT.
Mertens' third theorem is
where γ is the Euler–Mascheroni constant ( A001620 ).
An estimate of the probability of X {\displaystyle X} ( X ≫ n {\displaystyle X\gg n} ) having no factor ≤ n {\displaystyle \leq n} is given by
This is closely related to Mertens' third theorem which gives an asymptotic approximation of | https://en.wikipedia.org/wiki/Mertens'_theorems |
Mesangial cells are specialised cells in the kidney that make up the mesangium of the glomerulus . Together with the mesangial matrix, they form the vascular pole of the renal corpuscle . [ 1 ] The mesangial cell population accounts for approximately 30-40% of the total cells in the glomerulus. [ 2 ] Mesangial cells can be categorized as either extraglomerular mesangial cells or intraglomerular mesangial cells , based on their relative location to the glomerulus. The extraglomerular mesangial cells are found between the afferent and efferent arterioles towards the vascular pole of the glomerulus. [ 3 ] The extraglomerular mesangial cells are adjacent to the intraglomerular mesangial cells that are located inside the glomerulus and in between the capillaries . [ 4 ] The primary function of mesangial cells is to remove trapped residues and aggregated protein from the basement membrane thus keeping the filter free of debris. The contractile properties of mesangial cells have been shown to be insignificant in changing the filtration pressure of the glomerulus. [ citation needed ]
Mesangial cells have irregular shapes with flattened-cylinder-like cell bodies and processes at both ends containing actin , myosin and actinin , giving mesangial cells contractile properties. [ 5 ] The anchoring filaments from mesangial cells to the glomerular basement membrane can alter capillary flow by changing glomerular ultrafiltration surface area. [ 1 ] Extraglomerular mesangial cells are in close connection to afferent and efferent arteriolar cells by gap junctions , allowing for intercellular communication. [ 3 ] Mesangial cells are separated by intercellular spaces containing extracellular matrix called the mesangial matrix that is produced by the mesangial cells. [ 1 ] Mesangial matrix provides structural support for the mesangium. [ 1 ] Mesangial matrix is composed of glomerular matrix proteins such as collagen IV (α1 and α2 chains), collagen V, collagen VI, laminin A, B1, B2, fibronectin , and proteoglycans . [ 6 ]
It is unclear whether the mesangial cells originate from mesenchymal or stromal cells . However there is evidence suggesting that they originate elsewhere outside of the glomerulus and then migrate into the glomerulus during development. [ 7 ] Human foetal and infant kidneys stained for alpha smooth muscle actin (α-SMA), a marker for mesangial cells, demonstrated that α-SMA-positive mesenchymal cells migrate towards the glomerulus and during a later stage they can be found within the mesangium. [ 5 ] It is possible that they share the same origin as supporting cells such as pericytes and vascular smooth muscle cells, or even be a type of specialised vascular smooth muscle cell. [ 8 ]
During development mesangial cells are important in the formation of convoluted capillaries allowing for efficient diffusion to occur. Endothelial precursor cells secrete platelet-derived growth factor (PDGF)-B and mesangial cells have receptors for PDGF. This induces mesangial cells to attach to endothelial cells causing developing blood vessels to loop resulting in convoluted capillaries. [ 8 ] Mice lacking the growth factor PDGF-B or PDGFRβ do not develop mesangial cells. [ 8 ] When mesangial cells are absent the blood vessel becomes a single dilated vessel with up to 100-fold decrease in surface area. [ 8 ] The transcription factor for PDGFRβ, Tbx18, is crucial for the development of mesangial cells. Without Tbx18 the development of mesangial cells is compromised and results in the formation of dilated loops. [ 8 ] Mesangial cell progenitors are also a target of PDGF-B and can be selected for by the signal to then develop into mesangial cells. [ 9 ]
Mesangial cells form a glomerular functional unit with glomerular endothelial cells and podocytes through interactions of molecular signalling pathways which are essential for the formation of the glomerular tuft. [ 1 ] Mesangial cells aid filtration by constituting part of the glomerular capillary tuft structure that filters fluids to produce urine. [ 10 ] Communication between mesangial cells and vascular smooth muscle cells via gap junctions helps regulate the process of tubuloglomerular feedback and urine formation. [ 11 ] Damage to mesangial cells using Thy 1-1 antibody specific to mesangial cells causes the vasoconstriction of arterioles mediated by tubuloglomerular feedback to be lost. [ 11 ]
Mesangial cells can contract and relax to regulate capillary flow. [ 1 ] This is regulated by vasoactive substances . [ 12 ] Contraction of mesangial cells is dependent on cell membrane permeability to calcium ions and relaxation is mediated by paracrine factors, hormones and cAMP . [ 12 ] In response to capillary stretching, mesangial cells can respond by producing several growth factors: TGF -1, VEGF and connective tissue growth factor . [ 1 ]
The mesangium is exposed to macromolecules from the capillary lumen as they are separated only by fenestrated endothelium without basement membrane. [ 2 ] Mesangial cells play a role in restricting macromolecules from accumulating in the mesangial space by receptor- independent uptake processes of phagocytosis , micro- and macro- pinocytosis , or receptor-dependent processes and then transported along the mesangial stalk. [ 1 ] Size, charge, concentration, and affinity for mesangial cell receptors of the macromolecule affects how the macromolecule is removed. [ 13 ] Triglycerides may undergo pinocytosis and antibody IgG complexes may lead to activation of adhesion molecules and chemokines by mesangial cells. [ 1 ] They also regulate glomerular filtration.
The expansion of mesangial matrix is one characteristic of diabetic nephropathy although it also involves other cells in interaction including podocytes and endothelial cells. [ 14 ] Mesangial expansion occurs due to increased deposition of extracellular matrix proteins, for example fibronectin, into the mesangium. [ 6 ] Accumulation of extracellular matrix proteins then occurs due to insufficient degradation by matrix metalloproteinases . [ 6 ]
Increased glucose levels results in the activation of metabolic pathways leading to increased oxidative stress . [ 2 ] This in turn results in the over-production and accumulation of advanced glycosylation end products responsible for enhancing the risk of developing glomerular diseases. [ 15 ] Mesangial cells grown on advanced glycosylation end product-modified matrix proteins demonstrate increased production of fibronectin and a decrease in proliferation. [ 15 ] These factors eventually lead to the thickening of the glomerular basement membrane, mesangial matrix expansion then glomerulosclerosis and fibrosis . [ 16 ]
Mesangial pathologies may also develop during the early phase of diabetes. Glomerular hypertension causes mesangial cells to stretch which causes induced expression of GLUT1 leading to increased cellular glucose. [ 16 ] The repetition of stretching and relaxation cycle of mesangial cells due to hypertension increases mesangial cell proliferation and the production of extracellular matrix which can then accumulate and lead to glomerular disease. [ 16 ] | https://en.wikipedia.org/wiki/Mesangial_cell |
Mesembrine is an alkaloid primarily derived from the plant Sceletium tortuosum , commonly known as kanna. This compound is noted for its psychoactive properties, particularly as a serotonin reuptake inhibitor , which contributes to its potential use in treating mood disorders and anxiety . Mesembrine has garnered interest in both traditional medicine and modern pharmacology, where it is explored for its effects on enhancing mood and cognitive function.
Kanna itself has a long history of use by indigenous peoples in southern Africa, who utilized it for its mood-enhancing and stress-relieving effects, often consuming it in various forms such as teas or chews. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
Mesembrine has also been identified in Mesembryanthemum cordifolium , Delosperma echinatum , and Oscularia deltoides . [ 5 ]
Mesembrine has been shown to act as a serotonin reuptake inhibitor (K i = 1.4 nM), and has also been found to behave as a weak inhibitor of the enzyme phosphodiesterase 4 (PDE4) (K i = 7,800 nM). [ 6 ] A concentrated mesembrine extract of Sceletium tortuosum may exert antidepressant effects by acting as a monoamine releasing agent . [ 7 ] As such, mesembrine likely plays a dominant role in the antidepressant effects of kanna. [ 8 ]
Rat studies have evaluated effects of kanna extract, finding analgesic and antidepressant potential. [ 9 ] No adverse results were noted for a commercial extract up to 5000 mg/kg daily in rats. [ 10 ]
Mesembrine was first isolated and characterized in 1957. [ 11 ] It is a tricyclic molecule with two bridgehead chiral carbons located between the five-membered and six-membered rings. The naturally occurring form of mesembrine produced by plants is the levorotatory isomer, (−)-mesembrine, where the carbon atoms at positions 3a and 7a both have the S configuration (3aS,7aS). [ 12 ]
Because of its structure and bioactivity, mesembrine has been a target for total synthesis over the past 40 years. Over 40 total syntheses have been reported for mesembrine, most of which focused on different approaches and strategies for the construction of the bicyclic ring system and the quaternary carbon .
The first total synthesis of mesembrine was reported in 1965. [ 13 ] This route has 21 steps, which was among the longest synthetic routes for mesembrine. Key steps involve the construction of the six-membered ketone ring by Diels–Alder reaction , α-allylation for synthesis of the quaternary carbon, and conjugate addition reaction for the final five-membered ring closure. The final product from this route is a racemic mixture of (+)- and (-)-mesembrine.
In 1971, first asymmetric total synthesis of (+)-mesembrine was reported. [ 14 ] This synthesis introduced the quaternary carbon atom through an asymmetric Robinson annulation reaction, which was mediated by a chiral auxiliary derived from L -proline . In the final step, an intramolecular aza- Michael addition produced the fused pyrrolidine ring system. | https://en.wikipedia.org/wiki/Mesembrine |
A mesenchymal–epithelial transition ( MET ) is a reversible biological process that involves the transition from motile, multipolar or spindle-shaped mesenchymal cells to planar arrays of polarized cells called epithelia . MET is the reverse process of epithelial–mesenchymal transition (EMT) and it has been shown to occur in normal development, induced pluripotent stem cell reprogramming, [ 1 ] cancer metastasis [ 2 ] and wound healing. [ 3 ]
Unlike epithelial cells – which are stationary and characterized by an apico-basal polarity with binding by a basal lamina , tight junctions , gap junctions , adherent junctions and expression of cell-cell adhesion markers such as E-cadherin , [ 4 ] mesenchymal cells do not make mature cell-cell contacts, can invade through the extracellular matrix , and express markers such as vimentin , fibronectin , N-cadherin , Twist , and Snail . [ 4 ] MET plays also a critical role in metabolic switching and epigenetic modifications . In general, epithelium-associated genes are upregulated and mesenchyme -associated genes are downregulated in the process of MET. [ 5 ]
During embryogenesis and early development, cells switch back and forth between different cellular phenotypes via MET and its reverse process, epithelial–mesenchymal transition (EMT). Developmental METs have been studied most extensively in embryogenesis during somitogenesis [ 6 ] and nephrogenesis [ 7 ] and carcinogenesis during metastasis , [ 8 ] but it also occurs in cardiogenesis [ 9 ] or foregut development. [ 10 ] MET is an essential process in embryogenesis to gather mesenchymal-like cells into cohesive structures. [ 1 ] Although the mechanism of MET during various organs morphogenesis is quite similar, each process has a unique signaling pathway to induce changes in gene expression profiles.
One example of this, the most well described of the developmental METs, is kidney ontogenesis . The mammalian kidney is primarily formed by two early structures: the ureteric bud and the nephrogenic mesenchyme, which form the collecting duct and nephrons respectively (see kidney development for more details). During kidney ontogenesis, a reciprocal induction of the ureteric bud epithelium and nephrogenic mesenchyme occurs. As the ureteric bud grows out of the Wolffian duct, the nephrogenic mesenchyme induces the ureteric bud to branch. Concurrently, the ureteric bud induces the nephrogenic mesenchyme to condense around the bud and undergo MET to form the renal epithelium, which ultimately forms the nephron . [ 7 ] Growth factors , integrins , cell adhesion molecules, and protooncogenes , such as c-ret , c-ros , and c-met , mediate the reciprocal induction in metanephrons and consequent MET. [ 11 ]
Another example of developmental MET occurs during somitogenesis . Vertebrate somites, the precursors of axial bones and trunk skeletal muscles, are formed by the maturation of the presomitic mesoderm (PSM). The PSM, which is composed of mesenchymal cells, undergoes segmentation by delineating somite boundaries (see somitogenesis for more details). Each somite is encapsulated by an epithelium, formerly mesenchymal cells that had undergone MET. Two Rho family GTPases – Cdc42 and Rac1 – as well as the transcription factor Paraxis are required for chick somitic MET. [ 12 ]
Development of heart is involved in several rounds of EMT and MET. While development splanchnopleure undergo EMT and produce endothelial progenitors, these then form the endocardium through MET. Pericardium is formed by sinus venosus mesenchymal cells that undergo MET. [ 1 ] Quite similar processes occur also while regeneration in the injured heart. Injured pericardium undergoes EMT and is transformed into adipocytes or myofibroblasts which induce arrhythmia and scars. MET than leads to the formation of vascular and epithelial progenitors that can differentiate into vasculogenic cells which lead to regeneration of heart injury. [ 9 ]
[ 13 ]
While relatively little is known about the role MET plays in cancer when compared to the extensive studies of EMT in tumor metastasis, MET is believed to participate in the establishment and stabilization of distant metastases by allowing cancerous cells to regain epithelial properties and integrate into distant organs. Between these two states, cells occur in 'intermediate‐state', or so‐called partial EMT. [ 8 ]
In recent years, researchers have begun to investigate MET as one of many potential therapeutic targets in the prevention of metastases. [ 14 ] This approach to preventing metastasis is known as differentiation-based therapy or differentiation therapy and it can be used for development of new anti-cancer therapeutic strategies. [ 1 ]
A number of different cellular processes must take place in order for somatic cells to undergo reprogramming into induced pluripotent stem cells (iPS cells). iPS cell reprogramming, also known as somatic cell reprogramming, can be achieved by ectopic expression of Oct4 , Klf4 , Sox2 , and c-Myc (OKSM). [ 15 ] Upon induction, mouse fibroblasts must undergo MET to successfully begin the initiation phase of reprogramming. Epithelial-associated genes such as E-cadherin/ Cdh1 , Cldns −3, −4, −7, −11, Occludin (Ocln), Epithelial cell adhesion molecule (Epcam), and Crumbs homolog 3 (Crb3), were all upregulated before Nanog , a key transcription factor in maintaining pluripotency , was turned on. Additionally, mesenchymal-associated genes such as Snail, Slug, Zeb −1, −2, and N-cadherin were downregulated within the first 5 days post-OKSM induction. [ 16 ] Addition of exogenous TGF-β1 , which blocks MET, decreased iPS reprogramming efficiency significantly. [ 17 ] These findings are all consistent with previous observations that embryonic stem cells resemble epithelial cells and express E-cadherin. [ 18 ]
Recent studies have suggested that ectopic expression of Klf4 in iPS cell reprogramming may be specifically responsible for inducing E-cadherin expression by binding to promoter regions and the first intron of CDH1 (the gene encoding for E-cadherin). [ 17 ] | https://en.wikipedia.org/wiki/Mesenchymal–epithelial_transition |
Mesentoblasts , also called 4d cells , are the cells from which the mesoderm originates. Mesentoblasts are found in the blastopore area between the endoderm and the ectoderm . In protostomes the embryos are mosaic, so mesentoblast removal will result in failure of formation of the mesoderm and other structures related to the mesoderm, which in turn will give abnormal embryos. The mesentoblast migrates to the blastocoel where it reproduces to form a mass of cells that becomes the mesoderm. [ 1 ]
This developmental biology article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Mesentoblast |
A mesh is a barrier made of interlaced strands of metal , fiber or other flexible or ductile materials. A mesh is similar to a web or a net in that it has many interwoven strands. | https://en.wikipedia.org/wiki/Mesh |
Mesh analysis (or the mesh current method ) is a circuit analysis method for planar circuits ; planar circuits are circuits that can be drawn on a plane surface with no wires crossing each other. A more general technique, called loop analysis (with the corresponding network variables called loop currents ) can be applied to any circuit, planar or not [ citation needed ] .
Mesh analysis and loop analysis both make systematic use of Kirchhoff’s voltage law (KVL) to arrive at a set of equations guaranteed to be solvable if the circuit has a solution. [ 1 ] Similarly, nodal analysis is a systematic application of Kirchhoff's current law (KCL). Mesh analysis is usually easier to use when the circuit is planar, compared to loop analysis. [ 2 ]
Mesh analysis works by arbitrarily assigning mesh currents in the essential meshes (also referred to as independent meshes). An essential mesh is a loop in the circuit that does not contain any other loop. Figure 1 labels the essential meshes with one, two, and three. [ 3 ]
A mesh current is a current that loops around the essential mesh and the equations are solved in terms of them. A mesh current may not correspond to any physically flowing current, but the physical currents are easily found from them. [ 2 ] It is usual practice to have all the mesh currents loop in the same direction. This helps prevent errors when writing out the equations. The convention is to have all the mesh currents looping in a clockwise direction. [ 3 ] Figure 2 shows the same circuit from Figure 1 with the mesh currents labeled.
Solving for mesh currents instead of directly applying Kirchhoff's current law and Kirchhoff's voltage law can greatly reduce the amount of calculation required. This is because there are fewer mesh currents than there are physical branch currents. In figure 2 for example, there are six branch currents but only three mesh currents.
Each mesh produces one equation. These equations are the sum of the voltage drops in a complete loop of the mesh current. [ 3 ] For problems more general than those including current and voltage sources , the voltage drops will be the impedance of the electronic component multiplied by the mesh current in that loop. [ 4 ]
If a voltage source is present within the mesh loop, the voltage at the source is either added or subtracted depending on if it is a voltage drop or a voltage rise in the direction of the mesh current. For a current source that is not contained between two meshes (for example, the current source in essential mesh 1 in the circuit above), the mesh current will take the positive or negative value of the current source depending on if the mesh current is in the same or opposite direction of the current source . [ 3 ] The following is the same circuit from above with the equations needed to solve for all the currents in the circuit.
{ Mesh 1: I 1 = I s Mesh 2: − V s + R 1 ( I 2 − I 1 ) + 1 s C ( I 2 − I 3 ) = 0 Mesh 3: 1 s C ( I 3 − I 2 ) + R 2 ( I 3 − I 1 ) + s L I 3 = 0 {\displaystyle {\begin{cases}{\text{Mesh 1: }}I_{1}=I_{s}\\{\text{Mesh 2: }}-V_{s}+R_{1}(I_{2}-I_{1})+{\frac {1}{sC}}(I_{2}-I_{3})=0\\{\text{Mesh 3: }}{\frac {1}{sC}}(I_{3}-I_{2})+R_{2}(I_{3}-I_{1})+sLI_{3}=0\\\end{cases}}\,}
Once the equations are found, the system of linear equations can be solved by using any technique to solve linear equations .
There are two special cases in mesh current: currents containing a supermesh and currents containing dependent sources .
A supermesh occurs when a current source is contained between two essential meshes. The circuit is first treated as if the current source is not there. This leads to one equation that incorporates two mesh currents. Once this equation is formed, an equation is needed that relates the two mesh currents with the current source . This will be an equation where the current source is equal to one of the mesh currents minus the other. The following is a simple example of dealing with a supermesh. [ 2 ]
{ Mesh 1, 2: − V s + R 1 I 1 + R 2 I 2 = 0 Current source: I s = I 2 − I 1 {\displaystyle {\begin{cases}{\text{Mesh 1, 2: }}-V_{s}+R_{1}I_{1}+R_{2}I_{2}=0\\{\text{Current source: }}I_{s}=I_{2}-I_{1}\end{cases}}\,}
A dependent source is a current source or voltage source that depends on the voltage or current of another element in the circuit. When a dependent source is contained within an essential mesh, the dependent source should be treated like an independent source. After the mesh equation is formed, a dependent source equation is needed. This equation is generally called a constraint equation. This is an equation that relates the dependent source’s variable to the voltage or current that the source depends on in the circuit. The following is a simple example of a dependent source. [ 2 ]
{ Mesh 1: − V s + R 1 I 1 + R 3 ( I 1 − I 2 ) = 0 Mesh 2: R 2 I 2 + 3 I x + R 3 ( I 2 − I 1 ) = 0 Dependent variable: I x = I 1 − I 2 {\displaystyle {\begin{cases}{\text{Mesh 1: }}-V_{s}+R_{1}I_{1}+R_{3}(I_{1}-I_{2})=0\\{\text{Mesh 2: }}R_{2}I_{2}+3I_{x}+R_{3}(I_{2}-I_{1})=0\\{\text{Dependent variable: }}I_{x}=I_{1}-I_{2}\end{cases}}\,} | https://en.wikipedia.org/wiki/Mesh_analysis |
In graph theory , Meshulam's game is a game used to explain a theorem of Roy Meshulam [ 1 ] related to the homological connectivity of the independence complex of a graph, which is the smallest index k such that all reduced homological groups up to and including k are trivial. The formulation of this theorem as a game is due to Aharoni, Berger and Ziv. [ 2 ] [ 3 ]
The game-board is a graph G. It is a zero-sum game for two players, CON and NON. CON wants to show that I( G ), the independence complex of G , has a high connectivity ; NON wants to prove the opposite.
At his turn, CON chooses an edge e from the remaining graph. NON then chooses one of two options:
The score of CON is defined as follows:
For every given graph G , the game value on G (i.e., the score of CON when both sides play optimally) is denoted by Ψ ( G ).
Meshulam [ 1 ] proved that, for every graph G :
η H ( I ( G ) ) ≥ Ψ ( G ) {\displaystyle \eta _{H}(I(G))\geq \Psi (G)}
where η H ( I ( G ) ) {\displaystyle \eta _{H}(I(G))} is the homological connectivity of I ( G ) {\displaystyle I(G)} plus 2.
To illustrate the connection between Meshulam's game and connectivity, we prove it in the special case in which η H ( I ( G ) ) = 1 {\displaystyle \eta _{H}(I(G))=1} , which is the smallest possible value of η H ( I ( G ) ) {\displaystyle \eta _{H}(I(G))} . We prove that, in this case, Ψ ( G ) ≤ 1 {\displaystyle \Psi (G)\leq 1} , i.e., NON can always destroy the entire graph using at most one explosion.
η H ( I ( G ) ) = 1 {\displaystyle \eta _{H}(I(G))=1} means that I ( G ) {\displaystyle I(G)} is not connected. This means that there are two subsets of vertices, X and Y , where no edge in I ( G ) {\displaystyle I(G)} connects any vertex of X to any vertex of Y. But I ( G ) {\displaystyle I(G)} is the independence complex of G ; so in G , every vertex of X is connected to every vertex of Y . Regardless of how CON plays, he must at some step select an edge between a vertex of X and a vertex of Y . NON can explode this edge and destroy the entire graph.
In general, the proof works only one way, that is, there may be graphs for which η H ( I ( G ) ) > Ψ ( G ) {\displaystyle \eta _{H}(I(G))>\Psi (G)} . | https://en.wikipedia.org/wiki/Meshulam's_game |
A meso compound or meso isomer is an optically inactive isomer in a set of stereoisomers , at least two of which are optically active. [ 1 ] [ 2 ] This means that despite containing two or more stereocenters , the molecule is not chiral . A meso compound is superposable on its mirror image (not to be confused with superimposable, as any two objects can be superimposed over one another regardless of whether they are the same). Two objects can be superposed if all aspects of the objects coincide and it does not produce a "(+)" or "(-)" reading when analyzed with a polarimeter . [ 3 ] The name is derived from the Greek mésos meaning “middle”.
For example, tartaric acid can exist as any of three stereoisomers depicted below in a Fischer projection . Of the four colored pictures at the top of the diagram , the first two represent the meso compound (the 2 R ,3 S and 2 S ,3 R isomers are equivalent), followed by the optically active pair of levotartaric acid (L-( R,R )-(+)-tartaric acid) and dextrotartaric acid (D-( S,S )-(-)-tartaric acid). The meso compound is bisected by an internal plane of symmetry that is not present for the non-meso isomers (indicated by an X). That is, on reflecting the meso compound through a mirror plane perpendicular to the screen, the same stereochemistry is obtained; this is not the case for the non-meso tartaric acid, [ 3 ] which generates the other enantiomer . The meso compound must not be confused with a 50:50 racemic mixture of the two optically-active compounds, although neither will rotate light in a polarimeter .
It is a requirement for two of the stereocenters in a meso compound to have at least two substituents in common (although having this characteristic does not necessarily mean that the compound is meso). For example, in 2,4-pentanediol, both the second and fourth carbon atoms, which are stereocenters, have all four substituents in common.
Since a meso isomer has a superposable mirror image, a compound with a total of n chiral centers cannot attain the theoretical maximum of 2 n stereoisomers if one of the stereoisomers is meso. [ 4 ]
A meso isomer need not have a mirror plane. It may have an inversion or a rotoreflexion symmetry such as S 4 . For example, there are two meso isomers of 1,4-difluoro-2,5-dichlorocyclohexane but neither has a mirror plane, and there are two meso isomers of 1,2,3,4-tetrafluorospiropentane (see figure).
1,2-substituted cyclopropane has a meso cis -isomer (molecule has a mirror plane) and two trans -enantiomers:
The two cis stereoisomers of 1,2-substituted cyclohexanes behave like meso compounds at room temperature in most cases. At room temperature, most 1,2-disubstituted cyclohexanes undergo rapid ring flipping (exceptions being rings with bulky substituents), and as a result, the two cis stereoisomers behave chemically identically with chiral reagents. [ 5 ] At low temperatures, however, this is not the case, as the activation energy for the ring-flip cannot be overcome, and they therefore behave like enantiomers. Also noteworthy is the fact that when a cyclohexane undergoes a ring flip, the absolute configurations of the stereocenters do not change. | https://en.wikipedia.org/wiki/Meso_compound |
The Mesoamerican Society for Ecological Economics ( SMEE ) is a regional chapter of the International Society for Ecological Economics (ISEE). After its foundation in 2008 at Guatemala City, the organization has already celebrated its first International Conference in 2010 at Mexico City and will carry out the second International Conference, EcoEco Alternatives, between March 4 and 8 2014 at the main campus of the University of Costa Rica . [ 1 ]
This branch of the ISEE has a unique emphasis within ecological economics . Topics like social justice and the human value in environmental conservation prevail in this region. As a consequence of the strong influence from Joan Martinez Alier's "environmentalism of the poor or social environmentalism", major attention is given to ecological-distributive conflicts. Alier insists that in the South a struggle exists against these conflicts generated by economic growth, mainly by the North. These endeavors "attempt to preserve the access of the communities to natural resources and services." [ 2 ]
On top of the negative effects on the environment by economic distribution, the cultural influence is also widely debated. For instance, the anthropologist Arturo Escobar suggests that culturally-driven preferences are one of the main factors degrading the environment. For example, society naturally gives privilege to the capitalist model that distributes natural resources with the purposes of production and profit, instead of endorsing the agroforestal ecosystem model, which is less harmful to the environment . [ 3 ] As part of this alternate perception in Mesoamerica , Ecological economics doesn't consider that the economic valuation of natural resources nor environmental norms are effective solutions to these social-environmental conflicts. [ 4 ] On the other hand, an alternative based on community-based conservation and the management of sustainability is more advocated upon. By adding the latter cultural perspective, the three pillars of sustainable development (the social, environmental, and economic) [ 5 ] end up being addressed by these proponents.
After the second biennial meeting of the International Society for Ecological Economics in 1994 at San José, Costa Rica , several professionals in the region became interested in creating a branch of this organization in their own countries to respond to the increasing development and worsening of social-environmental conflicts by the conventional-economics -based policies.
However, it wasn't until 2008 that the efforts of the Latin American Social Sciences Institute in Guatemala, the International Center for Political Economy at the National University of Costa Rica, and the Metropolitan Autonomous University of Mexico resulted in the Ecological Economics Forum, May 26 and 27 2008 at Guatemala City , with the participation of zealous youth, students, and 50 professionals of Mesoamerica .
Highly-recognized experts spoke, such as Alejandro Nadal , Coordinator of the working group on the Environment, Macroeconomics, Trade and Investments of the International Union for Conservation of Nature (IUCN); David Barkin and Roberto Constantino from the Metropolitan Autonomous University of Mexico; Eduardo García Frapolli from the Center for Ecosystem Research of the National Autonomous University of Mexico ; Bernardo Aguilar of Prescott College , AZ , U.S. and Executive Director of the Fundacion Neotropica in Costa Rica ; Miguel Martínez of the World Wide Fund for Nature (WWF Guatemala), and Juan Pablo Castañeda World Bank WAVES and GPS programs.
This meeting undertook the writing of the organization's Constitution and the election of the first Board of Directors. Under the lead of the first President, M. Sc. Iliana Monterroso of Guatemala, the consolidation of the legal inscription and statutes took place. [ 6 ]
According to the Forum's participants, the main objectives of the SMEE are to create an open field for discussion of the methodological and theoretical development of Ecological economics , to promote interdisciplinary, multidisciplinary, and transdisciplinary scientific research, and to support academic initiatives related to this thematic in the region.
In 2010, the First International (and Biennial) Conference of the SMEE was celebrated at the Ecological Park of Xochimilco, Mexico City , from November 22 to the 26th. Very influential speakers such as Fander Falconí ; David Barkin; Mario Pérez, and Carlos Muñóz Piña lectured about and discussed the Ecological economics platform for the advancement of social justice , environmental justice , and the principles of sustainability .
Since then, the new board of directors has had to deal with a shortage of memberships and the last financial crisis ; but despite these challenges, it has achieved important progress in creating its website and completing several ecological economics studies and projects with the Fundacion Neotropica . Furthermore, the organization foresees the inauguration of a professional Master's program on ecological economics and political ecology at the University for International Cooperation and the proceeding of the 2014 EcoEco Alternatives Biennial Conference.
We agree with the premise that the methodological pluralism characteristic of ecological economics must be understood beyond the philosophy of "everything counts." It must respond to epistemology required by the necessities established by the specific context of the conflict within that region.
The Board of Directors rotates every two years and is usually elected around the biennial conference. The next election will take place in March, 2014.
First Board of Directors (2008-2011):
Second Board of Directors (2011-2014):
Current Board of Directors (2014-2016):
The Second International Conference of the Mesoamerican Society for Ecological Economics will take place at the Rodrigro Facio campus of the University of Costa Rica from March 4 to the 8th, 2014, with the support of the School of Biology and the Fundacion Neotropica . Its purpose is to further the debate on Ecological economics and to sensitize more people about the importance of the ecological crisis and the solutions proposed by this school of thought.
The Conference's thematic will be "Advancing Towards Alternatives for People and Ecosystems in Latin America ". It will include multidisciplinary, interdisciplinary, and transdisciplinary debates about the resolution of social-environmental conflicts, the alternatives within the ecological economics model for handling production and services , and the social conflicts related to the distribution of wealth and gender . | https://en.wikipedia.org/wiki/Mesoamerican_Society_for_Ecological_Economics |
A mesocrystal is a material structure composed of numerous small crystals of similar size and shape, which are arranged in a regular
periodic pattern. It is a form of oriented aggregation, where the small crystals have parallel crystallographic alignment but are spatially separated. [ 2 ]
When the sizes of individual components are at the nanoscale, mesocrystals represent a new class of nanostructured solids made from crystiallographically oriented nanoparticles. The sole criterion for determining whether a material is mesocrystal is the unique crystallographically hierarchical structure, not its formation mechanism. [ 3 ]
Helmut Cölfen discovered and named mesocrystals in 2005 during his studies on biominerals. [ 4 ] He suggested that their growth was due to a non-classical, self-assembly based process. [ 3 ]
Mesocrystal is an abbreviation for mesoscopically structured crystal, where individual subunits often form a perfect 3D order, as in a traditional crystal where the subunits are individual atoms. [ 3 ]
This is when a mesocrystal is formed by filling organic matrix compartments with crystalline matter. This crystalline matter would be oriented by the organic matrix. This is the process of biomineralization and this is how mesocrystals are produced in nature. [ 3 ]
In most cases mesocrystals form nanoparticles in solution. These nanoparticles aggregate and arrange in crystallographic formation, without any additives. [ 3 ] The main causes of this ordering are tensorial polarization forces and dipole fields. [ 5 ]
Formation with mineral bridges occur with the formation of nanocrystals. Growth is quenched at this stage by the absorption of a polymer into the nanoparticle surface. Now mineral bridges can nucleate at the defect site, within the growing inhibition layer on the nanocrystal. Through this, a new nanocrystal grows on the mineral bridge, and the growth is again stopped by the polymer. This process is repeated until the crystal builds up. [ 3 ]
This argument for formation of mesocrystals requires only a confined space that the reaction takes place in. As the nanoparticle grow into crystals, they have no choice but to align with each other in such a confined space. [ 3 ]
Mesocrystals have unique structural features and the physical and physiochemical properties that come from that structure have made them become a subject of interest. Mesocrystals are expected to have a role in many different applications. These include heterogeneous photocatalysts , electrodes , optoelectronics , biomedical materials, and lightweight structural materials. [ 5 ]
The properties that make mesocrystals viable for future applications are their shared properties with nanoparticulate, mesoporous , and single-crystal materials. Because mesocrystals are made up of nanoparticles , the properties of the nanoparticles themselves are, in some cases, passed to the whole mesocrystal structure. This allows for the practical application of mesocrystals because they are "potentially more stable analogues of nanoparticulate materials." High porosity is generally a quality of mesocrystals, this is the property shared with mesoporous materials. Closed, internal pores are good for thermal and dielectric insulation and the open pores then aid in absorption and could be utilized for medical delivery. Alternatively, a mesocrystal could have its pores filled and then it would be similar to a single-crystal material and have some unusual electronic and optical properties. The diversity of the properties of mesocrystals could allow them to be effectively utilized in many applications. [ 5 ]
The spines of sea urchins are composed of mesocrystals of calcite nano-crystals (92%) in a matrix of non-crystalline calcium carbonate (8%). This structure makes the spines hard but also shock-absorbing, which special property makes them effective defences against predators. [ 6 ] Mesocrystals also appear in the shells of some eggs , coral , chitin , and the shells of mussels . [ 3 ] | https://en.wikipedia.org/wiki/Mesocrystal |
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm , and the inner layer is the endoderm . [ 1 ] [ 2 ]
The mesoderm forms mesenchyme , mesothelium and coelomocytes . Mesothelium lines coeloms . Mesoderm forms the muscles in a process known as myogenesis , septa (cross-wise partitions) and mesenteries (length-wise partitions); and forms part of the gonads (the rest being the gametes ). [ 1 ] [ unreliable source? ] Myogenesis is specifically a function of mesenchyme .
The mesoderm differentiates from the rest of the embryo through intercellular signaling , after which the mesoderm is polarized by an organizing center . [ 3 ] The position of the organizing center is in turn determined by the regions in which beta-catenin is protected from degradation by GSK-3. Beta-catenin acts as a co-factor that alters the activity of the transcription factor tcf-3 from repressing to activating, which initiates the synthesis of gene products critical for mesoderm differentiation and gastrulation. Furthermore, mesoderm has the capability to induce the growth of other structures, such as the neural plate , the precursor to the nervous system.
The mesoderm is one of the three germinal layers that appears in the third week of embryonic development . It is formed through a process called gastrulation . There are four important components, which are the axial , paraxial , intermediate , and lateral plate mesoderms . The axial mesoderm gives rise to the notochord . The paraxial mesoderm forms the somitomeres , which give rise to mesenchyme of the head, and organize into somites in occipital and caudal segments, and give rise to sclerotomes (cartilage and bone), and dermatomes (subcutaneous tissue of the skin). [ 1 ] [ 2 ] Signals for somite differentiation are derived from surroundings structures, including the notochord, neural tube , and epidermis . The intermediate mesoderm connects the paraxial mesoderm with the lateral plate. Eventually it differentiates into urogenital structures that consist of the kidneys, gonads, their associated ducts, and the adrenal cortex. The lateral plate mesoderm gives rise to the heart, blood vessels, and blood cells of the circulatory system, as well as to the mesodermal components of the limbs. [ 4 ]
Some of the mesoderm derivatives include the muscle (smooth, cardiac, and skeletal), the muscles of the tongue (occipital somites), the pharyngeal arches muscle (muscles of mastication, muscles of facial expressions), connective tissue, the dermis and subcutaneous layer of the skin , bone and cartilage, dura mater, the endothelium of blood vessels , red blood cells , white blood cells , microglia , the dentin of teeth, the kidneys, and the adrenal cortex. [ 5 ]
During the third week, a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm. This process begins with the formation of a primitive streak on the surface of the epiblast. [ 6 ] The cells of the layers move between the epiblast and the hypoblast, and begin to spread laterally and cranially. The cells of the epiblast move toward the primitive streak and slip beneath it, in a process called "invagination". Some of the migrating cells displace the hypoblast and create the endoderm, and other cells migrate between the endoderm and the epiblast to create the mesoderm. The remaining cells form the ectoderm. After that, the epiblast and the hypoblast establish contact with the extraembryonic mesoderm until they cover the yolk sac and amnion. They move onto either side of the prechordal plate . The prechordal cells migrate to the midline to form the notochordal plate. The chordamesoderm is the central region of trunk mesoderm. [ 4 ] This forms the notochord, which induces the formation of the neural tube, and establishes the anterior-posterior body axis. The notochord extends beneath the neural tube from the head to the tail. The mesoderm moves to the midline until it covers the notochord. When the mesoderm cells proliferate, they form the paraxial mesoderm. In each side, the mesoderm remains thin, and is known as the lateral plate. The intermediate mesoderm lies between the paraxial mesoderm and the lateral plate.
Between days 13 and 15, the proliferation of extraembryonic mesoderm, primitive streak, and embryonic mesoderm take place. The notochord process occurs between days 15 and 17. Eventually, the development of the notochord canal and the axial canal takes place between days 17 and 19, when the first three somites are formed. [ 7 ]
During the third week, the paraxial mesoderm is organized into segments. If they appear in the cephalic region and grow with cephalocaudal direction, they are called somitomeres. If they appear in the cephalic region but establish contact with the neural plate, they are known as neuromeres , which later will form the mesenchyme in the head. The somitomeres organize into somites which grow in pairs. In the fourth week, the somites lose their organization and cover the notochord and spinal cord to form the backbone. In the fifth week, there are 4 occipital somites, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8 to 10 coccygeal that will form the axial skeleton. Somitic derivatives are determined by local signaling between adjacent embryonic tissues, in particular the neural tube, notochord, surface ectoderm and the somitic compartments themselves. [ 8 ] The correct specification of the deriving tissues, skeletal, cartilage, endothelia and connective tissue is achieved by a sequence of morphogenic changes of the paraxial mesoderm, leading to the three transitory somitic compartments: dermomyotome, myotome and sclerotome. These structures are specified from dorsal to ventral and from medial to lateral. [ 8 ] Each somite will form its own sclerotome that will differentiate into the tendon cartilage and bone component. Its myotome will form the muscle component and the dermatome that will form the dermis of the back. The myotome and dermatome have a nerve component. [ 1 ] [ 2 ]
Surrounding structures such as the notochord, neural tube, epidermis and lateral plate mesoderm send signals for somite differentiation [ 1 ] [ 2 ] Notochord protein accumulates in presomitic mesoderm destined to form the next somite and then decreases as that somite is established. The notochord and the neural tube activate the protein SHH, which helps the somite to form its sclerotome. The cells of the sclerotome express the protein PAX1 that induces the cartilage and bone formation. The neural tube activates the protein WNT1 that expresses PAX 2 so the somite creates the myotome and dermatome. Finally, the neural tube also secretes neurotrophin 3, so that the somite creates the dermis. Boundaries for each somite are regulated by retinoic acid and a combination of FGF8 and WNT3a. [ 1 ] [ 2 ] [ 9 ] So retinoic acid is an endogenous signal that maintains the bilateral synchrony of mesoderm segmentation and controls bilateral symmetry in vertebrates. The bilaterally symmetric body plan of vertebrate embryos is obvious in somites and their derivates, such as the vertebral column. Therefore, asymmetric somite formation correlates with a left-right desynchronization of the segmentation oscillations. [ 10 ]
Many studies with Xenopus and zebrafish have analyzed the factors of this development and how they interact in signaling and transcription. However, there are still some doubts in how the prospective mesodermal cells integrate the various signals they receive and how they regulate their morphogenic behaviours and cell-fate decisions. [ 8 ] Human embryonic stem cells for example have the potential to produce all of the cells in the body and they are able to self-renew indefinitely so they can be used for a large-scale production of therapeutic cell lines. They are also able to remodel and contract collagen and were induced to express muscle actin. This shows that these cells are multipotent cells. [ 11 ]
The intermediate mesoderm connects the paraxial mesoderm with the lateral plate mesoderm, and differentiates into urogenital structures . [ 12 ] In upper thoracic and cervical regions, this forms the nephrotomes. In caudal regions, it forms the nephrogenic cord. It also helps to develop the excretory units of the urinary system and the gonads. [ 4 ]
The lateral plate mesoderm splits into the parietal (somatic) and visceral (splanchnic) layers. The formation of these layers starts with the appearance of intercellular cavities. [ 12 ] The somatic layer depends upon a continuous layer with mesoderm that covers the amnion. The splanchnic layer depends upon a continuous layer that covers the yolk sac. The two layers cover the intraembryonic cavity. The parietal layer, together with overlying ectoderm, forms the lateral body wall folds. The visceral layer forms the walls of the gut tube. Mesoderm cells of the parietal layer form the mesothelial membranes or serous membranes, which line the peritoneal, pleural, and pericardial cavities. [ 1 ] [ 2 ] | https://en.wikipedia.org/wiki/Mesoderm |
The Mesodinium nuclear code (translation table 29) is a genetic code used by the nuclear genome of the ciliates Mesodinium and Myrionecta . [ 1 ]
Bases: adenine (A), cytosine (C), guanine (G) and thymine (T) or uracil (U).
Amino acids: Alanine (Ala, A), Arginine (Arg, R), Asparagine (Asn, N), Aspartic acid (Asp, D), Cysteine (Cys, C), Glutamic acid (Glu, E), Glutamine (Gln, Q), Glycine (Gly, G), Histidine (His, H), Isoleucine (Ile, I), Leucine (Leu, L), Lysine (Lys, K), Methionine (Met, M), Phenylalanine (Phe, F), Proline (Pro, P), Serine (Ser, S), Threonine (Thr, T), Tryptophan (Trp, W), Tyrosine (Tyr, Y), and Valine (Val, V).
This article incorporates text from the United States National Library of Medicine , which is in the public domain . [ 2 ] | https://en.wikipedia.org/wiki/Mesodinium_nuclear_code |
Halteria rubra Lohmann, 1908 Myrionecta rubra Lohmann, 1908 Cyclotrichium meunieri Powers, 1932 Mesodinium pulex Bakker, 1966
Mesodinium rubrum (or Myrionecta rubra ) is a species of ciliates . [ 1 ] It constitutes a plankton community and is found throughout the year, most abundantly in spring and fall, in coastal areas. Although discovered in 1908, its scientific importance came into light in the late 1960s when it attracted scientists by the recurrent red colouration it caused by forming massive blooms, [ 2 ] that cause red tides in the oceans. [ 3 ] [ 4 ]
Unlike typical protozoans, M. rubrum can make its own nutrition by photosynthesis . The unusual autotrophic property was discovered in 2006 when genetic sequencing revealed that the photosynthesising organelles, plastids , were derived from the ciliate's principal food, the autotrophic algae called cryptomonads (or cryptophytes), which contain endosymbiont red algae whose internal chloroplasts (evolved via endosymbiosis with cyanobacteria ) indirectly enable M. rubrum to photosynthesize using sunlight . [ 5 ] The ciliate is thus both autotrophic and heterotrophic at the same time. This also indicates that it is an example of multiple-stage endosymbiosis in the form of kleptoplasty . [ 6 ] Moreover, these “stolen” plastids can be further transferred to additional hosts, as seen in the case of predation of M. rubrum by dinoflagellate planktons of the genus Dinophysis . [ 7 ] [ 8 ]
In 2009, a new species of Gram-negative bacteria called Maritalea myrionectae was discovered from a cell culture of M. rubrum . [ 9 ]
M. rubrum is a free-living marine ciliate. It is reddish in colour and form dark-red mass during blooming. Its body is almost spherical, looking like a miniature sunflower with its radiating hair-like cilia on its body surface. It measures up to 100 μm in length and 75 μm in width. The body is superficially divided into two lobes due to formation of a constriction at the centre. The constriction gives rise to a larger anterior lobe and a smaller posterior lobe. The cilia arise from the constriction. Using the cilia it can jump about 10-20 times its body length in one movement. [ 10 ] Its nucleus is prominently situated at the centre, and is surrounded by organelles mostly derived from algae. For example, its cytoplasm contains numerous plastids, mitochondria and other nuclei . These organelles are properly separated such that the mitochondria are fully enclosed in a vacuole membrane and two endoplasmic reticulum membranes of the ciliate. [ 6 ] This indicates that the ciliate is primarily a heterotroph, but after acquiring algal plastid, it transforms into an autotroph.
Genetic analysis showed that in the American coastal areas, the primary food of M. rubrum is the algae most closely related to the free-living Geminigera cryophila . [ 5 ] But in Japanese coasts, the major algal species is Teleaulax amphioxeia . [ 8 ] When these plastid-containing algae are ingested by the ciliate, they are not digested. The plastids remain functional and provide nutrition to the ciliate by photosynthesis. In order for the plastids to be normally active, they still require enzymes, which are synthesised by the sequestered algal nuclei. The single nucleus can survive and remain genetically active up to 30 days in the cytoplasm of the ciliate. [ 11 ] As the retention time of the prey nuclei is short, an average M. rubrum cell may contain eight algal plastids per single prey nucleus and the nuclei need to be replaced by continuous feeding on fresh algae. Thus, the algal organelles are not permanently integrated. [ 5 ] | https://en.wikipedia.org/wiki/Mesodinium_rubrum |
Mesohabitat simulation model (MesoHABSIM) , created by Dr. Piotr Parasiewicz , addresses the requirements of watershed -based management of running waters and is designed to predict an aquatic community's response to habitat modification.
MesoHABSIM builds upon pre-existing physical habitat simulation models (e.g. PHABSIM) which is an essential component of the United States government's methods for establishing minimum stream flow requirements.
MesoHABSIM is an augmentation of this system, designed during a restoration study of the Quinebaug River . [ 1 ] The changing spatial distributions of physical attributes of a river as a result of variations in flow and the biological responses of aquatic species to these changes, provide the basis for simulating the consequences of ecosystem alteration, and consequently the justification of restoration measures. MesoHABSIM modifies the data acquisition technique and analytical approach of similar models by changing the scale of resolution from micro- to meso-scales. Due to this increase in scale, the model takes variations in stream morphology along the river into account and allows for application in larger-scale projects.
Mesohabitat types are defined by their hydromorphological units (HMUs), such as pools and rapids , geomorphology , land cover and other hydrological characteristics. Mesohabitats are mapped under multiple flow conditions at extensive sites along the river. Fish data is collected in randomly distributed mesohabitats where habitat surveys are also conducted. This allows modeling of available fish habitat at a range of flows. Rating curves represent the changes in relative area of suitable habitat in response to flow and allow for the determination of habitat quantity at any given flow within the range of surveys. These rating curves can be developed for river units of any size allowing conclusions to be drawn about the suitability of channel patterns or habitat structures for various species of fish for specific sections as well as for the entire river. Rating curves can also be used to evaluate the benefits of various restoration measures on the entire fish community. In combination with hydrologic time series, rating curves are used to create Continuous Under Threshold (CUT) curves for the analysis of frequency, magnitude and duration of significant habitat events. The CUT curve technique described by Capra et al. (1995) [ 2 ] defines critical thresholds and determines what habitat variability and availability is necessary to support the target river fauna . CUT curves evaluate durations of unsuitable habitat under a specified threshold by comparing continuous durations in days under this threshold to the cumulative durations in the study period. A highly useful product of the CUT curves are reference tables that can be used to determine how long a given species can tolerate unsuitable conditions depending on its life stage.
To use physical habitat models to analyze and predict ecosystem potential, compositions must also be determined of the native fish community and a subset of species must be selected for model development. The development of a Reference Fish Community (RFC) is based on the Target Fish Community approach, described by Bain and Meixler (2000). [ 3 ] A comprehensive list of species is generated from literature sources and available regional data collected on relatively intact river reaches. The species are ranked on the basis of abundance in long-term fish collection data from multiple rivers of similar character. Securing habitat for naturally occurring dominant species (ecology) should preserve the most profound characteristics of the ecosystem, providing survival conditions for the majority of the aquatic community and therefore a reference for restoration efforts. The simplest way to create a river habitat model is therefore to select the five to ten highest ranking species for model development. It can then be assumed that community structure reflects habitat structure; therefore, the most common species should indicate the most common habitat. Since habitat availability forms the structure of aquatic fauna, the affinity between the structure of the river habitat and the structure of the fish community can be used as a measure of habitat quality.
The results of MesoHABSIM creates the framework for integrative analysis of many aspects of the ecosystem. It also allows managers to recreate reference conditions and evaluate possible instream and watershed restoration measures or alterations (such as dam removals or changes in water withdrawals). From the perspective of resource managers, it not only allows for quantitative measures of ecological integrity, but also creates a basis for making decisions where trade-offs between resource use and river restoration need to be considered. | https://en.wikipedia.org/wiki/Mesohabitat_simulation_model |
In chemistry , mesoionic carbenes ( MICs ) are a type of reactive intermediate that are related to N-heterocyclic carbenes (NHCs); thus, MICs are also referred to as abnormal N-heterocyclic carbenes ( aNHCs ) or remote N-heterocyclic carbenes ( rNHCs ). Unlike simple NHCs, the canonical resonance structures of these carbenes are mesoionic : an MIC cannot be drawn without adding additional charges to some of the atoms.
A variety of free carbenes can be isolated and are stable at room temperature. Other free carbenes are not stable and are susceptible to intermolecular decomposition pathways. MICs do not dimerize according to Wanzlick equilibrium as do normal NHCs. This results in relaxed steric requirements for mesoionic carbenes as compared to NHCs. [ 1 ] [ 2 ] [ 3 ]
There are several mesoionic carbenes that cannot be generated as free compounds, but can be synthesized as a ligand in a transition metal complex . Most MIC transition metal complexes are less sensitive to air and moisture than phosphine or normal NHC complexes. They are also resistant to oxidation . The robust nature of MIC complexes is due to the ligand’s strong σ-donating ability. They are stronger σ-donors than phosphines, as well as normal N-heterocyclic carbenes due to decreased heteroatom stabilization. The strength of carbene ligands is attributed to the electropositive carbon center that forms strong bonds of a covalent nature with the metal. [ 1 ] [ 2 ] They have been shown to lower the frequency of CO stretching vibrations in metal complexes [ 4 ] [ 5 ] and exhibit large trans effects . [ 4 ] [ 6 ]
The most studied mesoionic carbenes are based on imidazole and are referred to as imidazolin-4-ylidenes. These complexes were first reported by Crabtree in 2001. [ 7 ] The formation of imidazolin-4-ylidenes (MIC) instead of imidazolin-2-ylidenes (NHC) is typically a matter of blocking the C2 position. Most imidazolin-4-ylidenes are trisubstituted in the N1, C2, and N3 positions or tetrasubstituted. Electron-withdrawing groups in the N3 and C5 positions stabilize the carbenes more than electron-donating groups. [ 8 ] Free carbenes [ 8 ] [ 9 ] [ 10 ] as well as numerous transition metal complexes have been synthesized.
Also well studied are the mesoionic carbenes based on 1,2,3-triazole , referred to as 1,2,3-triazol-4(or 5)-ylidenes. The first triazolylidenes were reported by Albrecht in 2008. [ 11 ] These carbenes are typically trisubstituted with alkyl groups in the N1 and N3 positions and an aryl group in the C4 or C5 position. Free carbenes as well as numerous transition metal complexes have been reported. Free carbenes that are alkylated at N3 tend to undergo decomposition reactions in which the alkyl group participates in a nucleophilic attack at the carbene position. If N3 is substituted with a bulky alkyl group or an aryl group, the stability of the carbene increases significantly.
The first mesoionic carbenes based on pyrazole have been reported by Huynh in 2007. [ 12 ] These carbenes are referred to as pyrazolin-3(or 4)-ylidenes. Pyrazolin-4-ylidenes are often tetrasubstituted with alkyl or aryl groups; however, the C3 and C5 positions could be substituted with nitrogen- or oxygen-based groups. The electronic properties of the groups in the C3 and C5 positions affect the overall electron properties of the ligand and influence catalytic activity. Free carbene have been produced as well as transition metal complexes. [ 13 ] [ 14 ]
Examples of tetrazol-5-ylidenes based on tetrazole have been prepared by Araki. [ 15 ] The N1 and N3 positions are substituted with alkyl or aryl groups. Transition metal complexes of these carbenes have been generated in situ . Mesoionic carbenes based on isoxazole and thiazole have been reported by Albrecht [ 16 ] and Bertrand [ 17 ] respectively. The isoxazol-4-ylidenes are trisubstituted in the N2, C3, and C5 positions with alkyl groups. The thiazol-5-ylidenes are trisubstituted in the C2, N3, and C4 positions with aryl groups. Transition metal complexes of both types of carbenes have been generated in situ . Bertrand also reported a 1,3-dithiol-5-ylidene based on 1,3-dithiolane , but it can only be isolated as a transition metal complex. [ 3 ]
Many free mesoionic carbenes are synthesized from their protonated salt form by deprotonation using strong potassium bases, such as potassium bis(trimethylsilyl)amide (KHMDS) or potassium tert -butoxide (KO t -Bu). Potassium bases are used because they do not form stable carbene-alkali metal adducts. [ 1 ] [ 8 ] [ 13 ] [ 14 ] [ 18 ]
Imidazolin-4-ylidenes (MIC) would form rather than imidazolin-2-ylidenes (NHC) due to blocking the C2 position. The C2 carbenes are thermodynamically more stable than their C4 counterparts due to resonance and inductive carbon-nitrogen interactions. Also, calculations show that the C4 hydrogen is less acidic than the C2 hydrogen of imidazole. This data suggests that the C2 position should be activated preferentially to the C4 position unless the C2 position is blocked. Aryl and bulky alkyl groups (such as isopropyl) are good at blocking the C2 position from being activated. [ 4 ] [ 18 ]
Many mesoionic carbenes may not be able to be isolated as a free carbene; however, these MICs can be generated as a ligand for transition metal complexes. Numerous mesoionic carbene transition metal complexes are known with metals including Fe, Os, Rh, Ir, Ni, Pd, Pt, Cu, and Ag. Metal complexes with Sm and Y are also known. MIC complexes are formed by a variety of mechanisms. [ 4 ] [ 18 ] [ 19 ]
Mesoionic carbenes may be generated in situ with addition of a strong base to their salt forms. The carbenes immediately form complexes with metals present in the reaction mixture through ligand exchange. [ 9 ] [ 17 ]
Direct metalation through C-H bond activation [ 1 ] [ 4 ] [ 11 ] [ 18 ] [ 19 ] [ 20 ] [ 21 ] [ 22 ] or C-H oxidative addition [ 4 ] [ 18 ] [ 23 ] is one method often utilized. Activation of a C‒H bond leads to oxidative addition of the carbene ligand to the metal center. Typically, direct metalation requires the blockage of sites that would lead to normal NHC complexes — phenyl and isopropyl groups are good blocking substituents, as discussed earlier. Smaller substituents may be cleaved. Direct metalation by silver(I) with imidazolium salts can cause cleavage at the C2 position if methyl is used as the blocking group. The result is formation of normal NHC carbenes. n -alkyl and benzyl groups may undergo the same fate as the methyl group. Steric bulk may also influence the formation of MIC complexes over NHC complexes. For imidazolium salts, the C2 position may not need to be blocked if the nitrogen substituents (N1 or N3) are sterically-demanding. Interactions between the nitrogen substituents and the metal center prevent normal NHC complexes from forming. If the carbene is part of a bidentate ligand with a forced geometry, the MIC complex may form preferentially as well. The counteranion of imidazolium salts participates in NHC vs. MIC formation. NHC formation typically occurs by heterolytic bond cleavage, so small, coordinating anions favor this pathway. MIC formation typically occurs by an oxidative addition pathway, so non-coordinating and apolar anions are preferred, such as BF 4 − or SbF 6 − . [ 4 ] Other techniques focus on the activation of the desired carbon rather than blocking undesired carbons. A carbon may be activated by a halogen. A C-X bond (X = halide) is more favorable for activation than a C-H bond. This pathway results in the oxidative addition of the MIC carbene halide to a low valent metal center. [ 4 ] [ 18 ]
Transmetalation is another method commonly utilized. [ 4 ] [ 11 ] [ 18 ] [ 19 ] [ 22 ] [ 24 ] [ 25 ] Typically, a silver carbene complex is produced by direct metalation. This silver complex is reacted via transmetalation with a salt of the desired metal. The metal MIC complex is produced and silver salts generally precipitate.
Since mesoionic carbene ligands are very strong σ-donors and make it easier for a metal center to undergo oxidative addition, MIC ligands have the potential to be useful in catalysis. [ 18 ] MIC transition metal complexes have been tested as catalysts in olefin metathesis , ring closure metathesis , and ring opening polymerization metathesis . [ 26 ] [ 27 ] The MIC complexes work very well, and in many cases, they outperform their NHC counterparts. MIC complexes have been successful as catalysts for Suzuki-Miyaura and Heck-Mizoroki cross-coupling reactions. [ 4 ] [ 9 ] [ 28 ] [ 29 ] [ 30 ] Again, in many cases, MIC catalysts are superior to their NHC counterparts. For example, in olefin metathesis, MIC catalysts are active at room temperature after simply addition of a Brønsted acid, such as hydrochloric acid or trifluoroacetic acid , compared to the large amount of thermal activation required for NHC catalysts. [ 27 ] MIC complexes have found use as catalysts in olefin hydrogenation. They have been shown to hydrogenate terminal and cis-alkenes. [ 4 ] [ 5 ] They work better than their NHC counterparts due to the MIC ligand’s stronger electron-donating properties. They are better able to provide electron density to promote hydrogen gas oxidative addition to the metal. MIC complexes have been used in transfer hydrogenation reactions. For example, they have been used to hydrogenate a diaryl ketone using isopropanol as a hydrogen source., [ 4 ] [ 21 ] MIC complexes are being considered as green chemistry catalysts. They act as catalysts for base- and oxidant-free oxidation of alcohols and amines. Some complexes have also been shown to synthesize certain aryl amides. [ 31 ] Other MIC complexes have been used in hydroarylation, involving the addition of an electron-rich aryl group and a hydrogen across a multiple bond. [ 32 ] The reactions that mesoionic carbene complexes catalyze will continue to expand as more research is done. [ 18 ] [ 33 ] | https://en.wikipedia.org/wiki/Mesoionic_carbene |
A mesokaryote or mesokaryotic organism is a single-celled eukaryote that shows intermediate resemblance to both prokaryotes and 'higher' eukaryotes. The term originates from a 1965 hypothesis by John David Dodge, who proposed that certain eukaryotes (mainly dinoflagellates ) with closed mitosis and other traits considered 'primitive' were an intermediate step between prokaryotes and the remaining eukaryotes. This idea originated in the late 20th century, and was later disproven by more detailed ultrastructural studies in the following decades.
The first investigations of the dinoflagellate nucleus , during the 1950s-1960s, revealed a fine nucleus and chromosome structure that was completely different from other nucleated organisms or eukaryotes , lacking histones and with a permanently condensed chromatin . [ 1 ] [ 2 ] Based on these findings, the phycologist John David Dodge proposed in 1965 the concept of Mesocaryota (or mesokaryotes ) under the hypothesis that these features were an intermediate nuclear organization between prokaryotes and eukaryotes. This hypothesis led to the theory that dinoflagellates were the first to evolve from the split with prokaryotes, followed by the remaining eukaryotes. [ 3 ] [ 4 ] The traits considered by Dodge to define Mesocaryota were: lack of detectable histones; absence of a mitotic spindle ; continuous DNA synthesis ; chromatin fibrils arranged in arched swirls as in bacterial nucleoids ; and chromosomes permanently condensed persistently adhered to the nuclear envelope , which remains intact throughout mitosis (i.e., it is a closed mitosis ). [ 5 ]
The mesokaryote hypothesis was disproven in the following decades through more detailed observations of the criteria listed above. [ 6 ] For example, detailed studies on the parasitic Syndinium demonstrated the presence of an unconventional type of basic histone-like proteins and of an extranuclear mitotic spindle in dinoflagellates, similarly to 'higher' eukaryotes. [ 5 ] Dinoflagellates remained considered a group of ancient but true eukaryotes. [ 6 ] With the improvement of molecular phylogenetics , dinoflagellates, like other groups that exhibited closed mitosis, were instead revealed to be derived, branching within the Alveolata , whose members have conventional nuclei. Thus, these traits were reinterpreted as highly derived . Due to its short lifespan, the mesokaryote hypothesis has had little impact. [ 2 ] | https://en.wikipedia.org/wiki/Mesokaryote |
In chemistry , the mesomeric effect (or resonance effect ) is a property of substituents or functional groups in a chemical compound . It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and lone pair of electrons present on an adjacent atom. [ 1 ] This change in electron arrangement results in the formation of resonance structures that hybridize into the molecule's true structure. The pi electrons then move away from or toward a particular substituent group. The mesomeric effect is stronger in compounds with a lower ionization potential . This is because the electron transfer states will have lower energies.
The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant resonance structures and is symbolized by the letter M . [ 2 ] The mesomeric effect is negative ( −M ) when the substituent is an electron-withdrawing group , and the effect is positive ( +M ) when the substituent is an electron donating group . Below are two examples of the +M and −M effect. Additionally, the functional groups that contribute to each type of resonance are given below.
The +M effect, also known as the positive mesomeric effect, occurs when the substituent is an electron donating group. The group must have one of two things: a lone pair of electrons, or a negative charge. In the +M effect, the pi electrons are transferred from the group towards the conjugate system, increasing the density of the system. Due to the increase in electron density , the conjugate system will develop a more negative charge. As a result, the system under the +M effect will be more reactive towards electrophiles , which can take away the negative charge, than a nucleophile . [ citation needed ]
+M effect order: [ 1 ]
The −M effect, also known as the negative mesomeric effect, occurs when the substituent is an electron-withdrawing group. In order for a negative mesomeric (−M) effect to occur the group must have a positive charge or an empty orbital in order to draw the electrons towards it. In the −M effect, the pi electrons move away from the conjugate system and towards the electron drawing group. In the conjugate system, the density of electrons decreases and the overall charge becomes more positive. With the −M effect the groups and compounds become less reactive towards electrophiles, and more reactive toward nucleophiles, which can give up electrons and balance out the positive charge. [ 3 ]
−M effect order:
The net electron flow from or to the substituent is determined also by the inductive effect . [ 3 ] The mesomeric effect as a result of p - orbital overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect has purely to do with the electronegativity of the atoms and their topology in the molecule (which atoms are connected to which). Specifically the inductive effect is the tendency for the substituents to repel or attract electrons purely based on electronegativity and not dealing with restructuring. The mesomeric effect however, deals with restructuring and occurs when the electron pair of the substituents shift around. The inductive effect only acts on alpha carbons, while the mesomeric utilizes pi bonds between atoms. [ 4 ] While these two paths often lead to the similar molecules and resonance structures, the mechanism is different. As such, the mesomeric effect is stronger than the inductive effect. [ 5 ]
The concepts of mesomeric effect, mesomerism and mesomer were introduced by Ingold in 1938 as an alternative to Pauling's synonymous concept of resonance. [ 6 ] "Mesomerism" in this context is often encountered in German and French literature, but in English literature the term "resonance" dominates.
Mesomeric effect can be transmitted along any number of carbon atoms in a conjugated system . This accounts for the resonance stabilization of the molecule due to delocalization of charge. [ 7 ] It is important to note that the energy of the actual structure of the molecule, i.e. the resonance hybrid, may be lower than that of any of the contributing canonical structures. The difference in energy between the actual inductive structure and the (most stable contributing structures) worst kinetic structure is called the resonance energy or resonance stabilization energy. [ 8 ] For the quantitative estimation of the mesomeric/resonance effect strength various substituent constants are used, i.e. Swain-Lupton resonance constant, Taft resonance constant or Oziminski and Dobrowolski pEDA parameter.
Additionally, the resulting resonance structures can give the molecule properties that are not inherently evident from looking at one structure. Some of these properties include different reactivities, local diamagnetic shielding in aromatics, deshielding, and acid and base strengths. [ 9 ] | https://en.wikipedia.org/wiki/Mesomeric_effect |
In chemistry and chemical physics , a mesophase or mesomorphic phase is a phase of matter intermediate between solid and liquid . Gelatin is a common example of a partially ordered structure in a mesophase. Further, biological structures such as the lipid bilayers of cell membranes are examples of mesophases. Mobile ions in mesophases are either orientationally or rotationally disordered while their centers are located at the ordered sites in the crystal structure. Mesophases with long-range positional order but no orientational order are plastic crystals , whereas those with long-range orientational order but only partial or no positional order are liquid crystals . [ 1 ] [ 2 ]
Georges Friedel (1922) called attention to the "mesomorphic states of matter" [ 3 ] in his scientific assessment of observations of the so-called liquid crystals . Conventionally a crystal is solid, and crystallization converts liquid to solid. The oxymoron of the liquid crystal is resolved through the notion of mesophases. The observations noted an optic axis persisting in materials that had been melted and had begun to flow . The term liquid crystal persists as a colloquialism , but use of the term was criticized in 1993: In The Physics of Liquid Crystals [ 4 ] the mesophases are introduced from the beginning:
Further, "The classification of mesophases (first clearly set out by G. Friedel in 1922) is essentially based on symmetry." [ 4 ] : 10
Molecules that demonstrate mesophases are called mesogens .
In technology, molecules in which the optic axis is subject to manipulation during a mesophase have become commercial products as they can be used to manufacture display devices , known as liquid-crystal displays (LCDs). The susceptibility of the optical axis, called a director , to an electric or magnetic field produces the potential for an optical switch that obscures light or lets it pass. Methods used include the Freedericksz transition , the twisted nematic field effect and the in-plane switching effect . From early liquid crystal displays the buying public has embraced the low-power optical switch facility of mesophases with director.
Consider a solid consisting of a single molecular species and subjected to melting . Ultimately it is rendered to an isotropic state classically referred to as liquid. Mesophases occur before then when an intermediate state of order is still maintained as in the nematic , smectic , and columnar phases of liquid crystals. Mesophases thus exhibit anisotropy . LCD devices work as an optical switch which is turned off and on by an electric field applied to the mesogen with director. The response of the director to the field is expressed with viscosity parameters, as in the Ericksen-Leslie theory in continuum mechanics developed by Jerald Ericksen and Frank Matthews Leslie . LCD devices work only up to the transition temperature when the mesophase changes to the isotropic liquid phase at the so-called clearing point . [ 5 ]
Mesophase phenomena are important in many scientific fields. The publishing arms of professional societies have academic journals as needed. For instance, the American Chemical Society has both Macromolecules and Langmuir , while Royal Society of Chemistry has Soft Matter , and American Physical Society has Physical Review E , and Elsevier has Advances in Colloid and Interface Science . | https://en.wikipedia.org/wiki/Mesophase |
A mesophile is an organism that grows best in moderate temperature , neither too hot nor too cold, with an optimum growth range from 20 to 45 °C (68 to 113 °F). [ 1 ] The optimum growth temperature for these organisms is 37 °C (about 99 °F). [ 2 ] The term is mainly applied to microorganisms . Organisms that prefer extreme environments are known as extremophiles . Mesophiles have diverse classifications, belonging to two domains : Bacteria , Archaea , and to kingdom Fungi of domain Eucarya . Mesophiles belonging to the domain Bacteria can either be gram-positive or gram-negative . Oxygen requirements for mesophiles can be aerobic or anaerobic . There are three basic shapes of mesophiles: coccus , bacillus , and spiral .
The habitats of mesophiles can include cheese and yogurt . They are often included during fermentation of beer and wine making. Since normal human body temperature is 37 °C , the majority of human pathogens are mesophiles, as are most of the organisms comprising the human microbiome .
Mesophiles are the opposite of extremophiles . Extremophiles that prefer cold environments are termed psychrophilic , those preferring warmer temperatures are termed thermophilic or thermotropic and those thriving in extremely hot environments are hyperthermophilic .
A genome-wide computational approach has been designed by Zheng, et al. to classify bacteria into mesophilic and thermophilic. [ 3 ]
All bacteria have their own optimum environmental surroundings and temperatures in which they thrive. Many factors are responsible for a given organism's optimal temperature range, but evidence suggests that the expression of particular genetic elements ( alleles ) can alter the temperature-sensitive phenotype of the organism. A study published in 2016 demonstrated that mesophilic bacteria could be genetically engineered to express certain alleles from psychrophilic bacteria, consequently shifting the restrictive temperature range of the mesophilic bacteria to closely match that of the psychrophilic bacteria. [ 4 ]
Due to the less stable structure of mesophiles, it has reduced flexibility for protein synthesis . [ 5 ] Mesophiles are not able to synthesize proteins in low temperatures. It is more sensitive to temperature changes, and the fatty acid composition of the membrane does not allow for much fluidity . [ 6 ] Decreasing the optimal temperature of 37 °C to 0 °C to 8 °C leads to a gradual decrease in protein synthesis. Cold-induced proteins (CIPs) are induced during low temperatures, which then allows cold-shock proteins (CSPs) to synthesize. The shift back to the optimal temperature sees an increase, indicating that mesophiles are highly dependent on temperature. [ 7 ] Oxygen availability also affects microorganism growth. [ 8 ]
There are two explanations for thermophiles being able to survive at such high temperatures whereas mesophiles can not. The most evident explanation is that thermophiles are believed to have cell components that are relatively more stable than the cell components of mesophiles which is why thermophiles are able to live at higher temperatures than mesophiles. [ 9 ] "A second school of thought, as represented by the writings of Gaughran (21) and Allen (3), believes that rapid resynthesis of damaged or destroyed cell constituents is the key to the problem of biological stability to heat." [ 9 ]
Due to the diversity of mesophiles, oxygen requirements greatly vary. Aerobic respiration requires the use of oxygen and anaerobic does not. There are three types of anaerobes . Facultative anaerobes grow in the absence of oxygen, using fermentation instead. During fermentation, sugars are converted to acids , alcohol , or gases . If there is oxygen present, it will use aerobic respiration instead. Obligate anaerobes cannot grow in the presence of oxygen. Aerotolerant anaerobes can withstand oxygen.
Microorganisms play an important role in decomposition of organic matter and mineralization of nutrients . In aquatic environments, the diversity of the ecosystem allows for the diversity of mesophiles. The functions of each mesophile rely on the surroundings, most importantly temperature range. [ 10 ] Bacteria such as mesophiles and thermophiles are used in the cheesemaking due to their role in fermentation . "Traditional microbiologists use the following terms to indicate the general (slightly arbitrary) optimum temperature for the growth of bacteria: psychrophiles (15–20 °C), mesophiles (30–37 °C), thermophiles (50–60 °C) and extreme thermophiles (up to 122 °C)". [ 11 ] Both mesophiles and thermophiles are used in cheesemaking for the same reason; however, they grow, thrive and die at different temperatures. Psychrotrophic bacteria contribute to dairy products spoiling, getting mouldy or going bad due to their ability to grow at lower temperatures such as in a refrigerator.
Some notable mesophiles include Listeria monocytogenes , Staphylococcus aureus , and Escherichia coli . Other examples of species of mesophiles are Clostridium kluyveri , Pseudomonas maltophilia , Thiobacillus novellus , Streptococcus pyogenes , and Streptococcus pneumoniae . Different types of diseases and infections typically have pathogens from mesophilic bacteria such as the ones listed above.
Listeria monocytogenes is a gram-positive bacterium. It is closely related to Bacillus and Staphylococcus . It is a rod-shaped, facultative anaerobe that is motile by peritrichous flagella . L. monocytogenes motility is limited from 20 °C to 25 °C. [ 12 ] At the optimal temperature, it loses its motility. This bacterium is responsible for listeriosis which derives from contaminated food. [ 12 ]
Staphylococcus aureus was first identified in 1880. [ 13 ] It is responsible for different infections stemming from an injury. The bacterium overcomes the body's natural mechanisms. Long lasting infections of S. aureus includes pneumonia , meningitis , and osteomyelitis . S. aureus is commonly contracted in hospital settings. [ 13 ]
Escherichia coli is a gram-negative, rod-shaped facultative anaerobic bacterium that does not produce spores . [ 14 ] The bacterium is a member of Enterobacteriaceae . It is capable of producing enterotoxins which are thermolabile or thermostable . [ 14 ] Other characteristics of E. coli are that it is oxidase -negative, citrate -negative, methyl-red positive, and Voges-Proskauer -negative. To sum up E. coli , it is a coliform . It is able to use glucose and acetate as a carbon source for fermentation. E. coli is commonly found in the gut of living organisms. [ 15 ] E. coli has many capabilities such as being a host for recombinant DNA and being a pathogen. [ 15 ] | https://en.wikipedia.org/wiki/Mesophile |
A mesoporous material (or super nanoporous [ 2 ] ) is a nanoporous material containing pores with diameters between 2 and 50 nm, according to IUPAC nomenclature. [ 3 ] [ 4 ] For comparison, IUPAC defines microporous material as a material having pores smaller than 2 nm in diameter and macroporous material as a material having pores larger than 50 nm in diameter.
Typical mesoporous materials include some kinds of silica and alumina that have similarly-sized mesopores. Mesoporous oxides of niobium , tantalum , titanium , zirconium , cerium and tin have also been reported. However, the flagship of mesoporous materials is mesoporous carbon, which has direct applications in energy storage devices. [ 5 ] Mesoporous carbon has porosity within the mesopore range and this significantly increases the specific surface area. Another very common mesoporous material is activated carbon which is typically composed of a carbon framework with both mesoporosity and microporosity depending on the conditions under which it was synthesized.
According to IUPAC, a mesoporous material can be disordered or ordered in a mesostructure. In crystalline inorganic materials, mesoporous structure noticeably limits the number of lattice units, and this significantly changes the solid-state chemistry. For example, the battery performance of mesoporous electroactive materials is significantly different from that of their bulk structure. [ 6 ]
A procedure for producing mesoporous materials (silica) was patented around 1970, [ 7 ] [ 8 ] [ 9 ] and methods based on the Stöber process from 1968 [ 10 ] were still in use in 2015. [ 11 ] It went almost unnoticed [ 12 ] and was reproduced in 1997. [ 13 ] Mesoporous silica nanoparticles (MSNs) were independently synthesized in 1990 by researchers in Japan. [ 14 ] They were later produced also at Mobil Corporation laboratories [ 15 ] and named Mobil Crystalline Materials , or MCM-41. [ 16 ] The initial synthetic methods did not allow to control the quality of the secondary level of porosity generated. It was only by employing quaternary ammonium cations and silanization agents during the synthesis that the materials exhibited a true level of hierarchical porosity and enhanced textural properties. [ 17 ] [ 18 ] Mesoporous materials have been also produced in the form of thin films via evaporation induced self-assembly, in different organized mesostructures and compositions. [ 19 ]
Since then, research in this field has steadily grown. Notable examples of prospective industrial applications are catalysis , sorption, gas sensing, batteries, [ 20 ] ion exchange, optics , and photovoltaics . In the field of catalysis, zeolites is an emerging topic where the mesoporosity as a function of the catalyst is studied to improve its performance for use in Fluid catalytic cracking .
It should be taken into account that this mesoporosity refers to the classification of nanoscale porosity, and mesopores may be defined differently in other contexts; for example, mesopores are defined as cavities with sizes in the range 30 μm–75 μm in the context of porous aggregations such as soil. [ 21 ] | https://en.wikipedia.org/wiki/Mesoporous_material |
Mesoporous organosilica ( periodic mesoporous organosilicas , PMO ) are a type of silica containing organic groups that give rise to mesoporosity . They exhibit pore size ranging from 2 nm - 50 nm, depending on the organic substituents. In contrast, zeolites exhibit pore sizes less than a nanometer. PMOs have potential applications as catalysts , adsorbents , trapping agents, drug delivery agents, stationary phases in chromatography and chemical sensors .
The breakthrough report in this area described the use of surfactants to produce periodic mesoporous silicas (PMS) in 1992 with pores larger than that of zeolites. [ 1 ]
Early mesoporous organosilicas developed had organic groups attached terminally to the silica surface. They were prepared either by grafting of organic group onto the channel walls or by template-directed co-condensation. [ 2 ] For example, by modifying the channels of PMSs with alkanethiol groups that could sequester heavy metals . However, there were some major limitations like, inhomogeneity of the pores compared to PMSs, and limited organic content (around 25% with respect to the silicon wall sites). [ 3 ]
In 1999, reports described mesoporous organosilicas with organic groups located within the pore channel walls as "bridges" between Si centers. [ 4 ] [ 5 ] [ 6 ] Since these materials had both organic and inorganic groups as integral part of the porous framework, they were considered as composites of organic and inorganic material and designated as periodic mesoporous organosilicas (PMOs). This family of porous materials had high degree of order and uniformity of pores compared to those with terminal organic groups. [ 3 ]
The framework of PMOs consists of inorganic components (polysilsesquioxanes) uniformly bridged by organic linkers. Most of the bridged polysilsesquioxane can be generically represented by the formula O 1.5 Si-R-SiO 1.5 . where R represents the organic bridging group. Each individual organic group is covalently bonded to two or more silicon atoms in the framework. The pores in the material are periodically ordered with diameter in the range 2 -30 nm. [ 7 ]
Depending on the synthetic conditions used to make mesoporous organosilicas, the mesoscale structure can either be amorphous or crystalline. [ 8 ] Most of the mesoporous organosilicas that have been synthesized are amorphous. Although, x-ray diffraction of these materials indicate periodicity in the structure, sharp peaks in the medium scattering angle representative of crystalline materials are usually absent, except for (00l) reflections. [ 9 ] However, few crystalline mesoporous organosilica have been reported,. [ 9 ] [ 10 ] [ 11 ] [ 12 ]
The primary methods used to make mesoporous organosilicas are evaporation-induced self-assembly, surfactant-mediated synthesis, post-synthetic grafting, and co-condensation. Organosilicas with amorphous structures are typically made by functionalizing organic groups rather than directly integrating the functional groups in the framework, which produces a periodic structure. [ 8 ] Furthermore, basic hydrolytic conditions typically produce a periodic structure because of hydrophobic and hydrophilic interactions between hydrolyzed precursors that then self-assemble.
Evaporation-induced self-assembly usually causes random alignment of the material pores. [ 13 ] This method of synthesis uses the difference in vapor pressure of solvents to vary the rate of evaporation and therefore the assembly of the organosilica framework.
Surfactant-mediated synthesis has been widely used for the production of mesoporous materials in general, and PMOs specifically,. [ 8 ] [ 14 ] It involves the addition of a surfactant or copolymer to a specific molecular precursor. The surfactant directs the structure of the material by interacting with the precursor in such a way that is dependent on the properties of the precursor. After the bulk structure is assembled, the surfactant is removed, leaving pores, or channels, embedded in the material framework. The surfactant template can be removed by solvent extraction or ion-exchange mechanisms. An aging process is usually performed at high temperature before removal of the surfactant. [ 15 ] During surfactant-mediated synthesis, hydrolysis and polycondensation , or co-condensation, are used to fuse precursor molecules in a framework. [ 8 ] Acidic or basic conditions are used for the hydrolysis depending on the precursor being introduced.
The other two synthesis methods used for these materials are post-synthetic grafting and co-condensation. [ 16 ] In the case of post-synthetic grafting, organic functional groups, typically organosilanes or alkoxyorganosilanes, are reacted with the assembled silicon mesostructure with or without the surfactant template present. [ 17 ] If the template is still present, the grafting process will involve simultaneously removing the template and attaching the functional group. However, the pores of the material can be blocked during this process so a one-pot synthesis using the necessary components is more advantageous. This one-pot synthesis is known as co-condensation, in which the desired organosilyl functional groups are combined with the surfactant or other structure-directing agent. In this method, the material becomes structured and functionalized. Co-condensation gives rise to periodicity with the mesostructure, and it accommodates larger organic groups as well as larger pore sizes because of the one-step assembly process. Most PMOs have been made using the co-condensation method. The most recent method developed builds on co-condensation by combining multiple reactive organic precursors to form a new functional group, which is still combined with the framework molecule and copolymer. [ 16 ]
Mesoporous organosilicate materials have been made using bridged organic precursors, in which an organic fragment is positioned between silicon-containing fragments. [ 8 ] Single precursor syntheses are typically done with bridged organosilane groups. When only one bridged organic precursor is used, there is a homogeneous distribution of the molecule in the framework. [ 18 ] This phenomenon is referred to as molecular-scale periodicity. Chiral precursors can also be introduced into the material framework, and using acidic conditions in the hydrolysis and condensation process proves better for chiral precursors because no racemization occurs. Co-condensation of multiple organosilane precursors can create multi-functional organosilica materials. Tetraethoxysilane (TEOS) is a common silicon precursor used in co-condensation reactions. [ 8 ]
Highly porous compounds are potential catalysts , [ 19 ] adsorption , [ 20 ] and separation . These have been the roles of zeolites, but their small pore size limits them to work with small molecules. The larger pore size (2-50 nm) of mesoporous materials gives them wider application – larger molecules can be admitted, and guest molecules can migrate faster. [ 21 ]
To effect catalytic transformations using mesoporous organosilicas, it is necessary to functionalize them. The two major methods [ 21 ] are to add a group or heteroatom , such as a metal center, to the organic framework, and to anchor [ 22 ] an organic or organometallic group to the pore surface.
Anchoring a homogeneous catalyst onto a mesoporous organosilicas framework has two primary disadvantages: the bulky group in the pore can block travel of guest molecules through it, and preparation of candidate molecules for anchoring to the framework is difficult. However, anchoring can create heterogeneous catalysts for a wide variety of chemical transformations: acid catalysis , base catalysis , [ 23 ] coupling and condensation reaction catalysis, and even asymmetric catalysis . [ 24 ] Anchored functional groups often have higher catalytic activity than does the bulk material, as one study [ 23 ] showed for Nafion , or even than groups incorporated into the organosilica framework, as with sulfonic acid. [ 21 ]
Mesoporous organosilicas can be functionalized give adsorbants, for removal specific contaminants from air and water. Candidate adsorbants include toxic heavy metals, radioactive material, and various organic pollutants have been synthesized. [ 20 ]
Mesoporous organosilicas have been functionalized with fluorescent probes. The advantage of this material as a sensor is its high surface area combined with the high specificity achievable by careful functionalization. Mesoporous organosilicas have been used to sense a wide variety of analytes: [ 25 ] metals, industrial pollutants, small organic molecules, and large biological molecules.
Mesoporous organosilicas have been tested as potential materials for separation using HPLC . Froba et al. have shown that by using benzene PMO microspheres as stationary phases better separation can be achieved in the HPLC system. The theory was that the π-π interaction between the aromatic analytes and the phenylene bridge of the PMO framework leads to stronger retention and hence better separation. [ 26 ]
Controlled drug release is another aspect in which PMOs have been shown promise. [ 27 ] The hydrophobic nature of the PMO walls allow for better control in drug release. In this respect, it is not just the mesoporosity of the PMOs make them advantageous, the tunability of the organic groups also play an important role. [ 7 ]
It has been proposed that the periodicity of PMOs may produce anisotropic mechanical, electrical and optical responses, in the same manner that periodicity magnifies anisotropy in the unit cell of conventional crystals. [ 7 ] Also, studies that have shown that dendrimers , [ 28 ] polyhedral oligomeric silsesquioxanes, [ 29 ] and carbon nanomaterials like C60 [ 30 ] can be incorporated into the pore walls of PMOs offers new directions in the possible applications of these materials. It has been shown that PMOs are more suitable for the construction of organic donor–acceptor systems for photocatalysis than periodic mesoporous silica because organic donor or acceptor groups within the framework provide larger empty spaces for mass transfer in photocatalysis than in mesoporous silicas. [ 31 ] Recent investigations on charge transfer systems based on PMOs [ 32 ] are suggestive of possible applications of PMOs in areas as such as heterojunction solar cells , photodetectors and light emitting diodes . More exciting applications can emerge by combining these materials with biological molecules such as lipids and proteins. [ 7 ]
PMOs with unconventional structures and properties have found high potential for future developments. [ 33 ] | https://en.wikipedia.org/wiki/Mesoporous_organosilica |
Mesoporous silicates are silicates with a special morphology .
Porous inorganic solids have found great utility as catalysts and sorption media because of their large internal surface area , i.e. the presence of voids of controllable dimensions at the atomic, molecular, and nanometer scales. With increasing environmental concerns worldwide, nanoporous materials have become more important and useful for the separation of polluting species and the recovery of useful ones. In recent years there has been great progress in applying environmentally friendly zeolites in heterogeneous reaction catalysis . The reason for their success is related to their specific features in converting molecules having kinetic diameter below 1 nm, but they become inadequate when reactants with sizes above the dimensions of the pores have to be processed. Research efforts to synthesize zeolites with larger pore diameter, high structural stability and catalytic activity have not given the expected results yet.
The discovery of a new family of mesoporous molecular sieves in the early 1990s by Kuroda et al., known as KSW-1 [ 1 ] and FSM-16, [ 2 ] and by ExxonMobil , called M41S, [ 3 ] opened new possibilities to prepare catalysts for reactions of relatively large molecules. The silicate wall of the pores is amorphous . Mesoporous silicates, such as MCM-41 and SBA-15 (the most common mesoporous silicates), are porous silicates with huge surface areas (normally ≥1000 m 2 /g), large pore sizes (2 nm ≤ size ≤ 20 nm) and ordered arrays of cylindrical mesopores with very regular pore morphology. The large surface areas of these solids increase the probability that a reactant molecule in solution will come into contact with the catalyst surface and react. The large pore size and ordered pore morphology allow one to be sure that the reactant molecules are small enough to diffuse into the pores.
This article about materials science is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Mesoporous_silicate |
Mesopotamian divination was divination within the Mesopotamian period .
Perceptual elements utilized in the practice of a divinatory technique included the astronomical ( stars and meteorites ), weather and the calendar , the configuration of the earth and waterways and inhabited areas, the outward appearance of inanimate objects and also vegetation, elements stemming from the behavior and the birth of animals, especially humans. [ 1 ]
Magic was used to counter a negative fate foretold by divination. [ 2 ]
The earliest evidence for practice is ( dating is true to this article ) from the fourth millennia B.C. ( Sumeria ), 2100 to 2000 BC ( Neo-Sumeria ) and 7th century BC ( Babylonia ), except for circa 2100 via the Babylonian Epic of Gilgamesh .
The area of land known as Sumer , within Mesopotamia, had a settled population within the 5th millennia BCE . [ 3 ]
A seal from Sumer , (of Mudgala , [ 4 ] Lord of Edin , Minister to Uruas [ 5 ] ) shows the word Azu, which meant water-divinator (lit. water knower), and additionally, physician. [ 4 ] Lord Mudgala was the son of Uruas the Khad , [ 6 ] who was the first dynasty of Sumeria (via Phoenicia ) of the fourth millennium BCE . [ 7 ]
Another artifact from Sumerian culture , [ 3 ] [ 4 ] a death amulet seal, shows the name Uzu-as' and is a resurrection amulet for the slave and seer of the Temple of the Sun, Uzu-as'. The part of the name, the word Uzu, meant in Sumerian, diviner, magician, or seer. [ 4 ]
There is some suggestion people of this era knew of, and were experiencing, dreams as portents and sources for divination. [ 8 ] The Neo-Sumerian period was from circa the years 2100 to 2000 BC. [ 9 ]
Most of the extant material showing evidence of divination practice are from the 7th century BCE [ 10 ] and accordingly from Babylonian culture , which dates from 1850 BCE and later. [ 3 ]
The Sumerian version of the Epic of Gilgamesh (circa 2100 [ 11 ] ) has the mother of Gilgamesh interpreting a dream of Gilgamesh (a portent of the advent of Enkidu ). [ 8 ]
Divination practice evolved through time from abductive positions to reckonings by virtue of an a priori , and a tendency to make generalizations about causes. [ 12 ] [ 13 ]
Two types of divination existed in Mesopotamia, divine and human. [ 1 ] Mesopotamian diviners most often used a liver for divination, or else by observing the sky . [ 14 ]
Another difference delineated by Bottéro, is of two types of divination, both divine, but one artificial and the other natural; the artificial being divinations where through a process of "computation and constant observation" a future truth is gleaned; and natural, being a kind of gift from a god whereby direct inspired communication occurs from god to human. [ 15 ]
Bottéro and Bahrani assert Mesopotamian divination was not just divination, and not limited in development to a type of superstition, but was developed to the extent to which it was in fact a science . [ 1 ]
Study of portents from gods was vital within Mesopotamia throughout the entire time of its existence. [ 16 ] The gods Šamaš and Adad were associated most closely with divination, Šamaš related to divination in decisions , and Adad for oracles and omens . [ 17 ]
Celestial divination was conducted for the purposes of the king and the state. [ 10 ] Diviners observed the sun by day and the stars of the night sky , which they knew as šıṭır samé , or, šıṭır šamāmī , or, šıṭır burūmē (writing of the firmament [ 18 ] ). These three things refer to their thought of the stars of the sky interpreted as heavenly writing . [ 14 ] [ 19 ] By way of the celestial, this type of divination was one of Babylon's three related celestial sciences, along with astronomy and horoscopy . [ 18 ]
The descriptions šıṭır šamê and šıṭırti šamāmī are found sometimes within Neo-Babylonian royal inscriptions in special reference to those temples thought of a beautiful in a way of those temples being (lit.) like the heavenly writing . [ 18 ]
Impetration is a type of divination which involved a diviner asking a deity to control a medium for the diviner to foretell the future. Media might include smoke, lots , or drops of oil in, or on, water. [ 10 ]
Divination by way of deductive thought whereby people understood the significance of forms and/ or, changes in a medium as showing and revealing a truth, is attested to within Old Babylonia , at a date of 1950 BCE [ 1 ] [ 2 ]
Divination of this type involved using the liver , and possibly additionally the gall-bladder . [ 20 ]
Examinining internal organs to make predictions is known as extispicy . [ 16 ] [ 17 ] [ 21 ]
Extant sources reveal individuals were restricted from using extispicic means by a prohibitive cost for the performance of this divination so that royal members and nobles were mostly the only ones able to afford to know the future by this means. [ 17 ]
Existing sources for knowledge of hepatoscopy are clay models of divined livers. [ 22 ] [ 23 ]
Hepatoscopic practice and belief began during the third millennium BCE. [ 23 ] The practice is referred to in the Hebrew Bible in Ezekiel 21:21 .
To make predictions, diviners had two things to aid their making of a divinatory statement – lists of previous predictions and clay models made of previously interpreted livers. [ 23 ]
Hepatoscopic predictions were made on the entrails of slaughtered animals (Oppenheim) by observing any kind of abnormality within the organ, such as atrophy , hypertrophy , displacement, or any type of unusual marking. [ 17 ]
In Mesopotamian culture, the liver was thought of as being the centre of thought and feeling . [ 23 ]
Study of the human body and foretelling of an individual's fate from this study is known as physiognomics . Diviners (or perhaps associated others) made and circulated these texts to successive generations, handing down knowledge for nearly two millennia. [ 24 ]
Physiognomic divination omens, in the first extant recorded, date from a period 2000 - 1600 BCE [ 24 ]
The Mesopotamian dream interpreter was known as ša'il(t)u. [ 8 ]
Necromantic practice is shown by historical document to have begun from at least 900 BCE, and was relied upon for insight to a much greater extent within urban culture by the time of King Esarhaddon in the early 7th century BCE. [ 25 ]
In literature, Babylonian divination material very often does not appear in the contents within written introductories, making it difficult for any reader who might want to know the contents of the text. [ 24 ]
Enūma Anu Enlil is a text of conclusions of divination. [ 14 ]
Šumma alammdimmǔ is a series of omens made by physiognomics dating to the close of the second millennium BCE. They are inscribed upon 27 clay tablets . [ 24 ]
The study of divination [ 26 ] within Babylonian culture [ 27 ] belongs to the discipline of Assyriology and began in earnest sometime during the decade of the 1870s. [ 26 ] | https://en.wikipedia.org/wiki/Mesopotamian_divination |
A mesopredator is a predator that occupies a mid-ranking trophic level in a food web . [ 1 ] There is no standard definition of a mesopredator, but mesopredators are usually medium-sized carnivorous or omnivorous animals, such as raccoons , foxes , or coyotes . [ 2 ] [ 3 ] They are often defined by contrast from apex predators or prey in a particular food web. [ 3 ] [ 2 ] [ 4 ] Mesopredators typically prey on smaller animals. [ 2 ]
Mesopredators vary across different ecosystems . Sometimes, the same species is a mesopredator in one ecosystem and an apex predator in another ecosystem, depending on the composition of that ecosystem. [ 3 ] When new species are introduced into an ecosystem, the role of the mesopredator often changes; this can also happen if species are removed. [ 4 ]
The American Institute of Biological Sciences states that due to the fact that mesopredators are smaller than large carnivores, they are more abundant, and therefore have greater diversity of mesopredator species.[2] Due to their smaller size, mesopredators play a part in the ecosystem of dispersing seeds in open spaces, as well as driving community structure.[2] Mesopredators are also very diverse in comparison to larger carnivores in their behaviour and ecology, from being reclusive to highly social. Their diversity and small size allows them to thrive in a range of habitats than larger carnivores are able to.[2] The population of these smaller carnivores also increases when the presence of a larger carnivore decline. This is known as the 'mesocarnivore release.' According to the National Park Service, "Mesocarnivore release is defined as the expansion in range and/or abundance of a smaller predator following the reduction or removal of a larger predator."[6] One impact of this is that these mesopredators can act as scavengers cleaning up dead animal carcasses discarded by humans in urban areas.[7] Mesopredators' habitat have shifted and changed, due to urbanisation, leading to habitat fragmentation and disturbance, resulting in habitat loss for animals.
When populations of an apex predator decrease, populations of mesopredators in the area often increase due to decreased competition and conflict with the apex predator. [ 2 ] This is known as the mesopredator release effect , which refers to the release of mesopredators from the trophic cascade . [ 5 ] These mesopredator outbreaks can lead to declining prey populations, destabilized ecological communities , reduced biodiversity, and can even drive local extinctions . [ 2 ] [ 4 ]
Typically, mesopredators are in competition with apex predators for food and other resources. [ 2 ] Apex predators reduce mesopredator populations and change mesopredator behaviors and habitat choices by preying on and intimidating mesopredators. [ 6 ] When apex predator populations decline, mesopredators can access hunting and den areas once controlled by the apex predators, essentially assuming the role of an apex predator. [ 2 ] However, mesopredators often occupy different ecological niches than the former apex predator and will have different effects on the structure and stability of the ecosystem. [ 3 ] [ 4 ]
Mesopredator outbreaks are becoming more common in fragmented habitats , which are areas where a species' preferred environment is broken up by obstacles. [ 4 ] Fragmented habitats can be caused by geological or human activity, and particularly affect larger animals that roam and hunt across large territories, such as apex predators. [ 7 ] Fragmented habitats can drive these species to leave and find more suitable habitats. [ 4 ]
Additionally, in many fragmented habitats, apex predators have more encounters with humans, leaving them susceptible to harmful or deadly conflicts, sometimes resulting in eradication of the apex predator population entirely. [ 4 ] Human development also promotes mesopredator outbreaks through increasing access to resources such as pet food, trash, and crops. [ 4 ]
The mesopredator release effect is not entirely understood. Most research has been conducted on mammal species, with limited studies on non-mammal animal species. [ 3 ] Additionally, it is not well understood how these dynamics may play out in ecosystems with many mesopredator and apex predator species. [ 3 ] | https://en.wikipedia.org/wiki/Mesopredator |
Mesoscopic physics is a subdiscipline of condensed matter physics that deals with materials of an intermediate size. These materials range in size between the nanoscale for a quantity of atoms (such as a molecule ) and of materials measuring micrometres . [ 1 ] The lower limit can also be defined as being the size of individual atoms. At the microscopic scale are bulk materials. Both mesoscopic and macroscopic objects contain many atoms. Whereas average properties derived from constituent materials describe macroscopic objects, as they usually obey the laws of classical mechanics , a mesoscopic object, by contrast, is affected by thermal fluctuations around the average, and its electronic behavior may require modeling at the level of quantum mechanics . [ 2 ] [ 3 ]
A macroscopic electronic device, when scaled down to a meso-size, starts revealing quantum mechanical properties. For example, at the macroscopic level the conductance of a wire increases continuously with its diameter. However, at the mesoscopic level, the wire's conductance is quantized : the increases occur in discrete, or individual, whole steps. During research, mesoscopic devices are constructed, measured and observed experimentally and theoretically in order to advance understanding of the physics of insulators , semiconductors , metals , and superconductors . The applied science of mesoscopic physics deals with the potential of building nanodevices.
Mesoscopic physics also addresses fundamental practical problems which occur when a macroscopic object is miniaturized, as with the miniaturization of transistors in semiconductor electronics. The mechanical, chemical, and electronic properties of materials change as their size approaches the nanoscale, where the percentage of atoms at the surface of the material becomes significant. For bulk materials larger than one micrometre, the percentage of atoms at the surface is insignificant in relation to the number of atoms in the entire material. The subdiscipline has dealt primarily with artificial structures of metal or semiconducting material which have been fabricated by the techniques employed for producing microelectronic circuits. [ 2 ] [ 3 ]
There is no rigid definition for mesoscopic physics but the systems studied are normally in the range of 100 nm (the size of a typical virus ) to 1 000 nm (the size of a typical bacterium): 100 nanometers is the approximate upper limit for a nanoparticle . Thus, mesoscopic physics has a close connection to the fields of nanofabrication and nanotechnology . Devices used in nanotechnology are examples of mesoscopic systems. Three categories of new electronic phenomena in such systems are interference effects, quantum confinement effects and charging effects. [ 2 ] [ 3 ]
Quantum confinement effects describe electrons in terms of energy levels, potential wells , valence bands , conduction bands , and electron energy band gaps .
Electrons in bulk dielectric materials (larger than 10 nm) can be described by energy bands or electron energy levels. Electrons exist at different energy levels or bands. In bulk materials these energy levels are described as continuous because the difference in energy is negligible. As electrons stabilize at various energy levels, most vibrate in valence bands below a forbidden energy level, named the band gap . This region is an energy range in which no electron states exist. A smaller amount have energy levels above the forbidden gap, and this is the conduction band.
The quantum confinement effect can be observed once the diameter of the particle is of the same magnitude as the wavelength of the electron's wave function . [ 4 ] When materials are this small, their electronic and optical properties deviate substantially from those of bulk materials. [ 5 ] As the material is miniaturized towards nano-scale the confining dimension naturally decreases. The characteristics are no longer averaged by bulk, and hence continuous, but are at the level of quanta and thus discrete. In other words, the energy spectrum becomes discrete, measured as quanta, rather than continuous as in bulk materials. As a result, the bandgap asserts itself: there is a small and finite separation between energy levels. This situation of discrete energy levels is called quantum confinement .
In addition, quantum confinement effects consist of isolated islands of electrons that may be formed at the patterned interface between two different semiconducting materials. The electrons typically are confined to disk-shaped regions termed quantum dots . The confinement of the electrons in these systems changes their interaction with electromagnetic radiation significantly, as noted above. [ 6 ] [ 7 ]
Because the electron energy levels of quantum dots are discrete rather than continuous, the addition or subtraction of just a few atoms to the quantum dot has the effect of altering the boundaries of the bandgap. Changing the geometry of the surface of the quantum dot also changes the bandgap energy, owing again to the small size of the dot, and the effects of quantum confinement. [ 6 ]
In the mesoscopic regime, scattering from defects – such as impurities – induces interference effects which modulate the flow of electrons. The experimental signature of mesoscopic interference effects is the appearance of reproducible fluctuations in physical quantities. For example, the conductance of a given specimen oscillates in an apparently random manner as a function of fluctuations in experimental parameters. However, the same pattern may be retraced if the experimental parameters are cycled back to their original values; in fact, the patterns observed are reproducible over a period of days. These are known as universal conductance fluctuations .
Time-resolved experiments in mesoscopic dynamics: the observation and study, at nanoscales, of condensed phase dynamics such as crack formation in solids, phase separation, and rapid fluctuations in the liquid state or in biologically relevant environments; and the observation and study, at nanoscales, of the ultrafast dynamics of non-crystalline materials. [ 8 ] [ 9 ] | https://en.wikipedia.org/wiki/Mesoscopic_physics |
Mesowear is a method, used in different branches and fields of biology . This method can apply to both extant and extinct animals, according to the scope of the study. Mesowear is based on studying an animal's tooth wearing fingerprint. In brief, each animal has special feeding habits, which cause unique tooth wearing. Rough feeds cause serious tooth abrasion, while smooth one triggers moderate abrasion, so browsers have teeth with moderate abrasion and grazers have teeth with rough abrasion. Scoring systems can quantify tooth abrasion observations and ease comparisons between individuals.
The mesowear method or tooth wear scoring method [ 1 ] is a quick and inexpensive process of determining the lifelong diet of a taxon ( grazer or browser ) and was first introduced in the year 2000.
The mesowear technique can be extended to extinct and also extant animals.
Mesowear analyses require large sample populations (>20), which can be problematic for some localities, but the method yields an accurate depiction of an animal's average lifelong diet. [ 2 ] Mesowear analysis is based on the physical properties of ungulate foods as reflected in the relative amounts of attritive and abrasive wear that they cause on the dental enamel of the occlusal surfaces. Mesowear was recorded by examining the buccal apices of molar tooth cusps . Apices were characterized as sharp, rounded, or blunt, and the valleys between them either high or low. The method has been developed only for selenodont and trilophodont molars, but the principle is readily extendable to other crown types. In collecting the data the teeth are inspected at close range, a hand lens will be used. Mesowear analysis is insensitive to wear stage as long as the very early and very late stages are excluded. [ 3 ] Mesowear analysis follows standard protocols . Specimens are digitally photographed in labial view so that cusp shape and occlusal relief can be scored. [ 4 ] this method helps zoologists and nutritionists to prepare proper kind of hay for captive feral herbivores with unknown feed habits in zoos. [ 5 ] In collecting the data the teeth are inspected at close range, using a hand lens.
Gravity toward lower teeth causes more abrasion on lower teeth than upper teeth. This fact is base of mesowear method. [ 6 ] [ 7 ]
Sharp : A sharp cusp terminates to a point and has practically no rounded area between the
mesial and distal phase I facets,
Round : a rounded cusp has a distinctly rounded tip (apex) without planar
facet wear but retains facets on the lower slopes.
Blunt : blunt cusp lacks distinct facets altogether. [ 8 ]
The attrition : this kind of dental wearing is as a result of rubbing tooth to tooth and no external forces cause this enamel abrasion.usually browsers feed contains less food abrasive materials( such as silica because of feed selecting behavior in this animals so wearing type of browser ungulates will be this type in most cases.
The abrasion : rubbing food to tooth triggers this kind of tooth wearing more visible for grazer animals than browsers. | https://en.wikipedia.org/wiki/Mesowear |
Mesoxalic acid , also called oxomalonic acid or ketomalonic acid , is an organic compound with formula C 3 H 2 O 5 or HO−(C=O) 3 −OH.
Mesoxalic acid is both a dicarboxylic acid and a ketonic acid . It readily loses two protons to yield the divalent anion C 3 O 2− 5 , called mesoxalate , oxomalonate , or ketomalonate . These terms are also used for salts containing this anion, such as sodium mesoxalate , Na 2 C 3 O 5 ; and for esters containing the −C 3 O 5 − or −O−(C=O) 3 −O− moiety , such as diethyl mesoxalate , (C 2 H 5 ) 2 C 3 O 5 . Mesoxalate is one of the oxocarbon anions , which (like carbonate CO 2− 3 and oxalate C 2 O 2− 4 ) consist solely of carbon and oxygen .
Mesoxalic acid readily absorbs and reacts with water to form a product commonly called "mesoxalic acid monohydrate", more properly dihydroxymalonic acid , HO−(C=O)−C(OH) 2 −(C=O)−OH. [ 2 ] In product catalogs and other contexts, the terms "mesoxalic acid", "oxomalonic acid" and so on often refer to this "hydrated" compound. In particular, the product traded as "sodium mesoxalate monohydrate" is almost always sodium dihydroxymalonate .
Mesoxalic acid can be obtained synthetically by hydrolysis of alloxan with baryta water, [ 2 ] by warming caffuric acid [ 3 ] with lead acetate solution, [ 2 ] or from glyceryl diacetate and concentrated nitric acid in ice-cold water. The product can be obtained also by oxidation of tartronic acid [ 4 ] or glycerol . [ 5 ] Since they are carried out in water, these procedures generally give the dihydroxy derivative.
It is also prepared by the oxidation of glycerol with the help of bismuth(III) nitrate . | https://en.wikipedia.org/wiki/Mesoxalic_acid |
The Mesozoic–Cenozoic Radiation is the third major extended increase of biodiversity in the Phanerozoic , [ 1 ] after the Cambrian Explosion and the Great Ordovician Biodiversification Event , which appeared to have exceeded the equilibrium reached after the Ordovician radiation. Made known by its identification in marine invertebrates, this evolutionary radiation began in the Mesozoic , after the Permian extinctions, and continues to this date. This spectacular radiation affected both terrestrial and marine flora and fauna, [ 2 ] during which the "modern" fauna came to replace much of the Paleozoic fauna. [ 1 ] Notably, this radiation event was marked by the rise of angiosperms during the mid- Cretaceous , [ 3 ] and the K-Pg extinction , which initiated the rapid increase in mammalian biodiversity. [ 4 ] [ 5 ]
The exact causes of this extended increase in biodiversity are still being debated, however, the Mesozoic-Cenozoic radiation has often been related to large-scale paleogeographical changes. [ 6 ] [ 2 ] [ 7 ] The fragmentation of the supercontinent Pangaea has been related to an increase in both marine and terrestrial biodiversity. [ 8 ] [ 9 ] [ 10 ] The link between the fragmentation of supercontinents and biodiversity was first proposed by Valentine and Moores in 1972. They hypothesized that the isolation of terrestrial environments and the partitioning of oceanic water masses, as a result of the breaking up of Pangaea, resulted in an increase in allopatric speciation , which led to an increased biodiversity. [ 11 ] These smaller landmasses, while individually being less diverse than a supercontinent , contain a high degree of endemic species, resulting in an overall higher biodiversity than a single landmass of equivalent size. [ 8 ] It is therefore argued that, similarly to the Ordovician bio-diversification, the differentiation of biotas along environmental gradients caused by the fragmentation of a supercontinent, was a driving force behind the Mesozoic-Cenozoic radiation. [ 6 ] [ 7 ]
Part of the dramatic increase in biodiversity during this time was caused by the evolutionary radiation of flowering plants, or angiosperms , during the mid-Cretaceous. [ 3 ] Characteristics of this clade associated with reproduction have served as a key innovation for an entire clade, and led to a burst of evolution known as the Cretaceous Terrestrial Revolution . [ 12 ] These later diversified further and co-radiated with pollinating insects , increasing biodiversity. [ 13 ]
A third factor which played a role in the Mesozoic-Cenozoic radiation was the K-Pg extinction , which marked the end of the dinosaurs and, surprisingly, resulted in a massive increase in biodiversity of terrestrial tetrapods , which can almost entirely be attributed to the radiation of mammals. There are multiple things which could have caused this deviation from the equilibrium, one of which is that the before the K-Pg extinction an equilibrium was reached which limited biodiversity. [ 4 ] [ 5 ] The extinction event reorganized the fundamental ecology, on which diversity is built and maintained. After these reorganized ecosystems stabilized a new, higher, equilibrium was reached, which was maintained during the Cenozoic. [ 14 ] Cenozoic biodiversity reached a peak twice as high as the biodiversity peak during the Palaeozoic. [ 15 ]
One effect which has to be taken into account when estimating past biodiversity levels is the pull of the recent , which describes a phenomenon in the fossil record which causes biodiversity estimates to be skewed towards modern taxa. [ 16 ] [ 17 ] This bias towards recent taxa is caused by a better availability of more recent fossil records. In mammals it has also been argued that the complexity of teeth, allowing for precise taxonomic identification of fragmentary fossils, increases their perceived diversity when compared to other clades at the time. [ 4 ] [ 5 ] The contribution of this effect to the apparent increase in biodiversity is still unclear and heavily debated. [ 18 ] [ 5 ] [ 6 ] | https://en.wikipedia.org/wiki/Mesozoic–Cenozoic_radiation |
MessagEase is an input method and virtual keyboard for touchscreen devices. It relies on a new entry system designed by Saied B. Nesbat, formatted as a 3x3 matrix keypad where users may press or swipe up, down, left, right, or diagonally to access all keys and symbols. [ 1 ] It is a keyboard that was designed for devices like cell phones, mimicking the early cell phones' limited number of 12 keys. [ 2 ]
The most frequently used letters (the large letters in the illustration below) are accessed by a tap. Less common letters are accessed by a slide. Example: Tapping the center square generates an 'o'. Sliding to the left from the same square generates a 'c'. A green trail shows the path of the finger. [ 3 ] The keyboard supports multiple user dictionaries, used for word prediction and correction. [ 4 ]
The software is developed and patented by ExIdeas, based in Belmont, California . It was first released in 2002 for the Palm, along with a paper in 2003. [ 5 ]
The keyboard layout has a 3x3 matrix that allows for full-text entry. The letter placement is optimized for minimal movement distance between letters, allowing for faster typing. [ 6 ]
The layout is 67% more efficient than a standard QWERTY software keyboard, and 31% more than a multi-tap keyboard, when typing is modeled with Fitt's law .
The 9 most frequent letters in English texts: ETAONRISH, are placed on the keyboard so they can be accessed on a single click.
The next 17 less frequent letters: DLFCMUGYPWBVKJXQZ, are placed as to be triggered by a single move of the finger from or to the central key (O) (except for Z which is centered around the 'E' key together with some punctuation characters). For example, the letter V is typed by dragging the finger from A to O, and the letter D by moving from O to E.
The moves producing special characters, which includes 38 characters including accents and punctuation marks, are displayed on a complete keyboard showing up when the user drags the space bar upwards.
This is not an alternate keyboard in the sense that the key pair moves are valid on both keyboard. It is rather a mnemonic help, which is normally hidden to avoid overwhelming the user with spurious information.
A small vertical bar on the right (or on the left in left-handed mode) gives direct access to the cut/copy/paste operations, the numeric keypad, the uppercase/lowercase control, as well the usual F1-F12 control keys. This is also commanded by moving the finger from one cell to an other.
A set of small movements makes the life of the typist easier, like drawing a small circle or a back and forth movement to write a letter uppercase, or prolonging the movement to put accent on letters.
The keyboard can be resized to fit the need of the user, and is also provided in a double sized version with the numeric keypad on the side of the alphabetic keypad. [ 7 ]
The keyboard is currently available for Android devices, iOS devices and the Apple Watch. [ 8 ]
As of 2024-02, Exideas appears to intend to no longer offer the keyboard for free. A nag screen asks users to join a subscription model. This change is not described on Exideas' web page and has caused numerous users to change their five-star reviews to one star on Google Play. [ 9 ]
On Android, "Thumb-Key" is available on F-Droid [ 10 ] and Google Play. [ 11 ]
"FlickBoard," which enables some gestures that are not supported in Thumb-Key, is available for Android devices on Izzysoft [ 12 ] (and via F-Droid if the Izzysoft repository is added), as well as on Google Play. [ 13 ]
Currently supported input languages:
MessagEase was released in 2002 for the Palm. It was also originally a competitor to the T9 predictive input method, on a 12-button phone, with 9 number buttons. In this first iteration, each of the 9 primary characters needed to be pressed twice in a row, and secondary characters were entered by first pressing the main button, and then pressing one of the remaining 8 buttons. [ 14 ] In this first iteration, because many letters required two presses, it was not significantly faster than the Multi-tap input method. [ 15 ]
MessagEase is now exclusively for touch screens, and no longer has physical 12-button support. All characters are now entered by tapping or swiping. | https://en.wikipedia.org/wiki/MessagEase |
Message-oriented middleware ( MOM ) is software or hardware infrastructure supporting sending and receiving messages between distributed systems. Message-oriented middleware is in contrast to streaming-oriented middleware where data is communicated as a sequence of bytes with no explicit message boundaries. Note that streaming protocols are almost always built above protocols using discrete messages such as frames ( Ethernet ), datagrams ( UDP ), packets ( IP ), cells ( ATM ), et al.
MOM allows application modules to be distributed over heterogeneous platforms and reduces the complexity of developing applications that span multiple operating systems and network protocols. The middleware creates a distributed communications layer that insulates the application developer from the details of the various operating systems and network interfaces. Application programming interfaces ( APIs ) that extend across diverse platforms and networks are typically provided by MOM. [ 1 ]
This middleware layer allows software components (applications, servlets, and other components) that have been developed independently and that run on different networked platforms to interact with one another. Applications distributed on different network nodes use the application interface to communicate. In addition, by providing an administrative interface, this new, virtual system of interconnected applications can be made fault tolerant and secure. [ 2 ]
MOM provides software elements that reside in all communicating components of a client/server architecture and typically support asynchronous calls between the client and server applications. MOM reduces the involvement of application developers with the complexity of the master-slave nature of the client/server mechanism.
All these models make it possible for one software component to affect the behavior of another component over a network. They are different in that RPC- and ORB-based middleware create systems of tightly coupled components, whereas MOM-based systems allow for a loose coupling of components. In an RPC- or ORB-based system, when one procedure calls another, it must wait for the called procedure to return before it can do anything else. In these mostly synchronous messaging models, the middleware functions partly as a super-linker, locating the called procedure on a network and using network services to pass function or method parameters to the procedure and then to return results. [ 2 ] Note that Object request brokers also support fully asynchronous messaging via oneway invocations. [ 3 ]
Central reasons for using a message-based communications protocol include its ability to store (buffer), route, or transform messages while conveying them from senders to receivers.
Another advantage of messaging provider mediated messaging between clients is that by adding an administrative interface, you can monitor and tune performance. Client applications are thus effectively relieved of every problem except that of sending, receiving, and processing messages. It is up to the code that implements the MOM system and up to the administrator to resolve issues like interoperability, reliability, security, scalability, and performance.
Using a MOM system, a client makes an API call to send a message to a destination managed by the provider. The call invokes provider services to route and deliver the message. Once it has sent the message, the client can continue to do other work, confident that the provider retains the message until a receiving client retrieves it. The message-based model, coupled with the mediation of the provider, makes it possible to create a system of loosely coupled components.
MOM comprises a category of inter- application communication software that generally relies on asynchronous message-passing , as opposed to a request-response architecture. In asynchronous systems, message queues provide temporary storage when the destination program is busy or not connected. In addition, most asynchronous MOM systems provide persistent storage to back up the message queue. This means that the sender and receiver do not need to connect to the network at the same time ( asynchronous delivery ), and problems with intermittent connectivity are solved. It also means that should the receiver application fail for any reason, the senders can continue unaffected, as the messages they send will simply accumulate in the message queue for later processing when the receiver restarts.
Many message-oriented middleware implementations depend on a message queue system. Some implementations permit routing logic to be provided by the messaging layer itself, while others depend on client applications to provide routing information or allow for a mix of both paradigms. Some implementations make use of broadcast or multicast distribution paradigms.
In a message-based middleware system, the message received at the destination need not be identical to the message originally sent. A MOM system with built-in intelligence can transform messages and route to match the requirements of the sender or of the recipient. [ 4 ] In conjunction with the routing and broadcast/ multicast facilities, one application can send a message in its own native format, and two or more other applications may each receive a copy of the message in their own native format. Many modern MOM systems provide sophisticated message transformation (or mapping) tools which allow programmers to specify transformation rules applicable to a simple GUI drag-and-drop operation.
The primary disadvantage of many message-oriented middleware systems is that they require an extra component in the architecture , the message transfer agent ( message broker ). As with any system , adding another component can lead to reductions in performance and reliability, and can also make the system as a whole more difficult and expensive to maintain .
In addition, many inter-application communications have an intrinsically synchronous aspect, with the sender specifically wanting to wait for a reply to a message before continuing (see real-time computing and near-real-time for extreme cases). Because message-based communication inherently functions asynchronously, it may not fit well in such situations. That said, most MOM systems have facilities to group a request and a response as a single pseudo-synchronous transaction.
With a synchronous messaging system, the calling function does not return until the called function has finished its task. In a loosely coupled asynchronous system, the calling client can continue to load work upon the recipient until the resources needed to handle this work are depleted and the called component fails. Of course, these conditions can be minimized or avoided by monitoring performance and adjusting message flow, but this is work that is not needed with a synchronous messaging system. The important thing is to understand the advantages and liabilities of each kind of system. Each system is appropriate for different kinds of tasks. Sometimes, a combination of the two kinds of systems is required to obtain the desired behavior.
Historically, there was a lack of standards governing the use of message-oriented middleware that has caused problems. Most of the major vendors have their own implementations, each with its own application programming interface (API) and management tools.
One of the long-standing standards for message oriented middleware is X/Open group's XATMI specification (Distributed Transaction Processing: The XATMI Specification) which standardizes API for interprocess communications . Known implementations for this API is ATR Baltic's Enduro/X middleware and Oracle 's Tuxedo .
The Advanced Message Queuing Protocol (AMQP) is an approved OASIS [ 5 ] and ISO [ 6 ] standard that defines the protocol and formats used between participating application components, so implementations are interoperable. AMQP may be used with flexible routing schemes, including common messaging paradigms like point-to-point , fan-out , publish/subscribe , and request-response (these are intentionally omitted from v1.0 of the protocol standard itself, but rely on the particular implementation and/or underlying network protocol for routing). It also supports transaction management, queuing, distribution, security, management, clustering, federation and heterogeneous multi-platform support. Java applications that use AMQP are typically written in Java JMS. Other implementations provide APIs for C# , C++ , PHP , Python , Ruby , and other programming languages .
The High Level Architecture (HLA IEEE 1516) is an Institute of Electrical and Electronics Engineers (IEEE) and Simulation Interoperability Standards Organization (SISO) standard for simulation interoperability. It defines a set of services, provided through an API in C++ or Java. The services offer publish/subscribe based information exchange, based on a modular Federation Object Model. There are also services for coordinated data exchange and time advance, based on logical simulation time, as well as synchronization points. Additional services provide transfer of ownership, data distribution optimizations and monitoring and management of participating Federates (systems).
The MQ Telemetry Transport (MQTT) is an ISO standard (ISO/IEC PRF 20922) supported by the OASIS organization. It provides a lightweight publish/subscribe reliable messaging transport protocol on top of TCP/IP suitable for communication in M2M/IoT contexts where a small code footprint is required and/or network bandwidth is at a premium.
The Object Management Group 's Data Distribution Service (DDS) provides message-oriented Publish/Subscribe (P/S) middleware standard that aims to enable scalable, real-time, dependable, high performance and interoperable data exchanges between publishers and subscribers. [ 7 ] The standard provides interfaces to C++, C++11, C, Ada , Java, and Ruby.
The eXtensible Messaging and Presence Protocol ( XMPP ) is a communications protocol for message-oriented middleware based on Extensible Markup Language ( XML ). Designed to be extensible, the protocol has also been used for publish-subscribe systems, signalling for VoIP, video, file transfer, gaming, Internet of Things applications such as the smart grid, and social networking services. Unlike most instant messaging protocols, XMPP is defined in an open standard and uses an open systems approach of development and application, by which anyone may implement an XMPP service and interoperate with other organizations' implementations. Because XMPP is an open protocol, implementations can be developed using any software license; although many server, client, and library implementations are distributed as free and open-source software , many freeware and proprietary software implementations also exist. The Internet Engineering Task Force (IETF) formed an XMPP working group in 2002 to formalize the core protocols as an IETF instant messaging and presence technology. The XMPP Working group produced four specifications (RFC 3920, RFC 3921, RFC 3922, RFC 3923), which were approved as Proposed Standards in 2004. In 2011, RFC 3920 and RFC 3921 were superseded by RFC 6120 and RFC 6121 respectively, with RFC 6122 specifying the XMPP address format. In addition to these core protocols standardized at the IETF, the XMPP Standards Foundation (formerly Jabber Software Foundation) is active in developing open XMPP extensions. XMPP-based software is deployed widely across the Internet, according to the XMPP Standards Foundation, and forms the basis for the Department of Defense (DoD) Unified Capabilities Framework. [ 8 ]
The Java EE programming environment provides a standard API called Java Message Service (JMS), which is implemented by most MOM vendors and aims to hide the particular MOM API implementations; however, JMS does not define the format of the messages that are exchanged, so JMS systems are not interoperable.
A similar effort is with the actively evolving OpenMAMA project, which aims to provide a common API, especially to C clients. As of August 2012, it is mainly appropriate for distributing market-oriented data (e.g. stock quotes) over pub-sub middleware.
Message queues allow the exchange of information between distributed applications. A message queue can reside in memory or disk storage. Messages stay in the queue until the time they are processed by a service consumer. Through the message queue, the application can be implemented independently - they do not need to know each other's position, or continue to implement procedures to remove the need for waiting to receive this message. [ 9 ]
[ 14 ] | https://en.wikipedia.org/wiki/Message-oriented_middleware |
In telephony , a message-waiting indicator ( MWI ) is a Telcordia Technologies (formerly Bellcore ) term for an FSK -based telephone calling feature that illuminates an LED on selected telephones to notify a telephone user of waiting voicemail messages on most North American public telephone networks and PBXs .
As described in Telcordia Generic Requirements document GR-283-CORE , a Message_Waiting_Indicator (MWI) is a mechanism that informs the subscriber about the status of recorded messages. [ verification needed ] The subscriber may subscribe to a notification feature that makes use of the status of this MWI.
This feature is also frequently called (and abbreviated) as visual message waiting indicator (VMWI). A VMWI, as defined in Telcordia GR-1401-CORE , is a stored program controlled switching (SPCS) system feature that activates and deactivates a visual indicator on customer-premises equipment (CPE) to notify the customer that new messages are waiting. VMWI differs from existing features that use other message indicators, such as audible stuttered dial tone , in that it activates a visual indicator on the CPE. The visual indicator may be as simple as lighting or flashing a light-emitting diode (LED), or as advanced as displaying a special message on a liquid-crystal display (LCD).
This technology was invented by Jerome (Jerry) Schull and Wayne Howe at BellSouth's Advanced Technology R&D group in 1992 and was issued as US Patents #5,363,431 and #5,521,964 . It was introduced in 1995, with the introduction of CLASS-based calling features and ADSI . It was at one time only compatible with ADSI -compliant telephones but is now compatible with any customer premises equipment ( CPE ) that simply responds visually to visual FSK .
This service is often erroneously associated with the abilities of most Caller ID standalone set-top boxes. Caller ID boxes manufactured after 1998 feature an LED that blinks green to notify that new calls have been recorded and red to indicate that a subscriber has new voicemail messages waiting. Some units also display the text "MESSAGE WAITING" (similar to ADSI -compliant telephones). These units do not use visual FSK to activate their red LEDs, but instead, they briefly " pick-up " the line at certain intervals (normally, within two minutes of a new call) to check for a "stuttered" dial tone . The presence of a stutter dial tone activates a red LED; while absence deactivates it.
For mobile phones , the message-waiting indicator is sent via a Short Message Service (SMS) message — the same system used for texting . (SMS was actually invented for utilitarian uses like this, not for user conversation.) It not only indicates that a message is waiting, but also how many unheard messages there are on the voicemail server for that telephone number . In the event that a phone is deactivated, out of range, or otherwise removed from the network, there is often no way to clear this indicator until another message is recorded to the system, causing the MWI message to be sent again once the phone reconnects. The customer service call center for the mobile network operator may also be able to provide simple technical support to reset this by resending the MWI message manually. | https://en.wikipedia.org/wiki/Message-waiting_indicator |
MessageMe was a messaging app and platform for the iPhone and Android . It launched in March 2013 and grew to 5 million users within 3 months. [ 1 ] The app allowed users to send and receive videos, photos, stickers , [ 2 ] and voice messages in addition to text. [ 3 ]
MessageMe was acquired and shutdown by Yahoo in 2014 [ 4 ] for a price rumored between $30 million and $40 million. [ 5 ]
MessageMe was founded in 2012 in by Arjun Sethi , Alexander Chee, Justin Rosenthal and Vivek Tatineni and based out of San Francisco, California . It raised a $1.9 million seed round from First Round Capital , Google Ventures , SV Angel and Andreessen Horowitz , among others. [ 6 ]
It launched in March 2013 on iOS and Android and reached 1 million users within 10 days. [ 7 ] In May 2013, MessageMe announced that it raised an additional $10 million in funding from Greylock Partners and that Greylock partner John Lilly had joined the board of directors . [ 8 ]
MessageMe reached 5 million users within 75 days of launch. [ 7 ]
Shortly after MessageMe's March 2013 launch, Facebook revoked MessageMe's access to the Facebook Platform 's "Find Friends" functionality that allowed MessageMe users to connect to Facebook and find their Facebook friends on MessageMe. [ 9 ] [ 10 ] Facebook cited its policy that allowed it to revoke access to developers who "replicate[] a core Facebook product or service without [Facebook's] permission." [ 9 ] [ 11 ] Leaked internal discussions showed that Facebook revoked MessageMe's access over concern that MessageMe was becoming too popular and competitive with Facebook messaging. [ 12 ] [ 13 ]
In an internal March 2013 email thread, Justin Osofsky, former Facebook director of platform partnerships, wrote:
In the first week after launch, MessageMe actually didn't make any friends.get calls. However, MessageMe is now up to ~350K MAU and made 333K friends calls last week. We will restrict their access to friends.get shortly.
In terms of next steps, Monika is working with Mike Nowak to see if there are any other messenger apps which have hit the growth team's radar recently. If so, we'd like to restrict them at the same time to group this into one press cycle. [ 14 ]
In 2018, the question of Facebook's use of platform data in anticompetitive ways against companies like MessageMe, Vine [ 15 ] and Voxer resurfaced as part of the Facebook–Cambridge Analytica data scandal . [ 16 ] At the same time that Facebook cut off data to apps like MessageMe, it shared private user data with important Facebook partners like Amazon , Netflix and Spotify . [ 17 ] [ 18 ]
In December 2018, Facebook officially ended the platform policy it used to revoke MessageMe's platform access. [ 19 ]
MessageMe along with WhatsApp , Kik and others led the movement of mobile messaging in the United States away from text-based SMS and towards richer text messaging which included audio, video, doodles, stickers, location and more in addition to text. [ 20 ] Global startups that led this trend included WeChat in China, Line in Japan, and KakaoTalk in Korea. [ 21 ] [ 20 ]
Messaging apps like MessageMe, Path and Lango were the first to bring stickers to the United States mobile messaging market. [ 2 ] Line 's success with stickers in Asia inspired those apps to bring stickers to the United States. [ 22 ] [ 23 ] MessageMe offered free and paid sticker packs that could be purchased through the app. [ 24 ]
Yahoo acquired MessageMe for between $30 million and $40 million in October 2014. [ 4 ] Other potential acquirers were Snapchat and Truecaller . [ 25 ]
The acquisition happened as part of CEO Marissa Mayer 's strategy of reinvigorating legacy products by acquiring top startup talent via small acquisitions. [ 26 ] Yahoo bought startups like MessageMe, shut down their products and then put those teams to work on existing or new Yahoo products. [ 27 ]
In July 2015, Yahoo launched its first mobile messaging app, Livetext , which was built internally by the MessageMe team on top of MessageMe technology. [ 28 ] Yahoo shut down Livetext in March 2016. [ 29 ] | https://en.wikipedia.org/wiki/MessageMe |
Message-Oriented Text Interchange System ( MOTIS ) is an ISO messaging standard based on the ITU-T X.400 standards.
It plays a similar role to the Simple Mail Transfer Protocol (SMTP) in the TCP/IP protocol suite.
This software article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Message_Oriented_Text_Interchange_Systems |
The message loop is an obligatory section of code in every program that uses a graphical user interface under Microsoft Windows . [ 1 ] Windows programs that have a GUI are event-driven . Windows maintains an individual message queue for each thread that has created a window. Usually only the first thread creates windows. Windows places messages into that queue whenever mouse activity occurs on that thread's window, whenever keyboard activity occurs while that window has focus, and at other times. A process can also add messages to its own queue. To accept user input, and for other reasons, each thread with a window must continuously retrieve messages from its queue, and act on them. A programmer makes the process do that by writing a loop that calls GetMessage (which blocks for a message and retrieves it), and then calls DispatchMessage (which dispatches the message), and repeats indefinitely. This is the message loop. There usually is a message loop in the main program , which runs on the main thread , and additional message loop in each created modal dialog . Messages for every window of the process pass through its message queue, and are handled by its message loop. A message loop is one kind of event loop .
A basic message loop appears as follows:
It is conventional for the event loop to call TranslateMessage on each message which can translate virtual keystrokes into strings . Calling TranslateMessage is not technically required, but problems can result if it is not called. The message loop must call DispatchMessage .
The message loop does not directly act on the messages that it handles. It dispatches them by calling DispatchMessage , which transfers the message to the "window procedure" for the window that the message was addressed to. (The "window procedure" is a callback procedure, which got associated with the window class when it was registered.) (More than one window can use the same window procedure.)
Code can also send messages directly to a window procedure. These are called nonqueued messages.
A strict message loop is not the only option. Code elsewhere in the program can also accept and dispatch messages. PeekMessage is a non-blocking call that returns immediately, with a message if any are waiting, or no message if none is waiting. WaitMessage allows a thread to sleep until a message is in the queue.
Modern graphical interface frameworks , such as Windows Forms , Windows Presentation Foundation , MFC , Delphi , Qt , and others do not require applications to code a Windows message loop, because they automatically route events such as key presses and mouse clicks to their appropriate handlers as defined within the framework. However, each framework implements a message loop somewhere, and the message loop can usually be accessed or replaced when more direct control is required. | https://en.wikipedia.org/wiki/Message_loop_in_Microsoft_Windows |
Message passing is an inherent element of all computer clusters . All computer clusters, ranging from homemade Beowulfs to some of the fastest supercomputers in the world, rely on message passing to coordinate the activities of the many nodes they encompass. [ 1 ] [ 2 ] Message passing in computer clusters built with commodity servers and switches is used by virtually every internet service. [ 1 ]
Recently, the use of computer clusters with more than one thousand nodes has been spreading. As the number of nodes in a cluster increases, the rapid growth in the complexity of the communication subsystem makes message passing delays over the interconnect a serious performance issue in the execution of parallel programs . [ 3 ]
Specific tools may be used to simulate, visualize and understand the performance of message passing on computer clusters. Before a large computer cluster is assembled, a trace-based simulator can use a small number of nodes to help predict the performance of message passing on larger configurations. Following test runs on a small number of nodes, the simulator reads the execution and message transfer log files and simulates the performance of the messaging subsystem when many more messages are exchanged between a much larger number of nodes. [ 4 ] [ 5 ]
Historically, the two typical approaches to communication between cluster nodes have been PVM, the Parallel Virtual Machine and MPI, the Message Passing Interface . [ 6 ] However, MPI has now emerged as the de facto standard for message passing on computer clusters. [ 7 ]
PVM predates MPI and was developed at the Oak Ridge National Laboratory around 1989. It provides a set of software libraries that allow a computing node to act as a "parallel virtual machine". It provides run-time environment for message-passing, task and resource management, and fault notification and must be directly installed on every cluster node. PVM can be used by user programs written in C , C++, or Fortran , etc. [ 6 ] [ 8 ]
Unlike PVM, which has a concrete implementation, MPI is a specification rather than a specific set of libraries. The specification emerged in the early 1990 out of discussions between 40 organizations, the initial effort having been supported by ARPA and National Science Foundation . The design of MPI drew on various features available in commercial systems of the time. The MPI specifications then gave rise to specific implementations. MPI implementations typically use TCP/IP and socket connections. [ 6 ] MPI is now a widely available communications model that enables parallel programs to be written in languages such as C , Fortran, Python , etc. [ 8 ] The MPI specification has been implemented in systems such as MPICH and Open MPI . [ 8 ] [ 9 ]
Computer clusters use a number of strategies for dealing with the distribution of processing over multiple nodes and the resulting communication overhead. Some computer clusters such as Tianhe-I use different processors for message passing than those used for performing computations. Tiahnhe-I uses over two thousand FeiTeng-1000 processors to enhance the operation of its proprietary message passing system, while computations are performed by Xeon and Nvidia Tesla processors. [ 10 ] [ 11 ]
One approach to reducing communication overhead is the use of local neighborhoods (also called locales ) for specific tasks. Here computational tasks are assigned to specific "neighborhoods" in the cluster, to increase efficiency by using processors which are closer to each other. [ 3 ] However, given that in many cases the actual topology of the computer cluster nodes and their interconnections may not be known to application developers, attempting to fine tune performance at the application program level is quite difficult. [ 3 ]
Given that MPI has now emerged as the de facto standard on computer clusters, the increase in the number of cluster nodes has resulted in continued research to improve the efficiency and scalability of MPI libraries. These efforts have included research to reduce the memory footprint of MPI libraries. [ 7 ]
From the earliest days MPI provided facilities for performance profiling via the PMPI "profiling system". [ 12 ] The use of the PMIPI- prefix allows for the observation of the entry and exit points for messages. However, given the high level nature of this profile, this type of information only provides a glimpse at the real behavior of the communication system. The need for more information resulted in the development of the MPI-Peruse system. Peruse provides a more detailed profile by enabling applications to gain access to state-changes within the MPI-library. This is achieved by registering callbacks with Peruse, and then invoking them as triggers as message events take place. [ 13 ] Peruse can work with the PARAVER visualization system. PARAVER has two components, a trace component and a visual component for analyze the traces, the statistics related to specific events, etc. [ 14 ] PARAVER may use trace formats from other systems, or perform its own tracing. It operates at the task level, thread level, and in a hybrid format. Traces often include so much information that they are often overwhelming. Thus PARAVER summarizes them to allow users to visualize and analyze them. [ 13 ] [ 14 ] [ 15 ]
When a large scale, often supercomputer level, parallel system is being developed, it is essential to be able to experiment with multiple configurations and simulate performance. There are a number of approaches to modeling message passing efficiency in this scenario, ranging from analytical models to trace-based simulation and some approaches rely on the use of test environments based on "artificial communications" to perform synthetic tests of message passing performance. [ 3 ] Systems such as BIGSIM provide these facilities by allowing the simulation of performance on various node topologies , message passing and scheduling strategies. [ 4 ]
At the analytical level, it is necessary to model the communication time T in term of a set of subcomponents such as the startup latency , the asymptotic bandwidth and the number of processors. A well known model is Hockney's model which simply relies on point to point communication , using T = L + (M / R) where M is the message size, L is the startup latency and R is the asymptotic bandwidth in MB/s. [ 16 ]
Xu and Hwang generalized Hockney's model to include the number of processors, so that both the latency and the asymptotic bandwidth are functions of the number of processors. [ 16 ] [ 17 ] Gunawan and Cai then generalized this further by introducing cache size , and separated the messages based on their sizes, obtaining two separate models, one for messages below cache size, and one for those above. [ 16 ]
Specific tools may be used to simulate and understand the performance of message passing on computer clusters. For instance, CLUSTERSIM uses a Java-based visual environment for discrete-event simulation . In this approach computed nodes and network topology is visually modeled. Jobs and their duration and complexity are represented with specific probability distributions allowing various parallel job scheduling algorithms to be proposed and experimented with. The communication overhead for MPI message passing can thus be simulated and better understood in the context of large-scale parallel job execution. [ 18 ]
Other simulation tools include MPI-sim and BIGSIM. [ 19 ] MPI-Sim is an execution-driven simulator that requires C or C++ programs to operate. [ 18 ] [ 19 ] ClusterSim, on the other hand uses a hybrid higher-level modeling system independent of the programming language used for program execution. [ 18 ]
Unlike MPI-Sim, BIGSIM is a trace-driven system that simulates based on the logs of executions saved in files by a separate emulator program. [ 5 ] [ 19 ] BIGSIM includes an emulator, and a simulator. The emulator executes applications on a small number of nodes and stores the results, so the simulator can use them and simulate activities on a much larger number of nodes. [ 5 ] The emulator stores information of sequential execution blocks (SEBs) for multiple processors in log files, with each SEB recording the messages sent, their sources and destinations, dependencies, timings, etc. The simulator reads the log files and simulates them, and may star additional messages which are then also stored as SEBs. [ 4 ] [ 5 ] The simulator can thus provide a view of the performance of very large applications, based on the execution traces provided by the emulator on a much smaller number of nodes, before the entire machine is available, or configured. [ 5 ] | https://en.wikipedia.org/wiki/Message_passing_in_computer_clusters |
In computer science , message queues and mailboxes are software-engineering components typically used for inter-process communication (IPC), or for inter- thread communication within the same process. They use a queue for messaging – the passing of control or of content. Group communication systems provide similar kinds of functionality.
The message queue paradigm is a sibling of the publisher/subscriber pattern, and is typically one part of a larger message-oriented middleware system. Most messaging systems support both the publisher/subscriber and message queue models in their API , e.g. Java Message Service (JMS).
Competing Consumers pattern enables multiple concurrent consumers to process messages on the same message queue. [ 1 ]
Message queues implement an asynchronous communication pattern between two or more processes/threads whereby the sending and receiving party do not need to interact with the message queue at the same time. Messages placed onto the queue are stored until the recipient retrieves them. Message queues have implicit or explicit limits on the size of data that may be transmitted in a single message and the number of messages that may remain outstanding on the queue. [ 2 ]
Many implementations of message queues function internally within an operating system or within an application . Such queues exist for the purposes of that system only. [ 3 ] [ 4 ] [ 5 ]
Other implementations allow the passing of messages between different computer systems, potentially connecting multiple applications and multiple operating systems. [ 6 ] These message queuing systems typically provide resilience functionality to ensure that messages do not get "lost" in the event of a system failure. Examples of commercial implementations of this kind of message queuing software (also known as message-oriented middleware ) include IBM MQ (formerly MQ Series) and Oracle Advanced Queuing (AQ). There is a Java standard called Java Message Service , which has several proprietary and free software implementations.
Real-time operating systems (RTOSes) such as VxWorks and QNX encourage the use of message queuing as the primary inter-process or inter-thread communication mechanism. This can result in integration between message passing and CPU scheduling. Early examples of commercial RTOSes that encouraged a message-queue basis to inter-thread communication also include VRTX and pSOS +, both of which date to the early 1980s. The Erlang programming language uses processes to provide concurrency; these processes communicate asynchronously using message queuing.
The message queue software can be either proprietary, open source or a mix of both. It is then run either on premise in private servers or on external cloud servers ( message queuing service ).
Examples on hardware-based messaging middleware vendors are Solace , Apigee , and IBM MQ .
In a typical message-queueing implementation, a system administrator installs and configures message-queueing software (a queue manager or broker), and defines a named message queue. Or they register with a message queuing service .
An application then registers a software routine that "listens" for messages placed onto the queue.
Second and subsequent applications may connect to the queue and transfer a message onto it.
The queue-manager software stores the messages until a receiving application connects and then calls the registered software routine. The receiving application then processes the message in an appropriate manner.
There are often numerous options as to the exact semantics of message passing, including:
These are all considerations that can have substantial effects on transaction semantics, system reliability, and system efficiency.
Historically, message queuing has used proprietary, closed protocols, restricting the ability for different operating systems or programming languages to interact in a heterogeneous set of environments.
An early attempt to make message queuing more ubiquitous was Sun Microsystems ' JMS specification, which provided a Java -only abstraction of a client API . This allowed Java developers to switch between providers of message queuing in a fashion similar to that of developers using SQL databases. In practice, given the diversity of message queuing techniques and scenarios, this wasn't always as practical as it could be.
Three standards have emerged which are used in open source message queue implementations:
These protocols are at different stages of standardization and adoption. The first two operate at the same level as HTTP , MQTT at the level of TCP/IP .
Some proprietary implementations also use HTTP to provide message queuing by some implementations, such as Amazon 's SQS . This is because it is always possible to layer asynchronous behaviour (which is what is required for message queuing) over a synchronous protocol using request-response semantics. However, such implementations are constrained by the underlying protocol in this case and may not be able to offer the full fidelity or set of options required in message passing above.
Many of the more widely known communications protocols in use operate synchronously . The HTTP protocol – used in the World Wide Web and in web services – offers an obvious example where a user sends a request for a web page and then waits for a reply.
However, scenarios exist in which synchronous behaviour is not appropriate. For example, AJAX ( Asynchronous JavaScript and XML ) can be used to asynchronously send text, JSON or XML messages to update part of a web page with more relevant information. Google uses this approach for their Google Suggest, a search feature which sends the user's partially typed queries to Google's servers and returns a list of possible full queries the user might be interested in the process of typing. This list is asynchronously updated as the user types.
Other asynchronous examples exist in event notification systems and publish/subscribe systems.
In both of the above examples it would not make sense for the sender of the information to have to wait if, for example, one of the recipients had crashed.
Applications need not be exclusively synchronous or asynchronous. An interactive application may need to respond to certain parts of a request immediately (such as telling a customer that a sales request has been accepted, and handling the promise to draw on inventory), but may queue other parts (such as completing calculation of billing, forwarding data to the central accounting system, and calling on all sorts of other services) to be done some time later.
In all these sorts of situations, having a subsystem which performs message-queuing (or alternatively, a broadcast messaging system) can help improve the behavior of the overall system.
There are two common message queue implementations in UNIX . One is part of the SYS V API, the other one is part of POSIX .
UNIX SYS V implements message passing by keeping an array of linked lists as message queues. Each message queue is identified by its index in the array, and has a unique descriptor. A given index can have multiple possible descriptors. UNIX gives standard functions to access the message passing feature. [ 7 ]
The POSIX.1-2001 message queue API is the later of the two UNIX message queue APIs. It is distinct from the SYS V API, but provides similar function. The unix man page mq_overview(7) provides an overview of POSIX message queues.
Graphical user interfaces (GUIs) employ a message queue, also called an event queue or input queue , to pass graphical input actions , such as mouse clicks , keyboard events, or other user inputs, to the application program . [ 9 ] The windowing system places messages indicating user or other events, such as timer ticks or messages sent by other threads, into the message queue. The GUI application removes these events one at a time by calling a routine called getNextEvent() or similar in an event loop , and then calling the appropriate application routine to process that event. [ 10 ] | https://en.wikipedia.org/wiki/Message_queue |
In molecular biology , messenger ribonucleic acid ( mRNA ) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene , and is read by a ribosome in the process of synthesizing a protein .
mRNA is created during the process of transcription , where an enzyme ( RNA polymerase ) converts the gene into primary transcript mRNA (also known as pre-mRNA ). This pre-mRNA usually still contains introns , regions that will not go on to code for the final amino acid sequence . These are removed in the process of RNA splicing , leaving only exons , regions that will encode the protein. This exon sequence constitutes mature mRNA . Mature mRNA is then read by the ribosome, and the ribosome creates the protein utilizing amino acids carried by transfer RNA (tRNA). This process is known as translation . All of these processes form part of the central dogma of molecular biology , which describes the flow of genetic information in a biological system.
As in DNA , genetic information in mRNA is contained in the sequence of nucleotides , which are arranged into codons consisting of three ribonucleotides each. Each codon codes for a specific amino acid , except the stop codons , which terminate protein synthesis. The translation of codons into amino acids requires two other types of RNA: transfer RNA, which recognizes the codon and provides the corresponding amino acid, and ribosomal RNA (rRNA), the central component of the ribosome's protein-manufacturing machinery.
The concept of mRNA was developed by Sydney Brenner and Francis Crick in 1960 during a conversation with François Jacob . In 1961, mRNA was identified and described independently by one team consisting of Brenner, Jacob, and Matthew Meselson , and another team led by James Watson . While analyzing the data in preparation for publication, Jacob and Jacques Monod coined the name "messenger RNA".
The brief existence of an mRNA molecule begins with transcription, and ultimately ends in degradation. During its life, an mRNA molecule may also be processed, edited, and transported prior to translation. Eukaryotic mRNA molecules often require extensive processing and transport, while prokaryotic mRNA molecules do not. A molecule of eukaryotic mRNA and the proteins surrounding it are together called a messenger RNP . [ citation needed ]
Transcription is when RNA is copied from DNA in the cytoplasm. During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. This process differs slightly in eukaryotes and prokaryotes. One notable difference is that prokaryotic RNA polymerase associates with DNA-processing enzymes during transcription so that processing can proceed during transcription. Therefore, this causes the new mRNA strand to become double stranded by producing a complementary strand known as the tRNA strand, which when combined are unable to form structures from base-pairing. Moreover, the template for mRNA is the complementary strand of tRNA, which is identical in sequence to the anticodon sequence that the DNA binds to. The short-lived, unprocessed or partially processed product is termed precursor mRNA , or pre-mRNA ; once completely processed, it is termed mature mRNA . [ citation needed ]
mRNA uses uracil (U) instead of thymine (T) in DNA. uracil (U) is the complementary base to adenine (A) during transcription instead of thymine (T). Thus, when using a template strand of DNA to build RNA, thymine is replaced with uracil. This substitution allows the mRNA to carry the appropriate genetic information from DNA to the ribosome for translation. Regarding the natural history, uracil came first then thymine; evidence suggests that RNA came before DNA in evolution. [ 1 ] The RNA World hypothesis proposes that life began with RNA molecules, before the emergence of DNA genomes and coded proteins. In DNA, the evolutionary substitution of thymine for uracil may have increased DNA stability and improved the efficiency of DNA replication. [ 2 ] [ 3 ]
Processing of mRNA differs greatly among eukaryotes , bacteria , and archaea . Non-eukaryotic mRNA is, in essence, mature upon transcription and requires no processing, except in rare cases. [ 4 ] Eukaryotic pre-mRNA, however, requires several processing steps before its transport to the cytoplasm and its translation by the ribosome.
The extensive processing of eukaryotic pre-mRNA that leads to the mature mRNA is the RNA splicing , a mechanism by which introns or outrons (non-coding regions) are removed and exons (coding regions) are joined. [ 5 ] [ 6 ]
A 5' cap (also termed an RNA cap, an RNA 7-methylguanosine cap, or an RNA m 7 G cap) is a modified guanine nucleotide that has been added to the "front" or 5' end of a eukaryotic messenger RNA shortly after the start of transcription. The 5' cap consists of a terminal 7-methylguanosine residue that is linked through a 5'-5'-triphosphate bond to the first transcribed nucleotide. Its presence is critical for recognition by the ribosome and protection from RNases . [ citation needed ]
Cap addition is coupled to transcription, and occurs co-transcriptionally, such that each influences the other. Shortly after the start of transcription, the 5' end of the mRNA being synthesized is bound by a cap-synthesizing complex associated with RNA polymerase . This enzymatic complex catalyzes the chemical reactions that are required for mRNA capping. Synthesis proceeds as a multi-step biochemical reaction. [ citation needed ]
In some instances, an mRNA will be edited , changing the nucleotide composition of that mRNA. An example in humans is the apolipoprotein B mRNA, which is edited in some tissues, but not others. The editing creates an early stop codon, which, upon translation, produces a shorter protein. Another well-defined example is A-to-I (adenosine to inosine) editing, which is carried out by double-strand specific adenosine-to inosine editing (ADAR) enzymes. This can occur in both the open reading frame and untranslated regions, altering the structural properties of the mRNA. Although essential for development, the exact role of this editing is not fully understood [ 7 ]
Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA molecule. In eukaryotic organisms most messenger RNA (mRNA) molecules are polyadenylated at the 3' end, but recent studies have shown that short stretches of uridine (oligouridylation) are also common. [ 8 ] The poly(A) tail and the protein bound to it aid in protecting mRNA from degradation by exonucleases. Polyadenylation is also important for transcription termination, export of the mRNA from the nucleus, and translation. mRNA can also be polyadenylated in prokaryotic organisms, where poly(A) tails act to facilitate, rather than impede, exonucleolytic degradation. [ citation needed ]
Polyadenylation occurs during and/or immediately after transcription of DNA into RNA. After transcription has been terminated, the mRNA chain is cleaved through the action of an endonuclease complex associated with RNA polymerase. After the mRNA has been cleaved, around 250 adenosine residues are added to the free 3' end at the cleavage site. This reaction is catalyzed by polyadenylate polymerase . Just as in alternative splicing , there can be more than one polyadenylation variant of an mRNA.
Polyadenylation site mutations also occur. The primary RNA transcript of a gene is cleaved at the poly-A addition site, and 100–200 A's are added to the 3' end of the RNA. If this site is altered, an abnormally long and unstable mRNA construct will be formed.
Another difference between eukaryotes and prokaryotes is mRNA transport. Because eukaryotic transcription and translation is compartmentally separated, eukaryotic mRNAs must be exported from the nucleus to the cytoplasm —a process that may be regulated by different signaling pathways. [ 9 ] Mature mRNAs are recognized by their processed modifications and then exported through the nuclear pore by binding to the cap-binding proteins CBP20 and CBP80, [ 10 ] as well as the transcription/export complex (TREX). [ 11 ] [ 12 ] Multiple mRNA export pathways have been identified in eukaryotes. [ 13 ]
In spatially complex cells, some mRNAs are transported to particular subcellular destinations. In mature neurons , certain mRNA are transported from the soma to dendrites . One site of mRNA translation is at polyribosomes selectively localized beneath synapses. [ 14 ] The mRNA for Arc/Arg3.1 is induced by synaptic activity and localizes selectively near active synapses based on signals generated by NMDA receptors . [ 15 ] Other mRNAs also move into dendrites in response to external stimuli, such as β-actin mRNA. [ 16 ] For export from the nucleus, actin mRNA associates with ZBP1 [ 17 ] and later with 40S subunit . The complex is bound by a motor protein and is transported to the target location ( neurite extension ) along the cytoskeleton . Eventually ZBP1 is phosphorylated by Src in order for translation to be initiated. [ 18 ] In developing neurons, mRNAs are also transported into growing axons and especially growth cones. Many mRNAs are marked with so-called "zip codes", which target their transport to a specific location. [ 19 ] [ 20 ] mRNAs can also transfer between mammalian cells through structures called tunneling nanotubes . [ 21 ] [ 22 ]
Because prokaryotic mRNA does not need to be processed or transported, translation by the ribosome can begin immediately after the end of transcription. Therefore, it can be said that prokaryotic translation is coupled to transcription and occurs co-transcriptionally . [ 23 ]
Eukaryotic mRNA that has been processed and transported to the cytoplasm (i.e., mature mRNA) can then be translated by the ribosome. Translation may occur at ribosomes free-floating in the cytoplasm, or directed to the endoplasmic reticulum by the signal recognition particle . Therefore, unlike in prokaryotes, eukaryotic translation is not directly coupled to transcription. It is even possible in some contexts that reduced mRNA levels are accompanied by increased protein levels, as has been observed for mRNA/protein levels of EEF1A1 in breast cancer . [ 24 ] [ non-primary source needed ]
Coding regions are composed of codons , which are decoded and translated into proteins by the ribosome; in eukaryotes usually into one and in prokaryotes usually into several. Coding regions begin with the start codon and end with a stop codon . In general, the start codon is an AUG triplet and the stop codon is UAG ("amber"), UAA ("ochre"), or UGA ("opal"). The coding regions tend to be stabilised by internal base pairs; this impedes degradation. [ 25 ] [ 26 ] In addition to being protein-coding, portions of coding regions may serve as regulatory sequences in the pre-mRNA as exonic splicing enhancers or exonic splicing silencers .
Untranslated regions (UTRs) are sections of the mRNA before the start codon and after the stop codon that are not translated, termed the five prime untranslated region (5' UTR) and three prime untranslated region (3' UTR), respectively. These regions are transcribed with the coding region and thus are exonic as they are present in the mature mRNA. Several roles in gene expression have been attributed to the untranslated regions, including mRNA stability, mRNA localization, and translational efficiency . The ability of a UTR to perform these functions depends on the sequence of the UTR and can differ between mRNAs. Genetic variants in 3' UTR have also been implicated in disease susceptibility because of the change in RNA structure and protein translation. [ 27 ]
The stability of mRNAs may be controlled by the 5' UTR and/or 3' UTR due to varying affinity for RNA degrading enzymes called ribonucleases and for ancillary proteins that can promote or inhibit RNA degradation. (See also, C-rich stability element .)
Translational efficiency, including sometimes the complete inhibition of translation, can be controlled by UTRs. Proteins that bind to either the 3' or 5' UTR may affect translation by influencing the ribosome's ability to bind to the mRNA. MicroRNAs bound to the 3' UTR also may affect translational efficiency or mRNA stability.
Cytoplasmic localization of mRNA is thought to be a function of the 3' UTR. Proteins that are needed in a particular region of the cell can also be translated there; in such a case, the 3' UTR may contain sequences that allow the transcript to be localized to this region for translation.
Some of the elements contained in untranslated regions form a characteristic secondary structure when transcribed into RNA. These structural mRNA elements are involved in regulating the mRNA. Some, such as the SECIS element , are targets for proteins to bind. One class of mRNA element, the riboswitches , directly bind small molecules, changing their fold to modify levels of transcription or translation. In these cases, the mRNA regulates itself.
The 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to the 3' end of the pre-mRNA. This tail promotes export from the nucleus and translation, and protects the mRNA from degradation.
An mRNA molecule is said to be monocistronic when it contains the genetic information to translate only a single protein chain (polypeptide). This is the case for most of the eukaryotic mRNAs. [ 28 ] [ 29 ] On the other hand, polycistronic mRNA carries several open reading frames (ORFs), each of which is translated into a polypeptide. These polypeptides usually have a related function (they often are the subunits composing a final complex protein) and their coding sequence is grouped and regulated together in a regulatory region, containing a promoter and an operator . Most of the mRNA found in bacteria and archaea is polycistronic, [ 28 ] as is the human mitochondrial genome. [ 30 ] Dicistronic or bicistronic mRNA encodes only two proteins .
In eukaryotes mRNA molecules form circular structures due to an interaction between the eIF4E and poly(A)-binding protein , which both bind to eIF4G , forming an mRNA-protein-mRNA bridge. [ 31 ] Circularization is thought to promote cycling of ribosomes on the mRNA leading to time-efficient translation, and may also function to ensure only intact mRNA are translated (partially degraded mRNA characteristically have no m7G cap, or no poly-A tail). [ 32 ]
Other mechanisms for circularization exist, particularly in virus mRNA. Poliovirus mRNA uses a cloverleaf section towards its 5' end to bind PCBP2, which binds poly(A)-binding protein , forming the familiar mRNA-protein-mRNA circle. Barley yellow dwarf virus has binding between mRNA segments on its 5' end and 3' end (called kissing stem loops), circularizing the mRNA without any proteins involved.
RNA virus genomes (the + strands of which are translated as mRNA) are also commonly circularized. [ 33 ] During genome replication the circularization acts to enhance genome replication speeds, cycling viral RNA-dependent RNA polymerase much the same as the ribosome is hypothesized to cycle.
Different mRNAs within the same cell have distinct lifetimes (stabilities). In bacterial cells, individual mRNAs can survive from seconds to more than an hour. However, the lifetime averages between 1 and 3 minutes, making bacterial mRNA much less stable than eukaryotic mRNA. [ 34 ] In mammalian cells, mRNA lifetimes range from several minutes to days. [ 35 ] The greater the stability of an mRNA the more protein may be produced from that mRNA. The limited lifetime of mRNA enables a cell to alter protein synthesis rapidly in response to its changing needs. There are many mechanisms that lead to the destruction of an mRNA, some of which are described below.
In general, in prokaryotes the lifetime of mRNA is much shorter than in eukaryotes. Prokaryotes degrade messages by using a combination of ribonucleases, including endonucleases , 3' exonucleases , and 5' exonucleases. In some instances, small RNA molecules (sRNA) tens to hundreds of nucleotides long can stimulate the degradation of specific mRNAs by base-pairing with complementary sequences and facilitating ribonuclease cleavage by RNase III . It was recently shown that bacteria also have a sort of 5' cap consisting of a triphosphate on the 5' end . [ 36 ] Removal of two of the phosphates leaves a 5' monophosphate, causing the message to be destroyed by the exonuclease RNase J, which degrades 5' to 3'.
Inside eukaryotic cells, there is a balance between the processes of translation and mRNA decay. Messages that are being actively translated are bound by ribosomes , the eukaryotic initiation factors eIF-4E and eIF-4G , and poly(A)-binding protein . eIF-4E and eIF-4G block the decapping enzyme ( DCP2 ), and poly(A)-binding protein blocks the exosome complex , protecting the ends of the message. The balance between translation and decay is reflected in the size and abundance of cytoplasmic structures known as P-bodies . [ 37 ] The poly(A) tail of the mRNA is shortened by specialized exonucleases that are targeted to specific messenger RNAs by a combination of cis-regulatory sequences on the RNA and trans-acting RNA-binding proteins. Poly(A) tail removal is thought to disrupt the circular structure of the message and destabilize the cap binding complex . The message is then subject to degradation by either the exosome complex or the decapping complex . In this way, translationally inactive messages can be destroyed quickly, while active messages remain intact. The mechanism by which translation stops and the message is handed-off to decay complexes is not understood in detail. The majority of mRNA decay was believed to be cytoplasmic; however, recently, a novel mRNA decay pathway was described, which starts in the nucleus. [ 38 ]
The presence of AU-rich elements in some mammalian mRNAs tends to destabilize those transcripts through the action of cellular proteins that bind these sequences and stimulate poly(A) tail removal. Loss of the poly(A) tail is thought to promote mRNA degradation by facilitating attack by both the exosome complex [ 39 ] and the decapping complex . [ 40 ] Rapid mRNA degradation via AU-rich elements is a critical mechanism for preventing the overproduction of potent cytokines such as tumor necrosis factor (TNF) and granulocyte-macrophage colony stimulating factor (GM-CSF). [ 41 ] AU-rich elements also regulate the biosynthesis of proto-oncogenic transcription factors like c-Jun and c-Fos . [ 42 ]
Eukaryotic messages are subject to surveillance by nonsense-mediated decay (NMD), which checks for the presence of premature stop codons (nonsense codons) in the message. These can arise via incomplete splicing, V(D)J recombination in the adaptive immune system , mutations in DNA, transcription errors, leaky scanning by the ribosome causing a frame shift , and other causes. Detection of a premature stop codon triggers mRNA degradation by 5' decapping, 3' poly(A) tail removal, or endonucleolytic cleavage . [ 43 ]
In metazoans , small interfering RNAs (siRNAs) processed by Dicer are incorporated into a complex known as the RNA-induced silencing complex or RISC. This complex contains an endonuclease that cleaves perfectly complementary messages to which the siRNA binds. The resulting mRNA fragments are then destroyed by exonucleases . siRNA is commonly used in laboratories to block the function of genes in cell culture. It is thought to be part of the innate immune system as a defense against double-stranded RNA viruses. [ 44 ]
MicroRNAs (miRNAs) are small RNAs that typically are partially complementary to sequences in metazoan messenger RNAs. [ 45 ] [ 46 ] Binding of a miRNA to a message can repress translation of that message and accelerate poly(A) tail removal, thereby hastening mRNA degradation. The mechanism of action of miRNAs is the subject of active research. [ 47 ] [ 48 ]
There are other ways by which messages can be degraded, including non-stop decay and silencing by Piwi-interacting RNA (piRNA), among others.
The administration of a nucleoside-modified messenger RNA sequence can cause a cell to make a protein, which in turn could directly treat a disease or could function as a vaccine ; more indirectly the protein could drive an endogenous stem cell to differentiate in a desired way. [ 49 ] [ 50 ]
The primary challenges of RNA therapy center on delivering the RNA to the appropriate cells. [ 51 ] Challenges include the fact that naked RNA sequences naturally degrade after preparation; they may trigger the body's immune system to attack them as an invader; and they are impermeable to the cell membrane . [ 50 ] Once within the cell, they must then leave the cell's transport mechanism to take action within the cytoplasm , which houses the necessary ribosomes . [ 49 ]
Overcoming these challenges, mRNA as a therapeutic was first put forward in 1989 "after the development of a broadly applicable in vitro transfection technique." [ 52 ] In the 1990s, mRNA vaccines for personalized cancer have been developed, relying on non-nucleoside modified mRNA. mRNA based therapies continue to be investigated as a method of treatment or therapy for both cancer as well as auto-immune, metabolic, and respiratory inflammatory diseases. Gene editing therapies such as CRISPR may also benefit from using mRNA to induce cells to make the desired Cas protein. [ 53 ]
Since the 2010s, RNA vaccines and other RNA therapeutics have been considered to be "a new class of drugs". [ 54 ] The first mRNA-based vaccines received restricted authorization and were rolled out across the world during the COVID-19 pandemic by Pfizer–BioNTech COVID-19 vaccine and Moderna , for example. [ 55 ] The 2023 Nobel Prize in Physiology or Medicine was awarded to Katalin Karikó and Drew Weissman for the development of effective mRNA vaccines against COVID-19. [ 56 ] [ 57 ] [ 58 ]
Several molecular biology studies during the 1950s indicated that RNA played some kind of role in protein synthesis, but that role was not clearly understood. For instance, in one of the earliest reports, Jacques Monod and his team showed that RNA synthesis was necessary for protein synthesis, specifically during the production of the enzyme β-galactosidase in the bacterium E. coli . [ 59 ] Arthur Pardee also found similar RNA accumulation in 1954 . [ 60 ] In 1953, Alfred Hershey , June Dixon, and Martha Chase described a certain cytosine-containing DNA (indicating it was RNA) that disappeared quickly after its synthesis in E. coli . [ 61 ] In hindsight, this may have been one of the first observations of the existence of mRNA but it was not recognized at the time as such. [ 62 ]
The idea of mRNA was first conceived by Sydney Brenner and Francis Crick on 15 April 1960 at King's College, Cambridge , while François Jacob was telling them about a recent experiment conducted by Arthur Pardee , himself, and Monod (the so-called PaJaMo experiment, which did not prove mRNA existed but suggested the possibility of its existence). With Crick's encouragement, Brenner and Jacob immediately set out to test this new hypothesis, and they contacted Matthew Meselson at the California Institute of Technology for assistance. During the summer of 1960, Brenner, Jacob, and Meselson conducted an experiment in Meselson's laboratory at Caltech which was the first to prove the existence of mRNA. That fall, Jacob and Monod coined the name "messenger RNA" and developed the first theoretical framework to explain its function. [ 62 ]
In February 1961, James Watson revealed that his Harvard -based research group had been right behind them with a series of experiments whose results pointed in roughly the same direction. Brenner and the others agreed to Watson's request to delay publication of their research findings. As a result, the Brenner and Watson articles were published simultaneously in the same issue of Nature in May 1961, while that same month, Jacob and Monod published their theoretical framework for mRNA in the Journal of Molecular Biology . [ 62 ] | https://en.wikipedia.org/wiki/Messenger_RNA |
In mathematics , the Mestre bound is a bound on the analytic rank of an elliptic curve in terms of its conductor, introduced by Mestre ( 1986 ).
This number theory -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Mestre_bound |
MetPetDB [ 1 ] is a relational database and repository for global geochemical data on and images collected from metamorphic rocks from the Earth's crust. MetPetDB is designed and built by a global community of metamorphic petrologists in collaboration with computer scientists at Rensselaer Polytechnic Institute as part of the National Cyberinfrastructure Initiative and supported by the National Science Foundation . MetPetDB is unique in that it incorporates image data collected by a variety of techniques, e.g. photomicrographs , backscattered electron images (SEM) , and X-ray maps collected by wavelength dispersive spectroscopy or energy dispersive spectroscopy .
MetPetDB was built for the purpose of archiving published data and for storing new data for ready access to researchers and students in the petrologic community. This database facilitates the gathering of information for researchers beginning new projects and permits browsing and searching for data relating to anywhere on the globe. MetPetDB provides a platform for collaborative studies among researchers anywhere on the planet, serves as a portal for students beginning their studies of metamorphic geology, and acts as a repository of vast quantities of data being collected by researchers globally. [ 2 ]
The basic structure of MetPetDB is based on a geologic sample and derivative subsamples. Geochemical data are linked to subsamples and the minerals within them, while image data can relate to samples or subsamples. MetPetDB is designed to store the distinct spatial/textural context of mineral analysis that is a crucial to petrologic interpretation. [ 3 ] A web-based user interface allows a user to become members and download their search results. Approved members may become contributors and upload data to catalogue and share with the public. More information about the data model and the design of the database is available on the MetPetDB Support Wiki . [ 4 ]
The database houses a wide range of information available for samples from all over the globe to be grouped into two categories: (a) observations and measurements (e.g. mineral data, images, chemical analyses), for which robust data models already exist, [ 2 ] and (b) interpretative results (e.g. P-T conditions, crystallization ages, cooling rates, etc.), which are conclusions based on the observational data. Development of a robust data model for interpretative data is currently underway as of December 2010. The database system is beginning to incorporate a number of tools for data analysis and calculation that adds considerable power to the researcher.
MetPetDB differs from other Geochemistry relational databases (e.g. GEOROC , NAVDAT , PetDB ) in that it incorporates unpublished data in addition to data published in peer-reviewed journals. The vast majority of data collected by metamorphic geologists is not presented in publication, and therefore a forum for sharing this data with the public is an objective of MetPetDB. [ 5 ] Contributors to MetPetDB also have the ability to store private data and create projects, or collections of private, public, and published data for sharing and organization. [ 4 ] A comprehensive list of the publications and their published samples are located at MetPetDB Published Samples | https://en.wikipedia.org/wiki/MetPetDB |
meta -Chloroperoxybenzoic acid ( mCPBA or m CPBA ) is a peroxycarboxylic acid . It is a white solid often used widely as an oxidant in organic synthesis . mCPBA is often preferred to other peroxy acids because of its relative ease of handling. [ 1 ] mCPBA is a strong oxidizing agent that may cause fire upon contact with flammable material. [ 2 ]
mCPBA can be prepared by reacting m-chlorobenzoyl chloride with a basic solution of hydrogen peroxide , followed by acidification. [ 3 ]
It is sold commercially as a shelf-stable mixture that is less than 72% mCPBA, with the balance made up of m -chlorobenzoic acid (10%) and water. [ 1 ] The peroxyacid can be purified by washing the commercial material with a sodium hydroxide and potassium phosphate solution buffered at pH = 7.5. [ 2 ] [ 4 ] Peroxyacids are generally slightly less acidic than their carboxylic acid counterparts, so the acid impurity can be extracted if the pH is carefully controlled. The purified material is reasonably stable against decomposition if stored at low temperatures in a plastic container.
In reactions where the exact amount of mCPBA must be controlled, a sample can be titrated to determine the exact amount of active oxidant.
The main areas of use are the conversion of ketones to esters ( Baeyer-Villiger oxidation ), epoxidation of alkenes ( Prilezhaev reaction ), conversion of silyl enol ethers to silyl α-hydroxy ketones ( Rubottom oxidation ), oxidation of sulfides to sulfoxides and sulfones , and oxidation of amines to produce amine oxides . The following scheme shows the epoxidation of cyclohexene with mCPBA.
The epoxidation mechanism is concerted: the cis or trans geometry of the alkene starting material is retained in the epoxide ring of the product. The transition state of the Prilezhaev reaction is given below: [ 5 ]
The geometry of the transition state, with the peracid bisecting the C-C double bond, allows the two primary frontier orbital interactions to occur: π C=C (HOMO) to σ* O-O (LUMO) and n O (HOMO, regarded as a filled p orbital on a sp 2 hybridized oxygen) to π* C=C (LUMO), corresponding, in arrow-pushing terms, to formation of one C-O bond and cleavage of the O-O bond and formation of the other C-O bond and cleavage of the C=C π bond. | https://en.wikipedia.org/wiki/Meta-Chloroperoxybenzoic_acid |
Meta-communication is a secondary communication (including indirect cues) about how a piece of information is meant to be interpreted. It is based on the idea that the same message accompanied by different meta-communication can mean something entirely different, including its opposite, as in irony . [ 1 ] The term was brought to prominence by Gregory Bateson to refer to "communication about communication", which he expanded to: "all exchanged cues and propositions about (a) codification and (b) relationship between the communicators". [ 2 ]
Gregory Bateson invented the term in 1951. [ 2 ] Bateson suggested the significance of metacommunication in 1951, and then elaborated upon one particular variation, the message "this is play," in 1956. [ 3 ] A critical fact for Bateson was that every message could have a metacommunicative element, and typically, each message held metacommunicative information about how to interpret other messages. He saw no distinction in type of message, only a distinction in function. [ 3 ]
Some metacommunicative signals are nonverbal. The term kinesics , referring to body motion communication and occasionally employed by Bateson, was first used in 1952 by Ray Birdwhistell , an anthropologist who wished to study how people communicate through posture, gesture, stance, and movement. [ 4 ] Part of Birdwhistell's work involved filming people in social situations and analyzing them to show different levels of communication not clearly seen otherwise. Birdwhistell's research was influenced by Margaret Mead and Gregory Bateson ; all three were participants in the Macy Conferences in Group Processes, [ 5 ] and both Birdwhistell and Bateson were part of a later multidisciplinary collaboration, The Natural History of an Interview . [ 6 ]
From 1952–1962, Bateson directed a research project on communication. This paid particular attention to logical paradoxes including Russell's paradox 1901 and to Bertrand Russell 's Theory of Types, Russell's solution to it. Bateson and his associates here pioneered the concept of meta-communication - something that means different (often contradictory) things at different levels. Meta-communication is thought to be a characteristic feature of complex systems . [ 7 ]
In 1975, Frits Staal related the term to the metalanguage concept that is found in logic both in Western and Indian traditions. [ 8 ] [ 9 ] Staal considered the term metalanguage, or its German or Polish equivalent, to have been introduced in 1933 by the logician Alfred Tarski , whom he credits with having made apparent its real significance. [ 8 ]
Russell's 1902 solution to his logical paradox [ 10 ] comes in large part from the so-called vicious circle principle , that no propositional function can be defined prior to specifying the function's scope of application. In other words, before a function can be defined, one must first specify exactly those objects to which the function will apply (the function's domain). For example, before defining that the predicate "is a prime number", one first needs to define the collection of objects that might possibly satisfy the predicate, namely the set, N, of natural numbers. [ 11 ] It functions as a formal definition of the function of meta-communication in communication.
Ivan Pavlov had learned that the ringing of the bell signaled "food is on the way" in his experiment in which dogs were trained to salivate upon hearing a bell ring. This was accomplished by ringing a bell just prior to feeding the dogs. After repeating this procedure for some time it was found that the dogs would salivate after hearing the bell - without the need for food being presented.
Something that is not often discussed in context with this experiment is the fact that the dogs would not salivate unless they were wearing a special harness. When exposed to the bell ringing without wearing the harness, the dogs did not salivate. The dogs only salivated upon hearing the bell while wearing the harness . [ 12 ] : 267 The bell ringing was direct communication of information, but the context of the communication also conveyed information.
The concept of metacommunication has also been related to Communication Theory. Mateus (2017), influenced by Derrida's Graphematic Structure of Communication, suggested to see metacommunication as a self-differentiating redundancy. The concept here "describes communication as an ad infinitum process in which every communication supposes always more communication. Metacommunication is the answer to the relationship level of communication and that's why we postulate metacommunication as a re-communicating communication" (Mateus, 2017).
In a 2001 study, it was used to discuss self-referentiality in mass media covering politics and was explained as a consequence of the political public relations ' presence in media themselves. [ 13 ]
In Bateson's works, metamessage was defined (1972) as a refinement of his earlier notion of "mood sign[al]"s from his works of the 1950s. Invoking Bertrand Russell 's Theory of Logical Types , Bateson envisaged a potentially infinite hierarchy of messages, metamessages, meta-metamessages and so forth, each metamessage deterministically providing the full context for the interpretation of subordinate messages. [ 12 ] : 247–248, 289 [ 14 ] Being rather technical, his definition was misunderstood, [ 15 ] and metamessage appropriated with the same meaning as subtext , especially in the field of business communication . [ 16 ] Additionally, Bateson's strictly hierarchical theory was criticized for not reflecting some real-world communication phenomena, where any signal (regardless of level) can be deceitful. [ 14 ]
Metacommunication as a concept has been picked up across a wide array of disciplines. A few representative citations follow:
Language in Society , 13(1), 1-28. | https://en.wikipedia.org/wiki/Meta-communication |
In theoretical physics , the word meta-operator is sometimes used to refer to a specific operation over a combination of operators, as in the example of path-ordering . A meta-operator is generally neither an operator (a linear transform on the vector space ) nor a superoperator (a linear transform on the space of operators).
This article about theoretical physics is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Meta-operator |
Meta-process modeling is a type of metamodeling used in software engineering and systems engineering for the analysis and construction of models applicable and useful to some predefined problems.
Meta-process modeling supports the effort of creating flexible process models . The purpose of process models is to document and communicate processes and to enhance the reuse of processes. Thus, processes can be better taught and executed. Results of using meta-process models are an increased productivity of process engineers and an improved quality of the models they produce. [ 2 ]
Meta-process modeling focuses on and supports the process of constructing process models . Its main concern is to improve process models and to make them evolve, which in turn, will support the development of systems. [ 2 ] This is important due to the fact that " processes change with time and so do the process models underlying them. Thus, new processes and models may have to be built and existing ones improved". [ 2 ] "The focus has been to increase the level of formality of process models in order to make possible their enactment in process-centred software environments". [ 3 ] [ 4 ]
A process meta-model is a meta model , "a description at the type level of a process model. A process model is, thus, an instantiation of a process meta-model. [..] A meta-model can be instantiated several times in order to define various process models. A process meta-model is at the meta-type level with respect to a process." [ 2 ]
There exist standards for several domains:
There are different techniques for constructing process models. "Construction techniques used in the information systems area have developed independently of those in software engineering . In information systems, construction techniques exploit the notion of a meta-model and the two principal techniques used are those of instantiation and assembly . In software engineering the main construction technique used today is language-based. However, early techniques in both, information systems and software engineering were based on the experience of process engineers and were, therefore, ad hoc in nature." [ 2 ]
"Traditional process models are expressions of the experiences of their developers. Since this experience is not formalised and is, consequently, not available as a fund of knowledge, it can be said that these process models are the result of an ad hoc construction technique. This has two major consequences: it is not possible to know how these process models were generated, and they become dependent on the domain of experience. If process models are to be domain independent and if they are to be rapidly generable and modifiable, then we need to go away from experience based process model construction. Clearly, generation and modifiability relate to the process management policy adopted (see Usage World). Instantiation and assembly, by promoting modularization, facilitate the capitalisation of good practice and the improvement of given process models." [ 2 ]
The assembly technique is based on the idea of a process repository from which process components can be selected. Rolland (1998) lists two selection strategies: [ 2 ]
For the assembly technique to be successful, it is necessary that process models are modular. If the assembly technique is combined with the instantiation technique then the meta-model must itself be modular. [ 2 ]
For reusing processes a meta-process model identifies "the common, generic features of process models and represents them in a system of concepts. Such a representation has the potential to 'generate' all process models that share these features. This potential is realised when a generation technique is defined whose application results in the desired process model." [ 2 ]
Process models are then derived from the process meta-models through instantiation . Rolland associates a number of advantages with the instantiation approach: [ 2 ]
"The instantiation technique has been used, for example, in NATURE, [ 9 ] Rolland 1993, [ 1 ] Rolland 1994, [ 12 ] and Rolland 1996. [ 13 ] The process engineer must define the instances of contexts and relationships that comprise the process model of interest." [ 2 ]
Rolland (1998) lists numerous languages for expressing process models used by the software engineering community: [ 2 ]
as well as further computational paradigms:
Languages are typically related to process programs whereas instantiation techniques have been used to construct process scripts. [ 2 ]
The meta-modeling process is often supported through software tools, called CAME tools (Computer Aided Method Engineering) or MetaCASE tools (Meta-level Computer Assisted Software Engineering tools).
Often the instantiation technique "has been utilised to build the repository of Computer Aided Method Engineering environments". [ 2 ] [ 21 ] [ 22 ] [ 23 ] [ 24 ]
Example tools for meta-process modeling are: [ 25 ]
Colette Rolland (1999) [ 3 ] provides an example of a meta-process model which utilizes the instantiation and assembly technique. In the paper the approach is called "Multi-model view" and was applied on the CREWS-L'Ecritoire method. The CREWS-L'Ecritoire method represents a methodical approach for Requirements Engineering , "the part of the IS development that involves investigating problems and requirements of the users community and developing a specification of the future system, the so-called conceptual schema.". [ 1 ] [ 26 ] [ 27 ]
Besides the CREWS-L'Ecritoire approach, the multi-model view has served as a basis for representing: [ 3 ]
Furthermore, the CREWS-L'Ecritoire utilizes process models and meta-process models in order to achieve flexibility for the situation at hand. The approach is based on the notion of a labelled graph of intentions and strategies called a map as well as its associated guidelines . [ 3 ] Together, map (process model) and the guidelines form the method.
The main source of this explanation is the elaboration of Rolland. [ 3 ]
The map is "a navigational structure which supports the dynamic selection of the intention to be achieved next and the appropriate strategy to achieve it"; it is "a process model in which a nondeterministic ordering of intentions and strategies has been included. It is a labelled directed graph with intentions as nodes and strategies as edges between intentions. The directed nature of the graph shows which intentions can follow which one." [ 3 ]
The map of the CREWS-L'Ecritoire method looks as follow:
The map consists of goals / intentions (marked with ovals) which are connected by strategies (symbolized through arrows). An intention is a goal, an objective that the application engineer has in mind at a given point of time. A strategy is an approach, a manner to achieve an intention. The connection of two goals with a strategy is also called section . [ 3 ]
A map "allows the application engineer to determine a path from Start intention to Stop intention. The map contains a finite number of paths, each of them prescribing a way to develop the product, i.e. each of them is a process model. Therefore the map is a multi-model. It embodies several process models, providing a multi-model view for modeling a class of processes. None of the finite set of models included in the map is recommended 'a priori'. Instead the approach suggests a dynamic construction of the actual path by navigating in the map. In this sense the approach is sensitive to the specific situations as they arise in the process. The next intention and strategy to achieve it are selected dynamically by the application engineer among the several possible ones offered by the map. Furthermore, the approach is meant to allow the dynamic adjunction of a path in the map, i.e. adding a new strategy or a new section in the actual course of the process. In such a case guidelines that make available all choices open to handle a given situation are of great convenience. The map is associated to such guidelines". [ 3 ]
A guideline "helps in the operationalisation of the selected intention"; [ 3 ] it is "a set of indications on how to proceed to achieve an objective or perform an activity." [ 33 ] The description of the guidelines is based on the NATURE project's contextual approach [ 9 ] [ 34 ] [ 35 ] and its corresponding enactment mechanism. [ 24 ] Three types of guidelines can be distinguished:
In our case, the following guidelines – which correspond with the map displayed above – need to be defined:
The following graph displays the details for the Intention Achievement Guideline 8 (IAG-8).
In the multi-model view as presented in the paper of C. Rolland, the meta-process (the instance of the meta-process model) is "a process for the generation of a path from the map and its instantaneous enactment for the application at hand." [ 3 ] While the meta-process model can be represented in many different ways, a map was chosen again as a means to do so. It is not to be mixed up with the map for the process model as presented above.
Colette Rolland describes the meta-model as follows: [ 3 ] (Meta-intentions are in bold, meta-strategies in italic – in green in the map.)
"The Start meta-intention starts the construction of a process by selecting a section in the method map which has map intention Start as source. The Choose Section meta-intention results in the selection of a method map section. The Enact Section meta-intention causes the execution of the method map section resulting from Choose Section . Finally, the Stop meta-intention stops the construction of the application process. This happens when the Enact Section meta-intention leads to the enactment of the method map section having Stop as the target.
As already explained in the previous sections, there are two ways in which a section of a method map can be selected, namely by selecting an intention or by selecting a strategy. Therefore, the meta-intention Choose Section has two meta-strategies associated with it, select intention and select strategy respectively. Once a method map section has been selected by Choose Section , the IAG to support its enactment must be retrieved; this is represented in [the graph] by associating the meta-strategy automated support with the meta-intention, Enact Section ."
The sample process "Eliciting requirements of a Recycling Machine" is about a method for designing the requirements of recycling facilities. The recycling facilities are meant for customers of a supermarket. The adequate method is obtained through instantiation of the meta-process model on the process model.
The following table displays the stepwise trace of the process to elicit requirements for the recycling machine (from [ 3 ] ): | https://en.wikipedia.org/wiki/Meta-process_modeling |
Meta-scheduling or super scheduling is a computer software technique of optimizing computational workloads by combining an organization's multiple job schedulers into a single aggregated view, allowing batch jobs to be directed to the best location for execution. [ clarification needed ]
Meta-scheduling technique is a solution for scheduling a set of dependent or independent faults with different scenarios that are mapping and modeling in an event-tree. It can be used as a dynamic or static scheduling method.
Scenario-based and multi-mode approaches are essential techniques in embedded-systems, e.g., design space exploration for MPSoCs and reconfigurable systems.
Optimization techniques for the generation of schedule graphs supporting such a SBMeS approach have been developed and implemented.
Scenario-based meta-scheduling can promise better performance by reducing dynamic scheduling overhead and recovering from faults.
The following is a partial list of noteworthy [ according to whom? ] open source and commercial meta-schedulers currently available.
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Meta-scheduling |
Meta -selective C–H functionalization refers to the regioselective reaction of a substituted aromatic ring on the C–H bond meta to the substituent.
Substituted aromatic ring is an important type of substructure in pharmaceuticals and industrial compounds. Thus, synthetic methods towards substituted aromatic rings are always of great interest to chemists.
Traditionally, regioselectivity on the aromatic ring is achieved by the electronic effect of substituents. Taking the well-known Friedel–Craft electrophilic aromatic substitution as example, electron donating groups direct the electrophile to ortho-/para- position while electron withdrawing groups direct the electrophile to meta -position. However, with complicated systems, electronic difference between different C–H bonds can be subtle and electronic directing effect alone could become less synthetically useful.
The fast development of C–H activation in the past few decades provides synthetic chemists with the powerful tools to synthesize functionalized aromatic compounds with high selectivity. The widely used approach to achieve ortho -selectivity involves metal-chelating directing groups, which forms a relatively stable 6- or 7-membered cyclic pre- transition state to bring the metal catalyst to the proximity of the ortho- hydrogen. [ 1 ] [ 2 ] [ 3 ] [ 4 ] [ 5 ] However, applying the same strategy to meta - or para - C-H functionalization does not work because the corresponding cyclophane -like cyclic pre-transition state is highly strained. [ 6 ] Thus, while ortho -selectivity has been achieved by numerous catalytic systems, meta - and para -selectivity remains a challenge.
In recent years, new strategies that override the electronic and steric bias have been developed to address meta -C–H functionalization. However, before these discoveries, synthesis of meta -substituted aromatic compounds could be either limited or cumbersome. For example, before the development of the C–H activation involving one-pot synthetic route to meta -substituted phenol derivatives by Maleczka and co-workers, the traditional synthesis requires 10 steps from TNT. [ 7 ] [ 8 ] [ 9 ] Some early attempts utilize steric and electronic effects to achieve meta -selectivity. [ 10 ] [ 11 ] [ 12 ] However, they are either limited to certain structure of substrates or are not highly selective. In recent years, several highly selective meta -C-H functionalization strategies have been reported which can override the intrinsic electronic and steric properties of the substrates and can apply to a wide range of substrate derivatives. The development of the modern meta -C-H functionalization strategies “open doors for numerous possibilities” for synthesis and catalyst development. [ 13 ]
In 2009, Gaunt's group reported a copper catalyzed meta -selective C–H arylation reaction on anilide derivatives. [ 14 ] Despite the intrinsic ortho-/para- selectivity of the amido group, the arylation occurs exclusively on the meta position on a variety of anilide substrates. Remarkably, the regioselectivity is totally different from the earlier reported Pd catalyzed C-H functionalizations, where the amido group serve as a powerful ortho -directing group. [ 7 ] [ 15 ] [ 16 ] The method is robust under mild reaction conditions. It is compatible with a spectrum of substituted anilide as well as different bisaryliodonium salts. However, the meta -selectivity is lost when highly ortho/para -directing methoxy group substitutes one of the meta -hydrogen of the anilide, which marks the limitation of this method.
Despite the limitation, the paper was of high impact. It has been highlighted in a number of journals and news and was voted as one of the top 12 papers of 2009 by Chemical and Engineering News. [ 17 ] In a more recent report from the same group, α -arylcarbonyl compounds were found to be good substrates for the copper catalyzed meta -selective C-H arylation. [ 18 ] The power of the meta-selectivity overrides the electronic effect of different substituents, including the strong ortho/para -directing m -methoxy group.
Although the copper catalyzed meta -selective C–H arylation is quite successful, the mechanism behind the meta -selectivity is not completely understood. There are generally two proposed mechanisms both involving a Cu(I)/Cu(III) catalyst cycle.
In Gaunt's original paper, he proposed a mechanism involving an anti-oxy-cupration step as the key to the meta-selectivity. [ 14 ] First, the Cu(II) salt generates the active Cu(I) species through either disproportionation or reduction by nucleophile . [ 19 ] The active Cu(I) species undergoes oxidative addition with diphenyliodonium salt to generate a highly electrophilic Cu(III) species. [ 14 ] [ 20 ] While the Cu(III) species activates the aromatic ring, [ 14 ] the amide oxygen attacks the ortho position, breaking the aromaticity and allowing cupration at the meta position. [ 14 ] [ 21 ] The intermediate then rearomatizes with base and undergoes reductive elimination to afford the meta -arylated product and regenerate the active Cu(I) catalyst. [ 14 ]
Alternatively, Li and Wu, based on DFT calculations, proposed a mechanism involving a "Heck-like four-membered-ring transition state". [ 19 ] The amide oxygen first coordinates to the Cu(III) species generated from oxidative addition of Cu(I) triflate and diphenyliodonium triflate. Then, the phenyl group bonded to copper interacts with the aromatic ring at the meta -position, forming a four-membered-ring transition state. According to their calculations, the aromaticity is not completely lost during the transformation. In the last step, the Cu(III)-C bond breaks to regenerate the Cu(I) catalyst while the triflate ion abstract the meta-hydrogen to recover the aromaticity and gives the product. [ 19 ]
In 2012, Yu and co-workers reported a pioneering meta -selective C-H olefination using nitrile -containing templates to deliver the palladium to the meta -position via a macrocyclic cyclophane -like pre-transition state. [ 6 ] The nitrile group is tethered to the aromatic ring by a removable linker. It coordinates weakly to palladium in an "end-on" fashion, which refers to the linear structure of C–CN–Pd bonds. [ 22 ]
The linear coordination is proposed to help overcome the high strain in the cyclophane-like pre-transition state that brings palladium to the vicinity of the meta -hydrogen. [ 6 ] The template is designed as such that the flat arene linker keeps the substrate aromatic ring and the nitrile group coplanar. Bulky substituents on the arene linker "lock" the nitrile tether in the desired position. The delicate design results in high regioselectivity towards the meta -C-H bond. The templates can be removed easily to give toluene derivatives or hydrocinnamic acid derivatives in high yield.
In their subsequent works, Yu and co-workers report the application of the same strategy in meta -selective C-H cross-coupling , [ 23 ] meta -C-H acetoxylation and meta -C–H olefination [ 24 ] in a broad substrate scope. It is demonstrated that the "end-on" template not only work with the Pd(0)/Pd(II) catalytic cycle but is also compatible with the Pd(II)/Pd(IV) cycle. [ 24 ] In all three works, addition of mono- N -protected amino acid (MPAA) such as N -acetyl glycine improves the reaction yields and enhances the regioselectivity. [ 6 ] [ 23 ] [ 24 ] [ 25 ]
The mechanistic study of the palladium-catalyzed meta -selective C–H bond activation with a nitrile-containing template was done by Yu, Wu, Houk and their co-workers. [ 25 ] The DFT calculations suggest that the regioselectivity is achieved in the C–H activation step, which is the rate-determining step . It proceeds via a concerted metalation-deprotonation (CMD) pathway, which means that palladation and deprotonation of the C–H bond happen at the same time. Surprisingly, calculations reveal that the Pd–Ag heterodimeric transition state leads to meta -selectivity while the Pd monomeric transition state leads to ortho -selectivity.
The role of mono- N -protected amino acid is proposed as a dianionic ligand which participates in the CMD step assisting the deprotonation of the C–H bond in the rate- and regio-determining step. [ 25 ]
Originally discovered by Frost and co-workers, the meta selective sulfonation of 2-phenylpyridine using a sulfonyl chloride coupling partner, [ 26 ] utilising a ruthenium(II) catalyst. This reaction has been proposed to proceed via a similar method to that of the meta alkylation reported by Frost and Ackermann which involves a meta -selective C-H bond alkylation reaction with secondary and tertiary alkyl halides catalyzed by ruthenium(II) carboxylate catalysts. [ 27 ] [ 28 ] The directing group first coordinate to the ruthenium catalyst. A reversible metalation takes place to generate the cycloruthenated complex as the key intermediates. The cycloruthenation activates the aromatic ring to undergo S E Ar type alkylation at the position para to the C–Ru bond.
In Gaunt's and Yu's works, some derivatives of drug molecules and biologically active compounds were successfully functionalized in their meta -position. For instance, meta -arylated derivatives of anti-inflammatory drugs ( S )- ibruprofen and ( S )- naproxen were synthesized with copper catalyzed C–H arylation. [ 18 ] Meta -olefinated biologically important biphenyl , amino acid and Baclofen derivatives have been accessed by remote C–H activation assisted by the "end-on" template. [ 6 ] These demonstrate the potential applications of meta-selective C–H functionalization in medicinal chemistry . | https://en.wikipedia.org/wiki/Meta-selective_C–H_functionalization |
In photonics , a meta-waveguide is a physical structures that guides electromagnetic waves with engineered functional subwavelength structures. [ 1 ] Meta-waveguides are the result of combining the fields of metamaterials and metasurfaces into integrated optics . [ 2 ] [ 3 ] The design of the subwavelength architecture allows exotic waveguiding phenomena to be explored. [ 3 ] [ 4 ]
Meta-waveguides can be classified by waveguide platforms or by design methods. [ 2 ] If classified by underlying waveguide platform, engineered subwavelength structures can be classified in combination with dielectric waveguides , optical fibers , or plasmonic waveguides . If classified by design methods, meta-waveguides can be classified as either using design primarily by physical intuition, or by computer algorithm based inverse design methods. [ 1 ] [ 5 ]
Meta-waveguides can provide new degrees of design freedom to the available structural library for optical waveguides in integrated photonics. [ 1 ] [ 3 ] Advantages can include enhancing the performance of conventional waveguide based integrated optical devices and creating novel device functionalities. [ 1 ] [ 3 ] Applications of meta-waveguides include beam/polarization splitting, [ 3 ] integrated waveguide mode converters, [ 4 ] versatile waveguide couplers, [ 6 ] lab-on-fiber sensing, [ 7 ] nano-optic endoscope imaging, [ 8 ] on-chip wavefront shaping, [ 9 ] structured-light generations, [ 10 ] and optical neural networks . [ 11 ] [ 12 ] The meta-structures can also be further integrated with van der Waals materials to add more functionalities and reconfigurability. [ 13 ] [ 14 ] | https://en.wikipedia.org/wiki/Meta-waveguide |
The MetaCyc database is one of the largest metabolic pathways and enzymes databases currently available. The data in the database is manually curated from the scientific literature, and covers all domains of life. MetaCyc has extensive information about chemical compounds, reactions, metabolic pathways and enzymes. The data have been curated from more than 58,000 publications. [ 1 ] [ 2 ] [ 3 ]
MetaCyc has been designed for multiple types of uses. It is often used as an extensive online encyclopedia of metabolism. In addition,
MetaCyc is used as a reference data set for computationally predicting the metabolic network of organisms from their sequenced genomes; it has been used to perform pathway predictions for thousands of organisms, including those in the BioCyc Database Collection . MetaCyc is also used in metabolic engineering and metabolomics research.
MetaCyc includes mini reviews for pathways and enzymes that provide background information as well as relevant literature references. It also provides extensive data on individual enzymes, describing their subunit structure, cofactors, activators and inhibitors, substrate specificity, and, when available, kinetic constants. MetaCyc data on metabolites includes chemical structures, predicted Standard energy of formation, and links to external databases. Reactions in MetaCyc are presented in a visual display that includes the structures of all components. The reactions are balanced and include EC numbers , reaction direction, predicted atom mappings that describe the correspondence between atoms in the reactant compounds and the product compounds, and computed Gibbs free energy .
All objects in MetaCyc are clickable and provide easy access to related objects. For example, the page for L-lysine lists all of the reactions in which L-lysine participates, as well as the enzymes that catalyze them and pathways in which these reactions take place. | https://en.wikipedia.org/wiki/MetaCyc |
MetaNetX is a database maintained by the SIB Swiss Institute of Bioinformatics for the automated model construction , and the genome annotation for large-scale metabolic networks. MetaNetX provides a number of tools to access, analyse and manipulate metabolic networks. [ 1 ]
MetaNetX provides a bunch of pre-mapped metabolic models.
To ease model comparison, MetaNetX has developed a resource to unify metabolites and biochemical reactions in the context of metabolic models. This unified namespace is called MetaNetX/ MNXref . [ 2 ] [ 3 ] [ 4 ]
MNXref reconciles chemical compounds by structural similarity and biochemical reaction context. Then reconciles biochemical reactions on the basis of the chemical compound reconciliation in an iterative way. Each reconciled group of chemical compounds, biochemical reactions and cellular compartments is a bag of similar items. MNXref sets a referent for each group.
MetaNetX allows search in MNXref by chemical compounds , biochemical reactions and cellular compartments .
Currently, MetaNetX/MNXref reconciles those resources: | https://en.wikipedia.org/wiki/MetaNetX |
MetaType1 , also stylized as METATYPE1 , is a tool for creating Type 1 fonts using MetaPost , developed by the Polish JNS team ( Bogusław Jackowski , Janusz Marian Nowacki and Piotr Strzelczyk). [ 1 ]
Since Metafont cannot produce outline fonts (vector-based), a new tool was needed to help creating such fonts, primarily for use with TeX , although the OpenType versions of the fonts might be used in any other program. It is less powerful than Metafont since no pens can be used, only filled paths, but it still allows creation of parametric fonts. [ 2 ]
Most important fonts produced with MetaType1 are: Latin Modern , Latin Modern Math , TeX Gyre , Antykwa Toruńska , Antykwa Półtawskiego , Kurier and Iwona .
This software article is a stub . You can help Wikipedia by expanding it .
This digital typography article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/MetaType1 |
The MetaWatch is a brand name of discontinued smartwatches developed by Meta Watch, Ltd. Strata MetaWatch and Frame MetaWatch are digital smartwatches released in 2012, funded by raising money via the crowd funding platform Kickstarter . [ 10 ] MetaWatch was a company founded by former Fossil engineers. [ 11 ]
The first generation of MetaWatch watches was released as a development system in September 2011. [ 12 ] Two models were sold - analog with OLED displays (model WDS111, style AU1000) and fully digital (model WDS112, style AU2000). Both were provided with a clip for charging, flashing, and debugging. [ 13 ] For wireless connection, Bluetooth 2.1 is used via a TI CC2560 module. [ 14 ] As a serial RAM Microchip 23A640 serial is used.
Unofficial first and half generation was extended of Bluetooth 4.0 capability. Style numbers for these watches were AU2001 and AU2004. [ 15 ] [ 16 ]
The second generation of MetaWatch had Bluetooth 4.0, a KXTI9 accelerometer instead of KXTF9 (using less active power), and larger battery. The 4 pins on the back of the case were multiplexed and could be either used with Spy-Bi-Wire (serialized version of JTAG ) or serial out/ground. External serial RAM was larger. The clip provided with watches was for charging and reflash only. A JTAG clip for development could be bought separately. [ 17 ]
The company has since ceased operations.
Second generation watches were sold under names Strata (model code SW12-1) and Frame (model code SW12-2). Strata has Water Resistant Mark 5 and Frame has Water Resistant Mark 3. [ 18 ] The Strata, which was focused more on value, was bulkier but more durable with its double injection-moulded polyurethane body. [ 19 ] It was also cheaper as the Kare model retailed for $140. [ 20 ] The Frame, on the other hand, was the variant targeted towards the fashion segment. The device featured a slimmer stainless steel build, smaller display, leather bands and a price tag of $199. [ 21 ] The Strata model was initially offered as an iOS -only device due to challenges in developing a version for the Android platform. [ 20 ] An Android-compatible version was later released.
In 2014, Metawatch announced Meta M1, [ 22 ] a premium smartwatch developed in collaboration with Frank Nuovo . The designer was known for his iconic works at Vertu and Nokia , where he served as a Vice President and Chief of Design. The device was launched at the 2014 Consumer Electronics Show . [ 23 ] | https://en.wikipedia.org/wiki/MetaWatch |
In chemistry , meta is a prefix , used for systematic names in IUPAC nomenclature . It has several meanings. [ 1 ] | https://en.wikipedia.org/wiki/Meta_(chemistry) |
Metabolic age is calculated by comparing one's basal metabolic rate to the average of one's chronological age group . [ 1 ] [ 2 ]
All the components in the body require various levels of energy to be maintained. Body fat requires much less energy than lean muscle , as lean muscle is much more metabolically active and therefore requires more energy expenditure to remain in homeostasis . If comparing two individuals, with all variables being equal, the person with more lean muscle mass will have a higher basal metabolic rate, and therefore, a lower metabolic age in comparison to those with the identical chronological age. [ 3 ]
The research on which the concept of metabolic age is based began with Alfred Joseph Clark in 1927. Clark found that the pulse rate of different species of animal varied with body size to the power of −0.27. Other researchers went on to find that other biological rates varied to the same, or a similar, coefficient. S. Brody developed a physiological age scale in 1945. In 1961 SCS Taylor used Brody's scale as a basis for a metabolic age scale along with taking 0.27 as a standard for the calculation. Taylor thus defines metabolic age, θ {\displaystyle \theta } as,
where A {\displaystyle A} is the standard adult body weight in kilograms t {\displaystyle t} is the time from conception λ {\displaystyle \lambda } is a dimensional coefficient that depends on the units of time being used
Taylor chose the term metabolic age as a nod to the work of Max Kleiber who used the term metabolic time . [ 4 ] | https://en.wikipedia.org/wiki/Metabolic_age |
In biochemistry , metabolic control analysis ( MCA ) is a mathematical framework for describing metabolic , signaling , and genetic pathways . MCA quantifies how variables, such as fluxes and species concentrations, depend on network parameters.
In particular, it is able to describe how network-dependent properties,
called control coefficients , depend on local properties called elasticities or elasticity coefficients . [ 1 ] [ 2 ] [ 3 ]
MCA was originally developed to describe the control in metabolic pathways
but was subsequently extended to describe signaling and genetic networks . MCA has sometimes also been referred to as Metabolic Control Theory, but this terminology was rather strongly opposed by Henrik Kacser , one of the founders [ citation needed ] .
More recent work [ 4 ] has shown that MCA can be mapped directly on to classical control theory and are as such equivalent.
Biochemical systems theory [ 5 ] (BST) is a similar formalism , though with rather different objectives. Both are evolutions of an earlier theoretical analysis by Joseph Higgins. [ 6 ]
Chemical reaction network theory is another theoretical framework that has overlap with both MCA and BST but is considerably more mathematically formal in its approach. [ 7 ] Its emphasis is primarily on dynamic stability criteria [ 8 ] and related theorems associated with mass-action networks. In more recent years the field has also developed [ 9 ] a sensitivity analysis which is similar if not identical to MCA and BST.
A control coefficient [ 10 ] [ 11 ] [ 12 ] measures the relative steady state change in a system variable, e.g. pathway flux (J) or metabolite concentration (S), in response to a relative change in a parameter , e.g. enzyme activity or the steady-state rate ( v i {\displaystyle v_{i}} ) of step i {\displaystyle i} . The two main control coefficients are the flux and concentration control coefficients. Flux control coefficients are defined by
and concentration control coefficients by
.
The flux control summation theorem was discovered independently by the Kacser/Burns group [ 10 ] and the Heinrich/Rapoport group [ 11 ] in the early 1970s and late 1960s. The flux control summation theorem implies that metabolic fluxes are systemic properties and that their control is shared by all reactions in the system. When a single reaction changes its control of the flux this is compensated by changes in the control of the same flux by all other reactions.
The elasticity coefficient measures the local response of an enzyme or other chemical reaction to changes in its environment. Such changes include factors such as substrates, products, or effector concentrations. For further information, please refer to the dedicated page at elasticity coefficients .
.
The connectivity theorems [ 10 ] [ 11 ] are specific relationships between elasticities and control coefficients. They are useful because they highlight the close relationship between the kinetic properties of individual reactions and the system properties of a pathway. Two basic sets of theorems exists, one for flux and another for concentrations. The concentration connectivity theorems are divided again depending on whether the system species S n {\displaystyle S_{n}} is different from the local species S m {\displaystyle S_{m}} .
Kacser and Burns [ 10 ] introduced an additional coefficient that described how a biochemical pathway would respond the external environment. They termed this coefficient the response coefficient and designated it using the symbol R. The response coefficient is an important metric because it can be used to assess how much a nutrient or perhaps more important, how a drug can influence a pathway. This coefficient is therefore highly relevant to the pharmaceutical industry. [ 15 ]
The response coefficient is related to the core of metabolic control analysis via the response coefficient theorem, which is stated as follows:
R m X = C i X ε m i {\displaystyle R_{m}^{X}=C_{i}^{X}\varepsilon _{m}^{i}}
where X {\displaystyle X} is a chosen observable such as a flux or metabolite concentration, i {\displaystyle i} is the step that the external factor targets, C i X {\displaystyle C_{i}^{X}} is the control coefficient of the target steps, and ε m i {\displaystyle \varepsilon _{m}^{i}} is the elasticity of the target step with respect to the external factor m {\displaystyle m} .
The key observation of this theorem is that an external factor such as a therapeutic drug, acts on the organism's phenotype via two influences: 1) How well the drug can affect the target itself through effective binding of the drug to the target protein and its effect on the protein activity. This effectiveness is described by the elasticity ε m i {\displaystyle \varepsilon _{m}^{i}} and 2) How well do modifications of the target influence the phenotype by transmission of the perturbation to the rest of the network. This is indicated by the control coefficient C i X {\displaystyle C_{i}^{X}} .
A drug action, or any external factor, is most effective when both these factors are strong. For example, a drug might be very effective at changing the activity of its target protein, however if that perturbation in protein activity is unable to be transmitted to the final phenotype then the effectiveness of the drug is greatly diminished.
If a drug or external factor, m {\displaystyle m} , targets multiple sites of action, for example n {\displaystyle n} sites, then the overall response in a phenotypic factor X {\displaystyle X} , is the sum of the individual responses:
R m X = ∑ i = 1 n C i X ε m i {\displaystyle R_{m}^{X}=\sum _{i=1}^{n}C_{i}^{X}\varepsilon _{m}^{i}}
It is possible to combine the summation with the connectivity theorems to obtain closed expressions that relate the control coefficients to the elasticity coefficients. For example, consider the simplest non-trivial pathway:
We assume that X o {\displaystyle X_{o}} and X 1 {\displaystyle X_{1}} are fixed boundary species so that the pathway can reach a steady state. Let the first step have a rate v 1 {\displaystyle v_{1}} and the second step v 2 {\displaystyle v_{2}} . Focusing on the flux control coefficients, we can write one summation and one connectivity theorem for this simple pathway:
Using these two equations we can solve for the flux control coefficients to yield
Using these equations we can look at some simple extreme behaviors. For example, let us assume that the first step is completely insensitive to its product (i.e. not reacting with it), S, then ε s v 1 = 0 {\displaystyle \varepsilon _{s}^{v_{1}}=0} . In this case, the control coefficients reduce to
That is all the control (or sensitivity) is on the first step. This situation represents the classic rate-limiting step that is frequently mentioned in textbooks. The flux through the pathway is completely dependent on the first step. Under these conditions, no other step in the pathway can affect the flux. The effect is however dependent on the complete insensitivity of the first step to its product. Such a situation is likely to be rare in real pathways. In fact the classic rate limiting step has almost never been observed experimentally. Instead, a range of limitingness is observed, with some steps having more limitingness (control) than others.
We can also derive the concentration control coefficients for the simple two step pathway:
Consider the simple three step pathway:
where X o {\displaystyle X_{o}} and X 1 {\displaystyle X_{1}} are fixed boundary species, the control equations for this pathway can be derived in a similar manner to the simple two step pathway although it is somewhat more tedious.
where D the denominator is given by
Note that every term in the numerator appears in the denominator, this ensures that the flux control coefficient summation theorem is satisfied.
Likewise the concentration control coefficients can also be derived, for S 1 {\displaystyle S_{1}}
And for S 2 {\displaystyle S_{2}}
Note that the denominators remain the same as before and behave as a normalizing factor.
Control equations can also be derived by considering the effect of perturbations on the system. Consider that reaction rates v 1 {\displaystyle v_{1}} and v 2 {\displaystyle v_{2}} are determined by two enzymes e 1 {\displaystyle e_{1}} and e 2 {\displaystyle e_{2}} respectively. Changing either enzyme will result in a change to the steady state level of x {\displaystyle x} and the steady state reaction rates v {\displaystyle v} . Consider a small change in e 1 {\displaystyle e_{1}} of magnitude δ e 1 {\displaystyle \delta e_{1}} . This will have a number of effects, it will increase v 1 {\displaystyle v_{1}} which in turn will increase x {\displaystyle x} which in turn will increase v 2 {\displaystyle v_{2}} . Eventually the system will settle to a new steady state. We can describe these changes by focusing on the change in v 1 {\displaystyle v_{1}} and v 2 {\displaystyle v_{2}} . The change in v 2 {\displaystyle v_{2}} , which we designate δ v 2 {\displaystyle \delta v_{2}} , came about as a result of the change δ x {\displaystyle \delta x} . Because we are only considering small changes we can express the change δ v 2 {\displaystyle \delta v_{2}} in terms of δ x {\displaystyle \delta x} using the relation
where the derivative ∂ v 2 / ∂ x {\displaystyle \partial v_{2}/\partial x} measures how responsive v 2 {\displaystyle v_{2}} is to changes in x {\displaystyle x} . The derivative can be computed if we know the rate law for v 2 {\displaystyle v_{2}} . For example, if we assume that the rate law is v 2 = k 2 x {\displaystyle v_{2}=k_{2}x} then the derivative is k 2 {\displaystyle k_{2}} . We can also use a similar strategy to compute the change in v 1 {\displaystyle v_{1}} as a result of the change δ e 1 {\displaystyle \delta e_{1}} . This time the change in v 1 {\displaystyle v_{1}} is a result of two changes, the change in e 1 {\displaystyle e_{1}} itself and the change in x {\displaystyle x} . We can express these changes by summing the two individual contributions:
We have two equations, one describing the change in v 1 {\displaystyle v_{1}} and the other in v 2 {\displaystyle v_{2}} . Because we allowed the system to settle to a new steady state we can also state that the change in reaction rates must be the same (otherwise it wouldn't be at steady state). That is we can assert that δ v 1 = δ v 2 {\displaystyle \delta v_{1}=\delta v_{2}} . With this in mind we equate the two equations and write
Solving for the ratio δ x / δ e 1 {\displaystyle \delta x/\delta e_{1}} we obtain:
In the limit, as we make the change δ e 1 {\displaystyle \delta e_{1}} smaller and smaller, the left-hand side converges to the derivative d x / d e 1 {\displaystyle dx/de_{1}} :
We can go one step further and scale the derivatives to eliminate units. Multiplying both sides by e 1 {\displaystyle e_{1}} and dividing both sides by x {\displaystyle x} yields the scaled derivatives:
The scaled derivatives on the right-hand side are the elasticities, ε x v {\displaystyle \varepsilon _{x}^{v}} and the scaled left-hand term is the scaled sensitivity coefficient or concentration control coefficient, C e x {\displaystyle C_{e}^{x}}
We can simplify this expression further. The reaction rate v 1 {\displaystyle v_{1}} is usually a linear function of e 1 {\displaystyle e_{1}} . For example, in the Briggs–Haldane equation, the reaction rate is given by v = e 1 k c a t x / ( K m + x ) {\displaystyle v=e_{1}k_{cat}x/(K_{m}+x)} . Differentiating this rate law with respect to e 1 {\displaystyle e_{1}} and scaling yields ε e 1 v 1 = 1 {\displaystyle \varepsilon _{e_{1}}^{v_{1}}=1} .
Using this result gives:
A similar analysis can be done where e 2 {\displaystyle e_{2}} is perturbed. In this case we obtain the sensitivity of x {\displaystyle x} with respect to e 2 {\displaystyle e_{2}} :
The above expressions measure how much enzymes e 1 {\displaystyle e_{1}} and e 2 {\displaystyle e_{2}} control the steady state concentration of intermediate x {\displaystyle x} . We can also consider how the steady state reaction rates v 1 {\displaystyle v_{1}} and v 2 {\displaystyle v_{2}} are affected by perturbations in e 1 {\displaystyle e_{1}} and e 2 {\displaystyle e_{2}} . This is often of importance to metabolic engineers who are interested in increasing rates of production. At steady state the reaction rates are often called the fluxes and abbreviated to J 1 {\displaystyle J_{1}} and J 2 {\displaystyle J_{2}} . For a linear pathway such as this example, both fluxes are equal at steady-state so that the flux through the pathway is simply referred to as J {\displaystyle J} . Expressing the change in flux as a result of a perturbation in e 1 {\displaystyle e_{1}} and taking the limit as before we obtain
The above expressions tell us how much enzymes e 1 {\displaystyle e_{1}} and e 2 {\displaystyle e_{2}} control the steady state flux. The key point here is that changes in enzyme concentration, or equivalently the enzyme activity, must be brought about by an external action.
The control equations can also be derived in a more rigorous fashion using the systems equation :
d x d t = N v ( x ( p ) , p ) {\displaystyle {\dfrac {\bf {dx}}{dt}}={\bf {N}}{\bf {v}}({\bf {x}}(p),p)}
where N {\displaystyle {\bf {N}}} is the stoichiometry matrix , x {\displaystyle {\bf {x}}} is a vector of chemical species, and p {\displaystyle {\bf {p}}} is a vector of parameters (or inputs) that can influence the system. In metabolic control analysis the key parameters are the enzyme concentrations. This approach was popularized by Heinrich, Rapoport, and Rapoport [ 16 ] and Reder and Mazat. [ 17 ] A detailed discussion of this approach can be found in Heinrich & Schuster [ 18 ] and Hofmeyr. [ 19 ]
A linear biochemical pathway is a chain of enzyme-catalyzed reaction steps. The figure below shows a three step pathway, with intermediates, S 1 {\displaystyle S_{1}} and S 2 {\displaystyle S_{2}} . In order to sustain a steady-state, the boundary species X o {\displaystyle X_{o}} and X 1 {\displaystyle X_{1}} are fixed.
At steady-state the rate of reaction is the same at each step. This means there is an overall flux from X_o to X_1.
Linear pathways possess some well-known properties: [ 20 ] [ 21 ] [ 22 ]
In all cases, a rationale for these behaviors is given in terms of how elasticities transmit changes through a pathway.
There are a number of software tools that can directly compute elasticities and control coefficients:
Classical Control theory is a field of mathematics that deals with the control of dynamical systems in engineered processes and machines. In 2004 Brian Ingalls published a paper [ 26 ] that showed that classical control theory and metabolic control analysis were identical. The only difference was that metabolic control analysis was confined to zero frequency responses when cast in the frequency domain whereas classical control theory imposes no such restriction. The other significant difference is that classical control theory [ 27 ] has no notion of stoichiometry and conservation of mass which makes it more cumbersome to use but also means it fails to recognize the structural properties inherent in stoichiometric networks which provide useful biological insights. | https://en.wikipedia.org/wiki/Metabolic_control_analysis |
A metabolic disorder is a disorder that negatively alters the body's processing and distribution of macronutrients , such as proteins , fats , and carbohydrates . Metabolic disorders can happen when abnormal chemical reactions in the body alter the normal metabolic process . [ 3 ] It can also be defined as inherited single gene anomaly, most of which are autosomal recessive . [ 4 ]
Some of the symptoms that can occur with metabolic disorders are lethargy , weight loss , jaundice and seizures . The symptoms expressed would vary with the type of metabolic disorder. There are four categories of symptoms: acute symptoms, late-onset acute symptoms, progressive general symptoms and permanent symptoms. [ 5 ]
Inherited metabolic disorders are one cause of metabolic disorders, and occur when a defective gene causes an enzyme deficiency. [ 6 ] These diseases, of which there are many subtypes, are known as inborn errors of metabolism. [ 7 ] Metabolic diseases can also occur when the liver or pancreas do not function properly. [ 3 ]
The principal classes of metabolic disorders are: [ 1 ] [ better source needed ]
Metabolic disorders can be present at birth, and many can be identified by routine screening. If a metabolic disorder is not identified early, then it may be diagnosed later in life, when symptoms appear. Specific blood and DNA tests can be done to diagnose genetic metabolic disorders. [ 2 ]
The gut microbiota , which is a population of microbes that live in the human digestive system , also has an important part in metabolism and generally has a positive function for its host. In terms of pathophysiological/mechanism interactions, an abnormal gut microbiota can play a role in metabolic disorder related obesity . [ 8 ] A 2023 meta-analysis of six cohort studies involving 484,994 participants found no significant overall association between metabolic syndrome (MetS) and Alzheimer's disease (AD) risk. However, individual components of MetS, such as hypertension and hyperglycemia, were linked to an increased risk of AD, highlighting the complex interplay between metabolic dysfunction and neurodegeneration. [ 9 ]
Metabolic disorder screening can be done in newborns via blood , skin , or hearing tests . [ 10 ]
Metabolic disorders can be treatable by nutrition management, especially if detected early. It is important for dieticians to have knowledge of the genotype to create a treatment that will be more effective for the individual. [ 11 ] Recent systematic reviews and meta-analyses have highlighted the effectiveness of medical nutrition therapy (MNT) in managing metabolic disorders, especially when tailored to individual genotypes. For instance, a 2023 study demonstrated that MNT provided by dietitians significantly improved glycemic control, anthropometric measures, lipid profiles, and blood pressure in adults with prediabetes. These findings underscore the importance of personalized nutrition interventions in the effective management of metabolic disorders. [ 12 ] | https://en.wikipedia.org/wiki/Metabolic_disorder |
Metabolic ecology is a field of ecology aiming to understand constraints on metabolic organization as important for understanding almost all life processes. [ 1 ] [ 2 ] Main focus is on the metabolism of individuals , emerging intra- and inter-specific patterns, and the evolutionary perspective.
Two main metabolic theories that have been applied in ecology are Kooijman's Dynamic energy budget (DEB) theory and the West, Brown, and Enquist (WBE) metabolic scaling theory . [ 2 ] Both theories have an individual-based metabolic underpinning but have fundamentally different assumptions. [ 3 ] [ 4 ] [ 5 ] [ 6 ] Metabolic Scaling Theory is based more in first principles and makes several simplifying assumptions to better reveal the generalities of the role of metabolism in shaping organismal form and function and its impact on ecology and evolution. In many ways, DEB is a more parameterized species-level version of the WBE theory.
Models of individual's metabolism follow the energy uptake and allocation, and can focus on mechanisms and constraints of energy transport (transport models), or dynamic use of stored metabolites (energy budget models). [ 1 ] [ 7 ] | https://en.wikipedia.org/wiki/Metabolic_ecology |
Metabolic engineering is the practice of optimizing genetic and regulatory processes within cells to increase the cell's production of a certain substance. These processes are chemical networks that use a series of biochemical reactions and enzymes that allow cells to convert raw materials into molecules necessary for the cell's survival. Metabolic engineering specifically seeks to mathematically model these networks, calculate a yield of useful products, and pin point parts of the network that constrain the production of these products. [ 1 ] Genetic engineering techniques can then be used to modify the network in order to relieve these constraints. Once again this modified network can be modeled to calculate the new product yield.
The ultimate goal of metabolic engineering is to be able to use these organisms to produce valuable substances on an industrial scale in a cost-effective manner. Current examples include producing beer , wine , cheese , pharmaceuticals , and other biotechnology products. [ 2 ] Another possible area of use is the development of oil crops whose composition has been modified to improve their nutritional value. [ 3 ] Some of the common strategies used for metabolic engineering are (1) overexpressing the gene encoding the rate-limiting enzyme of the biosynthetic pathway, (2) blocking the competing metabolic pathways, (3) heterologous gene expression, and (4) enzyme engineering. [ 4 ]
Since cells use these metabolic networks for their survival, changes can have drastic effects on the cells' viability. Therefore, trade-offs in metabolic engineering arise between the cells ability to produce the desired substance and its natural survival needs. Therefore, instead of directly deleting and/or overexpressing the genes that encode for metabolic enzymes, the current focus is to target the regulatory networks in a cell to efficiently engineer the metabolism. [ 5 ]
In the past, to increase the productivity of a desired metabolite , a microorganism was genetically modified by chemically induced mutation , and the mutant strain that overexpressed the desired metabolite was then chosen. [ 6 ] However, one of the main problems with this technique was that the metabolic pathway for the production of that metabolite was not analyzed, and as a result, the constraints to production and relevant pathway enzymes to be modified were unknown. [ 6 ]
In 1990s, a new technique called metabolic engineering emerged. This technique analyzes the metabolic pathway of a microorganism , and determines the constraints and their effects on the production of desired compounds. It then uses genetic engineering to relieve these constraints. Some examples of successful metabolic engineering are the following: (i) Identification of constraints to lysine production in Corynebacterium glutamicum and insertion of new genes to relieve these constraints to improve production [ 7 ] (ii) Engineering of a new fatty acid biosynthesis pathway, called reversed beta oxidation pathway, that is more efficient than the native pathway in producing fatty acids and alcohols which can potentially be catalytically converted to chemicals and fuels [ 8 ] (iii) Improved production of DAHP an aromatic metabolite produced by E. coli that is an intermediate in the production of aromatic amino acids. [ 9 ] It was determined through metabolic flux analysis that the theoretical maximal yield of DAHP per glucose molecule utilized, was 3/7. This is because some of the carbon from glucose is lost as carbon dioxide, instead of being utilized to produce DAHP. Also, one of the metabolites (PEP, or phosphoenolpyruvate ) that are used to produce DAHP, was being converted to pyruvate (PYR) to transport glucose into the cell, and therefore, was no longer available to produce DAHP. In order to relieve the shortage of PEP and increase yield, Patnaik et al. used genetic engineering on E. coli to introduce a reaction that converts PYR back to PEP. Thus, the PEP used to transport glucose into the cell is regenerated, and can be used to make DAHP. This resulted in a new theoretical maximal yield of 6/7 – double that of the native E. coli system.
At the industrial scale, metabolic engineering is becoming more convenient and cost-effective. According to the Biotechnology Industry Organization , "more than 50 biorefinery facilities are being built across North America to apply metabolic engineering to produce biofuels and chemicals from renewable biomass which can help reduce greenhouse gas emissions". Potential biofuels include short-chain alcohols and alkanes (to replace gasoline ), fatty acid methyl esters and fatty alcohols (to replace diesel ), and fatty acid -and isoprenoid -based biofuels (to replace diesel ). [ 10 ]
Metabolic engineering continues to evolve in efficiency and processes aided by breakthroughs in the field of synthetic biology and progress in understanding metabolite damage and its repair or preemption . Early metabolic engineering experiments showed that accumulation of reactive intermediates can limit flux in engineered pathways and be deleterious to host cells if matching damage control systems are missing or inadequate. [ 11 ] [ 12 ] Researchers in synthetic biology optimize genetic pathways, which in turn influence cellular metabolic outputs. Recent decreases in cost of synthesized DNA and developments in genetic circuits help to influence the ability of metabolic engineering to produce desired outputs. [ 13 ]
An analysis of metabolic flux can be found at Flux balance analysis
The first step in the process is to identify a desired goal to achieve through the improvement or modification of an organism's metabolism. Reference books and online databases are used to research reactions and metabolic pathways that are able to produce this product or result. These databases contain copious genomic and chemical information including pathways for metabolism and other cellular processes. Using this research, an organism is chosen that will be used to create the desired product or result. Considerations that are taken into account when making this decision are how close the organism's metabolic pathway is to the desired pathway, the maintenance costs associated with the organism, and how easy it is to modify the pathway of the organism. Escherichia coli ( E. coli ) is widely used in metabolic engineering to synthesize a wide variety of products such as amino acids because it is relatively easy to maintain and modify. [ 14 ] If the organism does not contain the complete pathway for the desired product or result, then genes that produce the missing enzymes must be incorporated into the organism.
The completed metabolic pathway is modeled mathematically to find the theoretical yield of the product or the reaction fluxes in the cell. A flux is the rate at which a given reaction in the network occurs. Simple metabolic pathway analysis can be done by hand, but most require the use of software to perform the computations. [ 15 ] These programs use complex linear algebra algorithms to solve these models. To solve a network using the equation for determined systems shown below, one must input the necessary information about the relevant reactions and their fluxes. Information about the reaction (such as the reactants and stoichiometry) are contained in the matrices G x and G m . Matrices V m and V x contain the fluxes of the relevant reactions. When solved, the equation yields the values of all the unknown fluxes (contained in V x ).
After solving for the fluxes of reactions in the network, it is necessary to determine which reactions may be altered in order to maximize the yield of the desired product. To determine what specific genetic manipulations to perform, it is necessary to use computational algorithms, such as OptGene or OptFlux. [ 16 ] They provide recommendations for which genes should be overexpressed, knocked out, or introduced in a cell to allow increased production of the desired product. For example, if a given reaction has particularly low flux and is limiting the amount of product, the software may recommend that the enzyme catalyzing this reaction should be overexpressed in the cell to increase the reaction flux. The necessary genetic manipulations can be performed using standard molecular biology techniques. Genes may be overexpressed or knocked out from an organism, depending on their effect on the pathway and the ultimate goal. [ 17 ]
In order to create a solvable model, it is often necessary to have certain fluxes already known or experimentally measured. In addition, in order to verify the effect of genetic manipulations on the metabolic network (to ensure they align with the model), it is necessary to experimentally measure the fluxes in the network. To measure reaction fluxes, carbon flux measurements are made using carbon-13 isotopic labeling . [ 18 ] The organism is fed a mixture that contains molecules where specific carbons are engineered to be carbon-13 atoms, instead of carbon-12. After these molecules are used in the network, downstream metabolites also become labeled with carbon-13, as they incorporate those atoms in their structures. The specific labeling pattern of the various metabolites is determined by the reaction fluxes in the network. Labeling patterns may be measured using techniques such as gas chromatography-mass spectrometry (GC-MS) along with computational algorithms to determine reaction fluxes.
Biotechnology Industry Organization(BIO) website: | https://en.wikipedia.org/wiki/Metabolic_engineering |
Metabolic flexibility is the capacity to alter metabolism in response to exercise or available fuel (especially fats and carbohydrates ). Metabolic inflexibility was first described as the ability to generate energy through either aerobic or anaerobic respiration [ 1 ] or as the inability of muscle to increase glucose oxidation in response to insulin. [ 2 ]
An organism can also be said to have metabolic flexibility if it is capable of metabolizing either carbohydrate or fat efficiently, depending on availability of those fuels. [ 3 ] By this definition, metabolic flexibility can be quantified using respiratory quotient . [ 4 ] This form of metabolic flexibility is reduced by insulin resistance . [ 5 ]
With aging there is a decrease in metabolic flexibility due to a decline in pyruvate dehydrogenase activity which results in pyruvate increasingly being anaerobically converted to lactate rather than aerobically converted to acetyl-CoA . [ 6 ] Similarly, a virus-induced cytokine storm can compromise metabolic flexibility by inactivating the pyruvate dehydrogenase complex and other enzymes. [ 7 ] | https://en.wikipedia.org/wiki/Metabolic_flexibility |
Metabolic flux analysis (MFA) is an experimental fluxomics technique used to examine production and consumption rates of metabolites in a biological system. At an intracellular level, it allows for the quantification of metabolic fluxes , thereby elucidating the central metabolism of the cell. [ 1 ] Various methods of MFA, including isotopically stationary metabolic flux analysis, isotopically non-stationary metabolic flux analysis, and thermodynamics-based metabolic flux analysis, can be coupled with stoichiometric models of metabolism and mass spectrometry methods with isotopic mass resolution to elucidate the transfer of moieties containing isotopic tracers from one metabolite into another and derive information about the metabolic network. Metabolic flux analysis (MFA) has many applications such as determining the limits on the ability of a biological system to produce a biochemical such as ethanol , [ 2 ] predicting the response to gene knockout , [ 3 ] [ 4 ] and guiding the identification of bottleneck enzymes in metabolic networks for metabolic engineering efforts. [ 5 ]
Metabolic flux analysis may use 13 C -labeled isotope tracers for isotopic labeling experiments. Nuclear magnetic resonance ( NMR ) techniques and mass spectrometry may then be used to measure metabolite labeling patterns to provide information for determination of pathway fluxes. [ 6 ] [ 1 ] [ 7 ] Because MFA typically requires rigorous flux calculation of complex metabolic networks, publicly available software tools have been developed to automate MFA and reduce its computational burden.
Although using a stoichiometric balance and constraints of the metabolites comprising the metabolic network can elucidate fluxes, this approach has limitations including difficulty in stimulating fluxes through parallel, cyclic, and reversible pathways. [ 8 ] Moreover, there is limited insight on how metabolites interconvert in a metabolic network without the use of isotope tracers. [ 8 ] Thus, the use of isotopes has become the dominant technique for MFA. [ 9 ]
Isotope labeling experiments are optimal for gathering experimental data necessary for MFA. Because fluxes determine the isotopic labeling patterns of intracellular metabolites, measuring these patterns allows for inference of fluxes. [ 10 ] The first step in the workflow of isotope labeling experiments is cell culture on labeled substrates. A substrate such as glucose is labeled by isotope(s), most often 13 C, and is introduced into the culture medium. The medium also typically contains vitamins and essential amino acids to facilitate cells' growth. [ 11 ] The labeled substrate is then metabolized by the cells, leading to the incorporation of the 13 C tracer in other intracellular metabolites. After the cells reach steady-state physiology (i.e., constant metabolite concentrations in culture), cells are then lysed to extract metabolites. For mammalian cells, extraction involves quenching of cells using methanol to stop their cellular metabolism and subsequent extraction of metabolites using methanol and water extraction. [ 12 ] Concentrations of metabolites and labeled isotope in metabolites of the extracts are measured by instruments like liquid chromatography-mass spectrometry or NMR, which also provide information on the position and number of labeled atoms on the metabolites. [ 11 ] This data are necessary for gaining insight into the dynamics of intracellular metabolism and metabolite turnover rates to infer metabolic flux.
A predominant method for metabolic flux analysis is isotopically stationary MFA. This technique for flux quantitation is applicable under metabolic and isotopic steady-state, [ 13 ] two conditions that assume that metabolite concentrations and isotopomer distributions are not changing over time, respectively. Knowledge of the stoichiometric matrix (S) comprising the consumption and production of metabolites within biochemical reactions is needed to balance fluxes (v) around the assumed metabolic network model. [ 13 ] Assuming metabolic steady-state, metabolic fluxes can thus be quantitated by solving the inverse of the following simple linear algebra equation:
S × v = 0 {\displaystyle S\times v=0}
To reduce the possible solution space for flux distributions, isotopically stationary MFA requires additional stoichiometric constraints such as growth rates, substrate secretion and uptake, and product accumulation rates as well as upper and lower bounds for fluxes. [ 14 ] Although isotopically stationary MFA allows precise deduction of metabolic fluxes through mathematical modeling, the analysis is limited to batch cultures during the exponential phase. [ 15 ] Moreover, after addition of a labeled substrate, the time-point for when metabolic and isotopic steady-state may be accurately assumed can be difficult to determine. [ 13 ]
When isotope labeling is transient and has not yet equilibrated, isotopically non-stationary MFA (INST-MFA) is advantageous in deducing fluxes, particularly for systems with slow labeling dynamics. Similar to isotopically stationary MFA, this method requires mass and isotopomer balances to characterize the stoichiometry and atom transitions of the metabolic network. Unlike traditional MFA methods, however, INST-MFA requires applying ordinary differential equations to examine how isotopic labeling patterns of metabolites change over time; such examination can be accomplished by measuring changing isotopic labeling patterns over different time points to input into INST-MFA. [ 16 ] INST-MFA is thus a powerful method for elucidating fluxes of systems with pathway bottlenecks and revealing metabolic phenotypes of autotrophic organisms. [ 16 ] Although INST-MFA's computationally intensive demands previously hindered its widespread use, newly developed software tools have streamlined INST-MFA to decrease computational time and demand. [ 17 ]
Thermodynamics-Based Metabolic Flux Analysis (TMFA) [ 18 ] is a specialized type of metabolic flux analysis which utilizes linear thermodynamic constraints in addition to mass balance constraints to generate thermodynamically feasible fluxes and metabolite activity profiles. TMFA takes into consideration only pathways and fluxes that are feasible by using the Gibbs free energy change of the reactions and activities of the metabolites that are part of the model. By calculating Gibbs free energies of metabolic reactions and consequently their thermodynamic favorability, TMFA facilitates identification of limiting pathway bottleneck reactions that may be ideal candidates for pathway regulation.
Simulation algorithms are needed to model the biological system and calculate the fluxes of all pathways in a complex network. Several computational software exist to meet the need for efficient and precise tools for flux quantitation. Generally, the steps for applying modeling software towards MFA include metabolic reconstruction to compile all desired enzymatic reactions and metabolites, provide experimental information such as the labeling pattern of the substrate, define constraints such as growth equations, and minimizing the error between the experimental and simulated results to obtain final fluxes. [ 19 ] Examples of MFA software include 13CFLUX2 [ 20 ] and OpenFLUX, [ 21 ] which evaluate 13 C labeling experiments for flux calculation under metabolic and isotopically stationary conditions. The increasing interest in developing computation tools for INST-MFA calculation has also led to the development of software applications such as INCA, which was the first software capable of performing INST-MFA and simulating transient isotope labeling experiments. [ 22 ]
Metabolic flux analysis has been used to guide scale-up efforts for fermentation of biofuels. [ 23 ] By directly measuring enzymatic reaction rates, MFA can capture the dynamics of cells' behavior and metabolic phenotypes in bioreactors during large-scale fermentations. [ 23 ] For example, MFA models were used to optimize the conversion of xylose into ethanol in xylose-fermenting yeast by using calculated flux distributions to determine maximal theoretical capacities of the selected yeast towards ethanol production. [ 24 ]
Identification of bottleneck enzymes determines rate-limiting reactions that limit the productivity of a biosynthetic pathway. Moreover, MFA can help predict unexpected phenotypes of genetically engineered strains by constructing a fundamental understanding of how fluxes are wired in engineered cells. [ 25 ] For example, by calculating the Gibbs free energies of reactions in Escherichia coli metabolism, TMFA facilitated identification of a thermodynamic bottleneck reaction in a genome-scale model of Escherichia coli. [ 18 ] | https://en.wikipedia.org/wiki/Metabolic_flux_analysis |
Metabolic imprinting refers to the long-term physiological and metabolic effects that an offspring's prenatal and postnatal environments have on them. [ 1 ] Perinatal nutrition has been identified as a significant factor in determining an offspring's likelihood of it being predisposed to developing cardiovascular disease , obesity , and type 2 diabetes amongst other conditions. [ 2 ]
During pregnancy , maternal glucose can cross the blood-placental barrier [ 3 ] meaning maternal hyperglycaemia is associated with foetal hyperglycaemia. [ 4 ] Despite maternal glucose being able to cross the blood-placental barrier, maternal insulin is not able and the foetus has to make its own. [ 5 ] [ 6 ] As a result, if a mother is hyperglycaemic the foetus is likely to be hyperinsulinaemic which leads to it having increased levels of growth and adiposity. [ 4 ]
Maternal undernutrition has been linked with low birth weight and also a number of diseases, including Cardiovascular disease , stroke , hypertension and diabetes . [ 7 ] When a foetus is in the womb and is not receiving sufficient nutrition, it can adapt to prioritize organ growth and increased metabolic efficiency to prepare itself for life in an energy deficient environment. Postnatally, when given the correct nutrition, babies exhibit ‘catch up growth’, potentially leading to obesity and other related complications. Studies based around restricting animals food intake throughout gestation have discovered that a reduction of just 30% of normal intake can cause low birth weight and increase sensitivity to high-fat-diet induced obesity. [ 8 ]
In animal models, intrauterine undernutrition has been shown to be associated with hypertension later in life. This is because the formation of the kidneys is inhibited, which decreases filtration and flow rate through the nephrons, leading to increased blood pressure. [ 9 ]
More extreme prenatal conditions such as famine have been shown to have effects on the neurodevelopment of a foetus. [ 7 ] After the Dutch Famine of the winter of 1944–1945, it was found that the risk of schizophrenia was significantly higher in those conceived at the height of the famine, as was the prevalence of schizoid personality . [ 10 ]
Maternal overnutrition can have detrimental effects on the health of the offspring later in life. This area is less well studied and understood but some progress has been made in identifying specific genes that are affected. [ 7 ] Studies have investigated hypermethylation of DNA and found it to be higher in obese mothers to those of a healthy BMI . [ 11 ] More specific studies have investigated Leptin (LEP) as a possible gene which is altered via metabolic imprinting in response to overnutrition in utero , and found hypermethylation of LEP in the placenta of those born to overly nourished mothers. [ 12 ] This hypermethylation has been found to cause changes in the levels of circulating Leptin, as well as to leptin sensitivity and the development of neural circuits involved in the control of homeostasis which causes the higher risk of metabolic disease.
Upon investigation it was found that a mother who was obese before conception was likely to have a higher level of placental LEP than the placenta of a mother of a healthy weight. [ 12 ] One strategy for overcoming obesity is the use of gastric bypass and other such surgeries, while this does not entirely alleviate the risk of altered metabolic imprinting it has been found that siblings born post maternal surgery are less likely to have as high body fat percentages than over nutrition as siblings born before the surgery. [ 13 ]
Paternal overnutrition can also have a detrimental effect and new-borns have shown changes in methylation of DNA generally, with substantial hypomethylation at the gene Insulin-like Growth factor 2 (IGF2). However, this topic is much less studied than maternal nutrition. [ 13 ]
An increase in certain hormones such as oestrogen , progesterone , human placental lactogen , human placental growth hormone and cortisol during the second and third trimester of pregnancy cause an increase in insulin resistance . This increase in insulin resistance and following increase in insulin secretion ensures that the foetus develops a normal glucose tolerance. [ 14 ] Gestational Diabetes Mellitus (GDM) arises when beta cells do not secrete enough insulin to adopt to the insulin resistance triggered by pregnancy, which leads to mild hyperglycaemia. [ 15 ]
Although the mechanisms are still largely unknown, foetus exposure to GDM and maternal diabetes has been shown to lead to lifelong metabolic complications because of metabolic imprinting. The risk of Type II diabetes developing in offspring is significantly higher in offspring where the mother was diagnosed with Type II diabetes before pregnancy rather than after. In addition, the age at which offspring are diagnosed with Type 2 diabetes is significantly younger in offspring exposed to maternal diabetes/GDM than those who are not. It is suggested that this is a result of DNA methylation during foetal development. [ 14 ] | https://en.wikipedia.org/wiki/Metabolic_imprinting |
Metabolic intermediates are compounds produced during the conversion of substrates (starting molecules) into final products in biochemical reactions within cells . [ 1 ]
Although these intermediates are of relatively minor direct importance to cellular function, they can play important roles in the allosteric regulation of enzymes , glycolysis , the citric acid cycle , and amino acid synthesis .
Metabolic pathways consist of a series of enzymatically catalyzed reactions where each step transforms a substrate into a product that serves as the substrate for the next reaction . Metabolic intermediates are compounds that form during these steps, and they are neither the starting substrate nor the final product of the pathway. These intermediates are crucial because they allow for regulation, energy storage, and extraction of chemical energy in a controlled manner. [ 2 ]
Metabolic intermediates can belong to different biochemical classes based on the type of pathway they are involved in. Some examples include: [ 3 ]
Some can be useful in measuring rates of metabolic processes (for example, 3,4-dihydroxyphenylacetic acid or 3-aminoisobutyrate ).
Because they can represent unnatural points of entry into natural metabolic pathways, some (such as AICA ribonucleotide ) are of interest to researchers in developing new therapies.
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metabolic_intermediate |
A metabolic network is the complete set of metabolic and physical processes that determine the physiological and biochemical properties of a cell . As such, these networks comprise the chemical reactions of metabolism , the metabolic pathways , as well as the regulatory interactions that guide these reactions.
With the sequencing of complete genomes , it is now possible to reconstruct the network of biochemical reactions in many organisms, from bacteria to human. Several of these networks are available online:
Kyoto Encyclopedia of Genes and Genomes ( KEGG ), [ 1 ] EcoCyc , [ 2 ] BioCyc [ 3 ] and metaTIGER. [ 4 ] Metabolic networks are powerful tools for studying and modelling metabolism.
Metabolic networks can be used to detect comorbidity patterns in diseased patients. [ 5 ] Certain diseases, such as obesity and diabetes , can be present in the same individual concurrently, sometimes one disease being a significant risk factor for the other disease. [ 6 ] The disease phenotypes themselves are normally the consequence of the cell's inability to breakdown or produce an essential substrate. However, an enzyme defect at one reaction may affect the fluxes of other subsequent reactions. These cascading effects couple the metabolic diseases associated with subsequent reactions resulting in comorbidity effects. Thus, metabolic disease networks can be used to determine if two disorders are connected due to their correlated reactions. [ 5 ]
This molecular biology article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Metabolic_network |
Metabolic network modelling , also known as metabolic network reconstruction or metabolic pathway analysis , allows for an in-depth insight into the molecular mechanisms of a particular organism. In particular, these models correlate the genome with molecular physiology . [ 1 ] A reconstruction breaks down metabolic pathways (such as glycolysis and the citric acid cycle ) into their respective reactions and enzymes, and analyzes them within the perspective of the entire network. In simplified terms, a reconstruction collects all of the relevant metabolic information of an organism and compiles it in a mathematical model. Validation and analysis of reconstructions can allow identification of key features of metabolism such as growth yield, resource distribution, network robustness, and gene essentiality. This knowledge can then be applied to create novel biotechnology .
In general, the process to build a reconstruction is as follows:
The related method of flux balance analysis seeks to mathematically simulate metabolism in genome-scale reconstructions of metabolic networks.
A metabolic reconstruction provides a highly mathematical, structured platform on which to understand the systems biology of metabolic pathways within an organism. [ 2 ] The integration of biochemical metabolic pathways with rapidly available, annotated genome sequences has developed what are called genome-scale metabolic models. Simply put, these models correlate metabolic genes with metabolic pathways. In general, the more information about physiology, biochemistry and genetics is available for the target organism, the better the predictive capacity of the reconstructed models. Mechanically speaking, the process of reconstructing prokaryotic and eukaryotic metabolic networks is essentially the same. Having said this, eukaryote reconstructions are typically more challenging because of the size of genomes, coverage of knowledge, and the multitude of cellular compartments. [ 2 ] The first genome-scale metabolic model was generated in 1995 for Haemophilus influenzae . [ 3 ] The first multicellular organism, C. elegans , was reconstructed in 1998. [ 4 ] Since then, many reconstructions have been formed. For a list of reconstructions that have been converted into a model and experimentally validated, see http://sbrg.ucsd.edu/InSilicoOrganisms/OtherOrganisms .
Because the timescale for the development of reconstructions is so recent, most reconstructions have been built manually. However, now, there are quite a few resources that allow for the semi-automatic assembly of these reconstructions that are utilized due to the time and effort necessary for a reconstruction. An initial fast reconstruction can be developed automatically using resources like PathoLogic or ERGO in combination with encyclopedias like MetaCyc, and then manually updated by using resources like PathwayTools. These semi-automatic methods allow for a fast draft to be created while allowing the fine tune adjustments required once new experimental data is found. It is only in this manner that the field of metabolic reconstructions will keep up with the ever-increasing numbers of annotated genomes.
A reconstruction is built by compiling data from the resources above. Database tools such as KEGG and BioCyc can be used in conjunction with each other to find all the metabolic genes in the organism of interest. These genes will be compared to closely related organisms that have already developed reconstructions to find homologous genes and reactions. These homologous genes and reactions are carried over from the known reconstructions to form the draft reconstruction of the organism of interest. Tools such as ERGO, Pathway Tools and Model SEED can compile data into pathways to form a network of metabolic and non-metabolic pathways. These networks are then verified and refined before being made into a mathematical simulation. [ 2 ]
The predictive aspect of a metabolic reconstruction hinges on the ability to predict the biochemical reaction catalyzed by a protein using that protein's amino acid sequence as an input, and to infer the structure of a metabolic network based on the predicted set of reactions. A network of enzymes and metabolites is drafted to relate sequences and function. When an uncharacterized protein is found in the genome, its amino acid sequence is first compared to those of previously characterized proteins to search for homology. When a homologous protein is found, the proteins are considered to have a common ancestor and their functions are inferred as being similar. However, the quality of a reconstruction model is dependent on its ability to accurately infer phenotype directly from sequence, so this rough estimation of protein function will not be sufficient. A number of algorithms and bioinformatics resources have been developed for refinement of sequence homology-based assignments of protein functions:
Once proteins have been established, more information about the enzyme structure, reactions catalyzed, substrates and products, mechanisms, and more can be acquired from databases such as KEGG , MetaCyc and NC-IUBMB . Accurate metabolic reconstructions require additional information about the reversibility and preferred physiological direction of an enzyme-catalyzed reaction which can come from databases such as BRENDA or MetaCyc database. [ 24 ]
An initial metabolic reconstruction of a genome is typically far from perfect due to the high variability and diversity of microorganisms. Often, metabolic pathway databases such as KEGG and MetaCyc will have "holes", meaning that there is a conversion from a substrate to a product (i.e., an enzymatic activity) for which there is no known protein in the genome that encodes the enzyme that facilitates the catalysis. What can also happen in semi-automatically drafted reconstructions is that some pathways are falsely predicted and don't actually occur in the predicted manner. [ 24 ] Because of this, a systematic verification is made in order to make sure no inconsistencies are present and that all the entries listed are correct and accurate. [ 1 ] Furthermore, previous literature can be researched in order to support any information obtained from one of the many metabolic reaction and genome databases. This provides an added level of assurance for the reconstruction that the enzyme and the reaction it catalyzes do actually occur in the organism.
Enzyme promiscuity and spontaneous chemical reactions can damage metabolites. This metabolite damage, and its repair or pre-emption , create energy costs that need to be incorporated into models. It is likely that many genes of unknown function encode proteins that repair or pre-empt metabolite damage, but most genome-scale metabolic reconstructions only include a fraction of all genes. [ 25 ] [ 26 ]
Any new reaction not present in the databases needs to be added to the reconstruction. This is an iterative process that cycles between the experimental phase and the coding phase. As new information is found about the target organism, the model will be adjusted to predict the metabolic and phenotypical output of the cell. The presence or absence of certain reactions of the metabolism will affect the amount of reactants /products that are present for other reactions within the particular pathway. This is because products in one reaction go on to become the reactants for another reaction, i.e. products of one reaction can combine with other proteins or compounds to form new proteins/compounds in the presence of different enzymes or catalysts . [ 1 ]
Francke et al. [ 1 ] provide an excellent example as to why the verification step of the project needs to be performed in significant detail. During a metabolic network reconstruction of Lactobacillus plantarum , the model showed that succinyl-CoA was one of the reactants for a reaction that was a part of the biosynthesis of methionine . However, an understanding of the physiology of the organism would have revealed that due to an incomplete tricarboxylic acid pathway, Lactobacillus plantarum does not actually produce succinyl-CoA, and the correct reactant for that part of the reaction was acetyl-CoA .
Therefore, systematic verification of the initial reconstruction will bring to light several inconsistencies that can adversely affect the final interpretation of the reconstruction, which is to accurately comprehend the molecular mechanisms of the organism. Furthermore, the simulation step also ensures that all the reactions present in the reconstruction are properly balanced. To sum up, a reconstruction that is fully accurate can lead to greater insight about understanding the functioning of the organism of interest. [ 1 ]
A metabolic network can be broken down into a stoichiometric matrix where the rows represent the compounds of the reactions, while the columns of the matrix correspond to the reactions themselves. Stoichiometry is a quantitative relationship between substrates of a chemical reaction. In order to deduce what the metabolic network suggests, recent research has centered on a few approaches, such as extreme pathways, elementary mode analysis, [ 27 ] flux balance analysis , and a number of other constraint-based modeling methods. [ 28 ] [ 29 ]
Price, Reed, and Papin, [ 30 ] from the Palsson lab, use a method of singular value decomposition (SVD) of extreme pathways in order to understand regulation of a human red blood cell metabolism. Extreme pathways are convex basis vectors that consist of steady state functions of a metabolic network. [ 31 ] For any particular metabolic network, there is always a unique set of extreme pathways available. [ 27 ] Furthermore, Price, Reed, and Papin, [ 30 ] define a constraint-based approach , where through the help of constraints like mass balance and maximum reaction rates , it is possible to develop a ‘solution space’ where all the feasible options fall within. Then, using a kinetic model approach, a single solution that falls within the extreme pathway solution space can be determined. [ 30 ] Therefore, in their study, Price, Reed, and Papin, [ 30 ] use both constraint and kinetic approaches to understand the human red blood cell metabolism. In conclusion, using extreme pathways, the regulatory mechanisms of a metabolic network can be studied in further detail.
Elementary mode analysis closely matches the approach used by extreme pathways. Similar to extreme pathways, there is always a unique set of elementary modes available for a particular metabolic network. [ 27 ] These are the smallest sub-networks that allow a metabolic reconstruction network to function in steady state. [ 32 ] [ 33 ] [ 34 ] According to Stelling (2002), [ 33 ] elementary modes can be used to understand cellular objectives for the overall metabolic network. Furthermore, elementary mode analysis takes into account stoichiometrics and thermodynamics when evaluating whether a particular metabolic route or network is feasible and likely for a set of proteins/enzymes. [ 32 ]
In 2009, Larhlimi and Bockmayr presented a new approach called "minimal metabolic behaviors" for the analysis of metabolic networks. [ 35 ] Like elementary modes or extreme pathways, these are uniquely determined by the network, and yield a complete description of the flux cone. However, the new description is much more compact. In contrast with elementary modes and extreme pathways, which use an inner description based on generating vectors of the flux cone, MMBs are using an outer description of the flux cone. This approach is based on sets of non-negativity constraints. These can be identified with irreversible reactions, and thus have a direct biochemical interpretation. One can characterize a metabolic network by MMBs and the reversible metabolic space.
A different technique to simulate the metabolic network is to perform flux balance analysis . This method uses linear programming , but in contrast to elementary mode analysis and extreme pathways, only a single solution results in the end. Linear programming is usually used to obtain the maximum potential of the objective function that you are looking at, and therefore, when using flux balance analysis, a single solution is found to the optimization problem. [ 33 ] In a flux balance analysis approach, exchange fluxes are assigned to those metabolites that enter or leave the particular network only. Those metabolites that are consumed within the network are not assigned any exchange flux value. Also, the exchange fluxes along with the enzymes can have constraints ranging from a negative to positive value (ex: -10 to 10).
Furthermore, this particular approach can accurately define if the reaction stoichiometry is in line with predictions by providing fluxes for the balanced reactions. Also, flux balance analysis can highlight the most effective and efficient pathway through the network in order to achieve a particular objective function. In addition, gene knockout studies can be performed using flux balance analysis. The enzyme that correlates to the gene that needs to be removed is given a constraint value of 0. Then, the reaction that the particular enzyme catalyzes is completely removed from the analysis.
In order to perform a dynamic simulation with such a network it is necessary to construct an ordinary differential equation
system that describes the rates of change in each metabolite's concentration or amount. To this end, a rate law, i.e., a kinetic equation that determines the rate of reaction based on the concentrations of all reactants is required for each reaction. Software packages that include numerical integrators, such as COPASI or SBMLsimulator , are then able to simulate the system dynamics given an initial condition. Often these rate laws contain kinetic parameters with uncertain values. In many cases it is desired to estimate these parameter values with respect to given time-series data of metabolite concentrations. The system is then supposed to reproduce the given data. For this purpose the distance between the given data set and the result of the simulation, i.e., the numerically or in few cases analytically obtained solution of the differential equation system is computed. The values of the parameters are then estimated to minimize this distance. [ 36 ] One step further, it may be desired to estimate the mathematical structure of the differential equation system because the real rate laws are not known for the reactions within the system under study. To this end, the program SBMLsqueezer allows automatic creation of appropriate rate laws for all reactions with the network. [ 37 ]
Synthetic accessibility is a simple approach to network simulation whose goal is to predict which metabolic gene knockouts are lethal. The synthetic accessibility approach uses the topology of the metabolic network to calculate the sum of the minimum number of steps needed to traverse the metabolic network graph from the inputs, those metabolites available to the organism from the environment, to the outputs, metabolites needed by the organism to survive. To simulate a gene knockout, the reactions enabled by the gene are removed from the network and the synthetic accessibility metric is recalculated. An increase in the total number of steps is predicted to cause lethality. Wunderlich and Mirny showed this simple, parameter-free approach predicted knockout lethality in E. coli and S. cerevisiae as well as elementary mode analysis and flux balance analysis in a variety of media. [ 38 ]
Reconstructions and their corresponding models allow the formulation of hypotheses about the presence of certain enzymatic activities and the production of metabolites that can be experimentally tested, complementing the primarily discovery-based approach of traditional microbial biochemistry with hypothesis-driven research. [ 41 ] The results these experiments can uncover novel pathways and metabolic activities and decipher between discrepancies in previous experimental data. Information about the chemical reactions of metabolism and the genetic background of various metabolic properties (sequence to structure to function) can be utilized by genetic engineers to modify organisms to produce high value outputs whether those products be medically relevant like pharmaceuticals; high value chemical intermediates such as terpenoids and isoprenoids; or biotechnological outputs like biofuels, [ 42 ] or polyhydroxybutyrates also known as bioplastics. [ 43 ]
Metabolic network reconstructions and models are used to understand how an organism or parasite functions inside of the host cell. For example, if the parasite serves to compromise the immune system by lysing macrophages , then the goal of metabolic reconstruction/simulation would be to determine the metabolites that are essential to the organism's proliferation inside of macrophages. If the proliferation cycle is inhibited, then the parasite would not continue to evade the host's immune system. A reconstruction model serves as a first step to deciphering the complicated mechanisms surrounding disease. These models can also look at the minimal genes necessary for a cell to maintain virulence. The next step would be to use the predictions and postulates generated from a reconstruction model and apply it to discover novel biological functions such as drug-engineering and drug delivery techniques. | https://en.wikipedia.org/wiki/Metabolic_network_modelling |
Hematopoietic stem cells (HSCs) have high regenerative potentials and are capable of differentiating into all blood and immune system cells. Despite this impressive potential, HSCs have limited potential to produce more multipotent stem cells. [ 1 ] This limited self-renewal potential is protected through maintenance of a quiescent state in HSCs. Stem cells maintained in this quiescent state are known as long term HSCs (LT-HSCs). During quiescence, HSCs maintain a low level of metabolic activity and do not divide. [ 2 ] [ 3 ] [ 4 ] LT-HSCs can be signaled to proliferate, producing either myeloid or lymphoid progenitors. Production of these progenitors does not come without a cost: When grown under laboratory conditions that induce proliferation, HSCs lose their ability to divide and produce new progenitors. [ 5 ] Therefore, understanding the pathways that maintain proliferative or quiescent states in HSCs could reveal novel pathways to improve existing therapeutics involving HSCs. [ 6 ]
All adult stem cells can undergo two types of division: symmetric and asymmetric. When a cell undergoes symmetric division, it can either produce two differentiated cells or two new stem cells. When a cell undergoes asymmetric division, it produces one stem and one differentiated cell. Production of new stem cells is necessary to maintain this population within the body. [ 7 ] Like all cells, hematopoietic stem cells undergo metabolic shifts to meet their bioenergetic needs throughout development. [ 1 ] These metabolic shifts play an important role in signaling, generating biomass, and protecting the cell from damage. Metabolic shifts also guide development in HSCs and are one key factor in determining if an HSC will remain quiescent, symmetrically divide, or asymmetrically divide. [ 1 ] [ 8 ] [ 9 ] [ 10 ] As mentioned above, quiescent cells maintain a low level of oxidative phosphorylation and primarily rely on glycolysis to generate energy. Fatty acid beta-oxidation has been shown to influence fate decisions in HSCs. [ 11 ] In contrast, proliferative HSCs primarily depend on oxidative phosphorylation. This switch is accompanied by an increase in intracellular reactive oxygen species (ROS) levels and increased anabolic activity in cells [ 3 ] [ 12 ] [ 13 ] [ 14 ]
It is well understood that quiescent HSCs have very low levels of metabolic activity. LT-HSCs primarily rely on anaerobic glycolysis to generate energy. Unlike other types of HSCs, little energy is produced from mitochondrial oxidative respiration. The reason from this is likely two-fold: LT-HSCs reside within the hypoxic niche of the bone marrow, and low levels of mitochondrial respiration protect quiescent cells from damage induced ROS. [ 15 ] [ 16 ] When excessive levels of ROS are present, LT-HSCs undergo differentiation or apoptosis , losing their ability to self-renew. [ 17 ] This suggests that dependence on glycolysis is not only an environmental adaptation, but also a necessity for LT-HSCs to preserve their stemness.
LT-HSC preference for glycolysis is encoded by the transcription factor MEIS1 and, to a lesser extent, the protein CBP/p300-interacting transactivator 2 ( CITED2 ). [ 18 ] [ 19 ] [ 20 ] Both enzymes up regulate hypoxia-inducible factor 1α ( HIF1α ). Under hypoxic conditions, HIF1α dimerizes with HIF1ß to increase expression of several glycolytic enzymes to lead to an enhanced rate of glycolysis. [ 19 ] HIF1α also activates pyruvate dehydrogenase kinases (PDK) 2 and 4. [ 15 ] These enzymes inhibit pyruvate dehydrogenase (PDH). PDH converts pyruvate into acetyl-CoA , a crucial first step for metabolite entry into the TCA cycle and oxidative phosphorylation . Because this system inhibits mitochondrial metabolism and activates glycolysis, it is thought that the metabolic reprogramming by HIF1α is a main driver of LT-HSC quiescence.
Metabolic reprogramming by HIF1α does not always happen through action on PDKs. HIF1α can also promote expression of the cytosolic protein CRIPTO . CRIPTO then interacts with its cell surface receptor GRP78 to activate glycolytic enzymes. [ 21 ] [ 22 ] Extracellular cytokines and chemokines may also contribute to HIF1α activity, but further work is required to elucidate the exact contribution of these signaling molecules.
In addition to HIF1α, MEIS1 induces transcription of HIF2α . Though this enzyme is structurally similar to HIF1α, HIF2α has distinct functions. HIF2α is thought to protect HSCs from mitochondrial ROS production. An accumulation of ROS in HSCs causes stress at the endoplasmic reticulum, eventually inducing the unfolded protein response and apoptosis. [ 23 ] HIF2α protects the cell from ROS accumulation by up regulating several genes involved in ROS quenching, including catalase , glutathione peroxidase type I , and superoxide dismutases . [ 8 ] Activation of HIF2α is therefore necessary to maintain cellular health during quiescence.
Despite low levels of mitochondrial respiration, emerging evidence shows that LT-HSCs with the highest regenerative potential also have a high number of mitochondria. [ 25 ] Despite this, quiescent HSC mitochondria have a low membrane potential and low rates of oxidative phosphorylation. This again highlights the dependence of LT-HSCs on glycolysis to generate energy. Despite their inactivity, possessing many mitochondria may indicate that the quiescent HSCs are prepared for proliferation once an appropriate signal is received [ 24 ]
Recently, it has been discovered that fatty acid oxidation (FAO) is a major determinant in whether a stem cell will symmetrically or asymmetrically divide. [ 26 ] Transport of fatty acids into the mitochondria and their subsequent metabolism must be efficient in order for cells to maintain the ability to self-renew. In HSCs, transcriptional activation of nuclear genes involved in fatty acid transport and β-oxidation through a promyelocytic leukemia protein (PML)/peroxisome proliferation-activated receptor-gamma coactivator 1α ( PGC-1α )/peroxisome proliferator-activating receptor type δ ( PPARδ ) mediates efficiency of these processes. This pathway is also essential for HSC self-renewal because it promotes maintenance of the stem cell population. [ 26 ] FAO promotes asymmetric HSC division to produce one progenitor and one stem cell. Inhibition of FAO has been shown to expand the population of progenitor cells, thus decreasing the stem cell population. [ 27 ] Despite correlations between FAO and asymmetrical HSC divisions, the exact mechanism by which FAO governs stem cell fate decisions is still unclear.
Though maintenance of quiescence is important to HSCs to preserve their self-renewal capacity, proliferation is necessary to regenerate blood cells and immune cells for the body. During divisions, HSCs leave the hypoxic niche and begin circulating. Under these normoxic conditions, HIF1α is hydroxylated by prolyl hydroxylases PHD1, 2 and 3. [ 29 ] [ 30 ] [ 31 ] This hydroxylation triggers the cell to degrade HIF1α through the von Hippel-Lindau ( VHL ) ubiquitin ligases. Degradation of HIF1α prevents dimerization with HIF1ß, impeding the transcription of glycolytic genes. Degrading HIF1α also prevents activation of PDK2 and 4, thus resuming function of PDH in the mitochondria. Because the cell is now able to catalyze the production of acetyl-CoA, mitochondrial metabolism is able to resume. Restoration of this mitochondrial metabolism is coordinated by reentry into the cell cycle. Concurrent with reinitiation of mitochondrial metabolism is an upregulation in transcription of cell cycle genes and genes involved in anabolic activities. [ 32 ] [ 33 ] As expected, HSCs with a high mitochondrial membrane potential have higher rates of expression for genes related to the cell cycle and metabolism. [ 33 ] The accompanying increase in ROS levels in these proliferating HSCs may in part drive differentiation of HSCs, but more work is needed to fully elucidate the role of ROS in this process [ 27 ]
Accompanying the processes driven by HIF1α is an activation of mitochondrial oxidative phosphorylation through inactivation of the protein tyrosine phosphatase mitochondrial 1 ( PTPMT1 ) enzyme. [ 27 ] PTPMT-1 is essential for differentiation of HSCs into progenitors, and loss of this enzyme results in failure to produce blood cells in mice. [ 34 ] Targets of PTPMT-1 include phosphatidylinositol phosphates (PIPs). When PIPs are acted upon by PTPMT-1, the mitochondrial membrane potential decreases. This decrease inhibits glucose entry into the TCA cycle and subsequent ATP generation through the electron transport chain. [ 34 ] Thus, PTPMT-1 activity is crucial for HSCs to differentiate.
Another important suppressor of mitochondrial metabolism during quiescence is mitochondrial carrier homolog 2 ( MTCH2 ). [ 16 ] Loss of MTCH2 increases oxidative phosphorylation and triggers HSC differentiation. As expected, this increase in oxidative phosphorylation increases ROS levels, ATP levels, and mitochondrial size. These phenotypes highlight the importance of MTCH2 in directing HSC fate.
Upregulation of glycolysis in proliferative HSCs may drive the pentose phosphate pathway (PPP) to maintain redox balance upon mitochondrial activation. [ 35 ] [ 36 ] The PPP generates nicotinamide adenine dinucleotide phosphate ( NADPH ), which is a powerful cellular reducing agent. Production of NADPH may protect cells against accumulation of ROS because it is a key component in the glutathione-reductase system. [ 36 ] Additionally, NADPH is required for synthesis of nucleic acids and lipids. Thus, high intracellular NADPH may be essential to generate biomass for HSCs as they reenter the cell cycle. [ 35 ] Work in ex- vivo HSC expansion systems supports this idea, but further work is needed to characterize the role of the PPP in vivo [ 35 ]
Several signaling pathways also have roles in mediating the metabolic shift from quiescent to proliferative HSCs. For example, purine metabolism is upregulated and thus promotes entry into the cell cycle through signaling in the p38 MAPK pathway. ERK and mTOR , other major signaling pathways, are also activated during cell cycle entry. Among other functions, these pathways promote protein, nucleotide, and lipid synthesis. Active ERK and mTOR pathways also lead to increased nutrient uptake in HSCs. In addition to this biosynthetic role, mTOR can also increase the rate of ATP production in cells. [ 28 ] | https://en.wikipedia.org/wiki/Metabolic_regulation_of_hematopoiesis |
Organizations:
Metabolic rift is a theory of ecological crisis tendencies under the capitalist mode of production that sociologist John Bellamy Foster ascribes to Karl Marx . Quoting Marx, Foster defines this as the "irreparable rift in the interdependent process of social metabolism ". [ 1 ] : 949 Foster argues that Marx theorized a rupture in the metabolic interaction between humanity and the rest of nature emanating from capitalist agricultural production and the growing division between town and country.
Foster, rather than Marx, coined the term “metabolic rift”. Foster argues the theory develops from Marx's earlier work in the Economic and Philosophical Manuscripts on species-being and the relationship between humans and nature. Metabolism is Marx's "mature analysis of the alienation of nature" [ 2 ] : ix and presents "a more solid—and scientific—way in which to depict the complex, dynamic interchange between human beings and nature, resulting from human labor." [ 3 ]
As opposed to those who have attributed to Marx a disregard for nature and responsibility for the environmental problems of the Soviet Union and other purportedly communist states , Foster sees in the theory of metabolic rift evidence of Marx's ecological perspective. The theory of metabolic rift "enable[ed] [Marx] to develop a critique of environmental degradation that anticipated much of present-day ecological thought", [ 2 ] : 142 including questions of sustainability as well as the limits of agricultural production using concentrated animal feeding operations . [ 4 ] Researchers building on the original Marxist concept have developed other similar terms like carbon rift .
Marx's writings on metabolism were developed during England's "second" agricultural revolution (1815–1880), a period which was characterized by the development of soil chemistry and the growth of the use of chemical fertilizer . [ 2 ] : 148 The depletion of soil fertility , or "soil exhaustion", had become a key concern for capitalist society, and demand for fertilizer was such that Britain and other powers initiated explicit policies for the importation of bone and guano , including raiding of Napoleonic battlefields [ 3 ] : 375 and catacombs, [ 2 ] : 150 British monopolization of Peruvian guano supplies, [ 3 ] : 377 and, in the United States, "the imperial annexation of any islands thought to be rich in [guano]" through the Guano Islands Act (1856). [ 2 ] : 151 [ 3 ] : 377 [ 4 ]
Foster argues that Marx's theory drew heavily on contemporary advances in agricultural chemistry unknown to earlier classical economists such as Ricardo and Malthus . For them, different levels of soil fertility (and thus rent ) was attributed "almost entirely to the natural or absolute productivity of the soil," [ 3 ] : 374 with improvement (or degradation) playing only a minor role.
German agricultural chemist Justus von Liebig , in his Organic Chemistry in Its Applications to Agriculture and Physiology (1840), presented the first convincing explanation of the role of soil nutrients in the growth of plants. [ 3 ] : 376 In 1842 , Liebig expanded the use of the term metabolism ( Stoffwechsel ), from referring to material exchanges in the body, up to the biochemical processes of natural systems. [ 3 ] : 374
Foster argues that Liebig's work became more critical of capitalist agriculture as time went on. From the standpoint of nutrient cycling, the socio-economic relationship between rural and urban areas was self-evidently contradictory, hindering the possibility of sustainability:
If it were practicable to collect, with the least loss, all the solid and fluid excrements of the inhabitants of the town, and return to each farmer the portion arising from produce originally supplied by him to the town, the productiveness of the land might be maintained almost unimpaired for ages to come, and the existing store of mineral elements in every fertile field would be amply sufficient for the wants of increasing populations. [ 5 ] : 261
Marx rooted his theory of social-ecological metabolism in Liebig's analysis but connected it to his understanding of the labor process. [ 3 ] : 380 Marx understood that, throughout history, it was through labor that humans appropriated nature to satisfy their needs. [ 2 ] : 141 Thus the metabolism, or interaction, of society with nature is "a universal and perpetual condition." [ 6 ] : 145
In Capital , Marx integrated his materialist conception of nature with his materialist conception of history . [ 2 ] : 141 Fertility, Marx argued, was not a natural quality of the soil, but was rather bound up with the social relations of the time. By conceptualizing the complex, interdependent processes of material exchange and regulatory actions that link human society with non-human nature as "metabolic relations," Marx allowed these processes to be both "nature-imposed conditions" and subject to human agency , [ 3 ] : 381 a dynamic largely missed, according to Foster, by the reduction of ecological questions to issues of value . [ 2 ] : 11
The central contribution of the metabolic rift perspective is to locate socio-ecological contradictions internal to the development of capitalism. Later socialists expanded upon Marx's ideas, including Nikolai Bukharin in Historical Materialism (1921) and Karl Kautsky in The Agrarian Question (1899), which developed questions of the exploitation of the countryside by the town and the "fertilizer treadmill" that resulted from metabolic rift. [ 2 ] : 239
Contemporary eco-socialist theorists aside from Foster have also explored these directions, including James O'Connor , who sees capitalist undervaluing of nature as leading to economic crisis, what he refers to as the second contradiction of capitalism . [ 7 ]
Scholars from a variety of disciplines have drawn on Marx's metabolic approach and the concept of metabolic rift in analyzing the relation of society to the rest of nature. [ 8 ] With increasing amounts of carbon dioxide being released into the environment from capitalist production, the theory of a carbon rift has also emerged. [ 9 ]
The metabolic rift is characterized in different ways by historical materialists. For Jason W. Moore, the distinction between social and natural systems is empirically false and theoretically arbitrary; following a different reading of Marx, Moore views metabolisms as relations of human and extra-human natures. In this view, capitalism's metabolic rift unfolds through the town-country division of labor, itself a "bundle" of relations between humans and the rest of nature. Moore sees it as constitutive of the endless accumulation of capital. [ 10 ] Moore's perspective, although also rooted in historical materialism, produces a widely divergent view from that of Foster and others about what makes ecological crisis and how it relates to capital accumulation.
Nine months after Foster's groundbreaking article appeared, Moore argued that the origins of the metabolic rift were not found in the 19th century but in the rise of capitalism during the "long" 16th century. [ 11 ] The metabolic rift was not a consequence of industrial agriculture but capitalist relations pivoting on the law of value. Moore consequently focuses attention on the grand movements of primitive accumulation, colonialism, and the globalization of town-country relations that characterized early modern capitalism. There were, in this view, not one but many metabolic rifts; every great phase of capitalist development organized nature in new ways, each one with its own metabolic rift. In place of agricultural revolutions, Moore emphasizes recurrent agro-ecological revolutions, assigned the historical task of providing cheap food and cheap labor, in the history of capitalism, an interpretation that extends the analysis to the food crises of the early 21st century. [ 12 ] [ 13 ]
Up until the 16th or 17th century, cities' metabolic dependency upon surrounding countryside (for resources, etc.), coupled with the technological limitations to production and extraction, prevented extensive urbanization. Early urban centers were bioregionally defined, and had relatively light " footprints ," recycling city nightsoils back into the surrounding areas. [ 6 ] : 410–411
However, with the rise of capitalism, cities expanded in size and population. Large-scale industry required factories, raw material, workers, and large amounts of food. As urban economic security was dependent upon its metabolic support system, [ 6 ] : 411 cities now looked further afield for their resource and waste flows. As spatial barriers were broken down, capitalist society "violated" what were previously "nature-imposed conditions of sustainability." [ 2 ] : 153–156
With trade and expansion, food and fiber were shipped longer distances. The nutrients of the soil were sent to cities in the form of agricultural produce, but these same nutrients, in the form of human and animal waste, were not returned to the land. Thus there was a one-way movement, a "robbing of the soil" in order to maintain the socio-economic reproduction of society. [ 2 ] : 153–156
Marx thus linked the crisis of pollution in cities with the crisis of soil depletion. The rift was a result of the antagonistic separation of town and country, and the social-ecological relations of production created by capitalism were ultimately unsustainable. [ 11 ] From Capital , volume 1, on "Large-scale Industry and Agriculture":
Capitalist production collects the population together in great centres, and causes the urban population to achieve an ever-growing preponderance. This has two results. On the one hand it concentrates the historical motive force of society; on the other hand, it disturbs the metabolic interaction between man and the earth, i.e. it prevents the return to the soil of its constituent elements consumed by man in the form of food and clothing; hence it hinders the operation of the eternal natural condition for the lasting fertility of the soil ... But by destroying the circumstances surrounding that metabolism... it compels its systematic restoration as a regulative law of social production, and in a form adequate to the full development of the human race... All progress in capitalist agriculture is a progress in the art, not only of robbing the worker, but of robbing the soil; all progress in increasing the fertility of the soil for a given time is a progress toward ruining the more long-lasting sources of that fertility... Capitalist production, therefore, only develops the techniques and the degree of combination of the social process of production by simultaneously undermining the original sources of all wealth—the soil and the worker (emphasis added). [ 14 ] : 637–638
The concept of metabolic rift captures "the material estrangement of human beings within capitalist society from the natural conditions which formed the basis for their existence." [ 2 ] : 163 However, Marx also emphasizes the importance of historical change. It was both necessary and possible to rationally govern human metabolism with nature, but this was something "completely beyond the capabilities of bourgeois society." [ 2 ] : 141 In a future society of freely associated producers , however, humans could govern their relations with nature via collective control, rather than through the blind power of market relations. [ 2 ] : 159 In Capital , volume 3 , Marx states:
Freedom, in this sphere...can consist only in this, that socialized man, the associated producers, govern the human metabolism with nature in a rational way, bringing it under their own collective control rather than being dominated by it as a blind power; accomplishing it with the least expenditure of energy and in conditions most worthy and appropriate for their human nature. [ 1 ] : 959
However, Marx did not argue that a sustainable relation to the Earth was an automatic result of the transition to socialism . [ 3 ] : 386 Rather, there was a need for planning and measures to address the division of labor and population between town and country and for the restoration and improvement of the soil. [ 2 ] : 169 [ 15 ] : 40–41
Despite Marx's assertion that a concept of ecological sustainability was "of very limited practical relevance to capitalist society," as it was incapable of applying rational scientific methods and social planning due to the pressures of competition, [ 2 ] : 164 the theory of metabolic rift may be seen as relevant to, if not explicitly invoked in, many contemporary debates and policy directions of environmental governance .
There is a rapidly growing body of literature on social-ecological metabolism. While originally limited to questions of soil fertility—essentially a critique of capitalist agriculture—the concept of metabolic rift has since been taken up in numerous fields and its scope expanded. For example, Clausen and Clark have extended the use of metabolic rift to marine ecology , [ 16 ] while Moore uses the concept to discuss the broader concerns of global environmental crises and the viability of capitalism itself. [ 11 ] [ 10 ] Fischer-Kowalski discusses the application of "the biological concept of metabolism to social systems," tracing it through several contributing scientific traditions, including biology , ecology, social theory , cultural anthropology , and social geography . [ 17 ] A social metabolism approach has become "one of the most important paradigms for the empirical analysis of the society-nature-interaction across various disciplines," [ 17 ] particularly in the fields of industrial metabolism and material flow analysis .
David Harvey points out that much of the environmental movement has held (and in some areas continues to hold) a profound anti-urban sentiment, seeing cities as "the highpoint of plundering and pollution of all that is good and holy on planet earth." [ 6 ] : 426 The problem is that such a perspective focuses solely on a particular form of nature, ignoring many people's lived experience of the environment and the importance of cities in ecological processes and as ecological sites in their own right. [ 6 ] : 427
In contrast, Erik Swyngedouw and other theorists have conceptualized the city as an ecological space through urban political ecology , which connects material flows within cities and between the urban and non-urban [ citation needed ] .
In city planning policy circles, there has been a recent movement toward urban sustainability . Hodson and Marvin discuss a "new eco-urbanism" that seeks to integrate environment and infrastructure, "bundling" architecture , ecology and technology in order to "internalize" energy, water, food, waste and other material flows. [ 18 ] Unlike previous efforts to integrate nature into the city, which, according to Harvey, were primarily aesthetic and bourgeois in nature, [ 6 ] : 427 these new efforts are taking place in the context of climate change, resource constraints and the threat of environmental crises. [ 18 ]
In contrast to the traditional approach of capitalist urbanization, which sought more and more distant sources for material resources and waste sinks (as seen in the history of Los Angeles water ), eco-urban sites would re-internalize their own resources and re-circulate wastes. The goal is autarky and greater ecological and infrastructural self-reliance through " closed-loop systems " that reduce reliance on external networks. [ 18 ] Although difficult given the reliance on international supply chains, urban food movements are working to reduce the commodification of food and individual and social forms of alienation from food within cities. [ 19 ] This takes place within actually existing conditions of neoliberalization, suggesting that healing metabolic rifts will be a process that requires both social and ecological transformations.
However, critics link these efforts to "managerial environmentalism," [ 6 ] : 427 and worry that eco-urbanism too closely falls into an "urban ecological security" approach, [ 18 ] echoing Mike Davis ' analysis of securitization and fortress urbanism. A Marxist critique might also question the feasibility of sustainable cities within the context of a global capitalist system. | https://en.wikipedia.org/wiki/Metabolic_rift |
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