text stringlengths 11 320k | source stringlengths 26 161 |
|---|---|
The TK85 was a ZX81 clone made by Microdigital Eletrônica , a computer company located in Brazil . [ 1 ] [ 2 ] [ 3 ] It came with 16 or 48 KB RAM , and had a ZX Spectrum –style case, similar to a Timex Sinclair 1500 . [ 4 ] [ 5 ]
Unlike the ZX81, the TK85 used standard logic components rather than a gate array ("ULA"), and during manufacture several of them were scraped so that competitors couldn't easily copy the circuit. The circuit board had space for a AY-3-8912 sound generator chip (compatible with the ZonX-81 sound board), and although none came factory installed, it is possible to add the necessary circuits. [ 6 ]
The TK85 came with a copy of the 8K ZX81 floating point BASIC , and an additional 2K EPROM , mapped to addresses 8192-10240, containing machine code routines for use with tape files. These routines could save with HISAVE , load with HILOAD and verify with HIVERIFY in "Hi-Speed" (4200 bps); [ 5 ] save and load, BASIC variables in 300 bps (standard ZX81 speed) using SAVE and DLOAD functions and 4200 bps (Hi-Speed) using DHSAVE and DHLOAD . These routines were all accessible using RAND USR commands. The save to variable function could be used to make copies of programs on tape. [ 2 ]
The expansion port on the back of the computer is compatible with the ZX81, although some peripherals may not work due to conflicts with the 2K of extra ROM.
The rear of the computer featured a TV output (without video back porch ), "EAR" and "MIC" sockets for connecting to an external tape recorder, a joystick port using a DIN socket (that simulated the 5 , 6 , 7 , 8 and 0 keys), a ZX81 compatible expansion port, space for a sound output socket, and a socket for the 9V external power supply.
Since the joystick used the cursor keys , and due to the circuitry for the keyboard, it wasn't possible to detect diagonal directions correctly.
This computing article is a stub . You can help Wikipedia by expanding it .
This computer hardware article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TK85 |
TK is an experimental cell therapy which may be used to treat high-risk leukemia . It is currently undergoing a Phase III clinical trial to determine efficacy and clinical usefulness. [ 1 ] [ 2 ]
TK is currently being investigated in patients with acute leukemia in first or subsequent complete remission and at high risk of relapse or in patients with relapsed disease who are candidates for haploidentical transplantation of hemopoietic stem cells (taken from a partially HLA-compatible family donor). [ 3 ]
TK is a cellular therapy based on the genetical engineering of donor T lymphocytes in order to express a suicide gene (thymidine kinase of the Herpes simplex virus, namely TK). [ 4 ] [ 5 ]
Once the lymphocytes donated by partially compatible family donors (haplo-transplant) have been genetically modified, they can be infused in patients in need of hematopoietic cell transplantation.
The infusion of lymphocytes expressing the TK suicide gene, has the aim to prevent or treat leukemic relapse and promote immune reconstitution, necessary to protect patients from infections that often limit transplant efficacy. [ 6 ]
The presence of TK allows for retention of immune protection and anti-leukaemic effects of donor T lymphocytes and at the same time to control and annul possible harmful reactions between these lymphocytes and healthy tissues of the patient, reaction known as graft-versus-host disease .
Activation of the cell suicide system is obtained by the administration of ganciglovir, an antiviral drug, which only leads to the death of cells expressing TK in patients with graft-versus-host disease. TK has been granted Orphan Drug [ 7 ] designation both in the EU and the United States. [ 8 ] | https://en.wikipedia.org/wiki/TK_cell_therapy |
199964
329909
ENSG00000143001
ENSMUSG00000085933
Q8N0U2
n/a
NM_182532
NM_001370848
NP_872338
n/a
Transmembrane protein 61 ( TMEM61) is a protein that is encoded by the TMEM61 gene in humans. It is located on the first chromosome in humans [ 5 ] and is highly expressed in the intestinal regions predominantly the kidney, adrenal gland and pituitary tissues. [ 6 ] The protein, unlike other transmembrane protein in the region does not promote cancer growth. [ 7 ] However, the TMEM61 protein when inhibited by secondary factors restricts normal activity in the kidney. [ 8 ] The human protein shares many Orthologs and has been prevalent on Earth for millions of years.
There are no known aliases of TMEM61 . The human protein can be identified with any tool that uses UniProt by Q8N0U2. [ 9 ]
TMEM61 is located on the plus strand of the human chromosome 1 at the locus 1 p32.3. [ 5 ] The gene is 11, 661 base pairs long, it ranges from position 54,980,628 to 54,992,288 on chromosome 1. TMEM61 lies between LOC124904184 and BSND. [ 10 ]
NCBI RefSeq contains seven mRNA transcript variants for TMEM61. Transcription variants X1, X2, and X2 both are splices of the original protein, but all three isoforms have their own variants. None of the variants share similar exon boundaries, domain or disordered regions.
There are six known Isoforms of the TMEM61 protein, Isoform X1 is encoded by transcript variant X1, and Isoform X2 with variant X2 and so on. There are two different X2 isoforms, but both have the same amino acid sequence, both the X2 have five less amino acids in the start of the protein, which differs from isoform X1 with same protein sequence and size as the original protein.
The Isoform 1 of the TMEM61 protein is made up of 210 amino acids. [ 12 ] The protein has a predicted molecular weight of about 22.2 KDa and a theoretical isoelectric point of about 4.54. [ 13 ] In terms of amino acid composition, TMEM61 is relatively rich in both the hydrophobic Proline and hydrophilic Serine. The protein is relatively poor in both hydrophilic Asparagine and Lysine. It is also poor in both hydrophobic Isoleucine and Phenylalanine. [ 14 ] The protein indicates acid components from it addition of Arginine and Lysine subtracted to the addition of Glutamic Acid and Aspartic Acid. [ 14 ]
TMEM61 Isoform 1 contains two transmembrane domains one of encompasses a DUF domain. TMEM61 also contains a MTP domain, unlike the transmembrane domain this domains located in the Golgi Apparatus and involves spanning transportation. All four domain regions had low value scores except the second TMEM domain was not able to be scored.
The Ali2D, and I-TASSER models predicted that the secondary structure of TMEM61 has both alpha helices and beta strands.
No confident model for tertiary structure for TMEM61.
While the modification are few, phosphorylation will not result in a change oil the amino acid for TMEM61, this is a result of the lack of glycosylation that takes place in the sequence. Results are represented by graph on bottom right.
Immunofluorescent standing experiments have detected the TMEM61 protein in the endocrine tissues, kidney and Urinary bladder, and proximal digestive tract. The experiment also found slight expression in the brain tissues. [ 17 ]
469
According to HumanAtlas, Geoprofile, and NCBI, TMEM61 is highly expressed in the Kidney, Pituitary gland , Salivary gland , Adrenal , and brain tissues in a decreasing order. [ 17 ] [ 10 ] [ 18 ]
In situ hybridization staining a mouse embryo discovered high levels of TMEM61 in Kidneys and found no other tissues to express the protein. [ 17 ]
TMEM61 was found to be very abundant in the human body in comparison to other proteins. [ 19 ]
Western blotting showed an over expression of lysate in mammalian, in this case rabbit. [ 20 ]
The staining of the human pancreas shows cytoplasmic positivity in exocrine cells. [ 20 ]
The IntAct, String, and BioGrid database found eight relevant interacting protein to the TMEM61 . [ 21 ] [ 22 ] [ 23 ] Other TMEM protein such as TMEM124 are closely monitored together for the cancer expression both in the same region but both did not promote cancer growth.
InTACT
10.1016/j.celrep.2020.108050
TMEM61 has orthologs in mammals, reptiles, aves, amphibians, and fish. A table of orthologs is beside to the right. There is no known paralog of TMEM61 .
West African lungfish is the furthest-from-human known organism to express TMEM61 approximately 408 million years ago. [ 24 ] The expression of TMEM61 protein throughout its closely related orthologs all indicate high expression in the Kidney.Based on a molecular clock analysis, the protein sequence of TMEM61 has on average evolved faster than Cytochrome C but slower than Fibrinogen alpha.
TMEM61 was anticipated to be associated with the formation of brain tumors but was later debunked as there was low levels expressed, however the test did indicate its location to be in the mitochondrial neural membrane region. [ 7 ] [ 25 ] The TMEM61 has been hoped to promote cancer or tumor growth but there has been no clinical research that proves this idea.
The information obtained about the TMEM61 does show expression on the kidney beyond it human organism [ clarification needed ] and the studies show MIF limiting the expression of TMEM61 . [ 8 ]
The Aquaporin-11 deficiency, closing or breaking of water channels limits the protein expression in the membrane and restricts TMEM61 expression and inhibits kidney function. [ 8 ]
Very close to other TMEM protein such as TMEM124 was closely monitored for the cancer expression both in the same region but both did not promote cancer growth. PMP22, YAP1. | https://en.wikipedia.org/wiki/TMEM61 |
Trimethoprim-Halotag (TMP-HTag) is a small molecule chemical linker developed for the rapid and reversible control of protein localization in living cells (Ballister). TMP is an dihydrofolate reductase (DHFR) inhibitor chosen for its specificity in binding to the bacterial form of DHFR. [ 1 ] [ 2 ] The other half of the linker is a Halotag , a self labelling bacterial globular protein ligand that can bind covalently and irreversibly to the chloroalkane group of a Haloenzyme. [ 2 ] [ 3 ] Positioned between the TMP group and HaloTag is a flexible linker that can be modified to optimize protein linking efficiency. [ 2 ] [ 3 ] The modular structure of TMP-HaloTag makes it an ideal heterobifunctional tool for use in chemically induced dimerization (CID). [ 1 ] [ 2 ] Additionally, TMP- HTag can be modified to include photo-cleavable groups that allow for the control of CID using light. [ 1 ] [ 2 ] | https://en.wikipedia.org/wiki/TMP-HTag |
2X29
8744
21950
ENSG00000125657
ENSMUSG00000035678
P41273
P41274
NM_003811
NM_009404
NP_003802
NP_033430
Tumor necrosis factor ligand superfamily member 9 also known as 4-1BB ligand or 4-1BBL or CD137L is a protein that in humans is encoded by the TNFSF9 gene . [ 5 ]
4-1BBL is a type 2 transmembrane glycoprotein receptor that is found on APCs (antigen presenting cells) and binds to 4-1BB (also known as CD137). The 4-1BB/4-1BBL complex belongs to the TNFR : TNF superfamily, [ 6 ] which is expressed on activated T Lymphocytes . [ 7 ]
TNFSF9 consists of an extracellular domain responsible for receptor binding, a transmembrane region, and a short intracellular domain, and can also exist in a soluble form when cleaved from the membrane. This structural organization enables TNFSF9 to function as a bidirectional signal transducer, facilitating costimulatory signaling crucial for T cell activation and immune response modulation. [ 8 ]
TNFSF9 forms a trimeric complex on the cell surface, which interacts with the 4-1BB (CD137) receptor on activated T lymphocytes. Each 4-1BB monomer binds to two 4-1BBL subunits via cysteine-rich domains (CRDs), with the CRD2 and CRD3 regions of 4-1BB engaging specific loops on 4-1BBL to stabilize the interaction through multiple hydrogen bonds. [ 8 ]
The 4-1BB/4-1BBL complex consists of three monomeric 4-1BBs bound to a trimeric 4-1BBL. Each 4-1BB monomer binds to two 4-1BBLs via cysteine-rich domains (CRDs). The interaction between 4-1BB and the second 4-1BBL is required to stabilize their interactions. [ 9 ] The link with 4-1BBL is largely made up of amino acids from the dynamic loops of the CRD2 and the β sheet of CRD3 of 4-1BB, according to a detailed study of the binding between the 4-1BB and 4-1BBL interface. CRD2 amino acids (T61, Q67, and K69) interact with the AA′ loop (Y110 and G114) and the intra-H-strand loop (Q227 and Q230) of 4-1BBL to form various hydrogen bond interactions. [ 10 ]
TNFSF9 plays a key role in immune cell interactions. TNFSF9 forms a trimeric complex on the cell surface, which interacts with the 4-1BB (CD137) receptor on activated T lymphocytes. Complex formation with its receptor enables TNFSF9 to function as a bidirectional signal transducer, facilitating costimulatory signaling crucial for T cell activation and immune response modulation. [ 8 ]
Early studies using the poorly immunogenic Ag104A sarcoma and highly tumorigenic P815 mastocytoma models provided the first systematic evidence that anti-4-1BB antibodies exert strong anti-tumor effects. These antibodies were found to significantly suppress tumor growth by enhancing cytotoxic T lymphocyte (CTL) activity. Subsequent research has consistently confirmed the role of 4-1BB signaling in promoting anti-tumor immunity. [ 11 ]
The 4-1BB/4-1BBL interaction delivers costimulatory signals that enhance T-cell responses, a mechanism with significant implications for cancer immunotherapy. When combined with T-cell receptor signaling, this interaction stimulates both CD4 + and CD8 + T cells, contributing to effective anti-tumor responses. [ 6 ] However, in human CD28 − T cells, 4-1BB signaling can promote expansion of this subset, which is associated with adverse outcomes in cancer and other diseases. As a result, modulating this pathway represents a promising therapeutic strategy. [ 12 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TNFSF9 |
A TNF inhibitor is a pharmaceutical drug that suppresses the physiologic response to tumor necrosis factor (TNF), which is part of the inflammatory response . TNF is involved in autoimmune and immune-mediated disorders such as rheumatoid arthritis , ankylosing spondylitis , inflammatory bowel disease , psoriasis , hidradenitis suppurativa and refractory asthma , so TNF inhibitors may be used in their treatment. The important side effects of TNF inhibitors include lymphomas, infections (especially reactivation of latent tuberculosis ), congestive heart failure , demyelinating disease , a lupus-like syndrome, induction of auto-antibodies , injection site reactions , and systemic side effects. [ 1 ]
The global market for TNF inhibitors in 2008 was US$13.5 billion , [ 2 ] in 2009 US$22 billion , [ 3 ] and in 2024 US$44 billion . [ 4 ]
Inhibition of TNF effects can be achieved with a monoclonal antibody such as infliximab , [ 5 ] adalimumab , certolizumab pegol , and golimumab , or with a circulating receptor fusion protein such as etanercept .
While most clinically useful TNF inhibitors are monoclonal antibodies, some are simple molecules such as xanthine derivatives [ 6 ] (e.g. pentoxifylline ) [ 7 ] and bupropion . [ 8 ]
Thalidomide and its derivatives lenalidomide and pomalidomide are also active against TNF.
Several 5-HT 2A agonist hallucinogens including ( R )- DOI , TCB-2 , LSD and LA-SS-Az have unexpectedly also been found to act as potent inhibitors of TNF, with DOI being the most active, showing TNF inhibition in the picomolar range, an order of magnitude more potent than its action as a hallucinogen. [ 9 ] [ 10 ] [ 11 ]
The role of TNF as a key player in the development of rheumatoid arthritis was originally demonstrated by Kollias and colleagues in proof of principle studies in transgenic animal models. [ 12 ] [ 13 ]
TNF levels have been shown to be raised in both the synovial fluid and synovium of patients with rheumatoid arthritis. This leads to local inflammation through the signalling of synovial cells to produce metalloproteinases and collagenase . [ 14 ]
Clinical application of anti-TNF drugs in rheumatoid arthritis was demonstrated by Marc Feldmann and Ravinder N. Maini , who won the 2003 Lasker Award for their work. [ 15 ] Anti-TNF compounds help eliminate abnormal B cell activity. [ 16 ] [ 17 ]
Therapy which combines certain anti-TNF agents such as etanercept with DMARDs such as methotrexate has been shown to be more effective at restoring quality of life to sufferers of rheumatoid arthritis than using either drug alone. [ 14 ]
Clinical trials regarding the effectiveness of these drugs on hidradenitis suppurativa are ongoing. [ 18 ]
The National Institute of Clinical Excellence (NICE) has issued guidelines for the treatment of severe psoriasis using the anti-TNF drugs etanercept and adalimumab as well as the anti- IL12 / 23 biological treatment ustekinumab . In cases where more conventional systemic treatments such as psoralen combined with ultraviolet A treatment ( PUVA ), methotrexate , and ciclosporin have failed or can not be tolerated, these newer biological agents may be prescribed. Infliximab may be used to treat severe plaque psoriasis if aforementioned treatments fail or can not be tolerated. [ 19 ]
In 2010 The National Institute of Clinical Excellence (NICE) in the UK issued guidelines for the treatment of severe Crohn's Disease with infliximab and adalimumab. [ 20 ]
Anti-TNF therapy has shown only modest effects in cancer therapy. Treatment of renal cell carcinoma with infliximab resulted in prolonged disease stabilization in certain patients. Etanercept was tested for treating patients with breast cancer and ovarian cancer showing prolonged disease stabilization in certain patients via downregulation of IL-6 and CCL2 . On the other hand, adding infliximab or etanercept to gemcitabine for treating patients with advanced pancreatic cancer was not associated with differences in efficacy when compared with placebo. [ 21 ]
The U.S. Food and Drug Administration continues to receive reports of a rare cancer of white blood cells (known as hepatosplenic T-cell lymphoma or HSTCL), primarily in adolescents and young adults being treated for Crohn's disease and ulcerative colitis with TNF blockers, as well as with azathioprine , and/or mercaptopurine . [ 22 ]
TNF inhibitors put patients at increased risk of certain opportunistic infections. The FDA has warned about the risk of infection from two bacterial pathogens, Legionella and Listeria . People taking TNF blockers are at increased risk for developing serious infections that may lead to hospitalization or death due to certain bacterial, mycobacterial , fungal, viral, and parasitic opportunistic pathogens. [ 23 ]
In patients with latent Mycobacterium tuberculosis infection, active tuberculosis (TB) may develop soon after the initiation of treatment with infliximab. [ 24 ] Before prescribing a TNF inhibitor, physicians should screen patients for latent tuberculosis. The anti-TNF monoclonal antibody biologics infliximab, golimumab, certolizumab and adalimumab, and the fusion protein etanercept , which are all currently approved by the FDA for human use, have warnings which state that patients should be evaluated for latent TB infection, and if it is detected, preventive treatment should be initiated prior to starting therapy with these medications.
The FDA issued a warning on September 4, 2008, that patients on TNF inhibitors are at increased risk of opportunistic fungal infections such as pulmonary and disseminated histoplasmosis , coccidioidomycosis , and blastomycosis . They encourage clinicians to consider empiric antifungal therapy in certain circumstances to all patients at risk until the pathogen is identified. [ 25 ] A recent review showed that anti-TNFα agents associate with increased infection risks for both endemic and opportunistic invasive fungal infections, particularly when given late in the overall course of treatment of the underlying disease, and in young patients receiving concomitant cytotoxic or augmented immunosuppressive therapy. [ 26 ]
In 1999 a randomized control trial was conducted testing a TNF-alpha inhibitor prototype, Lenercept, for the treatment of multiple sclerosis (MS). However, the patients in the study who received the drug had significantly more exacerbations and earlier exacerbations of their disease than those who did not. [ 27 ] [ 28 ]
Case reports have also come out suggesting the possibility that anti-TNF-alpha agents not only may worsen, but may cause new-onset Multiple Sclerosis or other demyelinating disorders in some patients. [ 28 ] A 2018 case report described an Italian man with plaque psoriasis who developed MS after starting entanercept. Their literature review at that time identified 34 other cases of demyelinating disease developing after the initiation of an anti-TNF drug. [ 29 ] Thus, anti-TNF-alpha drugs are contraindicated in patients with MS, and the American Academy of Dermatology recommends avoiding their use in those with a first degree relative with MS. [ 30 ] [ 28 ]
Several other monoclonal antibodies like adalimumab , [ 31 ] [ 32 ] pembrolizumab , [ 33 ] nivolumab , and infliximab [ 34 ] have been reported to trigger MS as an adverse event. [ 35 ] [ 28 ]
The risk of anti-TNF-associated demyelination is not associated with genetic variants of multiple sclerosis. In some studies, there were clinical differences to multiple sclerosis as 70% of the patients with anti-TNF-induced demyelination. The symptoms of demyelination do not resolve with corticosteroids, intravenous immunoglobulin or plasma exchange, and is not clear whether MS therapies are effective in anti-TNF-induced demyelination. [ 36 ]
Despite their good safety profile, one of the common adverse events and side effects associated with TNF-α inhibitors is the occurrence of paradoxical psoriasis . [ 37 ] [ 38 ] [ 39 ] Paradoxical psoriasis is defined as the development of psoriatic lesions or as an exacerbation of pre-existent psoriatic lesions, in patients with or without a prior history of psoriasis , while undergoing treatment with TNF-α inhibitors, such as infliximab , adalimumab , and etanercept for their underlying inflammatory disease. [ 37 ] [ 38 ] [ 39 ] The first case of paradoxical psoriasis induced by TNF-α inhibitors was reported in a patient suffering from inflammatory bowel disease . [ 37 ] [ 38 ] [ 39 ] Subsequently, an increasing number of cases were reported in IBD cohorts and in patients suffering from other chronic immune-mediated inflammatory diseases such as rheumatoid arthritis . [ 37 ] [ 38 ] [ 39 ] This increase was positively correlated with the increasing use of TNF-α inhibitors. [ 37 ] [ 38 ] [ 39 ] The rates of paradoxical psoriasis reported across observational studies range from 2% to 5%, with higher rates observed in female patients. [ 37 ] [ 38 ] [ 39 ] The time to onset from induction therapy can range anywhere from a few days to a few months; with the most common clinical presentations being pustular psoriasis, plaque psoriasis and guttate psoriasis, with nail and scalp involvement. [ 37 ] [ 38 ] [ 39 ] Moreover, some patients may experience more than one type of lesion and/or have lesions across multiple locations. [ 37 ] [ 38 ] [ 39 ]
TNF or its effects are inhibited by several natural compounds, including curcumin [ 40 ] [ 41 ] [ 42 ] [ 43 ] (a compound present in turmeric ), and catechins (in green tea ). Cannabidiol [ 44 ] and Echinacea purpurea also seem to have anti-inflammatory properties through inhibition of TNF-α production, although this effect may be mediated through cannabinoid CB 1 or CB 2 receptor-independent effects. [ 45 ]
5-HT 2A receptor agonists have also been shown to have potent inhibitory effects on TNF-α, including psilocybin found in many species of mushrooms. [ 46 ] [ 47 ]
Thymoquinone , a compound found in the flower Nigella sativa , has been studied for possible TNF-α inhibition and related benefits for autoimmune disorder treatment. [ 48 ] [ 49 ] [ 50 ] [ 51 ]
Early experiments associated TNF with the pathogenesis of bacterial sepsis. Thus, the first preclinical studies using polyclonal antibodies against TNF-alpha were performed in animal models of sepsis in 1985 and showed that anti-TNF antibodies protected mice from sepsis. [ 52 ] [ 53 ] However, subsequent clinical trials in patients with sepsis showed no significant benefit. It wasn't until 1991 that studies in a transgenic mouse model of overexpressed human TNF provided the pre-clinical rationale for a causal role of TNF in the development of polyarthritis and that anti-TNF treatments could be effective against human arthritides. [ 12 ] This was later confirmed in clinical trials [ 54 ] and led to the development of the first biological therapies for rheumatoid arthritis. | https://en.wikipedia.org/wiki/TNF_inhibitor |
TNF receptor associated factors ( TRAFs ) are a family of proteins primarily involved in the regulation of inflammation , antiviral responses and apoptosis . [ 1 ]
Currently, seven TRAF proteins have been characterized in mammals : TRAF1 , TRAF2 , TRAF3 , TRAF4 , TRAF5 , TRAF6 and TRAF7 .
Except for TRAF7, these proteins share a relatively conserved secondary structure , including a namesake C-terminal TRAF domain that mediates interactions with other signaling components such as the transmembrane TNF receptors and CD40 .
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TNF_receptor_associated_factor |
TNO (gastro-) Intestinal Models (“TIM”) is a system of models mimicking the digestive tract. The system was developed by TNO, the Netherlands Organisation for Applied Scientific Research . [ 1 ]
The models are dynamic computer controlled multi-compartmental systems with adjustable parameters for the physiological conditions of the stomach and intestine. Temperature, peristalsis, bile secretion, secretion of saliva, stomach and pancreas enzymes are all fully adjustable.
The TIM systems are being used to study the behavior of oral products during transit through the stomach, the small intestine and large intestine. Commonly performed studies concern the digestibility of food and food components, the bioaccessibility for absorption of pharmaceutical compounds, proteins, fat, minerals and (water- and fat-soluble) vitamins. [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ]
There are different models for the stomach and small intestine ( TIM-1 and Tiny-TIM ) and a model simulating the physiological conditions of the colon ( TIM-2 ).
The TIM-1 system consists of a stomach compartment and 3 compartments for the small intestine , the duodenum, jejunum and ileum. The Tiny-TIM system consists of a stomach compartment and one single compartment for the small intestine. Samples can be harvested for analysis from these models from any compartment at any time.
TIM-2 simulates the colon, containing the microbiota as found in human colon. This model serves as a tool to study fermentation of non-digestible food components (fibers and prebiotics) and the release of drugs specifically targeted for the colon. [ 9 ] | https://en.wikipedia.org/wiki/TNO_intestinal_model |
TNP-ATP is a fluorescent molecule that is able to determine whether a protein binds to ATP , and the constants associated with that binding. It is primarily used in fluorescence spectroscopy , but is also very useful as an acceptor molecule in FRET , and as a fluorescent probe in fluorescence microscopy and X-ray crystallography . [ 1 ]
TNP refers to the chemical compound 2,4,6-trinitrophenol, also known as Picric acid . [ 2 ] It is a primary constituent of many unexploded landmines, and is a cousin to TNT , but less stable. [ 2 ] It is recognized as an environmental contaminant and is toxic to many organisms. [ 2 ] It is still commonly used in the manufacturing of fireworks , explosives , and rocket fuels , as well as in leather, pharmaceutical, and dye industries. [ 2 ]
ATP is an essential mediator of life. [ 1 ] It is used to overcome unfavorable energy barriers to initiate and fuel chemical reactions. [ 1 ] It is also used to drive biological machinery and regulate a number of processes via protein- phosphorylation . [ 1 ] However, the proteins that bind ATP for both regulation and enzymatic reactions are very diverse—many yet undiscovered—and for many proteins their relationship to ATP in terms of number of binding sites , binding constants , and dissociation constants remain unclear. [ 1 ]
Conjugating TNP to ATP renders this nucleotide triphosphate fluorescent and colored whilst allowing it to retain its biological activity. [ 1 ] TNP-ATP is thus a fluorescent analog of ATP. [ 3 ] This conjugation is very useful in providing information about interactions between ATP and an ATP-binding protein because TNP-ATP interacts with proteins and enzymes as a substitute for its parent nucleotide, and has a strong binding affinity for most systems that require ATP. [ 1 ]
TNP is excited at a wavelength of 408 and 470 nm, and fluoresces in the 530–560 nm range. [ 1 ] [ 2 ] [ 4 ] [ 5 ] This is a very useful range of excitation because it is far from where proteins or nucleotides absorb. [ 1 ] When TNP-ATP is in water or other aqueous solutions, this emission is very weak. [ 1 ] [ 6 ] However, once TNP-ATP binds to a protein , there is a dramatic increase in fluorescent intensity. [ 1 ] [ 3 ] [ 5 ] [ 6 ] This property enables researchers to study various proteins’ binding interaction with ATP. Thus, with enhanced fluorescence, it can be seen whether a protein binds to ATP. [ 1 ]
When TNP-ATP in water is excited at 410 nm, TNP-ATP shows a single fluorescence maximum at 561 nm. [ 6 ] This maximum shifts as the fluid's viscosity changes. For example, in N,N-dimethylformamide , instead of having its maxima at 561 nm as in water, the maxima is instead at 533 nm. [ 6 ]
Binding to a protein will also change the wavelength of maximal emission, as well as a change in fluorescent intensity. [ 6 ] For example, binding to the chemotaxis protein CheA indicates a severalfold enhancement of fluorescence intensity and a blue-shift in wavelength of the maximal emission. [ 6 ]
Using this TNP nucleotide analog has been shown in many instances to be superior to traditional radionucleotide-labelling based techniques. [ 1 ] The health concerns and the cost associated with the use of radioactive isotopes makes TNP-ATP an attractive alternative. [ 1 ]
The first fluorescent ribose -modified ATP is 2’,3’-O-(2,4,7-trinitrocyclohexadienylidene) adenosine 5’triphosphate (TNP-ATP), and was introduced in 1973 by Hiratsuka and Uchida. [ 1 ] [ 4 ] TNP-ATP was originally synthesized to investigate the ATP binding site of myosin ATPase . [ 1 ] [ 3 ] Reports of TNP-ATP’s success in the investigation of this motor protein extended TNP-ATP’s use to other proteins and enzymes. [ 1 ] TNP-ATP has now been used as a spectroscopic probe for numerous proteins suspected to have ATP interactions. [ 1 ] These include several protein kinases , ATPases , myosin , and other nucleotide binding proteins. [ 1 ] Over the past twenty years, there have been hundreds of papers describing TNP-ATP’s use and applications. [ 1 ] Many applications involving this fluorescently labeled nucleotide have helped to clarify structure-function relationships of many ATP-requiring proteins and enzymes. [ 1 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] There have also been a growing number of papers that display TNP-ATP use as a means of assessing the ATP-binding capacity of various mutant proteins. [ 1 ] [ 6 ]
Preparing TNP-ATP is a one-step synthesis that is relatively safe and easy. [ 1 ] Adenosine’s ribose moiety can be trinitrophenylated by 2,4,6-trinitrobenzene-1-sulfonate ( TNBS ). [ 4 ] The resulting compound assumes a bright orange color and has visible absorption characteristics, as is characteristic of a Meiseinheimer spiro complex compound linking . [ 1 ] [ 4 ]
To see the exact method of preparion, please refer to T. Hiratsuka's and K. Uchida's paper "Preparation and Properties of 2'(r 3')-O(2,4,6-trinitrophenyl) Adenosine 5'-triphosphate, an Analog of Adenosine Triphosphate," found in the reference section.
To revert TNP-ATP back to its constituent parts, or in other words to hydrolyze TNP-ATP to give equilmolar amounts of picric acid (TNP) and ATP, TNP-ATP should be treated with 1 M HCl at 100 degrees Celsius for 1.5 hours. [ 4 ] This is because if TNP-ATP is acidified under mild conditions, it results in the opening of the dioxolane ring attached to the 2’-oxygen, leaving a 3’O-TNP derivative as the only product. [ 1 ]
TNP-ATP should be stored at −20 °C, in the dark, and used under minimal lighting conditions. [ 6 ] When in solution, TNP-ATP has a shelf life of about 30 days.
When absorption was measured against wavelength at various pH values, the changes at wavelength 408 nm and 470 nm yielded a sigmoidal line with a midpoint at 5.1. [ 4 ] This indicated that the absorbance at these two wavelengths depends upon the ionization of the chromophoric portion of TNP-ATP and is unaffected by ionization of ATP. [ 4 ] Although this ionization constant of 5.1 is not in physiological range, it has been shown that the absorbance of TNP-ATP is sensitive enough to detect changes due to slight shifts in neutral pH. [ 4 ] Spectroscopic superposition indicated TNP-ATP’s isosbestic point to be 339 nm. [ 4 ]
At low concentrations of TNP-ATP (≤1 μM), fluorescent intensity is proportional to the concentration of TNP added. [ 6 ] However, at concentrations exceeding 1 μM, inner filter effects cause this relationship to no longer be linear. [ 6 ] To correct this, researchers must determine the ratio of the predicted theoretical fluorescence intensity (assuming linearity) to the observed fluorescence intensity and then apply this correction factor. [ 6 ] However, in most cases, researchers will try to keep the concentration of TNP to lower than 1 μM. [ 1 ] [ 2 ] [ 3 ] [ 5 ] [ 6 ]
To determine binding affinities, TNP-ATP is added to a solution and then titrated with protein. [ 5 ] [ 6 ] This produces a saturation curve from which the binding affinity can be determined. [ 5 ] [ 6 ] The number of binding sites may also be determined through this saturation curve by looking to see if there are sudden changes in slope. [ 5 ] One can also titrate a fixed amount of protein with increasing additions of TNP-ATP to obtain a saturation curve. [ 6 ] To do so, however, may get complicated due to the inner filter effects that will need to be corrected for. [ 6 ]
To determine dissociation constants, TNP-ATP can be competed off of a protein with ATP. [ 5 ] [ 6 ] The value of the dissociation constant K d for a single-site binding can then be obtained by applying the Langmuir equation for a curve fit:
R F U o b s = R F U f r e e + ( R F U b o u n d − R F U f r e e ) × ( ( [ p r o t e i n ] t o t a l + [ T N P ] t o t a l + K d ) − ( [ p r o t e i n ] t o t a l + [ T N P ] t o t a l + K d ) 2 − ( 4 × [ p r o t e i n ] t o t a l × [ T N P ] ) ) 2 [ T N P ] t o t a l {\displaystyle \mathrm {RFU_{obs}} =\mathrm {RFU_{free}} +{\frac {(\mathrm {RFU_{bound}} -\mathrm {RFU_{free}} )\times \left((\mathrm {[protein]_{total}} +\mathrm {[TNP]_{total}} +\mathrm {K_{d}} )-{\sqrt {(\mathrm {[protein]_{total}} +\mathrm {[TNP]_{total}} +\mathrm {K_{d}} )^{2}-(4\times \mathrm {[protein]_{total}} \times \mathrm {[TNP]} )}}\right)}{2\mathrm {[TNP]_{total}} }}}
where RFU is relative fluorescent units, RFU obs is the fluorescence observed, RFU free is the fluorescence of free TNP-ATP, and RFU bound is the fluorescence of TNP-ATP when completely bound to a protein. [ 5 ]
To measure an ATP competitor, one can add competitor to pre-incubated samples of protein:TNP-ATP. The fraction of TNP-ATP bound to the protein can be calculated via:
θ = R F U o b s − R F U f r e e R F U m a x − R F U f r e e {\displaystyle \theta ={\frac {\mathrm {RFU_{obs}} -\mathrm {RFU_{free}} }{\mathrm {RFU_{max}} -\mathrm {RFU_{free}} }}}
where θ is that fraction, and RFU max is the value of fluorescence intensity at saturation, meaning when 100% of TNP-ATP is bound. [ 5 ]
The dissociation constants for TNP and competitor can then be calculated through the equation: [ 5 ]
θ = 1 2 [ T N P ] × ( K T N P + K T N P K c o m p e t i t o r × [ c o m p e t i t o r ] + [ T N P ] + [ p r o t e i n ] − ( K T N P + K T N P K c o m p e t i t o r × [ c o m p e t i t o r ] + [ T N P ] + [ p r o t e i n ] ) 2 − 4 × [ T N P ] × [ p r o t e i n ] ) {\displaystyle \theta ={\frac {1}{2}}\mathrm {[TNP]} \times \left(\mathrm {K_{TNP}} +{\frac {\mathrm {K_{TNP}} }{\mathrm {K_{competitor}} }}\times \mathrm {[competitor]} +\mathrm {[TNP]} +\mathrm {[protein]} -{\sqrt {\left(\mathrm {K_{TNP}} +{\frac {\mathrm {K_{TNP}} }{\mathrm {K_{competitor}} }}\times \mathrm {[competitor]} +\mathrm {[TNP]} +\mathrm {[protein]} \right)^{2}-4\times \mathrm {[TNP]} \times \mathrm {[protein]} }}\right)}
For reasons not yet fully understood, TNP-ATP typically binds the ATP binding sites of proteins and enzymes anywhere from one to three times tighter than regular ATP. [ 1 ] [ 6 ] The dissociation constants are usually around 0.3–50 μM. [ 1 ]
In addition to using TNP-ATP to determine whether a protein binds ATP, its binding affinity and dissociation constants, and number of binding sites, TNP-ATP can also be used in ligand binding studies. [ 1 ] To do this, titrations of the protein are added to TNP-ATP. Then, ligand is added to displace the bound analog. [ 1 ] This is measured by decreases in fluorescence. [ 1 ] One can also do this by titrating protein with TNP-ATP in the presence and absence of varying concentrations of the ligand of interest. [ 1 ] Using either experiment will allow the binding affinity of the ligand to protein to be measured.
TNP-ATP is also valuable fluorescence acceptor. [ 1 ] [ 2 ] This is because, as with any good acceptor, TNP-ATP absorbs over a wide wavelength range that corresponds to the range of emission of common FRET donors. [ 2 ] Thus, TNP-ATP can be used to look at the conformational changes that proteins undergo. [ 2 ] For example, Na+/K+ ATPase, the distance between the active site and Cys457 was shown to change from 25 Angstroms to 28 Angstroms in changing from the Na+ conformation to the K+ conformation. [ 1 ]
In addition to fluorescent spectroscopy, TNP-ATP is very useful in fluorescent microscopy . [ 1 ] This is because it greatly increases the sensitivity of the observations when bound to proteins—the enhanced fluorescence greatly reduces the problem of background fluorescence. [ 1 ] This is especially true under epifluorescent illumation (illumination and light are both on the same side of the specimen). [ 1 ]
TNP-ATP has also been used in X-ray crystallography because it can be used to determine binding constants of crystallized substrates. This technique also demonstrates the structure of proteins in the presence or absence of TNP-ATP, which may or may not correspond to the structure of proteins when they bind ATP. [ 1 ] [ 6 ] | https://en.wikipedia.org/wiki/TNP-ATP |
Trinitrotoluene ( / ˌ t r aɪ ˌ n aɪ t r oʊ ˈ t ɒ lj u iː n / ), [ 5 ] [ 6 ] more commonly known as TNT (and more specifically 2,4,6-trinitrotoluene , and by its preferred IUPAC name 2-methyl-1,3,5-trinitrobenzene ), [ 1 ] is a chemical compound with the formula C 6 H 2 (NO 2 ) 3 CH 3 . TNT is occasionally used as a reagent in chemical synthesis , but it is best known as an explosive material with convenient handling properties. The explosive yield of TNT is considered to be the standard comparative convention of bombs and asteroid impacts. In chemistry , TNT is used to generate charge transfer salts .
TNT was first synthesized in 1861 by German chemist Julius Wilbrand [ 7 ] and was originally used as a yellow dye. Its potential as an explosive was not recognized for three decades, mainly because it was so much less sensitive than other explosives known at the time. Its explosive properties were discovered in 1891 by another German chemist, Carl Häussermann. [ 8 ] TNT can be safely poured when liquid into shell cases, and is so insensitive that in 1910 it was exempted from the UK's Explosives Act 1875 and was not considered an explosive for the purposes of manufacture and storage. [ 9 ]
The German armed forces adopted it as a filling for artillery shells in 1902. TNT-filled armour-piercing shells would explode after they had penetrated the armour of British capital ships , whereas the British Lyddite -filled shells tended to explode upon striking armour, thus expending much of their energy outside the ship. [ 9 ] The British started replacing Lyddite with TNT in 1907. [ 10 ]
The United States Navy continued filling armour-piercing shells with explosive D after some other nations had switched to TNT, but began filling naval mines , bombs , depth charges , and torpedo warheads with burster charges of crude grade B TNT with the color of brown sugar and requiring an explosive booster charge of granular crystallized grade A TNT for detonation. High-explosive shells were filled with grade A TNT, which became preferred for other uses as industrial chemical capacity became available for removing xylene and similar hydrocarbons from the toluene feedstock and other nitrotoluene isomer byproducts from the nitrating reactions. [ 11 ]
In industry, TNT is produced in a three-step process. First, toluene is nitrated with a mixture of sulfuric and nitric acid to produce mononitrotoluene (MNT). The MNT is separated and then renitrated to dinitrotoluene (DNT). In the final step, the DNT is nitrated to trinitrotoluene (TNT) using an anhydrous mixture of nitric acid and oleum . Nitric acid is consumed by the manufacturing process, but the diluted sulfuric acid can be reconcentrated and reused.
After nitration, TNT can either be purified by crystallization from an organic solvent or stabilized by a process called sulfitation, where the crude TNT is treated with aqueous sodium sulfite solution to remove less stable isomers of TNT and other undesired reaction products. The rinse water from sulfitation is known as red water and is a significant pollutant and waste product of TNT manufacture. [ 12 ]
Control of nitrogen oxides in feed nitric acid is very important because free nitrogen dioxide can result in oxidation of the methyl group of toluene. This reaction is highly exothermic and carries with it the risk of a runaway reaction leading to an explosion. [ citation needed ]
In the laboratory, 2,4,6-trinitrotoluene is produced by a two-step process. A nitrating mixture of concentrated nitric and sulfuric acids is used to nitrate toluene to a mixture of mono- and di-nitrotoluene isomers, with careful cooling to maintain temperature. The nitrated toluenes are then separated, washed with dilute sodium bicarbonate to remove oxides of nitrogen, and then carefully nitrated with a mixture of fuming nitric acid and sulfuric acid. [ citation needed ]
X-ray crystallography determined that each of the three planar nitro groups is substantially rotated out of the plane of the benzene ring. [ 13 ]
TNT is one of the most commonly used explosives for military, industrial, and mining applications. TNT has been used in conjunction with hydraulic fracturing (popularly known as fracking), a process used to acquire oil and gas from shale formations. The technique involves displacing and detonating nitroglycerin in hydraulically induced fractures followed by wellbore shots using pelletized TNT. [ 14 ]
TNT is valued partly because of its insensitivity to shock and friction, with reduced risk of accidental detonation compared to more sensitive explosives such as nitroglycerin . TNT melts at 80 °C (176 °F), far below the temperature at which it will spontaneously detonate, allowing it to be poured or safely combined with other explosives. TNT neither absorbs nor dissolves in water, which allows it to be used effectively in wet environments. To detonate, TNT must be triggered by a pressure wave from a starter explosive, called an explosive booster . [ 15 ]
Although blocks of TNT are available in various sizes (e.g. 250 g, 500 g, 1,000 g), it is more commonly encountered in synergistic explosive blends comprising a variable percentage of TNT plus other ingredients. Examples of explosive blends containing TNT include:
Upon detonation , TNT undergoes a decomposition equivalent to the reaction
plus some of the reactions
and
The reaction is exothermic but has a high activation energy in the gas phase (~62 kcal/mol). The condensed phases (solid or liquid) show markedly lower activation energies of roughly 35 kcal/mol due to unique bimolecular decomposition routes at elevated densities. [ 23 ] Because of the production of carbon , TNT explosions have a sooty appearance. Because TNT has an excess of carbon, explosive mixtures with oxygen-rich compounds can yield more energy per kilogram than TNT alone. During the 20th century amatol , a mixture of TNT with ammonium nitrate , was a widely used military explosive. [ 24 ]
TNT can be detonated with a high velocity initiator or by efficient concussion. [ 25 ] For many years, TNT used to be the reference point for the Figure of Insensitivity . TNT had a rating of exactly 100 on the "F of I" scale. The reference has since been changed to a more sensitive explosive called RDX , which has an F of I rating of 80. [ 26 ]
The energy density of TNT is used as a reference point for many other explosives, including nuclear weapons, as their energy content is measured in equivalent tonnes (metric tons, t) of TNT. The energy used by NIST to define the equivalent is 4.184 GJ /t or exactly 1 kcal /g. [ 27 ]
For safety assessments, it has been stated that the detonation of TNT, depending on circumstances, can release 2.673–6.702 GJ/t. [ 28 ]
The heat of combustion however is 14.5 GJ/t (14.5 MJ/kg or 4.027 kWh/kg), which requires that the carbon in TNT fully react with atmospheric oxygen, which does not occur in the initial event. [ 29 ]
For comparison, gunpowder contains 3 MJ/kg, dynamite contains 7.5 MJ/kg, and gasoline contains 47.2 MJ/kg (though gasoline requires an oxidant , so an optimized gasoline and O 2 mixture contains 10.4 MJ/kg). [ citation needed ]
Various methods can be used to detect TNT, including optical and electrochemical sensors and explosive-sniffing dogs. In 2013, researchers from the Indian Institutes of Technology using noble-metal quantum clusters could detect TNT at the sub- zeptomolar (10 −18 mol/m 3 ) level. [ 30 ]
TNT is poisonous, and skin contact can cause skin irritation, causing the skin to turn a bright yellow-orange color. During the First World War , female munition workers who handled the chemical found that their skin turned bright yellow, which resulted in their acquiring the nickname " canary girls " or simply "canaries". [ 31 ]
People exposed to TNT over a prolonged period tend to experience anemia and abnormal liver functions. Blood and liver effects, spleen enlargement and other harmful effects on the immune system have also been found in animals that ingested or breathed trinitrotoluene. There is evidence that TNT adversely affects male fertility . [ 32 ] TNT is listed as a possible human carcinogen , with carcinogenic effects demonstrated in animal experiments with rats, although effects upon humans so far amount to none (according to IRIS of March 15, 2000). [ 33 ] Consumption of TNT produces red urine through the presence of breakdown products and not blood as sometimes believed. [ 34 ]
Some military testing grounds are contaminated with wastewater from munitions programs, including contamination of surface and subsurface waters which may be colored pink because of the presence of TNT. Such contamination, called "pink water", may be difficult and expensive to remedy . [ citation needed ]
TNT is prone to exudation of dinitrotoluenes and other isomers of trinitrotoluene when projectiles containing TNT are stored at higher temperatures in warmer climates. Exudation of impurities leads to formation of pores and cracks (which in turn cause increased shock sensitivity). Migration of the exudated liquid into the fuze screw thread can form fire channels , increasing the risk of accidental detonation. Fuze malfunction can also result from the liquid migrating into the fuze mechanism. [ 35 ] Calcium silicate is mixed with TNT to mitigate the tendency towards exudation. [ 36 ]
Pink water and red water are two distinct types of wastewater related to trinitrotoluene. [ 37 ] Pink water is produced from equipment washing processes after munitions filling or demilitarization operations, [ 38 ] [ 39 ] and as such is generally saturated with the maximum amount of TNT that will dissolve in water (about 150 parts per million (ppm).) However it has an indefinite composition that depends on the exact process; in particular, it may also contain cyclotrimethylenetrinitramine (RDX) if the plant uses TNT/RDX mixtures, or HMX if TNT/HMX is used. Red water (also known as "Sellite water") is produced during the process used to purify the crude TNT. It has a complex composition containing more than a dozen aromatic compounds, but the principal components are inorganic salts ( sodium sulfate , sodium sulfite , sodium nitrite and sodium nitrate ) and sulfonated nitroaromatics . [ citation needed ]
Pink and red water are colorless at the time of generation; the color is produced by photolytic reactions under the influence of sunlight. Despite the names, red and pink water are not necessarily different shades; the color depends mainly on the duration of solar exposure. If exposed long enough, "pink" water may turn various shades of pink, red, rusty orange, or black. [ 39 ] [ 40 ]
Because of the toxicity of TNT, the discharge of pink water to the environment has been prohibited in the US and many other countries for decades, but ground contamination may exist in very old plants. However, RDX and tetryl contamination is usually considered more problematic, as TNT has very low soil mobility. Red water is significantly more toxic and as such it has always been considered hazardous waste. It has traditionally been disposed of by evaporation to dryness (as the toxic components are not volatile), followed by incineration. Much research has been conducted to develop better disposal processes. [ citation needed ]
Because of its suitability in construction and demolition, TNT has become the most widely used explosive and thus its toxicity is the most characterized and reported. Residual TNT from manufacture, storage, and use can pollute water, soil, the atmosphere , and the biosphere . [ 41 ]
The concentration of TNT in contaminated soil can reach 50 g/kg of soil, where the highest concentrations can be found on or near the surface. In September 2001, the United States Environmental Protection Agency (USEPA) declared TNT a pollutant whose removal is a priority. [ 42 ] The USEPA maintains that TNT levels in soil should not exceed 17.2 milligrams per kilogram of soil and 0.01 milligrams per litre of water. [ 43 ]
Dissolution is a measure of the rate that solid TNT in contact with water is dissolved. The relatively low aqueous solubility of TNT causes solid particles to be continuously released to the environment over extended periods of time. [ 44 ] Studies have shown that TNT dissolves more slowly in saline water than in freshwater. However, when salinity is altered, TNT dissolves at the same speed. [ 45 ] Because TNT is moderately soluble in water, it can migrate through subsurface soil, and cause groundwater contamination. [ 46 ]
Adsorption is a measure of the distribution between soluble and sediment adsorbed contaminants following attainment of equilibrium. TNT and its transformation products are known to adsorb to surface soils and sediments, where they undergo reactive transformation or remained stored. [ 47 ] The movement or organic contaminants through soils is a function of their ability to associate with the mobile phase (water) and a stationary phase (soil). Materials that associate strongly with soils move slowly through soil. The association constant for TNT with soil is 2.7 to 11 L/kg of soil. [ 48 ] This means that TNT has a one- to tenfold tendency to adhere to soil particulates than not when introduced into the soil. [ 44 ] Hydrogen bonding and ion exchange are two suggested mechanisms of adsorption between the nitro functional groups and soil colloids.
The number of functional groups on TNT influences the ability to adsorb into soil. Adsorption coefficient values have been shown to increase with an increase in the number of amino groups. Thus, adsorption of the TNT decomposition product 2,4-diamino-6-nitrotoluene (2,4-DANT) was greater than that for 4-amino-2,6-dinitrotoluene (4-ADNT), which was greater than that for TNT. [ 44 ] Lower adsorption coefficients for 2,6-DNT compared to 2,4-DNT can be attributed to the steric hindrance of the NO 2 group in the ortho position .
Research has shown that in freshwater environments, with high abundances of Ca 2+ , the adsorption of TNT and its transformation products to soils and sediments may be lower than observed in a saline environment, dominated by K + and Na + . Therefore, when considering the adsorption of TNT, the type of soil or sediment and the ionic composition and strength of the ground water are important factors. [ 49 ]
The association constants for TNT and its degradation products with clays have been determined. Clay minerals have a significant effect on the adsorption of energetic compounds. Soil properties, such as organic carbon content and cation exchange capacity have significant impacts on the adsorption coefficients.
Additional studies have shown that the mobility of TNT degradation products is likely to be lower "than TNT in subsurface environments where specific adsorption to clay minerals dominates the sorption process." [ 49 ] Thus, the mobility of TNT and its transformation products are dependent on the characteristics of the sorbent. [ 49 ] The mobility of TNT in groundwater and soil has been extrapolated from "sorption and desorption isotherm models determined with humic acids , in aquifer sediments, and soils". [ 49 ] From these models, it is predicted that TNT has a low retention and transports readily in the environment. [ 42 ]
Compared to other explosives, TNT has a higher association constant with soil, meaning it adheres more with soil than with water. Conversely, other explosives, such as RDX and HMX with low association constants (ranging from 0.06 to 7.3 L/kg and 0 to 1.6 L/kg respectively) can move more rapidly in water. [ 44 ]
TNT is a reactive molecule and is particularly prone to react with reduced components of sediments or photodegradation in the presence of sunlight. TNT is thermodynamically and kinetically capable of reacting with a wide number of components of many environmental systems. This includes wholly abiotic reactants, like hydrogen sulfide , Fe 2+ , or microbial communities, both oxic and anoxic and photochemical degradation. [ citation needed ]
Soils with high clay contents or small particle sizes and high total organic carbon content have been shown to promote TNT transformation. Possible TNT transformations include reduction of one, two, or three nitro-moieties to amines and coupling of amino transformation products to form dimers . Formation of the two monoamino transformation products, 2-ADNT and 4-ADNT, is energetically favored, and therefore is observed in contaminated soils and ground water. The diamino products are energetically less favorable, and even less likely are the triamino products. [ citation needed ]
The transformation of TNT is significantly enhanced under anaerobic conditions as well as under highly reducing conditions. TNT transformations in soils can occur both biologically and abiotically. [ 49 ]
Photolysis is a major process that impacts the transformation of energetic compounds. The alteration of a molecule in photolysis occurs by direct absorption of light energy or by the transfer of energy from a photosensitized compound. Phototransformation of TNT "results in the formation of nitrobenzenes , benzaldehydes , azodicarboxylic acids, and nitrophenols , as a result of the oxidation of methyl groups , reduction of nitro groups , and dimer formation." [ 44 ]
Evidence of the photolysis of TNT has been seen due to the color change to pink of TNT-containing wastewaters when exposed to sunlight. Photolysis is more rapid in river water than in distilled water. Ultimately, photolysis affects the fate of TNT primarily in the aquatic environment but could also affect the fate of TNT in soil when the soil surface is exposed to sunlight. [ 49 ]
The ligninolytic physiological phase and manganese peroxidase system of fungi can cause a very limited amount of mineralization of TNT in a liquid culture, though not in soil. An organism capable of the remediation of large amounts of TNT in soil has yet to be discovered. [ 50 ] Both wild and transgenic plants can phytoremediate explosives from soil and water. [ 51 ] | https://en.wikipedia.org/wiki/TNT |
TNT equivalent is a convention for expressing energy , typically used to describe the energy released in an explosion. A ton of TNT equivalent is a unit of energy defined by convention to be 4.184 gigajoules ( 1 gigacalorie ). [ 1 ] It is the approximate energy released in the detonation of a metric ton (1,000 kilograms) of trinitrotoluene (TNT). In other words, for each gram of TNT exploded, 4.184 kilojoules (or 4184 joules ) of energy are released.
This convention intends to compare the destructiveness of an event with that of conventional explosive materials , of which TNT is a typical example, although other conventional explosives such as dynamite contain more energy.
A related concept is the physical quantity TNT-equivalent mass (or mass of TNT equivalent ), [ 2 ] [ 3 ] [ 4 ] [ 5 ] expressed in the ordinary units of mass and its multiples: kilogram (kg), megagram (Mg) or tonne (t), etc.
The " kiloton (of TNT equivalent)" is a unit of energy equal to 4.184 terajoules ( 4.184 × 10 12 J ). [ 6 ] A kiloton of TNT can be visualized as a cube of TNT 8.46 metres (27.8 ft) on a side.
The " megaton (of TNT equivalent)" is a unit of energy equal to 4.184 petajoules ( 4.184 × 10 15 J ). [ 7 ]
The kiloton and megaton of TNT equivalent have traditionally been used to describe the energy output, and hence the destructive power, of a nuclear weapon . The TNT equivalent appears in various nuclear weapon control treaties , and has been used to characterize the energy released in asteroid impacts . [ 8 ]
Alternative values for TNT equivalency can be calculated according to which property is being compared and when in the two detonation processes the values are measured. [ 9 ] [ 10 ] [ 11 ] [ 12 ]
Where for example the comparison is by energy yield, an explosive's energy is normally expressed for chemical purposes as the thermodynamic work produced by its detonation. For TNT this has been accurately measured as 4,686 J/g from a large sample of air blast experiments, and theoretically calculated to be 4,853 J/g. [ 13 ]
However, even on this basis, comparing the actual energy yields of a large nuclear device and an explosion of TNT can be slightly inaccurate. Small TNT explosions, especially in the open, do not tend to burn the carbon-particle and hydrocarbon products of the explosion. Gas-expansion and pressure-change effects tend to "freeze" the burn rapidly. A large, open explosion of TNT may maintain fireball temperatures high enough that some of those products do burn up with atmospheric oxygen. [ 14 ]
Such differences can be substantial. For safety purposes, a range as wide as 2,673–6,702 J has been stated for a gram of TNT upon explosion. [ 15 ] Thus one can state that a nuclear bomb has a yield of 15 kt ( 6.3 × 10 13 J ), but the explosion of an actual 15,000-ton pile of TNT may yield (for example) 8 × 10 13 J due to additional carbon/hydrocarbon oxidation not present with small open-air charges. [ 14 ]
These complications have been sidestepped by convention. The energy released by one gram of TNT was arbitrarily defined as a matter of convention to be 4,184 J, [ 16 ] which is exactly one kilocalorie .
1 ton of TNT equivalent is approximately:
The relative effectiveness factor (RE factor) relates an explosive's demolition power to that of TNT, in units of the TNT equivalent/kg (TNTe/kg). The RE factor is the relative mass of TNT to which an explosive is equivalent: The greater the RE, the more powerful the explosive.
This enables engineers to determine the proper masses of different explosives when applying blasting formulas developed specifically for TNT. For example, if a timber-cutting formula calls for a charge of 1 kg of TNT, then based on octanitrocubane 's RE factor of 2.38, it would take only 1.0/2.38 (or 0.42) kg of it to do the same job. Using PETN , engineers would need 1.0/1.66 (or 0.60) kg to obtain the same effects as 1 kg of TNT. With ANFO or ammonium nitrate , they would require 1.0/0.74 (or 1.35) kg or 1.0/0.32 (or 3.125) kg, respectively.
Calculating a single RE factor for an explosive is, however, impossible. It depends on the specific case or use. Given a pair of explosives, one can produce 2× the shockwave output (this depends on the distance of measuring instruments) but the difference in direct metal cutting ability may be 4× higher for one type of metal and 7× higher for another type of metal. The relative differences between two explosives with shaped charges will be even greater. The table below should be taken as an example and not as a precise source of data. | https://en.wikipedia.org/wiki/TNT_equivalent |
TNet is a secure top-secret-level intranet system in the White House , notably used to record information about telephone and video calls between the President of the United States and other world leaders. [ 1 ] [ 2 ] TNet is connected to Joint Worldwide Intelligence Communications System (JWICS), which is used more widely across different offices in the White House. Contained within TNet is an even more secure system known as NSC Intelligence Collaboration Environment (NICE). [ 3 ]
The NSC Intelligence Collaboration Environment ( NICE ) is a computer system operated by the United States National Security Council 's Directorate for Intelligence Programs. A subdomain of TNet, it was created to enable staff to produce and store documents, such as presidential findings or decision memos, on top secret codeword activities. Due to the extreme sensitivity of the material held on it, only about 20 percent of NSC staff can reportedly access the system. The documents held on the system are tightly controlled and only specific named staff are able to access files. [ 4 ]
The system became the subject of controversy during the Trump–Ukraine scandal , when a whistleblower complaint to the Inspector General of the Intelligence Community revealed that NICE had been used to store transcripts of calls between President Donald Trump , and foreign leaders, apparently to restrict access to them. [ 5 ] The system was reportedly used for this purpose from 2017 after leaks of conversations with foreign leaders. It was said to have been upgraded in the spring of 2018 to log, who had accessed particular files, as a deterrent against possible leaks. [ 6 ]
This computer networking article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TNet |
The TOLIMAN ( Telescope for Orbit Locus Interferometric Monitoring of our Astronomical Neighbourhood ) space telescope is a low-cost mission concept aimed at detecting of exoplanets via the astrometry method, and specifically targeting the Alpha Centauri system. [ 1 ] TOLIMAN will focus on stars within 10 parsecs (32.6 light years) of the Sun. [ 2 ] The telescope is still under construction. [ 3 ] [ 4 ] The mission will involve scientists of the University of Sydney , Saber Astronautics in Australia, Breakthrough Initiatives , and NASA's Jet Propulsion Laboratory . [ 5 ]
TOLIMAN will explore all three components of the Alpha Centauri system in search of planets in the habitable zone. [ citation needed ]
TOLIMAN is planned to be launched in 2026. [ 6 ]
This telescope -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TOLIMAN |
Topoisomerase-based cloning (TOPO cloning) is a molecular biology technique in which DNA fragments are cloned into specific vectors without the requirement for DNA ligases . Taq polymerase has a nontemplate-dependent terminal transferase activity that adds a single deoxyadenosine (A) to the 3'-end of the PCR products. This characteristic is exploited in "sticky end" TOPO TA cloning. [ 1 ] For "blunt end" TOPO cloning, the recipient vector does not have overhangs and blunt-ended DNA fragments can be cloned.
The technique utilizes the inherent biological activity of DNA topoisomerase I. The biological role of topoisomerase is to cleave and rejoin supercoiled DNA ends to facilitate replication. Vaccinia virus topoisomerase I specifically recognize DNA sequence 5´-(C/T)CCTT-3'. During replication , the enzyme digests DNA specifically at this sequence, unwinds the DNA and, re-ligates it again at the 3' phosphate group of the thymidine base. [ 1 ]
The vectors in commercially available TOPO kits have been added to the topoisomerase site embedded in a beta-galactosidase cassette allowing blue-white scanning. The vector ends thus self-assemble, resulting in the production of blue colonies that do not need to be selected and sequenced for potential positive clones.
TOPO vectors are designed in such a way that they carry this specific sequence 5´-(C/T)CCTT-3' at the two linear ends. The linear vector DNA already has the topoisomerase enzyme covalently attached to both of its strands' free 3' ends. This is then mixed with PCR products. When the free 5' ends of the PCR product strands attach to the topoisomerase 3' end of each vector strand, the strands are covalently linked by the already bound topoisomerase. This reaction proceeds efficiently when this solution is incubated at room temperature with the required salt. Different types of vectors are used for cloning fragments amplified by either Taq or Pfu polymerase as Taq polymerase (unlike Pfu) leaves an extra "A" nucleotide at the 3'end during amplification. [ 1 ]
The TA TOPO cloning technique relies on the ability of adenine (A) and thymine (T) (complementary base pairs) on different DNA fragments to hybridize and, in the presence of ligase or topoisomerase, become ligated together.
The insert is created by PCR using Taq DNA polymerase, a polymerase that lacks 3' to 5' proofreading activity and with a high probability adds a single, 3'-adenine overhang to each end of the PCR product. It is best if the PCR primers have guanines at the 5' end as this maximizes probability of Taq DNA polymerase adding the terminal adenosine overhang. Thermostable polymerases containing extensive 3´ to 5´ exonuclease activity should not be used as they do not leave the 3´ adenine-overhangs.
The target vector is linearized and cut with a blunt-end restriction enzyme. This vector is then tailed with dideoxythymidine triphosphate (ddTTP) using terminal transferase. It is important to use ddTTP to ensure the addition of only one T residue. This tailing leaves the vector with a single 3'-overhanging thymine residue on each blunt end. [ 1 ]
Polymerases (such as Phusion) or restriction enzymes that produce blunt ends can also be used for TOPO cloning. Rather than relying on sticky ends, the blunt end TOPO vector has blunt ends where the topoisomerase molecules are bound. Commercial kits, such as the Zero Blunt® Cloning Kit from Invitrogen, are available. [ 2 ] | https://en.wikipedia.org/wiki/TOPO_cloning |
Total Operations Processing System ( TOPS ) is a computer system for managing railway locomotives and rolling stock , known for many years of use in the United Kingdom .
TOPS was originally developed between the Southern Pacific Railroad (SP), Stanford University and IBM as a replacement for paper-based systems for managing rail logistics . A jointly-owned consultancy company, TOPS On-Line Inc. , was established in 1960 with the goal of implementing TOPS, as well as selling it to third parties. Development was protracted, requiring around 660 man-years of effort to produce a releasable build. During mid-1968, the first phase of the system was introduced on the SP, and quickly proved its advantages over the traditional methods practiced prior to its availability. [ 1 ]
In addition to SP, TOPS was widely adopted throughout North America and beyond. While it was at one point in widespread use across many of the United States railroads, the system has been perhaps most prominently used in the United Kingdom . During 1971, the country's nationalised rail operation, British Rail (BR), opted to procure and integrate TOPS into its operations. The acquisition of an existing system rather than develop an indigenous programme was reasoned to be both cheaper and quicker to implement; it was noted, however, that TOPS was not capable of performing all desired functions. Since its implementation during the mid 1970s, both BR and its successors have continued to operate the system. SP itself has developed a newer system called the Terminal Information Processing System (TIPS), which replaced TOPS entirely during 1980. [ 1 ]
During the 1950s and 1960s, it was increasingly recognised that the adoption of computer -based management systems could provide substantial benefits in various operations, particularly those involving logistics . [ 2 ] Consequently, by the 1960s, various railways in various countries, including Japan , Canada , and the United States had begun to develop and introduce such systems. Amongst the organisations that adopted the technology early on was the Southern Pacific Railroad (SP). [ 2 ]
During the late 1950s, SP entered into discussions with the American technology company IBM about implementing its technology for rail management purposes. [ 2 ] IBM repurposed much of their work on the US Air Force's SAGE project, designed to direct interceptor aircraft against approaching Soviet nuclear bombers, to instead serve the needs of the Southern Pacific. [ 3 ] The project gained the name Total Operations Processing System , or TOPS , and its development was handled by a specially established consultancy company, TOPS On-Line Inc. , which was 80 percent owned by SP with the remaining stake held by IBM. [ 2 ]
TOPS was to take all the paperwork associated with a locomotive or rolling stock - its maintenance history, its allocation to division and depot and duty, its status, its location, and much more - and keep it in computer form, constantly updated by terminals at every maintenance facility. [ 4 ] [ 2 ] On paper, this information was difficult to keep track of, awkward to keep up to date, and time-consuming to query, requiring many telephone calls. Computerizing this information enabled a railroad to keep better track of its assets, and thus to make better use of them. [ 4 ]
TOPS was a relatively complex system for the era, being not only comprehensive but required to operate in real time. [ 2 ] Accordingly, development was particularly time-consuming; according to BR Chief Operations Manager Robert Arnott, the first phase of TOPS involved around 660 man-years of effort, with eight years passing between the start of work and it being declared operational during mid-1968. Despite the lengthy development time, TOPS quickly proved to be a success for SP; clerks often observed that jobs which had taken half a day and dozens of telephone calls could instead be completed in under five minutes using TOPS. [ 2 ]
The success of TOPS with SP soon led to a quick succession of sales of the system to a variety of other American railroads, along with international customers, where it typically proved to be similarly beneficial. [ 2 ] Selling TOPS to other operators helped offset the systems' development costs, so SP was keen to sell TOPS to third parties. The company was also motivated to protect its reputation, and thus provided assistance to other railroads interested in TOPS, to improve its chances of success. [ 2 ] Some operators, such as the Canadian National Railway , opted to introduce TOPS as an interim measure while its own bespoke system was developed as a long-term successor. [ 2 ]
During the late 1960s, British Rail (BR) was looking for ways to increase efficiency, particularly of its declining freight operations, and identified a computer-based system as a key tool for improving both planning and control. [ 2 ] The specific requirements included the more effective utilisation of freight rolling stock, better pre-planning of terminal and marshalling yard operations, better alignment of specific consignments to specific services, prompt response to customer location-related requests. BR planners realised early on that it would be quicker and cheaper to buy an existing system, rather than develop one locally, even if that breached the British Government 's requirement for nationalised industries to ' Buy British '.
Various systems around the world were explored, such as France's Gestion Centralisée du Trafic Marchandises (GCTM) and Canada's Traffic Reporting and Control System (TRACS), but found this to be ill-suited to BR's requirements; in fact, no existing system in the world satisfied them in full. [ 2 ] However, SP's TOPS system met many - but not all - of the outstanding requirements. Starting in June 1970, several delegations from SP came to the UK to discuss and evaluate BR's existing practices, along with corresponding visits to the USA to witness SP's operations, before both sides concluded that TOPS was a viable option. [ 2 ]
Groundwork on TOPS' financial case had commenced in the summer of 1970, during which a four-year timescale for implementation had emerged as the preferred option. [ 2 ] From an analysis performed in 1971, it was found that, even in the event of the most pessimistic assumptions being true, TOPS' introduction retained a healthy gain in net value of £34m per annum. Suitably convinced of its benefits, BR's board opted to purchase the system, along with the source code (as was typical in those days for such a large mainframe-based system) during June 1971. [ 2 ] Due to its foreign origins, the purchase of an IBM System/360 mainframe to operate TOPS had to be approved by the Heath cabinet , which was given in October 1971. The decision was justified by a belief that TOPS would enable BR's freight operations to become profitable. [ 2 ]
The cost of BR's TOPS implementation included £5.6m of capital costs, development costs of £5.7m, and equipment rental costs of £22.5m between 1972 and 1980. [ 2 ] Aside from the computers themselves, and suitably trained staff to operate them, perhaps the most technically challenging aspect impacting implementation was telecommunications , necessary to bring the system's geographically disparate elements together. [ 2 ] The implementation phase was greatly assisted by data processing experts provided by SP. It was at the urging of SP's specialists that BR omitted the originally-sought volume acceptance feature, as it was considered to be impractical. [ 2 ]
The operational headquarters for TOPS was established in an existing railway office block in close proximity to BR's corporate headquarters. [ 2 ] The building had to be extensively refitted for the scheme, the top floor being turned into an open-plan office for housing planning and development work, while the computing equipment and telecommunications gear was accommodated across two separate floors below; the latter requiring a controlled climate for reliable operation. According to Amott, the implementation of TOPS was undertaken without any significant adverse reaction in terms of industrial relations or senior management. [ 2 ]
The adoption of the TOPS system during the early 1970s led to several changes in working practices across Britain's railway network. Hitherto, locomotives had been numbered in three different series. Steam locomotives carried unadorned numbers up to five digits long. Diesel locomotives carried one to four-digit numbers prefixed with a letter 'D', and electric locomotives with a letter 'E'. Thus, up to three locomotives could carry the same number. TOPS could not handle that, and it also required similar locomotives to be numbered in a consecutive series in terms of classification, so that they might be treated as a group.
Sequentiality was all that was required but, given the need to renumber, it was decided to adopt a logical system for classification, and the five- or six-digit TOPS number was divided into two parts. No class of locomotive or multiple unit numbered over 1,000 examples, so the last three digits were used for the individual number between 001 and 999 in that class, although Class 43 goes down to 000, that being the number of the only remaining HST prototype power car. The first two or three digits were used to denote the class of locomotive or multiple unit. The numbers were often written in two space separated groups, such as "47 401" to highlight that division, but the TOPS system actually stored and displayed them without the space: "47401". Sub-classifications were indicated in the TOPS system with a slash and a subclass number, e.g. "47/4". It was convention, though not enforced within the TOPS system, that subclass numbers were boundaries in the locomotive numbering system, such that class "47/4" started with number "47 401". If there were more than 99 numbers in a subclass, the number series extended to the next value of the third digit; thus, since there were more than 200 locomotives in class "47/4", subclasses "47/5" and "47/6" did not exist, and the next valid subclass by convention was "47/7" starting with "47 701". However, in some cases, the sequences do not match, e.g. 158/ 0 numbers start at 158 7 01.
Locomotives are assigned classes 01–98: diesel locomotives 01–79 (originally 01–69), AC electric locomotives 80–96, departmental locos (those not in revenue-earning use) 97, and steam locomotives 98. DC electric locomotives were originally allocated classes 70–79 but this was modified in 2011 (see British Rail locomotive and multiple unit numbering and classification ); the sole relic of this is Class 73 which continues unrenumbered, probably because it can be considered equally a diesel locomotive as it is a DC electric. One oddity was the inclusion of British Rail's shipping fleet in the system as Class 99. Diesel multiple units (DMUs) with mechanical or hydraulic transmission are classified 100–199, with electric transmission 200–299. Electric multiple units (EMUs) are given the subsequent classes; 300–399 are overhead AC units (including AC/DC dual-voltage units), while Southern Region DC third rail EMUs are 400–499, other DC EMUs 500–599. More recently, new electric multiple units and bi-mode multiple units have been given the 700 series and new high-speed units have been given the 800 series. Selected numbers in the 900 series have been used for departmental multiple units, mostly converted from former passenger units.
Coaching stock and individual multiple unit cars are allocated five-digit numbers; since the early 1980s, it has been forbidden for them to have the same numbers as locomotives, but before then duplication was possible because they carried a prefix letter, which was considered part of the number. More recent EMU deliveries have six-figure coach numbers.
TOPS has become outdated in recent decades [ when? ] . It is a text-terminal , mainframe-driven system; which is regarded as not very user-friendly, and hard to use compared with contemporary computer user-interfaces. In addition, it is written in its own programming language, TOPSTRAN (not strictly speaking a separate language but a set of IBM Assembler macros), and it is increasingly hard to find and train developers to maintain it. The division of British Rail and privatisation has also hurt TOPS, because it was not designed for that purpose; some freight operating companies do not keep information as up to date as they should. [ citation needed ]
Attempts have been made to 'skin' the system with a more user-friendly interface, called TOPS 2000; in addition, there are other parallel systems now, such as TRUST , Genius and the Mobile Consisting Application (since 2024 marketed as part of the Velociti Solutions RailSmart software suite ), [ 5 ] but none has yet fully supplanted the TOPS system.
This is a typical report that a TOPS clerk could generate. The train in question is a 25 wagon freight train travelling from Over & Wharton , near Winsford , to Reading West Junction, Berkshire . [ 6 ] | https://en.wikipedia.org/wiki/TOPS |
The Technique for Order of Preference by Similarity to Ideal Solution ( TOPSIS ) is a multi-criteria decision analysis method, which was originally developed by Ching-Lai Hwang and Yoon in 1981 [ 1 ] with further developments by Yoon in 1987, [ 2 ] and Hwang, Lai and Liu in 1993. [ 3 ] TOPSIS is based on the concept that the chosen alternative should have the shortest geometric distance from the positive ideal solution (PIS) and the longest geometric distance from the negative ideal solution (NIS). [ citation needed ] A dedicated book in the fuzzy context was published in 2021 [ 4 ]
It is a method of compensatory aggregation that compares a set of alternatives, normalising scores for each criterion and calculating the geometric distance between each alternative and the ideal alternative, which is the best score in each criterion. The weights of the criteria in TOPSIS method can be calculated using Ordinal Priority Approach , Analytic hierarchy process , etc. An assumption of TOPSIS is that the criteria are monotonically increasing or decreasing. Normalisation is usually required as the parameters or criteria are often of incongruous dimensions in multi-criteria problems. [ 5 ] [ 6 ] Compensatory methods such as TOPSIS allow trade-offs between criteria, where a poor result in one criterion can be negated by a good result in another criterion. This provides a more realistic form of modelling than non-compensatory methods, which include or exclude alternative solutions based on hard cut-offs. [ 7 ] An example of application on nuclear power plants is provided in. [ 8 ]
The TOPSIS process is carried out as follows:
Two methods of normalisation that have been used to deal with incongruous criteria dimensions are linear normalisation and vector normalisation.
Linear normalisation can be calculated as in Step 2 of the TOPSIS process above. Vector normalisation was incorporated with the original development of the TOPSIS method, [ 1 ] and is calculated using the following formula:
In using vector normalisation, the non-linear distances between single dimension scores and ratios should produce smoother trade-offs. [ 9 ] | https://en.wikipedia.org/wiki/TOPSIS |
The TOP Assay (Total Oxidizable Precursor Assay) is a laboratory method developed in 2012 [ 1 ] that oxidatively converts (unknown) precursor compounds of perfluorocarboxylic acids (PFCAs) into the latter. This makes quantification possible. Potassium peroxodisulfate is used. This sum parameter can be used to determine the concentration of precursor compounds present by comparing the sample before and after the application of the TOP Assay. [ 2 ]
This method is used, for example, in the analysis of fire-fighting foams ( aqueous film forming foam ), textiles or water samples. [ 3 ] [ 4 ] Blood serum can also be analyzed in this way. [ 5 ]
Neutral, anionic and cationic precursor compounds may be distinguished using an extension of the method published in 2025. [ 6 ]
In addition to fluorotelomer compounds, hydrogen-substituted perfluorosulfonic acids (H n -PFSAs), for example, can also be oxidized using the TOP Assay. Saturated and unsaturated perfluorosulfonic acids as well as perfluoroalkyl ether sulfonic acids, on the other hand, are stable. [ 7 ] | https://en.wikipedia.org/wiki/TOP_Assay |
The TOSCO II process is an above ground retorting technology for shale oil extraction , which uses fine particles of oil shale that are heated in a rotating kiln . The particularity of this process is that it use hot ceramic balls for the heat transfer between the retort and a heater. The process was tested in a 40 tonnes per hour test facility near Parachute, Colorado .
TOSCO II process is a refinement of the Swedish Aspeco process. [ 1 ] The Tosco Corporation purchased its patent rights in 1952. [ 2 ] In 1956, the Denver Research Institute performed research and development of this technology, including testing of a 24 ton per day pilot plant, which operated until 1966. Later the technology development was continued under Tosco's own directions. [ 3 ] In 1964 Tosco, Standard Oil of Ohio , and Cleveland Cliffs Iron Company formed Colony Development, a joint venture company to develop the Colony Shale Oil Project and to commercialize the TOSCO II technology. The project was ended in April 1972. [ 2 ]
The TOSCO II process is classified as a hot recycled solids technology. [ 4 ] It employs a horizontal rotating kiln -type retort. In this process, oil shale is crushed smaller than 0.5 inches (13 mm) and enters the system through pneumatic lift pipes in which oil shale is elevated by hot gas streams and preheated to about 500 °F (260 °C ). [ 5 ] After entering into retort, oil shale is mixed with hot ceramic balls with temperature from 1,200 °F (650 °C ) to 1,600 °F (870 °C ). This increases the oil shale temperature to between 900 °F (480 °C ) and 1,100 °F (590 °C ), in which pyrolysis occurs. In the pyrolysis process, kerogen decomposes to oil shale gas and oil vapors, while the remainder of the oil shale forms spent shale . Vapors are transferred to a condensor ( fractionator ) for separation into various fractions . [ 2 ] At the kiln passage, the spent shale and the ceramic balls are separated in a perforated rotating separation drum ( trommel ). [ 5 ] The crushed spent shale falls through holes in the trommel, while ceramic balls are transferred to the ball heater. [ 6 ] Combustible shale gas is burned in the ball heater to reheat the ceramic balls. [ 2 ]
The overall thermal efficiency of TOSCO II process is low because the energy of spent shale is not recovered and much of the produced shale gas is consumed by the process itself. The efficiency could be increased by burning char (carbonaceous residue in the spent shale) instead of shale gas as a fuel of the ball heater. The process' other disadvantages are mechanical complexity and large number of moving parts. Also the lifetime of ceramic balls is limited. Disposal of spent shale includes environmental problems because it is very finely crushed and contains carbon residue. [ 2 ] | https://en.wikipedia.org/wiki/TOSCO_II_process |
TOXMAP was a geographic information system (GIS) from the United States National Library of Medicine (NLM) that was deprecated on December 16, 2019. The application used maps of the United States to help users explore data from the United States Environmental Protection Agency 's (EPA) Toxics Release Inventory (TRI) and Superfund programs with visual projections and maps. [ 1 ] [ 2 ]
TOXMAP helped users create nationwide, regional, or local area maps showing where TRI chemicals are released on-site into the air, water, ground, and by underground injection, as reported by industrial facilities in the United States. [ 3 ] It also identified the releasing facilities, color-codes release amounts for a single year or year range, and provides multi-year aggregate chemical release data and trends over time, starting with 1988. Maps also can show locations of Superfund sites on the Agency for Toxic Substances and Disease Registry National Priorities List (NPL), which lists all chemical contaminants present at these sites. TOXMAP is a useful environmental health tool that makes epidemiological and environmental information available to the public.
There were two versions of TOXMAP available from its home page: the classic version of TOXMAP released in 2004 and, a newer version released in 2014 that is based on Adobe Flash / Apache Flex technology. In addition to many of the features of TOXMAP classic, the new version provides an improved map appearance and interactive capabilities as well as a more current GIS look-and-feel. This included seamless panning, immediate update of search results when zooming to a location, two collapsible side panels to maximize map size, and automatic size adjustment after a window resize. The new TOXMAP also improved U.S. Census layers and availability by Census Tract (2000 and 2010), Canadian National Pollutant Release Inventory (NPRI) data, U.S. commercial nuclear power plants , as well as improved and updated congressional district boundaries.
TOXMAP classic users may search the system by location (such as city, state, or ZIP code), chemical name, chemical name fragment, release medium, release amount, facility name and ID, and can filter results to those residing within a pre-defined or custom geographic region.
Search results may be brought up in Google Maps or Google Earth , or saved for use in other tools. TOXMAP also overlays map data such as U.S. Census population information, income figures from the Bureau of Economic Analysis, and health data from the National Cancer Institute and the National Center for Health Statistics . [ 4 ]
The data shown in TOXMAP comes from the following sources: [ 5 ]
The database was pulled from the internet by the Trump administration in December 2019. The NLM said in a statement that much of the information remained available from the original sources, and that thus the database could be removed; critics, such as the Environmental Data & Governance Initiative, [ 6 ] suggested it was part of a larger effort on the part of the administration to obfuscate the detrimental results of the rollback of Obama-era environmental regulations . [ 7 ]
The data underlying TOXMAP remains accessible through their original resources: Government of Canada National Pollutant Release Inventory (NPRI), [ 8 ] U.S. Census Bureau , U.S. EPA Clean Air Markets Program, [ 9 ] U.S. EPA Geospatial Applications, [ 10 ] U.S. EPA Facilities Registry System (FRS), [ 11 ] U.S. EPA Superfund Program, [ 12 ] U.S. EPA Toxics Release Program (TRI), [ 13 ] U.S. NIH NCI Surveillance, Epidemiology, and End Results Program (SEER), [ 14 ] U.S. Nuclear Regulatory Commission (NRC). [ 15 ] [ 16 ] | https://en.wikipedia.org/wiki/TOXMAP |
TP-Link is a Chinese company that manufactures network equipment and smart home products. The company was established in 1996 in Shenzhen . TP-Link's main headquarters is located in Nanshan, Shenzhen ; [ 2 ] there is a smaller headquarters in Irvine, California . [ 3 ] It has subsidiaries operating globally and owns several brands, including Deco, Tapo, Omada, Omada Pro, VIGI, Aginet, Kasa Smart, and Mercusys. [ 4 ] [ 5 ] [ 6 ] The company has been investigated by the governments of India and the United States for national security risks. [ 7 ] [ 8 ]
TP-Link was founded in 1996 by two brothers, Zhao Jianjun ( 赵建军 Zhào Jiànjūn ) and Zhao Jiaxing ( 赵佳兴 Zhào Jiāxīng ). [ 9 ] Zhao Jianjun runs the company's California operations. [ 7 ] The company name was based on the concept of " twisted pair link" invented by Alexander Graham Bell , a kind of cabling that reduces electromagnetic interference. [ 10 ]
TP-Link began its first international expansion in 2005. In 2007, the company moved into a new 100,000-square-meter headquarters and facilities at Shenzhen's Hi-Tech Industry Park. TP-Link USA was established in 2008. [ 11 ] In 2022, TP-Link Corporation began to separate from TP-LINK Technologies Co., Ltd. (TP-LINK) in China. After the separation, TP-Link Corporation became a standalone entity in all shareholdings and all operational aspects, such as workforce, research and development, production, marketing, and customer service. [ 12 ] [ non-primary source needed ]
In a 2023 patent dispute lawsuit, a U.S. federal judge rejected the company's argument that there was no link between its U.S. and China businesses. [ 7 ]
In May 2024, the government of India issued a warning saying that TP-Link routers present a security risk. [ 13 ] [ 7 ] [ 14 ]
In May 2024, TP-Link announced the completion of corporate restructuring, with secondary headquarters in the United States and Singapore. [ 15 ]
In August 2024, the United States House Select Committee on Strategic Competition between the United States and the Chinese Communist Party asked the United States Department of Commerce to investigate TP-Link and its affiliates for potential national security risks. [ 16 ] [ 17 ] [ 18 ] The U.S. Departments of Justice (DOJ), Commerce, and Defense opened investigations into the company, [ 8 ] with the DOJ probing whether TP-Link sells its routers below cost. [ 7 ] A spokesperson from TP-Link's United States subsidiary responded via The Wall Street Journal that they are welcome to engage with the government of the United States to demonstrate the security of their products, and the commitment to address national security risks. [ 19 ] TP-Link created a dual headquarters structure in the U.S. and China in response to increased scrutiny. [ 20 ] The majority of its employees, including those in research and development, are located in China. [ 20 ]
TP-Link products include high speed cable modems , mobile phones , ADSL , range extenders , routers , switches , IP cameras , power-line adapters , print servers , media converters , wireless adapters , power banks , USB hubs , smart home devices, and home robots . TP-Link also manufactured the OnHub router for Google . [ 21 ] TP-Link manufactures smart home devices under their Kasa Smart and Tapo product lines. [ 22 ] [ 23 ] The company also participates in several research and development projects with the Chinese government . [ 7 ]
TP-Link manufactures its products in-house, as opposed to outsourcing to original design manufacturers (ODMs). [ 24 ] | https://en.wikipedia.org/wiki/TP-Link |
4CNC , 4CNM
7162
21983
ENSG00000146242 ENSG00000283085
ENSMUSG00000035274
Q13641
Q9Z0L0
NM_001166392 NM_006670 NM_001376922
NM_001164792 NM_011627
NP_001159864 NP_006661 NP_001363851
NP_001158264 NP_035757
Trophoblast glycoprotein , also known as TPBG , 5T4 , Wnt-Activated Inhibitory Factor 1 or WAIF1 , is a human protein encoded by a TPBG gene . [ 5 ] TPBG is an antagonist of Wnt/β-catenin signalling pathway . [ 6 ]
5T4 is an antigen expressed in a number of carcinomas . [ 7 ] It is an N-glycosylated transmembrane 72 kDa glycoprotein containing eight leucine-rich repeats . [ 6 ] 5T4 is often referred to as an oncofetal antigen due to its expression in foetal trophoblast (where it was first discovered) or trophoblast glycoprotein (TPBG).
5T4 is found in tumors including the colorectal , ovarian , and gastric . Its expression is used as a prognostic aid in these cases. It has very limited expression in normal tissue but is widespread in malignant tumours throughout their development. One study found that 5T4 was present in 85% of a cohort of 72 colorectal carcinomas and in 81% of a cohort of 27 gastric carcinomas. [ 8 ]
Its confined expression appears to give 5T4 the potential to be a target for T cells in cancer immunotherapy . There has been extensive research into its role in antibody -directed immunotherapy through the use of the high-affinity murine monoclonal antibody , mAb5T4, to deliver response modifiers (such as staphylococcus aureus superantigen ) accurately to a tumor.
5T4 is also the target of the cancer vaccine TroVax which is in clinical trials for the treatment of a range of different solid tumour types.
TPBG has been shown to interact with GIPC1 . [ 9 ]
This article on a gene on human chromosome 6 is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TPBG |
TRACE is a high-precision orbit determination and orbit propagation program. It was developed by The Aerospace Corporation in El Segundo, California . An early version ran on the IBM 7090 computer in 1964. [ 1 ] The Fortran source code can be compiled for any platform with a Fortran compiler .
When Satellite Tool Kit 's high-precision orbit propagator and parameter and coordinate frame transformations underwent an Independent Verification and Validation effort in 2000, TRACE v2.4.9 was the standard against which STK was compared. [ 2 ]
As of 2013, TRACE is still used by the U.S. Government and some of its technical contractors.
This article about physics software is a stub . You can help Wikipedia by expanding it .
This astrophysics -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TRACE_(computer_program) |
1D0G , 1D2Q , 1D4V , 1DG6 , 1DU3 , 4N90
8743
22035
ENSG00000121858
ENSMUSG00000039304
P50591
P50592
NM_001190942 NM_001190943 NM_003810
NM_009425
NP_001177871 NP_001177872 NP_003801
NP_033451
In the field of cell biology , TNF-related apoptosis-inducing ligand (TRAIL) , is a protein functioning as a ligand that induces the process of cell death called apoptosis . [ 5 ] [ 6 ]
TRAIL is a cytokine that is produced and secreted by most normal tissue cells. It causes apoptosis primarily in tumor cells, [ 7 ] by binding to certain death receptors . TRAIL and its receptors have been used as the targets of several anti-cancer therapeutics since the mid-1990s, such as Mapatumumab . However, as of 2013, these have not shown significant survival benefit. [ 8 ] TRAIL has also been implicated as a pathogenic or protective factor in various pulmonary diseases, particularly pulmonary arterial hypertension . [ 9 ]
TRAIL has also been designated CD253 ( cluster of differentiation 253) and TNFSF10 ( tumor necrosis factor (ligand) superfamily, member 10). [ 7 ]
In humans, the gene that encodes TRAIL is located at chromosome 3q26 , which is not close to other TNF family members. [ 5 ] The genomic structure of the TRAIL gene spans approximately 20 kb and is composed of five exonic segments 222, 138, 42, 106, and 1245 nucleotides and four introns of approximately 8.2, 3.2, 2.3 and 2.3 kb.
The TRAIL gene lacks TATA and CAAT boxes and the promoter region contains putative response elements for transcription factors GATA , AP-1, C/EBP, SP-1, OCT-1 , AP3, PEA3, CF-1, and ISRE. [ citation needed ]
TIC10 (which causes expression of TRAIL) was investigated in mice with various tumour types. [ 8 ]
Small molecule ONC201 causes expression of TRAIL which kills some cancer cells. [ 10 ]
TRAIL shows homology to other members of the tumor necrosis factor superfamily. It is composed of 281 amino acids and has characteristics of a type II transmembrane protein . The N-terminal cytoplasmic domain is not conserved across family members, however, the C-terminal extracellular domain is conserved and can be proteolytically cleaved from the cell surface. TRAIL forms a homotrimer that binds three receptor molecules.
TRAIL binds to the death receptors DR4 (TRAIL-RI) and DR5 (TRAIL-RII). The process of apoptosis is caspase-8 -dependent. Caspase-8 activates downstream effector caspases including procaspase-3, -6, and -7, leading to activation of specific kinases. [ 11 ] TRAIL also binds the receptors DcR1 and DcR2, which do not contain a cytoplasmic domain (DcR1) or contain a truncated death domain (DcR2). DcR1 functions as a TRAIL-neutralizing decoy-receptor. The cytoplasmic domain of DcR2 is functional and activates NFkappaB .
In cells expressing DcR2, TRAIL binding therefore activates NFkappaB , leading to transcription of genes known to antagonize the death signaling pathway and/or to promote inflammation. Application of engineered ligands that have variable affinity for different death (DR4 and DR5) and decoy receptors (DCR1 and DCR2) may allow selective targeting of cancer cells by controlling activation of Type 1/Type 2 pathways of cell death and single cell fluctuations. Luminescent iridium complex-peptide hybrids, which mimic TRAIL, have recently been synthesized in vitro . These artificial TRAIL mimics bind to DR4/DR5 on cancer cells and induce cell death via both apoptosis and necrosis, which makes them a potential candidate for anticancer drug development. [ 12 ] [ 13 ]
In clinical trials only a small proportion of cancer patients responded to various drugs that targeted TRAIL death receptors. Many cancer cell lines develop resistance to TRAIL and limits the efficacy of TRAIL-based therapies. [ 14 ]
TRAIL has been shown to interact with TNFRSF10B . [ 15 ] [ 16 ] [ 17 ] | https://en.wikipedia.org/wiki/TRAIL |
A Transcoder and Rate Adaptation Unit ( TRAU ) performs transcoding function for speech channels and RA (Rate Adaptation) [ 1 ] [ 2 ] [ 3 ] for data channels in the GSM network . The Transcoder/Rate Adaptation Unit (TRAU) is the data rate conversion unit. The PSTN/ISDN switch is a switch for 64 kbit/s voice. Current technology permits to decrease the bit-rate (in GSM radio interface it is 16 kbit/s for full rate and 8 kbit/s for half rate). Since MSC is basically a PSTN/ISDN switch its bit-rate is still 64 kbit/s. That is why a rate conversion is required in between the BSC and MSC.
Transcoding is the compression of speech data from 64 kbit/s to 13/12.2/6.5 kbit/s in case FR/EFR/HR (respectively) speech coding.
Rate adaptation without transcoding allows Tandem Free Operation (TFO), allowing the original encoded speech data to be carried in a 64 kbit/s channel. TFO offers benefits because transcoding can lead to a degradation of speech quality and requires computational resources.
TRAU was also the term used for the frame format used in transport of the compressed bits from these speech coders.
For an MS-to-MS call, the transmission path covers the radio access network (RAN) as
well as the core network (CN). Since the transmission modes and coding standards are
different for RAN and CN, speech data is converted/transcoded at the transition points
from RAN to CN. This conversion is performed in the TRAU network element which
connects RAN and CN.
16 kbit/s for FR (Full Rate), Redundancy (Channel Coding)= 9.8 kbit/s => Gross data rate after adding redundancy = 22.8 kbit/s => 12.2 kbit/s for EFR (Enhanced Full rate) => Gross data rate after adding redundancy = 11.4 kbit/s.
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TRAU |
TRENDnet is a global manufacturer of computer networking products headquartered in Torrance, California , in the United States . It sells networking and surveillance products especially in the small to medium business (SMB) and home user market segments. [ 1 ]
The company was founded in 1990 by Pei Huang and Peggy Huang. [ 2 ]
In September 2013, the Federal Trade Commission (FTC) brought an enforcement action against TRENDnet alleging that the company marketed its SecurView IP cameras describing them as "secure", when in fact the software allowed online viewing by anyone with the camera's IP address. [ 3 ] The FTC approved a final settlement with TRENDnet in February 2014. [ 4 ] | https://en.wikipedia.org/wiki/TRENDnet |
TRESOR ( recursive acronym for "TRESOR Runs Encryption Securely Outside RAM", and also the German word for a safe ) is a Linux kernel patch which provides encryption using only the CPU to defend against cold boot attacks on computer systems by performing encryption inside CPU registers rather than random-access memory (RAM). It is one of two proposed solutions for general-purpose computers. The other, called "frozen cache" uses the CPU cache instead. [ 1 ] It was developed from its predecessor AESSE , presented at EuroSec 2010 and presented at USENIX Security 2011. [ 2 ] The authors state that it allows RAM to be treated as untrusted from a security viewpoint without hindering the system.
In computer security , a common problem for data security is how an intruder can access encrypted data on a computer. Modern encryption algorithms, correctly implemented and with strong passwords , are often unbreakable with current technology, so emphasis has moved to techniques that bypass this requirement, by exploiting aspects of data security where the encryption can be "broken" with much less effort, or else bypassed completely.
A cold boot attack is one such means by which an intruder can defeat encryption despite system security, if they can gain physical access to the running machine. It is premised on the physical properties of the circuitry within memory devices that are commonly used in computers. The concept is that when a computer system has encrypted data open, the encryption keys themselves used to read or write that data are usually stored on a temporary basis in physical memory, in a plain readable form. (Holding these keys in "plain" form during use is hard or impossible to avoid with usual systems since the system itself must be able to access the data when instructed by the authorized user). Usually this is no benefit to an unauthorised intruder, because they cannot access or use those keys—for example due to security built into the software or system. However, if the memory devices can be accessed outside the running system without loss of contents, for example by quickly restarting the computer or removing the devices to a different device, then the current contents—including any encryption keys in use—can be plainly read and used. This can be important if the system cannot be used to view, copy or access that data—for example the system is locked, or may have booby traps or other intrusion controls, or is needed in a guaranteed untouched form for forensic or evidentiary purposes.
Since this is a physical property of the hardware itself, and based on physical properties of memory devices, it cannot be defeated easily by pure software techniques, since all software running in memory at the point of intervention becomes accessible. As a result, any encryption software whose keys could be accessed this way is vulnerable to such attacks. Usually a cold boot attack involves cooling memory chips or quickly restarting the computer, and exploiting the fact that data is not immediately lost (or not lost if power is very quickly restored) and the data that was held at the point of intervention will be left accessible to examination.
Cold boot attacks can therefore be a means of unauthorized data theft, loss or access. Such attacks can be nullified if the encryption keys are not accessible at a hardware level to an intruder–i.e., the devices in which the keys are stored when in use are not amenable to cold boot attacks–but this is not the usual case.
TRESOR is a software approach that seeks to resolve this insecurity by storing and manipulating encryption keys almost exclusively on the CPU alone, and in registers accessible at ring 0 (the highest privilege level) only—the exception being the brief period of initial calculation at the start of a session. This ensures that encryption keys are almost never available to userspace code or following a cold boot attack. TRESOR is written as a patch to the kernel that stores encryption keys in the x86 debug registers , and uses on-the-fly round key generation, atomicity , and blocking of usual ptrace access to the debug registers for security.
TRESOR was foreshadowed by a 2010 thesis by Tilo Muller which analyzed the cold boot attack issue. He concluded that modern x86 processors had two register areas where CPU-based kernel encryption was realistic: the SSE registers which could in effect be made privileged by disabling all SSE instructions (and necessarily, any programs relying on them), and the debug registers which were much smaller but had no such issues. He left the latter for others to examine, and developed a proof of concept distribution called Paranoix based on the SSE register method. [ 3 ]
Its developers state that "running TRESOR on a 64-bit CPU that supports AES-NI , there is no performance penalty compared to a generic implementation of AES ", [ 4 ] and run slightly faster than standard encryption despite the need for key recalculation, a result which initially surprised the authors as well. [ 2 ]
The authors' paper notes the following: | https://en.wikipedia.org/wiki/TRESOR |
Prior to the availability of sensitive TSH assays , thyrotropin releasing hormone or TRH stimulation tests were relied upon for confirming and assessing the degree of suppression
in suspected hyperthyroidism . Typically, this stimulation test involves determining basal TSH levels and levels 15 to 30 minutes after an intravenous bolus of TRH. Normally,
TSH would rise into the concentration range measurable with less sensitive TSH assays.
Third generation TSH assays do not have this
limitation and thus TRH stimulation is generally not required when third generation TSH
assays are used to assess degree of suppression.
TRH-stimulation testing however
continues to be useful for the differential diagnosis of secondary (pituitary disorder) and tertiary (hypothalamic disorder) hypothyroidism. Patients with these conditions
appear to have physiologically inactive TSH in their circulation that is recognized by
TSH assays to a degree such that they may yield misleading, " euthyroid " TSH
results. Use and Interpretation: [ citation needed ]
• Helpful in diagnosis in patients with confusing TFTs. In primary hyperthyroidism
TSH are low and TRH administration induces little or no change in TSH levels
• In hypothyroidism due to end organ failure , administration of TRH produces a
prompt increase in TSH
• In hypothyroidism due to pituitary disease (secondary hypothyroidism) administration of TRH does not produce
an increase in TSH
• In hypothyroidism due to hypothalamic disease (tertiary hypothyroidism), administration of TRH produces a
delayed (60–120 minutes, rather than 15–30 minutes) increase in TSH
The TRH test involves administration of a small amount of TRH intravenously, [ 1 ] following which levels of TSH will be measured at several subsequent time points using samples of blood taken from a peripheral vein. [ citation needed ]
The test is used in the differential diagnosis of secondary and tertiary hypothyroidism. First, blood is drawn and a baseline TSH level is measured. Then, TRH is administered via a vein. After 30 minutes blood is drawn again and the levels of TSH are measured and compared to the baseline. Some authors recommend additional blood sampling at 15 minutes. In children, late blood sampling at 60 to 120 minutes is necessary.
An increase in the serum TSH level following TRH administration means that the cause of the hypothyroidism is in the hypothalamus (tertiary hypothyroidism), i.e. the hypothalamus is not producing TRH. Therefore, when TRH is given exogenously, TSH levels increase. If the increase in serum TSH level following TRH administration is absent or very slight, then the cause of the hypothyroidism is in the anterior pituitary gland , i.e. the pituitary is not secreting TSH. Therefore, even when TRH is given exogenously, TSH levels do not rise as the pituitary is diseased. [ citation needed ]
TRH may cause nausea, vomiting and some patients experience an urge to urinate. [ citation needed ]
Rarely, TRH may cause blood vessel constriction leading to hemorrhage in patients with pre-existing pituitary tumors . Accordingly, patients should be advised about the risks, albeit rare, of TRH testing. [ 2 ] | https://en.wikipedia.org/wiki/TRH_stimulation_test |
TRICS (Trip Rate Information Computer System) is a database of trip rates for developments used in the United Kingdom for transport planning purposes, [ 1 ] specifically to quantify the trip generation of new developments. [ 2 ]
The TRICS Consortium describes TRICS as follows:
The database was established in 1989 [ 3 ] by six county councils in South East England county councils ( Dorset , East Sussex , Hampshire , Kent , Surrey and West Sussex ). It is now maintained by TRICS Consortium Ltd, based in Barnet , London.
TRICS 7, a major update, was released in late 2013.
TRICS includes the following development categories:
TRICS have also developed SAM (Standard Assessment Methodology), [ 4 ] a system to measure the effectiveness of travel plans .
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TRICS |
2YYN , 3O33 , 3O34 , 3O35 , 3O36 , 3O37 , 4YAB , 4YAD , 4YAT , 4YAX , 4YBM , 4YBS , 4YBT , 4YC9 , 4ZQL
8805
21848
ENSG00000122779
ENSMUSG00000029833
O15164
Q64127
NM_003852 NM_015905
NM_001272064 NM_001272076 NM_145076
NP_003843 NP_056989
NP_001258993 NP_001259005 NP_659542
Tripartite motif-containing 24 ( TRIM24 ) also known as transcriptional intermediary factor 1α ( TIF1α ) is a protein that, in humans, is encoded by the TRIM24 gene . [ 5 ] [ 6 ] [ 7 ]
The protein encoded by this gene mediates transcriptional control by interaction with the activation function 2 (AF2) region of several nuclear receptors, including the estrogen , retinoic acid , and vitamin D 3 receptors . The protein localizes to nuclear bodies and is thought to associate with chromatin and heterochromatin-associated factors. The protein is a member of the tripartite motif (TRIM) family. The TRIM motif includes three zinc-binding domains – a RING, a B-box type 1 and a B-box type 2 – and a coiled-coil region. Two alternatively spliced transcript variants encoding different isoforms have been described for this gene. [ 5 ]
TRIM24 has been shown to interact with Mineralocorticoid receptor , [ 6 ] [ 8 ] TRIM33 , [ 9 ] Estrogen receptor alpha [ 6 ] [ 10 ] and Retinoid X receptor alpha . [ 6 ] [ 11 ]
This article incorporates text from the United States National Library of Medicine , which is in the public domain . | https://en.wikipedia.org/wiki/TRIM24 |
1FP0 , 2RO1 , 2YVR
10155
21849
ENSG00000130726
ENSMUSG00000005566
Q13263
Q62318
NM_005762
NM_011588
NP_005753
NP_035718
Tripartite motif-containing 28 ( TRIM28 ), also known as transcriptional intermediary factor 1β ( TIF1β ) and KRAB-associated protein-1 ( KAP1 ), is a protein that in humans is encoded by the TRIM28 gene . [ 5 ] [ 6 ]
The protein encoded by this gene mediates transcriptional regulation by interacting with the Krüppel -associated box ( KRAB ) repression domain found in many transcription factors . The protein localizes to the nucleus and is thought to associate with specific chromatin regions. TRIM28 is a member of the tripartite motif family . This tripartite motif includes three zinc-binding domains, a RING finger domain , a B-box type 1 and a B-box type 2, and a coiled-coil region. [ 7 ] TRIM28 additionally possesses a domain that interacts with heterochromatin protein 1 (HP1) and a bromodomain capable of recognizing acetylated lysine residues in other proteins.
TRIM28/KAP1 is a ubiquitously expressed protein involved in many critical functions including: transcriptional regulation, cellular differentiation and proliferation, DNA damage repair, viral suppression, and apoptosis. Its functionality is dependent upon post-translational modifications . SUMOylated TRIM28 can assemble epigenetic machinery for gene silencing, while phosphorylated TRIM28 is involved in DNA repair. [ 8 ]
Studies have shown that deletion of TRIM28/KAP1 in mice before gastrulation results in death (implicating it as a necessary protein in early development) while deletion in adult mice results in increased anxiety and stress-induced alterations in learning and memory. KAP1 has been shown to participate in the maintenance of pluripotency of embryonic stem cells and to promote and inhibit cellular differentiation of adult cell lines. Increased levels of KAP1 have been found in liver, gastric, breast, lung, and prostate cancers as well, indicating that it may play an important role in tumor cell proliferation (possibly by inhibiting apoptosis). [ 8 ]
KAP1 can regulate genomic transcription through a variety of mechanisms, many of which remain somewhat unclear. Studies have shown that KAP1 can repress transcription by binding directly to the genome (which can be sufficient in and of itself) or through the induction of heterochromatin formation via the Mi2α- SETDB1 -HP1 macromolecular complex. [ 9 ] [ 10 ] KAP1 recruits and interacts directly with the histone methyltransferase SETDB1 and with histone deacetylases via its C-terminal PHD finger and bromodomain. [ 8 ] It thus functions as a bridge between sequence-specific DNA-binding KRAB-ZFP transcription factors and various histone-modifying proteins responsible for silencing transcription via nucleosome remodeling, allowing precise epigenetic changes to be made at specific loci across the genome.
It has been shown that the kinase ATM phosphorylates KAP1 upon the discovery of damaged or broken DNA. Phosphorylated KAP1, along with many other DNA damage proteins, rapidly migrate to the site of the DNA damage. Its exact involvement in this pathway is somewhat unclear, but it has been implicated in triggering cell arrest, allowing for the damaged DNA to be repaired. [ 8 ]
KAP1 forms a complex with MDM2 (a ubiquitin E3 ligase ) that binds to p53 . This complex marks the bound p53 for degradation by proteasomes . p53 is a known precursor of apoptosis that facilitates the synthesis of proteins necessary for cell death, so its degradation accordingly results in apoptosis inhibition. [ 8 ]
KAP1 facilitates the establishment of viral latency in certain cell types for Human Cytomegalovirus (HCMV) and other endogenous retroviruses . [ 8 ] [ 9 ] KAP1 acts as a transcriptional corepressor of the viral genome. The protein binds to the histones of the viral chromatin and then recruits Mi2α and SETDB1. SETDB1 is a histone methyltransferase that recruits HP1, thus inducing heterochromatin formation and preventing the transcription of the viral genome. mTOR has been implicated in the phosphorylation of KAP1, resulting in a switch from latency to the lytic cycle . [ 9 ]
Ataxia telangiectasia mutated (ATM) is a kinase that (similar to mTOR) can phosphorylate KAP1, resulting in the switch from viral latency to the lytic cycle. Chloroquine , an ATM activator, has been shown to result in increases in transcription of the HCMV genome. This effect is augmented by the use of tumor necrosis factor . It has been proposed that this treatment (accompanied by antiretroviral treatment) has the potential to purge the virus from infected individuals. [ 9 ]
TRIM28/KAP1 has been shown to interact with:
This article incorporates text from the United States National Library of Medicine , which is in the public domain . | https://en.wikipedia.org/wiki/TRIM28 |
3U5M , 3U5N , 3U5O , 3U5P
51592
94093
ENSG00000197323
ENSMUSG00000033014
Q9UPN9
Q99PP7
NM_015906 NM_033020
NM_001079830 NM_053170
NP_056990 NP_148980
NP_001073299 NP_444400
E3 ubiquitin-protein ligase TRIM33 , also known as (ectodermin homolog and tripartite motif-containing 33) is a protein encoded in the human by the gene TRIM33 , a member of the tripartite motif family . [ 5 ] [ 6 ]
TRIM33 is thought to be a transcriptional corepressor . However unlike the related TRIM24 and TRIM28 proteins, few transcription factors such as SMAD4 that interact with TRIM33 have been identified. [ 7 ]
The protein is a member of the tripartite motif family . [ 8 ] This motif includes three zinc-binding domains :
and a coiled-coil region.
Three alternatively spliced transcript variants for this gene have been described, however, the full-length nature of one variant has not been determined. [ 7 ]
TRIM33 has been shown to interact with TRIM24 . [ 9 ]
TRIM33 acts as a tumor suppressor gene preventing the development chronic myelomonocytic leukemia. [ 10 ] TRIM33 regulates also the TRIM28 receptor and promotes physiological aging of hematopoietic stem cells. [ 11 ] TRIM33 acts as an oncogene by preventing apoptosis in B-cell leukemias. [ 12 ]
This article incorporates text from the United States National Library of Medicine , which is in the public domain . | https://en.wikipedia.org/wiki/TRIM33 |
TRISPHAT (full name tris(tetrachlorocatecholato)phosphate(1−) ) is an inorganic anion with the formula P(O 2 C 6 Cl 4 ) − 3 often prepared as the tributylammonium ( (C 4 H 9 ) 3 NH + ) or tetrabutylammonium ( (C 4 H 9 ) 4 N + salt . The anion features phosphorus (V) bonded to three tetrachlorocatecholate ( C 6 Cl 4 O 2− 2 ) ligands . This anion can be resolved into the axially chiral enantiomers , which are optically stable (the picture shows the Δ enantiomer).
The TRISPHAT anion has been used as a chiral shift reagent for cations . [ 1 ] It improves the resolution of 1 H NMR spectra by forming diastereomeric ion pairs.
The anion is prepared by treatment of phosphorus pentachloride with tetrachlorocatechol followed by a tertiary amine gives the anion:
Using a chiral amine, the anion can be readily resolved. [ 2 ] | https://en.wikipedia.org/wiki/TRISPHAT |
TROPI , or "Analysis of a Novel Sensory Mechanism in Root Phototropism", is an experiment on the International Space Station (ISS) to investigate the growth and development of plant seedlings under various gravity and lighting combinations. [ 1 ] It was launched on Space Shuttle Endeavour during the STS-130 mission and was performed on the ISS during Expedition 22 . Frozen plant samples from the TROPI experiment were returned on the landing of the STS-131 mission on Space Shuttle Discovery .
Arabidopsis thaliana seeds, (thale cress, the genome of which has been DNA sequenced as a reference organism for the study of plant biology and genetics), were germinated and grown under various lighting and gravity conditions, using centrifugal gravity-simulation [ 2 ] and LEDs of various wavelengths (colors) and intensities to model lighting conditions. The specific aim of this project was to investigate phototropism in plants grown in microgravity conditions without the complications of a 1-g environment. [ 3 ] Experiments performed were used to explore the mechanisms of both blue-light- and red-light-induced phototropism in plants.
John Z. Kiss of Miami University (Oxford OH) is Principal Investigator, Richard E. Edelmann of Miami University and Melanie J. Correll of the University of Florida are co-investigators; Kenny Vassigh of NASA is the project manager and Marianne Steele of NASA is the project scientist. The payload was developed by the NASA Ames Research Center , Moffett Field, CA. The experiment was performed in the European Modular Cultivation System (EMCS) [ 4 ] built by the European Space Agency (ESA). The Norwegian User Support Operation Centre (N-USOC), [ 5 ] located in Trondheim, Norway , controlled the EMCS during the TROPI experiments on the ISS.
In the long term, the results from TROPI will help in the development of future space, Moon , and Mars life-support systems, in which plants are used to help remove carbon dioxide and generate oxygen via photosynthesis for maintenance of atmospheric and other conditions, reducing the need for very expensive re-supply from Earth. [ 6 ]
Scientific research on the ISS | https://en.wikipedia.org/wiki/TROPI |
T-complex protein Ring Complex ( TRiC ), otherwise known as Chaperonin Containing TCP-1 ( CCT ), [ a ] is a multiprotein complex and the chaperonin of eukaryotic cells. Like the bacterial GroEL , the TRiC complex aids in the folding of ~10% of the proteome, and actin and tubulin are some of its best known substrates. [ 2 ] [ 3 ] TRiC is an example of a biological machine that folds substrates within the central cavity of its barrel-like assembly using the energy from ATP hydrolysis.
The human TRiC complex is formed by two rings containing 8 similar but non-identical subunits, each with molecular weights of ~60 kDa . The two rings are stacked in an asymmetrical fashion, forming a barrel-like structure with a molecular weight of ~1 MDa. [ 2 ] [ 3 ]
A Molecular weight of human subunits.
Counterclockwise from the exterior, each ring is made of the subunits in the following order: 6-8-7-5-2-4-1-3. [ 4 ]
The CCT evolved from the archaeal thermosome ~2Gya, with the two subunits diversifying into multiple units. The CCT changed from having one type of subunit, to having two, three, five, and finally eight types. [ 4 ] : fig. 4 | https://en.wikipedia.org/wiki/TRiC_(complex) |
TSE or Tris/Saline/EDTA , is a buffer solution containing a mixture of Tris base , Sodium chloride and EDTA .
In molecular biology, TSE buffers are often used in procedures involving nucleic acids . Tris-acid solutions are effective buffers for slightly basic conditions, which keep DNA deprotonated and soluble in water. The concentration of tris in the solution is kept near 25 mM . EDTA is a chelator of divalent cations , particularly of magnesium (Mg 2+ ). As these ions are necessary co-factors for many enzymes, including contaminant nucleases , the role of the EDTA is to protect the nucleic acids against enzymatic degradation. But since Mg 2+ is also a co-factor for many useful DNA-modifying enzymes such as restriction enzymes and DNA polymerases , its concentration in TSE buffers is generally kept low (typically at around 2.5 mM ). The sodium chloride is generally kept at a concentration of 0.05 M . [ 1 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TSE_buffer |
TSH-T 3 shunt is a feed-forward mechanism of the hypothalamic–pituitary–thyroid axis (HPT axis) in which pituitary thyroid-stimulating hormone (TSH) directly augments secretion of the biologically active thyroid hormone triiodothyronine (T 3 ) by the thyroid gland. [ 1 ] Besides stimulating production of the pro-hormone thyroxine (T 4 ), TSH up-regulates intrathyroidal deiodination of T 4 to T 3 via type II iodothyronine deiodinase (DIO2), effectively “shunting” part of the gland’s T 4 output into active T 3 before release. [ 2 ] [ 3 ]
The pathway buffers circulating free T 3 (FT 3 ) against fluctuations in thyroid output and contributes to the circadian rhythm of serum T 3 . [ 4 ] Recognition of a TSH-T 3 shunt has refined models of thyroid homeostasis and influences interpretation of thyroid function tests and the management of hypothyroidism. [ 5 ]
In euthyroid adults about 80 % of circulating T 3 arises from peripheral deiodination of T 4 , while ~20 % is secreted directly by the thyroid. [ 6 ] Under high TSH drive – for example in iodine deficiency or after exogenous TSH administration – the thyroidal fraction of T 3 secretion rises substantially. [ 7 ]
TSH activates cAMP-dependent signalling that stimulates expression and activity of DIO2 (and to a lesser extent DIO1) in follicular cells, promoting rapid conversion of stored T 4 to T 3 and increasing the T 3 :T 4 ratio of secreted hormone. [ 2 ] TSH concurrently shortens thyroglobulin residence time, favouring release of newly formed T 3 . [ 3 ]
Mathematical sensitivity analysis places the shunt gain (G T ) at 15–30 pmol s −1 / 10 9 thyrocytes, sufficient to stabilise FT 3 against ±30 % changes in T 4 output. [ 1 ]
The shunt acts as a buffering, feed-forward element complementing negative feedback by thyroid hormones on TSH secretion. [ 8 ] In population studies FT 3 varies little across a six-fold range of TSH, whereas FT 4 shows a strong log-linear inverse relationship with TSH. [ 9 ] The shunt therefore underlies the relative constancy of T 3 supply critical for metabolic stability.
Disruption of Dio2 in mice produced pituitary resistance to T 4 and reduced circulating T 3 , demonstrating the importance of thyroidal DIO2 for systemic hormone balance. [ 12 ]
In silico models incorporating a TSH-dependent T 3 secretion term replicate the observed circadian FT 3 rhythm and the stability of FT 3 across variable gland capacities, whereas models without the shunt do not. [ 1 ] Sensitivity analysis shows that shunt gain profoundly influences TSH responsiveness and FT 3 homeostasis. [ 13 ]
Standard LT 4 monotherapy cannot replace the thyroidal component of T 3 secretion; a subset of treated patients have low-normal FT 3 and persistent symptoms despite normal TSH. [ 14 ] Combination LT 4 + LT 3 or tailored TSH targets have been proposed to restore physiological FT 3 levels in these patients. [ 15 ]
Disproportionate thyroidal T 3 secretion under TSH stimulation was described in the 1960s, [ 16 ] but the term “TSH-T3 shunt” and its formal cybernetic treatment were introduced in 2012–2018. [ 17 ] [ 1 ] | https://en.wikipedia.org/wiki/TSH-T3_shunt |
Through-Focus Scanning Optical Microscopy ( TSOM ) is an imaging method that produces nanometer-scale three-dimensional measurement sensitivity using a conventional bright-field optical microscope. TSOM has been introduced and maintained by Ravikiran Attota [ 1 ] at NIST . It was given an R&D 100 Award in 2010. [ 2 ] In the TSOM method a target is scanned through the focus of an optical microscope, acquiring conventional optical images at different focal positions. The TSOM images are constructed using the through-focus optical images. A TSOM image is unique under given experimental conditions and is sensitive to changes in the dimensions of a target in a distinct way, which is very well applicable in nanoscale dimensional metrology . The TSOM method is alleged to have several nanometrology [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ] applications ranging from nanoparticles to through-silicon-vias (TSV).
The National Institute of Standards and Technology, USA, produced a short Video on YouTube on the TSOM method.
This optics -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TSOM |
6-Methoxy-(8- p -toluenesulfonamido)quinoline ( TSQ ) is one of the most efficient fluorescent stains for zinc (II). It was introduced by Soviet biochemists Toroptsev and Eshchenko in the early 1970s. The popularity of TSQ as physiological stain rose after seminal works by Christopher Frederickson two decades later. [ 1 ] TSQ forms a 2:1 (ligand-metal) complex with zinc and emits blue light upon excitation at 365 nanometers. TSQ has been extensively applied for determination of extracellular or intracellular levels of Zn 2+ in biological systems, also to study Zn 2+ in mossy fibers of the hippocampus .
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TSQ |
TSUBAME was a microsatellite developed by the Tokyo Institute of Technology and Tokyo University of Science from a student design concept in 2004. [ 1 ] The satellite was designed to demonstrate new technologies for rapid attitude control, observing gamma ray bursts , and Earth observation. [ 2 ] The name, TSUBAME, means swift in Japanese and was chosen both because of the experimental attitude control system and to invoke another gamma ray observatory, the Swift Gamma-Ray Burst Mission , [ 3 ] which launched shortly after TSUBAME's first design concept was published in 2004. [ 4 ]
TSUBAME was launched with four other satellites from Yasny Cosmodrome on a Dnepr rocket on November 6, 2014. [ 5 ] It was placed in a 500 km altitude Sun-synchronous orbit . A week after the launch, problems were reported with communication hardware and communication was lost with the satellite after three months of recovery efforts. [ 1 ]
The TSUBAME mission included both engineering and scientific objectives. Primarily engineering objectives were successful demonstration of new compact control moment gyroscopes for rapid changes in spacecraft attitude and demonstration of a compact, 14-meter resolution optical camera. In addition to these engineering objectives, the mission also included a primarily scientific objective to observe ephemeral, high-energy phenomena, such as gamma ray bursts , using polarimetry of hard X-rays . [ 6 ] [ 7 ] Early concepts of the mission also included tether formation control experiments [ 6 ] but this objective appears to have been dropped from the final design.
The Matunaga space systems lab at the Tokyo Institute of Technology had launched several student-designed satellites before TSUBAME, including CUTE-1 in 2003, CUTE-1.7 + APD in 2006, and CUTE-1.7 + APD II in 2007. [ 6 ] The first design concepts for the TSUBAME mission were entered into a student satellite design contest in 2004, where it won Grand Prix. [ 8 ] TSUBAME was one of four small satellites to launch piggyback with ASNARO 1 . The mission was launched from Yasny Cosmodrome on a Dnepr rocket on November 6, 2014. [ 5 ] It was deployed into a 500 km Sun synchronous orbit . [ 1 ]
Initially, TSUBAME's mission was expected to last one year. Early results from the check-in phase, where hardware is activated and vital functions checked, were positive, but a week after launch issues with RF command receiver (in the FM broadcast band ) were observed. Later, issues developed with the satellite spin rate, S-band communications, GPS receiver, and battery voltage and temperature. About three months after launch, all communication was lost with the failure of the continuous wave transceiver . Subsequent analysis of communications with the satellite and laboratory reproduction have revealed that the ultimate loss of contact and some of the other issues were caused by a failure in a DC-DC converter . Several other issues and design flaws were revealed in the failure analysis. [ 1 ]
The satellite bus was a rectangular prism 45 cm × 45 cm × 56 cm (1.48 ft × 1.48 ft × 1.84 ft) assembled from an internal structure of metal panels an exterior covered with carbon fibre reinforced polymer lattice. The spacecraft mass was approximately 49 kg (108 lb). [ 7 ]
The miniature control moment gyroscopes demonstrated in TSUBAME each had a rotating flywheel driven by a synchronous motor , and a single stepper motor , which controlled the gimbal . These control moment gyros were arranged in a pyramid. The flywheels were 7.35 cm in diameter and weighed 1 kg. In addition to the control moment gyros, a magnetorquer was also used for attitude control. [ 3 ] Navigation and attitude determination used a number of sensors including six Sun sensors , a three-direction magnetometer , three MEMS gyroscopes , and two star trackers . A field-programmable gate array contained the software for the attitude determination and control systems. [ 9 ]
TSUBAME had multiple communication systems to cope with the limitations of the radio equipment available on the ground. Similar to other satellites developed by the space systems laboratory at Tokyo Institute of Technology, TSUBAME had an FM band receiver and ultra high frequency transmitter which could communicate with the university's ground station. The UHF transmitter broadcast continuously after deployment so that amateur radio operators could assist in tracking the satellite. The volume of data that TSUBAME was expected to produce (on the order of tens of megabytes) could not be quickly broadcast by the radio transmitter, so an additional S-band transceiver was also included, even though an S-band antenna was not immediately available to the university; it was hoped that Fukui University of Technology would allow use of a 10m parabolic antenna after the launch. Poor design of the antennas on TSUBAME were found to be a likely cause of many of the failures that occurred on orbit. The first communication issues that were observed were attributed to interference from radio waves reflected off the solar panels. Subsequent failures were attributed to a failure of a DC-DC converter powering the communication systems. [ 1 ]
The primary science objective measuring polarization of gamma ray bursts required two instruments. The Wide-Field Burst Monitors (WBMs) were used to help detect and localize events so the spacecraft can be pointed in the correct direction, while the Hard X-ray Compton Polarimeter (HXCP) was designed to make observations once correctly oriented. [ 10 ] There were five WBM units at diverse locations on the TSUBAME. Each WBM was a tile of caesium iodide scintillator . When an increase in the count rate was detected by one or more scintillators, the main processor would determine the direction, initiate control systems of the satellite. [ 3 ] The HXCP could measure polarization for photons from 30 to 200 keV. It comprised tiles of plastic scintillator to scatter incident photons, photomultiplier tubes, and a second caesium iodide based scintillator used as an absorber. Each scintillator tile was attached to an avalanche photodiode , which converts photons to an electric signal. To mitigate background noise, the HXCP was surrounded by metal composite shielding, and coincidence between the two scintillating materials could be compared and processed by onboard computers. [ 10 ] The optical camera was developed by the Tokyo University of Science. It had a ground resolution of 14 meters per pixel and could take up to five images each second. [ 3 ] | https://en.wikipedia.org/wiki/TSUBAME_(satellite) |
The Time-Triggered Ethernet (SAE AS6802) (also known as TTEthernet or TTE ) standard defines a fault-tolerant synchronization strategy for building and maintaining synchronized time in Ethernet networks, and outlines mechanisms required for synchronous time-triggered packet switching for critical integrated applications and integrated modular avionics (IMA) architectures. SAE International released SAE AS6802 in November 2011.
Time-Triggered Ethernet network devices are Ethernet devices which at least implement:
TTEthernet network devices are standard Ethernet devices with additional capability to configure and establish robust synchronization, synchronous packet switching, traffic scheduling and bandwidth partitioning, as described in SAE AS6802. If no time-triggered traffic capability is configured or used, it operates as full duplex switched Ethernet devices compliant with IEEE802.3 and IEEE802.1 standards.
In addition, such network devices implement other deterministic traffic classes to enable mixed-criticality Ethernet networking. Therefore, TTEthernet networks are designed to host different Ethernet traffic classes without interference.
TTEthernet device implementation expands standard Ethernet with services to meet time-critical, deterministic or safety-relevant requirements in double- and triple-redundant configurations for advanced integrated systems. TTEthernet switching devices are used for integrated systems and safety-related applications primarily in the aerospace, industrial controls and automotive [ 1 ] applications.
TTEthernet has been selected by NASA and ESA as the technology for communications between the Orion MPCV and the European Service Module , and is described by the ESA as being "prime choice for future launchers allowing them to deploy distributed modular avionics concepts". [ 2 ] It has also been selected as the backbone network for NASA's Lunar Gateway [ 3 ] to which ESA is a key stakeholder.
As an increasingly used network architecture in the space industry, European Cooperation for Space Standardization published ECSS-E-ST-50-16C on September 30, 2021. [ 4 ]
TTEthernet network devices implement OSI Layer 2 services, and therefore it claims to be compatible with IEEE 802.3 standards and coexist with other Ethernet networks and services or traffic classes, such as IEEE 802.1Q, on the same device.
Three traffic classes and message types are provided in current TTEthernet switch implementations: [ 5 ]
Three traffic classes cover different types of determinism - from soft-time best-effort traffic to "more deterministic" to "very deterministic" (max.latency defined per VL) to "strictly deterministic" (fixed latency, μs-jitter), thus creating a deterministic unified Ethernet networking technology. While standard full duplex switched Ethernet is typically best effort or more deterministic, time-triggered traffic is bound only to the system time progression and traffic scheduling, and not to priorities. It can be considered the highest priority traffic, above the highest priority 802.1Q VLAN traffic.
TTEthernet (i.e. Ethernet switch with SAE AS6802) integrates a model of fault-tolerance and failure management [ citation needed ] . TTEthernet switch can implement a reliable redundancy management and dataflow (datastream) integration to assure message transmission even in case of a switch failure. The SAE AS6802 implemented on an Ethernet switch supports the design of synchronous system architectures with defined fault-hypothesis.
The single-failure hypothesis, dual-failure hypothesis, and tolerance against arbitrary synchronization disturbances define the basic fault-tolerance concept in a Time-Triggered Ethernet (SAE AS6802-based) network.
Under the single-failure hypothesis, Time-Triggered Ethernet (SAE AS6802) is intended to tolerate either the fail-arbitrary failure of an end system or the fail-inconsistent-omission failure of a switch. The switches in Time-Triggered Ethernet network can be configured to execute a central bus guardian function. The central bus guardian function ensures that even if a set of end systems becomes arbitrarily faulty, it masks the system-wide impact of these faulty end systems by transforming the fail-arbitrary failure mode into an inconsistent-omission failure mode. The arbitrarily faulty failure mode also includes so called "babbling-idiot" behavior. Time-Triggered Ethernet switches therefore establish fault-containment boundaries.
Under the dual-failure hypothesis, Time-Triggered Ethernet networks are intended to tolerate two fail-inconsistent-omission faulty devices. These devices may be two end systems, two switches, or an end system and a switch. The last failure scenario (i.e., end system and switch failure) means that Time-Triggered Ethernet network tolerates an inconsistent communication path between end systems. This failure mode is one of the most difficult to overcome.
Time-Triggered Ethernet networks are intended to tolerate transient synchronization disturbances, even in the presence of permanent failures. Under both single- and dual-failure hypothesis, Time-Triggered Ethernet provides self-stabilization properties. Self-stabilization means that synchronization can reestablish itself, even after a transient upset in a multitude of devices in the distributed computer network.
Time-triggered traffic is scheduled periodically, and depending on the architecture, line speed (e.g. 1GbE), topology and computing model with control loops operating at 0.1–5(+) kHz, using a time-triggered architecture (TTA) model of computation and communication. Hard real-time is possible at application level due to strict determinism, jitter control and alignment/synchronization between tasks and scheduled network messaging.
In L-TTA (Loosely TTA) architectures with synchronous TTEthernet network, but with local computer clocks decoupled from system/network time the performance of control loops may be limited. In this case, time-triggered transmissions are necessarily cyclically scheduled and thus delays between processes in the application layer can be large, e.g. with plesiochronous processes operating on their own local clock and execution cycle, as is observed in systems using cyclic MIL-STD-1553 B buses, up to twice the transmission interval due to released packets waiting for scheduled transmission at the source and for the receiving process to run at the destination.
Rate-constrained traffic is another periodic time-sensitive traffic class, and it shall be modeled to align with time-triggered traffic (and vice versa) in order to fulfill maximum latency and jitter requirements. However, even where the sum of the allocated bandwidths is less than the capacity provided at every point in the network, delivery is still not guaranteed due, e.g., to potential buffer overflows at switch queues, etc., which simple limitation of bandwidths does not guarantee are avoided.
Best effort traffic will utilize network bandwidth not used by rate-constrained and time-triggered traffic.
In TTEthernet devices, this traffic class cannot interfere with deterministic traffic, as it resides in its own separate buffer memory. Moreover, it implements internal architecture which isolates best effort traffic on partitioned ports, from the traffic assigned to other ports. This mechanism can be associated with fine-grained IP traffic policing, to enable traffic control which is much more robust than VLANs with FIFO buffering.
In 2008, it was announced Honeywell would apply the technology to applications in the aerospace and automation industry. [ 7 ] In 2010 a switch-based implementation was shown to perform better than shared bus systems such as FlexRay for use in automobiles. [ 8 ] Since then, Time-Triggered Ethernet has been implemented in different industrial, space and automotive programs and components. | https://en.wikipedia.org/wiki/TTEthernet |
TTFB (4,5,6,7-Tetrachloro-2-trifluoromethylbenzimidazole) is a halogenated benzimidazole derivative that acts as an uncoupling agent . [ 2 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it .
This article about an organic halide is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TTFB_(chemical) |
TTP/A is a time-triggered real-time fieldbus protocol used for the interconnection of low-cost smart transducer nodes. In contrast to the Time-Triggered Protocol (TTP), which is a high-speed, fault-tolerant communication network intended for safety critical applications ( SAE class C), TTP/A provides a low-speed (below 1 MB/s), low-cost protocol suitable for non-critical applications like car body electronics (SAE class A). [ 1 ]
Time-triggered means that all actions are scheduled at predefined times. The schedules are repeated periodically for a predictable repetitive behavior. This is for sensor or actuator applications and closed control loops, where arbitrary message delays worsen or even invalidate the result. [ 2 ]
The TTP/A protocol implements a communication layer and data access according to the Object Management Group smart transducer Interface standard which was published in late 2002. [ 3 ] The interface to each transducer is known as an Interface File System (IFS), that includes both real-time data, as well as configuration and node management features. The IFS makes it possible to share data between different TTP/A clusters in a transparent fashion, so that for the application no distinction between data from local and remote nodes is made. The TTP/A protocol does not require special controller hardware and can be implemented on any standard embedded microcontroller. An open-source implementation exists for the Atmel AVR architecture. Like the fault-tolerant time-triggered protocol TTP, TTP/A follows the time-triggered paradigm featuring a clock synchronization and predefined time-triggered schedules for communication and execution of tasks. Thus, TTP/A communication implements a strict time division multiple access (TDMA) scheduling. TTP/A can be used as a sub-bus for a higher-performance and more dependable real-time systems, [ 1 ] or to provide transducer data to a Linux system in a soft-real-time fashion. TTP/A was used in applications such as an intercommunication system for mobile robots.
TTP/A is a time-triggered protocol used for the communication of one active master with or among smart transducer nodes within a cluster. This cluster is controlled by the master, which establishes a common time base among the nodes. In case of a master failure, a shadow master can take over control. Every node in this cluster has a unique alias, an 8 bit (1 byte) integer, which can be assigned to the node a priori or set at any time via the configuration interface.
TTP/A communication is organized into rounds separated by inter round gaps (IRG). The TTP/A bus is inactive for at least 13 bit times during each gap. A TTP/A round consists of one or more frames. A frame is a sequence of bytes transmitted by one node. A byte is transmitted in a slot consisting of 13 bit cells: one start-bit, eight data-bits, one parity, one stop-bits and a two bit cell wide inter byte gap. The rounds are independent from each other. Every round starts with a fireworks byte sent by the master. The arrival of the fireworks byte is a synchronization event for every node in the cluster and identifies the round. According to the specification of the selected round, the fireworks frame is followed by data frames of specified length from the specified nodes. Each such frame is described by an entry in the round descriptor list (RODL) in the file-system of the sender and the receiver(s). Because the slot position at which each communication action takes place is defined a priori, no further bus arbitration is necessary. [ 4 ]
The transmission frames are based on universal asynchronous receiver/transmitter (UART) frames.
This allowed the protocol to be implemented using microcontrollers featuring a standard UART module. For low transmission rates, also a software-based implementation without a hardware-supported UART is possible.
Several papers were published by researchers at the Vienna University of Technology on TTP/A from 1994 through 2006 by Hermann Kopetz and Wilfried Elmenreich . [ 4 ] A related protocol called TTP/C was also described in 2003. [ 5 ] | https://en.wikipedia.org/wiki/TTP/A |
TUGSAT-1 , also known as BRITE-Austria and CanX-3B , is the first Austrian satellite . It is an optical astronomy spacecraft operated by the Graz University of Technology as part of the international BRIght-star Target Explorer programme.
TUGSAT-1 was manufactured by the University of Toronto based on the Generic Nanosatellite Bus , and had a mass at launch of 7 kilograms (15 lb) [ 2 ] (plus another 7 kg for the XPOD separation system). The spacecraft is cube-shaped, with each side measuring 20 centimetres (7.9 in). [ 3 ] The satellite will be used, along with five other spacecraft, to conduct photometric observations of stars with apparent magnitude of greater than 4.0 as seen from Earth. [ 4 ] TUGSAT-1 was one of the first two BRITE satellites to be launched, along with the Austro-Canadian UniBRITE-1 spacecraft. Four more satellites, two Canadian and two Polish, were launched at later dates.
The TUGSAT-1 spacecraft was launched through the University of Toronto's Nanosatellite Launch System programme, as part of the NLS-8 launch, along with UniBRITE-1 and AAUSAT3 . [ 5 ] The NLS-8 launch was subcontracted to the Indian Space Research Organisation (ISRO) , who placed the satellites into orbit using a Polar Satellite Launch Vehicle (PSLV) in the PSLV-CA configuration, flying from the First Launch Pad at the Satish Dhawan Space Centre . [ 6 ] The NLS spacecraft were secondary payloads on the rocket, whose primary mission was to deploy the Franco-Indian SARAL ocean research satellite. Canada's Sapphire and NEOSSat-1 spacecraft, and the United Kingdom's STRaND-1 , were also carried by the same rocket under separate launch contracts. [ 2 ] The launch took place at 12:31 UTC on 25 February 2013, and the rocket deployed all of its payloads successfully. [ 7 ] | https://en.wikipedia.org/wiki/TUGSAT-1 |
Terminal deoxynucleotidyl transferase dUTP nick end labeling ( TUNEL ) is a method for detecting DNA fragmentation by labeling the 3′- hydroxyl termini in the double-strand DNA breaks generated during apoptosis . [ 1 ] [ 2 ]
TUNEL is a method for detecting apoptotic DNA fragmentation, widely used to identify and quantify apoptotic cells, or to detect excessive DNA breakage in individual cells. [ 3 ] The assay relies on the use of terminal deoxynucleotidyl transferase (TdT), an enzyme that catalyzes attachment of deoxynucleotides, tagged with a fluorochrome or another marker , to 3'-hydroxyl termini of DNA double strand breaks. It may also label cells having DNA damage by other means than in the course of apoptosis. [ citation needed ]
The fluorochrome-based TUNEL assay applicable for flow cytometry , combining the detection of DNA strand breaks with respect to the cell cycle -phase position, was originally developed by Gorczyca et al. [ 4 ] Concurrently, the avidin-peroxidase labeling assay applicable for light absorption microscope was described by Gavrieli et al. [ 5 ] Since 1992 the TUNEL has become one of the main methods for detecting apoptotic programmed cell death. [ 6 ] However, for years there has been a debate about its accuracy, due to problems in the original assay which caused necrotic cells to be inappropriately labeled as apoptotic. [ 7 ] The method has subsequently been improved dramatically and if performed correctly should only identify cells in the last phase of apoptosis . [ 8 ] [ 9 ] New methods incorporate the dUTPs modified by fluorophores or haptens, including biotin or bromine , which can be detected directly in the case of a fluorescently-modified nucleotide (i.e., fluorescein-dUTP), or indirectly with streptavidin or antibodies , if biotin-dUTP or BrdUTP are used, respectively. The most sensitive of them is the method utilizing incorporation of BrdUTP by TdT followed by immunocytochemical detection of BrdU . [ 10 ] | https://en.wikipedia.org/wiki/TUNEL_assay |
TURBOMOLE is an ab initio computational chemistry program that implements various quantum chemistry methods. It was initially developed by the group of Prof. Reinhart Ahlrichs at the University of Karlsruhe . In 2007, TURBOMOLE GmbH, founded by R. Ahlrichs, F. Furche, C. Hättig, W. Klopper, M. Sierka, and F. Weigend, took over the responsibility for the coordination of the scientific development of TURBOMOLE program, for which the company holds all copy and intellectual property rights. In 2018 David P. Tew joined the TURBOMOLE GmbH. Since 1987, this program is one of the useful tools as it involves in many fields of research including heterogeneous and homogeneous catalysis , organic and inorganic chemistry , spectroscopy as well as biochemistry . This can be illustrated by citation records of Ahlrich's 1989 publication which is more than 6700 times as of 18 July 2020. [ 1 ] In the year 2014, [ 2 ] the second Turbomole article has been published. The number of citations from both papers indicates that the Turbomole's user base is expanding.
Turbomole was developed in 1987 and turned into a mature program system under the control of Reinhart Ahlrichs and his collaborators. Turbomole can perform a large-scale quantum chemical simulations of molecules, clusters, and later periodic solids. Gaussian basis sets are used in Turbomole. The functionality of the program concentrates extensively on the electronic structure methods with effective cost-performance characteristics such as density functional theory , [ 3 ] second–order Møller-Plesset [ 4 ] [ 5 ] and coupled cluster theory . Aside from energies and structures, an assortment of optical, electrical, and magnetic properties are available from analytical energy derivative for electronic ground and excited states . [ 2 ] However, up to the year 2000, Turbomole was only limited to the calculation of molecules in gas phase, thus, COSMO has been implemented in the Turbomole in a cooperative initiative of BASF AG and Bayer AG. [ 6 ] Turbomole version 6.5 releasing in the year 2013, comes with post-Kohn–Sham calculations within the random-phase approximation . Turbomole also comes with another significant additions including nonadiabatic molecular dynamics, ultra-efficient higher order CC methods , new density functionals and periodic calculations. [ 7 ] TmoleX is available as a graphical user interface for Turbomole allowing the user to perform the entire workflow of a quantum chemical investigation ranging from building of an initial structure to the interpretation of the results. [ 8 ]
The current version of Turbomole is V7.8 released in December 2023 [ 7 ] | https://en.wikipedia.org/wiki/TURBOMOLE |
Televisión Azteca, S.A.B. de C.V. , commonly known as TV Azteca , is a Mexican multimedia conglomerate owned by Grupo Salinas . It is the second-largest mass media company in Mexico after Televisa . [ 1 ] [ 2 ] It primarily competes with Televisa as well as some local operators. It owns two national television networks, Azteca Uno and Azteca 7 , and operates two other nationally distributed services, adn40 and A Más+ . All three of these networks have transmitters in most major and minor cities.
TV Azteca also operates Azteca Trece Internacional , reaching 13 countries in Central and South America , and formerly part of the Azteca América network in the United States . Its flagship program is the newscast Hechos .
In the early 1990s, the presidency of Carlos Salinas de Gortari privatized many government assets. Among them was the Instituto Mexicano de la Televisión, known as Imevisión , which owned two national television networks (Red Nacional 7 and Red Nacional 13) and three local TV stations. In preparation for the privatization, the Imevisión stations were parceled into a variety of newly created companies, the largest of which was named Televisión Azteca, S.A. de C.V. [ 3 ]
With the exception of Canal 22 , which was spun off to Conaculta , one bidder won all of the stations. On July 18, 1993, [ 4 ] Mexico's Finance Ministry, the Secretaría de Hacienda y Crédito Público (SHCP), announced that Radio Televisora del Centro, a group controlled by Ricardo Salinas Pliego , was the winner of the auction to acquire the "state-owned media package", which also included Imevisión's studios in the Ajusco area of Mexico City. The winning bid amounted to US$645 million. The new group soon took on the Televisión Azteca name for the entire operation and soon challenged Televisa, turning what had been a television monopoly into a television duopoly. The two conglomerates held 97 percent of the commercial television concessions in the country. [ 5 ]
In 1998, TV Azteca announced an investment of US$25 million in XHTVM-TV , which was owned by Javier Moreno Valle through concessionaire Televisora del Valle de México, S.A. de C.V. Under the deal, Azteca restructured TVM and took control of ad sales and most programming duties, while Moreno Valle's CNI news service retained some primetime space. However, in 2000, Moreno Valle broke the contract with Azteca, alleging Azteca of filling up time allotted to CNI and not fulfilling the obligations in the contract. In December 2002, Azteca used private security guards to retake control of the XHTVM facilities on Cerro del Chiquihuite in Mexico City. However, the Mexican government stepped into the dispute and forced Azteca to relinquish control of XHTVM. In 2005, an employee strike that crippled CNI, Moreno Valle's mounting legal troubles, and a deal with the 5% owner of the concessionaire allowed Azteca to buy the remainder of the station and retake control of XHTVM, under the name Proyecto 40, in 2006.
On March 7, 2011, TV Azteca changed its name to Azteca , reflecting its growth into a multimedia company. [ 6 ] However, in May 2016, the TV Azteca name was restored.
TV Azteca is the second largest mass media company in México after Televisa. [ 5 ] These two big organizations control the 97% of mass media in Mexico. [ 5 ] TV Azteca was funded in 1993 by Ricardo Salinas Pliego. TV Azteca has 31% of the 465 television concessions in México. [ 5 ] The auction of the state channels and the granting of further concessions to TV Azteca further strengthen their connection. It also owns Azteca banks, Azteca insurance, Iusacell, programing pay television, cinemas, live theater, news channels, newspapers, Azteca music, an acting school, Azteca consumer products, Azteca internet, Azteca series, Azteca sports, stadiums, etc. TV Azteca is another company which also serves the government however to a much lesser extent than Televisa. [ 7 ] TV Azteca also receives lucrative contracts from the Mexican government, and therefore the information that emits is also controlled by the actual government. The news that is normally emitted by TV Azteca is 25% news bulletins that come from advertising, and infotainment relying on celebrities and biased editorials. [ 5 ]
On March 21, 2023, creditors for the company pushed the company into an involuntary Chapter 11 bankruptcy petition in the U.S. [ 8 ] However, on April 26, TV Azteca asked the New York bankruptcy judge to dismiss its Chapter 11 case due to it being pointless to start reorganization proceedings for the company anywhere but Mexico. [ 9 ] On June 1, 2023, TV Azteca was suspended from the Mexico Stock Exchange. [ 10 ]
On 5 January 2005, the U.S. Securities and Exchange Commission (SEC) accused TV Azteca executives (including chairman Ricardo Salinas Pliego ) of having personally profited from a multimillion-dollar debt fraud committed by TV Azteca and another company in which they held stock. [ 13 ] The charges were among the first brought under the provisions of the Sarbanes-Oxley Act of 2002, [ 13 ] introduced in the wake of the corporate financial scandals of that year.
The Federal Radio and Television Law (known as the Ley Televisa) was a bill concerning the licensing and regulation of the electromagnetic spectrum. The LFRT was favorable to both TV Azteca and Televisa (who together control 95 percent of all television frequencies) because it allowed them to renew their licenses without paying for them. According to The Economist , the Ley Federal de Radio y Televisión "raced through Congress confirming the country's longstanding television duopoly" and constituted a "giveaway of radio spectrum and a provision that allows broadcasting licenses to be renewed more or less automatically". [ 14 ]
In February 2012, TV Azteca networks ( Azteca 7 , Azteca 13 , and Proyecto 40 ) were dropped by Mexican cable-TV carriers representing more than 4 million subscribers in a carriage dispute over terms. Cable operators claimed that Azteca wanted to charge a fee by packaging its over-the-air stations with cable networks, such as news and soap opera channels, which potentially represented a higher cost to subscribers. [ 15 ] After a nine-month absence, TV Azteca returned gradually to cable operators. [ 16 ]
In August 2018, American Tower 's Mexican Unit, MATC Infraestructura sued TV Azteca for $97 Million in a New York court for defaulting on a loan from the company. [ 17 ]
TV Azteca is part of the conglomerate Grupo Salinas , which includes the Grupo Elektra franchise of department stores, the Banco Azteca bank, and Seguros Azteca life insurance. TV Azteca also owns Liga MX soccer club, Monarcas Morelia .
The network has set up an acting school, Centro de Estudios y Formación Actoral (CEFAT). Alumni include Iliana Fox , Luis Ernesto Franco , Adriana Louvier , Fran Meric , Bárbara Mori , Laura Palma and Adrián Rubio . [ 18 ]
The network also owns a record label, Azteca Music, [ 19 ] which was founded in 1996. [ 20 ] | https://en.wikipedia.org/wiki/TV_Azteca |
A television accessory ( TV accessory ) is an accessory that is used in conjunction with a television (TV) or other compatible display devices and is intended to either improve the user experience or to offer new possibilities of using it.
The first TV accessory with which owners could actively influence the content displayed on the screen in real time was the Magnavox Odyssey , the first commercial home video game console , released in September 1972 by Magnavox for a list price of $99.95. [ 1 ]
One of the first TV accessories that could record TV programs available for consumers was the Clie Pega-VR100K by Sony , released on October 9, 2003, for a list price of $479.99. [ 2 ]
As of 2017, TV accessories are a rapidly growing market which is expected to grow even more rapidly in the near future. [ 3 ] Some of the most popular manufacturers of TV accessories include Sony, Magnavox, Apple , Nvidia , Amazon , Samsung , and Google , as well as many independent third-party suppliers .
A soundbar (also called sound bar or media bar) is a type of loudspeaker that projects audio from a wide enclosure . Soundbars are one of the most popular TV accessories because they are affordable, very easy to install and a relatively large upgrade compared to other accessories, offering much better sound than most integrated TV loudspeakers. [ 4 ] [ 5 ] [ 6 ]
A universal remote is a remote control that can be programmed to operate various brands of one or more types of consumer electronics devices.
On May 30, 1985, Philips introduced the first universal remote (U.S. Pat. #4774511) under the Magnavox brand name. [ 7 ] In 1985, Robin Rumbolt, William "Russ" McIntyre, and Larry Goodson with North American Philips Consumer Electronics (Magnavox, Sylvania, and Philco) developed the first universal remote control.
Streaming television is the digital distribution of television content, such as TV shows, as streaming video delivered over the Internet. Most TVs today are smart TVs , meaning that they can connect to the Internet to use different functions. [ 8 ] However, since there are many different TV manufacturers that use different inferfaces for these functions, this may be confusing for some users. [ 8 ] A dedicated streaming box like an Apple TV , Google Chromecast , Amazon Fire TV Stick or PlayStation TV offers a universal user experience across all TV brands. [ 8 ] An Android TV box like the Nvidia Shield TV can also run all Android apps on the Play Store and stream PC gaming content to the TV. [ 9 ]
An HDMI switch (also known as HDMI switcher or HDMI switching box) is a device that accepts input from multiple HDMI sources and sends the signal you select to your HDTV via an HDMI cable. [ 10 ] When they also support USB devices, they are KVM switches .
A home video game console is a type of video game console that is designed to be connected to a display device, such as a television, and an external power source as to play video games . In contrast to many other TV accessories that improve the user experience, a home video game console offers new possibilities of using a TV, meaning that users of such can not only determine what should be shown on the television screen, but also actively influence it in real time. | https://en.wikipedia.org/wiki/TV_accessory |
A TV gateway (also called network TV tuner) is a television headend to a network UPnP router that receives live digital video broadcast (DVB) MPEG transport streams (channels) from terrestrial aerials, satellite dishes, or cable feeds and converts them into IP streams for distribution over an IP network .
TV gateways allow users to stream broadcast live TV content to connected devices on the IP network, including tablets, smartphones, computers, gaming consoles and smart tvs . They also allow multiple users to watch and record different channels at the same time. [ 1 ]
The device offers multi-platform, multi-screen broadcast television with rich live TV content and high quality HD channels .
Most TV gateways support free-to-air (FTA) television services found in many countries. These include services such as Freeview and Freesat in the United Kingdom, [ 2 ] TNT in France and TDT in Spain, and basic cable packages in Germany, Switzerland, Austria, and others.
A few TV gateways also support third party conditional access modules (CAMs) for premium pay TV channels, which are transmitted by using a CAM CI card provided by the broadcasters or by third party manufacturers to access their TV service.
While many first-generation TV gateways support only one channel or a limited number of channels, modern TV gateways provide multiple TV tuners that can process several channels simultaneously. The more channels a TV gateway provides the more users it can service at the same time. Modern TV gateways also allow users to record TV programs to a USB flash drive, or external hard disk and in some cases, shared folders or network attached storage (NAS). [ 3 ]
An electronic program guide (EPG) is like a traditional TV listing magazine but available online or on a TV service like aerial, satellite or cable. It allows viewers to find out what shows will air and search for programs they’d like to watch. EPG's also allow users to set reminders and record shows automatically. [ 4 ]
Most TV gateways with PVR functionality offer EPG data. This can be free of charge for data processed from the broadcaster TV stream (according to standard DVB EN 300 468 and technical specification TS 101 211) or via a paid service provided by a third party online EPG provider. [ 5 ]
Whole house HD- digital video recorder allows users to record programs on a centralized TV gateway DVR and then watch them on any device connected to their home network. [ 6 ]
TV gateways with whole house HD-DVR require storage to record live TV programs or schedule future recordings using the EPG.
Whole house DVR TV gateways use a number of storage mediums to store recordings: [ 6 ]
Unicast (HTTP) protocols are mainly used in consumer grade TV gateways to provide a small number of simultaneous users with the flexibility to view multiple channels.
Multicast (UDP), is mainly used in professional-grade TV gateways to enable efficient broadcast of a preset number of channels to a large number of simultaneous viewers. Multicast TV gateways are used primarily by IPTV broadcasters, hotels, hospitals, and digital signage applications.
Universal plug and play (UPnP) is a set of communication protocols that permits network devices, to seamlessly discover each other's presence on the network and establish functional network services for data sharing, communications, and entertainment.
M3U - The m3u8 file format is a de facto standard playlist format suitable for carrying lists of media file URL. Advanced TV gateways use M3U in addition to UPnP to offer better application support, allowing for faster channel zapping and provide a preset channel list. | https://en.wikipedia.org/wiki/TV_gateway |
Two Wide-Angle Imaging Neutral-Atom Spectrometers ( TWINS ) are a pair of NASA instruments aboard two United States National Reconnaissance Office (NRO) satellites in Molniya orbits . TWINS was designed to provide stereo images of the Earth's ring current . The first instrument, TWINS-1 , was launched aboard USA-184 on 28 June 2006. TWINS-2 followed aboard USA-200 on 13 March 2008.
Each instrument consists of an energetic neutral atom imager and a Lyman alpha detector. The ENA imager provides indirect remote sensing of the ring current ions, and the Lyman alpha detector gives a measure of the neutral hydrogen cloud about the Earth , known as the geocorona . The TWINS prime mission lasted two years, from 2008 to 2010, and has been followed by an extended mission which is ongoing. [ 1 ]
Launched as missions of opportunity (MoO) aboard classified, non-NASA U.S. government spacecraft, TWINS conducts stereoscopic imaging of Earth's magnetosphere . By imaging the charge exchange neutral atoms over a broad energy range (~1-100 keV ) using two identical instruments on two widely spaced high-altitude, high-inclination spacecraft, TWINS enables the 3-dimensional visualization and the resolution of large scale structures and dynamics within the magnetosphere for the first time. In contrast to traditional space experiments, which make measurements at only one point in space, imaging experiments provide simultaneous viewing of different regions of the magnetosphere. Stereo imaging, as done by TWINS, takes the next step of producing 3-D images, and provides a leap ahead in our understanding of the global aspects of the terrestrial magnetosphere.
The ENA imagers observe energetic neutrals produced from the global magnetospheric ion population, over an energy range of 1 to 100 keV with high angular (4°) and time (about 1-minute) resolution. A Lyman-alpha geocoronal imager is used to monitor cold exospheric hydrogen atoms that produce ENAs from ions via charge exchange . Complementing these imagers are detectors that measure the local charged particle environment around the spacecraft. The offset in the orbital phases ( apogees at different times) of TWINS 1 and TWINS 2 means that in addition to stereo ENA imaging for several hours twice per day, the two TWINS instruments also obtain essentially continuous magnetospheric observations. The TWINS instrumentation is essentially the same as the MENA instrument on the IMAGE spacecraft. This instrumentation consists of a neutral atom imager covering the ~1-100 keV energy range with 4° x 4° angular resolution and 1-minute time resolution, and a simple Lyman-alpha imager to monitor the geocorona.
TWINS provides stereo imaging of the Earth's magnetosphere, the region surrounding the planet controlled by its magnetic field and containing the Van Allen radiation belts and other energetic charged particles. TWINS enables three-dimensional global visualization of this region, leading to greatly enhanced understanding of the connections between different regions of the magnetosphere and their relation to the solar wind . [ 2 ]
Routine stereo imaging by TWINS began on 15 June 2008, during an extremely weak geomagnetic storm whose Dst index never fell below -40 nT , as compared to a nominal Dst of -100 nT for classification as a storm. During the TWINS prime mission (2008–2010), an extended and unprecedented solar minimum (from solar cycle 23) prevailed, bringing with it very calm magnetospheric conditions ranging from dead quiet to mildly disturbed. During this time period TWINS observed numerous weak storms, roughly once every 27 days (corresponding to the solar rotation period and triggered by solar corotating interaction regions (CIRs). The strongest storm (which was still very mild) observed by TWINS during its prime mission was on 22 July 2009, with Dst reaching a moderate -79 nT. Throughout these extended quiet conditions TWINS images contained ENA signals from both high-altitude (ring current) and low-altitude emission (LAE) regions.
The TWINS Mission of Opportunity maintains a library of selected storm events. [ 3 ] | https://en.wikipedia.org/wiki/TWINS |
TYPSET is an early document editor that was used with the 1964-released RUNOFF program, one of the earliest text formatting programs to see significant use. [ 1 ]
Of two earlier print/formatting programs DITTO and TJ-2 , only the latter had, and introduced, text justification ; RUNOFF also added pagination .
The name RUNOFF, and similar names led to other formatting program implementations. By 1982, Runoff (a name not possible before lowercase letters were introduced to filenames) largely became associated with Digital Equipment Corporation and Unix computers. DEC used the terms VAX DSR and DSR to refer to VAX DIGITAL Standard Runoff . [ 2 ]
The original RUNOFF type-setting program for CTSS was written by Jerome H. Saltzer circa 1964. Bob Morris and Doug McIlroy translated that from MAD to BCPL . [ 3 ] Morris and McIlroy then moved the BCPL version to Multics when the IBM 7094 on which CTSS ran was being shut down.
Documentation for the Multics version of RUNOFF described it as "types out text segments in manuscript form." [ 4 ]
A later version of runoff for Multics was written in PL/I by Dennis Capps, in 1974. [ 5 ] This runoff code was the ancestor of roff that was written for the fledgling Unix in assembly language by Ken Thompson .
Other versions of Runoff were developed for various computer systems including Digital Equipment Corporation 's PDP-11 minicomputer systems running RT-11 , RSTS/E , RSX on Digital's PDP-10 [ 6 ] and for OpenVMS on VAX minicomputers, as well as UNIVAC Series 90 mainframes using the EDT text editor under the VS/9 operating system . These different releases of Runoff typically had little in common except the convention of indicating a command to Runoff by beginning the line with a period.
The origin of IBM's SCRIPT software began in 1968 when IBM contracted Stuart Madnick of MIT to write a simple document preparation tool [ 7 ] for CP/67 , [ 8 ] which he modelled on MIT 's CTSS RUNOFF. [ 9 ]
RUNOFF was written in 1964 for the CTSS operating system by Jerome H. Saltzer in MAD and FAP .
It actually consisted of a pair of programs, TYPSET (which was basically a document editor), and RUNOFF (the output processor). RUNOFF had support for pagination and headers, as well as text justification ( TJ-2 appears to have been the earliest text justification system, but it did not have the other capabilities).
RUNOFF is a direct predecessor of the runoff document formatting program of Multics , which in turn was the ancestor of the roff and nroff document formatting programs of Unix , and their descendants. It was also the ancestor of FORMAT for the IBM System/360 , and of course indirectly of every computerized word processing system.
Likewise, RUNOFF for CTSS was the predecessor of the various RUNOFFs for DEC 's operating systems, via the RUNOFF developed by the University of California, Berkeley 's Project Genie for the SDS 940 system. [ 10 ] [ 11 ]
The name is alleged to have come from the phrase at the time, I'll run off a copy . [ 12 ]
TYPESET contains features inspired by a variety of other programs including Colossal Typewriter and Expensive Typewriter . [ 13 ]
Input:
Output: | https://en.wikipedia.org/wiki/TYPSET_and_RUNOFF |
The T-Layout is an architectural and design concept for web applications , specifically tailored to improve the user experience on mobile devices. It features a horizontally scrollable container divided into three distinct sections, each spanning the full width of the screen, and was developed to optimise space usage and streamline navigation. [ 1 ] [ 2 ] [ 3 ]
The T-Layout introduces horizontal scrolling as a complementary method to the conventional pop-up-based navigation system in mobile web applications. [ 4 ] In this layout , the central section which is visible by default upon accessing the application, facilitates the main content of a URL address and is flanked by two "helper" sections. This approach minimises the need for extensive user movements, in order to reach navigation controls typically located at the top of the screen. [ 5 ] [ 6 ]
It is aimed at enhancing the user experience on mobile devices by providing an easier way to access essential content such as the main navigation, e-commerce related screens, or user account related information, ensuring that those elements are readily accessible while requiring minimal user effort. [ 7 ]
The T-Layout was first implemented by E (e-streetwear.com) in their mobile web app layout, and it was inspired by the interfaces of well-tested native mobile apps like Instagram and Revolut . [ 5 ] [ 1 ]
A study titled "Mobile Navigation and User Preferences Survey" indicated a preference among mobile app users for one-handed usage, primarily navigating with their thumb. [ 8 ] These insights led to the T-Layout Experiment, which compared the efficiency of using swipe gestures to access navigational elements against reaching traditional navigation controls. [ 2 ]
It was first released as the mobile layout of E in early 2023. It was originally developed based on six principles: user-centric functionality , lightweight filesize , HTML and CSS implementation with minimal or no use of JavaScript required, suitable both for browser and server-rendering architectures , intuitive design , and improved SEO . [ 4 ] [ 7 ]
The development of the T-Layout was driven by the necessity for more ergonomic and user-friendly interfaces in mobile web applications. Its design, reminiscent of the letter 'T', emerged as a solution to several usability challenges mobile device users face, emphasising ease of access and efficient screen space utilisation. [ 1 ]
In July 2023, E formalised the concept and its technical specifications , introducing it to the web design and development community. [ 7 ]
In October 2023 the "Mobile Navigation and User Preferences Survey" was conducted, establishing that the vast majority of individuals prefer to use mobile applications by holding the phone in a one-handed grip, utilising only the thumb for gestures when possible. [ 8 ]
The subsequent "T-Layout Experiment", designed to measure the time in seconds and the distance (user effort) in pixels, required to access navigational elements by traditionally tapping on fixed-positioned controls compared to swiping anywhere on the screen. The results proved that swipe gestures require less time and much less effort. [ 2 ]
The main characteristic of the T-Layout is its horizontal scrolling feature, which can improve navigation efficiency while preserving the functionality of traditionally structured user interfaces . [ 5 ]
Its Implementation can be achieved with a combination of HTML and styling with CSS as well as precompiled Scss and Sass , CSS-in-JS , and styled JSX . It can be either a purely HTML/CSS solution but JavaScript can be utilised as well to add more specific functionalities, while It can be implemented to both existing and new applications. Its application in server-side rendering architectures will ensure that all its underlying principles apply. [ 4 ] [ 7 ]
Although principally each section in the layout has a distinct role and facilitates specific types of content, the T-Layout as a concept is versatile, and it is adaptable allowing modifications in the layout or how it's implemented to cater to the specific needs of different applications. [ 8 ] | https://en.wikipedia.org/wiki/T_Layout |
T cells are one of the important types of white blood cells of the immune system and play a central role in the adaptive immune response . T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell surface .
T cells are born from hematopoietic stem cells , [ 1 ] found in the bone marrow . Developing T cells then migrate to the thymus gland to develop (or mature). T cells derive their name from the thymus . [ 2 ] [ 3 ] After migration to the thymus, the precursor cells mature into several distinct types of T cells. T cell differentiation also continues after they have left the thymus. Groups of specific, differentiated T cell subtypes have a variety of important functions in controlling and shaping the immune response .
One of these functions is immune-mediated cell death, and it is carried out by two major subtypes: CD8 + "killer" (cytotoxic) and CD4 + "helper" T cells. (These are named for the presence of the cell surface proteins CD8 or CD4 .) CD8 + T cells, also known as "killer T cells", are cytotoxic – this means that they are able to directly kill virus-infected cells, as well as cancer cells. CD8 + T cells are also able to use small signalling proteins, known as cytokines , to recruit other types of cells when mounting an immune response. A different population of T cells, the CD4 + T cells, function as "helper cells". Unlike CD8 + killer T cells, the CD4 + helper T (T H ) cells function by further activating memory B cells and cytotoxic T cells, which leads to a larger immune response. The specific adaptive immune response regulated by the T H cell depends on its subtype (such as T-helper1, T-helper2, T-helper17, regulatory T-cell), [ 4 ] which is distinguished by the types of cytokines they secrete. [ 2 ]
Regulatory T cells are yet another distinct population of T cells that provide the critical mechanism of tolerance , whereby immune cells are able to distinguish invading cells from "self". This prevents immune cells from inappropriately reacting against one's own cells, known as an " autoimmune " response. For this reason, these regulatory T cells have also been called "suppressor" T cells. These same regulatory T cells can also be co-opted by cancer cells to prevent the recognition of, and an immune response against, tumor cells.
All T cells originate from c-kit + Sca1 + haematopoietic stem cells (HSC) which reside in the bone marrow. In some cases, the origin might be the foetal liver during embryonic development . The HSC then differentiate into multipotent progenitors (MPP) which retain the potential to become both myeloid and lymphoid cells . The process of differentiation then proceeds to a common lymphoid progenitor (CLP), which can only differentiate into T, B or NK cells. [ 5 ] These CLP cells then migrate via the blood to the thymus, where they engraft: . Henceforth they are known as thymocytes , the immature stage of a T cell.
The earliest cells which arrived in the thymus are commonly termed double-negative , as they express neither the CD4 nor CD8 co-receptor. The newly arrived CLP cells are CD4 − CD8 − CD44 + CD25 − ckit + cells, and are termed early thymic progenitor (ETP) cells. [ 6 ] These cells will then undergo a round of division and downregulate c-kit and are termed double-negative one (DN1) cells. To become T cells, the thymocytes must undergo multiple DN stages as well as positive selection and negative selection.
Double negative thymocytes can be identified by the surface expression of CD2 , CD5 and CD7 . Still during the double negative stages, CD34 expression stops and CD1 is expressed. Expression of both CD4 and CD8 makes them double positive , and matures into either CD4 + or CD8 + cells.
A critical step in T cell maturation is making a functional T cell receptor (TCR). Each mature T cell will ultimately contain a unique TCR that reacts to a random pattern, allowing the immune system to recognize many different types of pathogens . This process is essential in developing immunity to threats that the immune system has not encountered before, since due to random variation there will always be at least one TCR to match any new pathogen.
A thymocyte can only become an active T cell when it survives the process of developing a functional TCR. The TCR consists of two major components, the alpha and beta chains. These both contain random elements designed to produce a wide variety of different TCRs, but due to this huge variety they must be tested to make sure they work at all. First, the thymocytes attempt to create a functional beta chain, testing it against a 'mock' alpha chain. Then they attempt to create a functional alpha chain. Once a working TCR has been produced, the cells then must test if their TCR will identify threats correctly, and to do this it is required to recognize the body’s major histocompatibility complex (MHC) in a process known as positive selection. The thymocyte must also ensure that it does not react adversely to "self" antigens , called negative selection. If both positive and negative selection are successful, the TCR becomes fully operational and the thymocyte becomes a T cell.
At the DN2 stage (CD44 + CD25 + ), cells upregulate the recombination genes RAG1 and RAG2 and re-arrange the TCRβ locus, combining V-D-J recombination and constant region genes in an attempt to create a functional TCRβ chain. As the developing thymocyte progresses through to the DN3 stage (CD44 − CD25 + ), the thymocyte expresses an invariant α-chain called pre-Tα alongside the TCRβ gene. If the rearranged β-chain successfully pairs with the invariant α-chain, signals are produced which cease rearrangement of the β-chain (and silence the alternate allele). [ 7 ] Although these signals require the pre-TCR at the cell surface, they are independent of ligand binding to the pre-TCR. If the chains successfully pair a pre-TCR forms, and the cell downregulates CD25 and is termed a DN4 cell (CD25 − CD44 − ). These cells then undergo a round of proliferation, and begin to re-arrange the TCRα locus during the double-positive stage.
The process of positive selection takes 3 to 4 days and occurs in the thymic cortex. [ 8 ] Double-positive thymocytes (CD4 + /CD8 + ) migrate deep into the thymic cortex , where they are presented with self- antigens . These self-antigens are expressed by thymic cortical epithelial cells on MHC molecules, which reside on the surface of cortical epithelial cells. Only thymocytes that interact well with MHC-I or MHC-II will receive a vital "survival signal", while those that cannot interact strongly enough will receive no signal and die from neglect . This process ensures that the surviving thymocytes will have an 'MHC affinity' that means they will exhibit stronger binding affinity for specific MHC alleles in that organism. [ 9 ] The vast majority of developing thymocytes will not pass positive selection, and die during this process. [ 10 ]
A thymocyte's fate is determined during positive selection. Double-positive cells (CD4 + /CD8 + ) that interact well with MHC class II molecules will eventually become CD4 + "helper" cells, whereas thymocytes that interact well with MHC class I molecules mature into CD8 + "killer" cells. A thymocyte becomes a CD4 + cell by down-regulating expression of its CD8 cell surface receptors. If the cell does not lose its signal, it will continue downregulating CD8 and become a CD4 + , both CD8 + and CD4 + cells are now single positive cells. [ 11 ]
This process does not filter for thymocytes that may cause autoimmunity . The potentially autoimmune cells are removed by the following process of negative selection, which occurs in the thymic medulla.
Negative selection removes thymocytes that are capable of strongly binding with "self" MHC molecules. Thymocytes that survive positive selection migrate towards the boundary of the cortex and medulla in the thymus. While in the medulla, they are again presented with a self-antigen presented on the MHC complex of medullary thymic epithelial cells (mTECs). [ 12 ] mTECs must be Autoimmune regulator positive (AIRE + ) to properly express tissue-specific antigens on their MHC class I peptides. Some mTECs are phagocytosed by thymic dendritic cells ; this makes them AIRE − antigen presenting cells (APCs), allowing for presentation of self-antigens on MHC class II molecules (positively selected CD4 + cells must interact with these MHC class II molecules, thus APCs, which possess MHC class II, must be present for CD4 + T-cell negative selection). Thymocytes that interact too strongly with the self-antigen receive an apoptotic signal that leads to cell death. However, some of these cells are selected to become Treg cells. The remaining cells exit the thymus as mature naive T cells , also known as recent thymic emigrants. [ 13 ] This process is an important component of central tolerance and serves to prevent the formation of self-reactive T cells that are capable of inducing autoimmune diseases in the host.
β-selection is the first checkpoint, where thymocytes that are able to form a functional pre-TCR (with an invariant alpha chain and a functional beta chain) are allowed to continue development in the thymus. Next, positive selection checks that thymocytes have successfully rearranged their TCRα locus and are capable of recognizing MHC molecules with appropriate affinity. Negative selection in the medulla then eliminates thymocytes that bind too strongly to self-antigens expressed on MHC molecules. These selection processes allow for tolerance of self by the immune system. Typical naive T cells that leave the thymus (via the corticomedullary junction) are self-restricted, self-tolerant, and single positive.
About 98% of thymocytes die during the development processes in the thymus by failing either positive selection or negative selection, whereas the other 2% survive and leave the thymus to become mature immunocompetent T cells. [ 14 ] The thymus contributes fewer cells as a person ages. As the thymus shrinks by about 3% [ 15 ] a year throughout middle age, a corresponding fall in the thymic production of naive T cells occurs, leaving peripheral T cell expansion and regeneration to play a greater role in protecting older people.
T cells are grouped into a series of subsets based on their function. CD4 and CD8 T cells are selected in the thymus, but undergo further differentiation in the periphery to specialized cells which have different functions. T cell subsets were initially defined by function, but also have associated gene or protein expression patterns.
T helper cells (T H cells) assist other lymphocytes, including the maturation of B cells into plasma cells and memory B cells , and activation of cytotoxic T cells and macrophages . These cells are also known as CD4 + T cells as they express the CD4 glycoprotein on their surfaces. Helper T cells become activated when they are presented with peptide antigens by MHC class II molecules, which are expressed on the surface of antigen-presenting cells (APCs). Once activated, they divide rapidly and secrete cytokines that regulate or assist the immune response. These cells can differentiate into one of several subtypes, which have different roles. Cytokines direct T cells into particular subtypes. [ 16 ]
Cytotoxic T cells (T C cells, CTLs, T-killer cells, killer T cells) destroy virus-infected cells and tumor cells, and are also implicated in transplant rejection. These cells are defined by the expression of the CD8 protein on their cell surface. Cytotoxic T cells recognize their targets by binding to short peptides (8-11 amino acids in length) associated with MHC class I molecules, present on the surface of all nucleated cells. Cytotoxic T cells also produce the key cytokines IL-2 and IFNγ. These cytokines influence the effector functions of other cells, in particular macrophages and NK cells.
Antigen-naive T cells expand and differentiate into memory and effector T cells after they encounter their cognate antigen within the context of an MHC molecule on the surface of a professional antigen presenting cell (e.g. a dendritic cell). Appropriate co-stimulation must be present at the time of antigen encounter for this process to occur. Historically, memory T cells were thought to belong to either the effector or central memory subtypes, each with their own distinguishing set of cell surface markers (see below). [ 20 ] Subsequently, numerous new populations of memory T cells were discovered including tissue-resident memory T (Trm) cells, stem memory TSCM cells, and virtual memory T cells. The single unifying theme for all memory T cell subtypes is that they are long-lived and can quickly expand to large numbers of effector T cells upon re-exposure to their cognate antigen. By this mechanism they provide the immune system with "memory" against previously encountered pathogens. Memory T cells may be either CD4 + or CD8 + and usually express CD45RO . [ 21 ]
Memory T cell subtypes:
Regulatory T cells are crucial for the maintenance of immunological tolerance . Their major role is to shut down T cell–mediated immunity toward the end of an immune reaction and to suppress autoreactive T cells that escaped the process of negative selection in the thymus.
Two major classes of CD4 + T reg cells have been described—FOXP3 + T reg cells and FOXP3 − T reg cells.
Regulatory T cells can develop either during normal development in the thymus, and are then known as thymic Treg cells, or can be induced peripherally and are called peripherally derived Treg cells. These two subsets were previously called "naturally occurring" and "adaptive" (or "induced"), respectively. [ 27 ] Both subsets require the expression of the transcription factor FOXP3 which can be used to identify the cells. Mutations of the FOXP3 gene can prevent regulatory T cell development, causing the fatal autoimmune disease IPEX .
Several other types of T cells have suppressive activity, but do not express FOXP3 constitutively. These include Tr1 and Th3 cells, which are thought to originate during an immune response and act by producing suppressive molecules. Tr1 cells are associated with IL-10, and Th3 cells are associated with TGF-beta . Recently, Th17 cells have been added to this list. [ 28 ]
Innate-like T cells or unconventional T cells represent some subsets of T cells that behave differently in immunity. They trigger rapid immune responses, regardless of the major histocompatibility complex (MHC) expression, unlike their conventional counterparts (CD4 T helper cells and CD8 cytotoxic T cells), which are dependent on the recognition of peptide antigens in the context of the MHC molecule. Overall, there are three large populations of unconventional T cells: NKT cells, MAIT cells, and gammadelta T cells. Now, their functional roles are already being well established in the context of infections and cancer. [ 29 ] Furthermore, these T cell subsets are being translated into many therapies against malignancies such as leukemia, for example. [ 30 ]
Natural killer T cells (NKT cells – not to be confused with natural killer cells of the innate immune system) bridge the adaptive immune system with the innate immune system . Unlike conventional T cells that recognize protein peptide antigens presented by major histocompatibility complex (MHC) molecules, NKT cells recognize glycolipid antigens presented by CD1d . Once activated, these cells can perform functions ascribed to both helper and cytotoxic T cells: cytokine production and release of cytolytic/cell killing molecules. They are also able to recognize and eliminate some tumor cells and cells infected with herpes viruses. [ 31 ]
Mucosal associated invariant T (MAIT) cells display innate , effector-like qualities. [ 32 ] [ 33 ] In humans, MAIT cells are found in the blood, liver, lungs, and mucosa , defending against microbial activity and infection. [ 32 ] The MHC class I -like protein, MR1 , is responsible for presenting bacterially-produced vitamin B metabolites to MAIT cells. [ 34 ] [ 35 ] [ 36 ] After the presentation of foreign antigen by MR1, MAIT cells secrete pro-inflammatory cytokines and are capable of lysing bacterially-infected cells. [ 32 ] [ 36 ] MAIT cells can also be activated through MR1-independent signaling. [ 36 ] In addition to possessing innate-like functions, this T cell subset supports the adaptive immune response and has a memory-like phenotype. [ 32 ] Furthermore, MAIT cells are thought to play a role in autoimmune diseases , such as multiple sclerosis , arthritis and inflammatory bowel disease , [ 37 ] [ 38 ] although definitive evidence is yet to be published. [ 39 ] [ 40 ] [ 41 ] [ 42 ]
Gamma delta T cells (γδ T cells) represent a small subset of T cells which possess a γδ TCR rather than the αβ TCR on the cell surface. The majority of T cells express αβ TCR chains. This group of T cells is much less common in humans and mice (about 2% of total T cells) and are found mostly in the gut mucosa , within a population of intraepithelial lymphocytes . In rabbits, sheep, and chickens, the number of γδ T cells can be as high as 60% of total T cells. The antigenic molecules that activate γδ T cells are still mostly unknown. However, γδ T cells are not MHC-restricted and seem to be able to recognize whole proteins rather than requiring peptides to be presented by MHC molecules on APCs . Some murine γδ T cells recognize MHC class IB molecules. Human γδ T cells that use the Vγ9 and Vδ2 gene fragments constitute the major γδ T cell population in peripheral blood. These cells are unique in that they specifically and rapidly respond to a set of nonpeptidic phosphorylated isoprenoid precursors, collectively named phosphoantigens , which are produced by virtually all living cells. The most common phosphoantigens from animal and human cells (including cancer cells) are isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMPP). Many microbes produce the active compound hydroxy-DMAPP ( HMB-PP ) and corresponding mononucleotide conjugates, in addition to IPP and DMAPP. Plant cells produce both types of phosphoantigens. Drugs activating human Vγ9/Vδ2 T cells comprise synthetic phosphoantigens and aminobisphosphonates , which upregulate endogenous IPP/DMAPP.
Activation of CD4 + T cells occurs through the simultaneous engagement of the T-cell receptor and a co-stimulatory molecule (like CD28 , or ICOS ) on the T cell by the major histocompatibility complex (MHCII) peptide and co-stimulatory molecules on the APC . Both are required for production of an effective immune response; in the absence of co-stimulation , T cell receptor signalling alone results in anergy . The signalling pathways downstream from co-stimulatory molecules usually engages the PI3K pathway generating PIP3 at the plasma membrane and recruiting PH domain containing signaling molecules like PDK1 that are essential for the activation of PKC-θ , and eventual IL-2 production. Optimal CD8 + T cell response relies on CD4 + signalling. [ 44 ] CD4 + cells are useful in the initial antigenic activation of naive CD8 T cells, and sustaining memory CD8 + T cells in the aftermath of an acute infection. Therefore, activation of CD4 + T cells can be beneficial to the action of CD8 + T cells. [ 45 ] [ 46 ] [ 47 ]
The first signal is provided by binding of the T cell receptor to its cognate peptide presented on MHCII on an APC. MHCII is restricted to so-called professional antigen-presenting cells , like dendritic cells, B cells, and macrophages, to name a few. The peptides presented to CD8 + T cells by MHC class I molecules are 8–13 amino acids in length; the peptides presented to CD4 + cells by MHC class II molecules are longer, usually 12–25 amino acids in length, [ 48 ] as the ends of the binding cleft of the MHC class II molecule are open.
The second signal comes from co-stimulation, in which surface receptors on the APC are induced by a relatively small number of stimuli, usually products of pathogens, but sometimes breakdown products of cells, such as necrotic -bodies or heat shock proteins . The only co-stimulatory receptor expressed constitutively by naive T cells is CD28, so co-stimulation for these cells comes from the CD80 and CD86 proteins, which together constitute the B7 protein, (B7.1 and B7.2, respectively) on the APC. Other receptors are expressed upon activation of the T cell, such as OX40 and ICOS, but these largely depend upon CD28 for their expression. The second signal licenses the T cell to respond to an antigen. Without it, the T cell becomes anergic , and it becomes more difficult for it to activate in future. This mechanism prevents inappropriate responses to self, as self-peptides will not usually be presented with suitable co-stimulation. Once a T cell has been appropriately activated (i.e. has received signal one and signal two) it alters its cell surface expression of a variety of proteins. Markers of T cell activation include CD69, CD71 and CD25 (also a marker for Treg cells), and HLA-DR (a marker of human T cell activation). CTLA-4 expression is also up-regulated on activated T cells, which in turn outcompetes CD28 for binding to the B7 proteins. This is a checkpoint mechanism to prevent over activation of the T cell. Activated T cells also change their cell surface glycosylation profile. [ 49 ]
The T cell receptor exists as a complex of several proteins. The actual T cell receptor is composed of two separate peptide chains, which are produced from the independent T cell receptor alpha and beta ( TCRα and TCRβ ) genes. The other proteins in the complex are the CD3 proteins: CD3εγ and CD3εδ heterodimers and, most important, a CD3ζ homodimer, which has a total of six ITAM motifs. The ITAM motifs on the CD3ζ can be phosphorylated by Lck and in turn recruit ZAP-70 . Lck and/or ZAP-70 can also phosphorylate the tyrosines on many other molecules, not least CD28, LAT and SLP-76 , which allows the aggregation of signalling complexes around these proteins.
Phosphorylated LAT recruits SLP-76 to the membrane, where it can then bring in PLC-γ , VAV1 , Itk and potentially PI3K . PLC-γ cleaves PI(4,5)P2 on the inner leaflet of the membrane to create the active intermediaries diacylglycerol ( DAG ), inositol-1,4,5-trisphosphate ( IP3 ); PI3K also acts on PIP2, phosphorylating it to produce phosphatidlyinositol-3,4,5-trisphosphate (PIP3). DAG binds and activates some PKCs. Most important in T cells is PKC-θ, critical for activating the transcription factors NF-κB and AP-1. IP3 is released from the membrane by PLC-γ and diffuses rapidly to activate calcium channel receptors on the ER , which induces the release of calcium into the cytosol. Low calcium in the endoplasmic reticulum causes STIM1 clustering on the ER membrane and leads to activation of cell membrane CRAC channels that allows additional calcium to flow into the cytosol from the extracellular space. This aggregated cytosolic calcium binds calmodulin, which can then activate calcineurin . Calcineurin, in turn, activates NFAT , which then translocates to the nucleus. NFAT is a transcription factor that activates the transcription of a pleiotropic set of genes, most notable, IL-2, a cytokine that promotes long-term proliferation of activated T cells.
PLC-γ can also initiate the NF-κB pathway . DAG activates PKC-θ, which then phosphorylates CARMA1, causing it to unfold and function as a scaffold. The cytosolic domains bind an adapter BCL10 via CARD (Caspase activation and recruitment domains) domains; that then binds TRAF6, which is ubiquitinated at K63. : 513–523 [ 50 ] This form of ubiquitination does not lead to degradation of target proteins. Rather, it serves to recruit NEMO, IKKα and -β, and TAB1-2/ TAK1. [ 51 ] TAK 1 phosphorylates IKK-β, which then phosphorylates IκB allowing for K48 ubiquitination: leads to proteasomal degradation. Rel A and p50 can then enter the nucleus and bind the NF-κB response element. This coupled with NFAT signaling allows for complete activation of the IL-2 gene. [ 50 ]
While in most cases activation is dependent on TCR recognition of antigen, alternative pathways for activation have been described. For example, cytotoxic T cells have been shown to become activated when targeted by other CD8 T cells leading to tolerization of the latter. [ 52 ]
In spring 2014, the T-Cell Activation in Space (TCAS) experiment was launched to the International Space Station on the SpaceX CRS-3 mission to study how "deficiencies in the human immune system are affected by a microgravity environment". [ 53 ]
T cell activation is modulated by reactive oxygen species . [ 54 ]
A unique feature of T cells is their ability to discriminate between healthy and abnormal (e.g. infected or cancerous) cells in the body. [ 55 ] Healthy cells typically express a large number of self derived pMHC on their cell surface and although the T cell antigen receptor can interact with at least a subset of these self pMHC, the T cell generally ignores these healthy cells. However, when these very same cells contain even minute quantities of pathogen derived pMHC, T cells are able to become activated and initiate immune responses. The ability of T cells to ignore healthy cells but respond when these same cells contain pathogen (or cancer) derived pMHC is known as antigen discrimination. The molecular mechanisms that underlie this process are controversial. [ 55 ] [ 56 ]
Causes of T cell deficiency include lymphocytopenia of T cells and/or defects on function of individual T cells. Complete insufficiency of T cell function can result from hereditary conditions such as severe combined immunodeficiency (SCID), Omenn syndrome , and cartilage–hair hypoplasia . [ 57 ] Causes of partial insufficiencies of T cell function include acquired immune deficiency syndrome (AIDS), and hereditary conditions such as DiGeorge syndrome (DGS), chromosomal breakage syndromes (CBSs), and B cell and T cell combined disorders such as ataxia-telangiectasia (AT) and Wiskott–Aldrich syndrome (WAS). [ 57 ]
The main pathogens of concern in T cell deficiencies are intracellular pathogens , including Herpes simplex virus , Mycobacterium and Listeria . [ 58 ] Also, fungal infections are also more common and severe in T cell deficiencies. [ 58 ]
Cancer of T cells is termed T-cell lymphoma , and accounts for perhaps one in ten cases of non-Hodgkin lymphoma . [ 59 ] The main forms of T cell lymphoma are:
T cell exhaustion is a poorly defined or ambiguous term. [ 60 ] [ 61 ] There are three approaches to its definition. [ 60 ] "The first approach primarily defines as exhausted the cells that present the same cellular dysfunction (typically, the absence of an expected effector response). The second approach primarily defines as exhausted the cells that are produced by a given cause (typically, but not necessarily, chronic exposure to an antigen). Finally, the third approach primarily defines as exhausted the cells that present the same molecular markers (typically, programmed cell death protein 1 [PD-1])." [ 60 ] Indeed, it is now starting to emerge that exhaustion might not be the only T cell dysfunctional state. [ 62 ] In fact, tolerization, anergy, cell death, ignorance, senesence and exclusion have recently emerged as additional sources and/or states of T cell dysfunction in cancer and chronic viral infection. [ 63 ]
Dysfunctional T cells are characterized by progressive loss of function, changes in transcriptional profiles and sustained expression of inhibitory receptors. At first, cells lose their ability to produce IL-2 and TNFα , which is followed by the loss of high proliferative capacity and cytotoxic potential, and eventually leads to their deletion. Exhausted T cells typically indicate higher levels of CD43 , CD69 and inhibitory receptors combined with lower expression of CD62L and CD127 . Exhaustion can develop during chronic infections, sepsis and cancer. [ 64 ] Exhausted T cells preserve their functional exhaustion even after repeated antigen exposure. [ 65 ]
T cell exhaustion can be triggered by several factors like persistent antigen exposure and lack of CD4 T cell help. [ 66 ] Antigen exposure also has effect on the course of exhaustion because longer exposure time and higher viral load increases the severity of T cell exhaustion. At least 2–4 weeks exposure is needed to establish exhaustion. [ 67 ] Another factor able to induce exhaustion are inhibitory receptors including programmed cell death protein 1 (PD1), CTLA-4 , T cell membrane protein-3 (TIM3), and lymphocyte activation gene 3 protein (LAG3). [ 68 ] [ 69 ] Soluble molecules such as cytokines IL-10 or TGF-β are also able to trigger exhaustion. [ 70 ] [ 71 ] Last known factors that can play a role in T cell exhaustion are regulatory cells. Treg cells can be a source of IL-10 and TGF-β and therefore they can play a role in T cell exhaustion. [ 72 ] Furthermore, T cell exhaustion is reverted after depletion of Treg cells and blockade of PD1. [ 73 ] T cell exhaustion can also occur during sepsis as a result of cytokine storm. Later after the initial septic encounter anti-inflammatory cytokines and pro-apoptotic proteins take over to protect the body from damage. Sepsis also carries high antigen load and inflammation. In this stage of sepsis T cell exhaustion increases. [ 74 ] [ 75 ] Currently there are studies aiming to utilize inhibitory receptor blockades in treatment of sepsis. [ 76 ] [ 77 ] [ 78 ]
While during infection T cell exhaustion can develop following persistent antigen exposure after graft transplant similar situation arises with alloantigen presence. [ 79 ] It was shown that T cell response diminishes over time after kidney transplant. [ 80 ] These data suggest T cell exhaustion plays an important role in tolerance of a graft mainly by depletion of alloreactive CD8 T cells. [ 75 ] [ 81 ] Several studies showed positive effect of chronic infection on graft acceptance and its long-term survival mediated partly by T cell exhaustion. [ 82 ] [ 83 ] [ 84 ] It was also shown that recipient T cell exhaustion provides sufficient conditions for NK cell transfer. [ 85 ] While there are data showing that induction of T cell exhaustion can be beneficial for transplantation it also carries disadvantages among which can be counted increased number of infections and the risk of tumor development. [ 86 ]
During cancer T cell exhaustion plays a role in tumor protection. According to research some cancer-associated cells as well as tumor cells themselves can actively induce T cell exhaustion at the site of tumor. [ 87 ] [ 88 ] [ 89 ] T cell exhaustion can also play a role in cancer relapses as was shown on leukemia. [ 90 ] Some studies have suggested that it is possible to predict relapse of leukemia based on expression of inhibitory receptors PD-1 and TIM-3 by T cells. [ 91 ] Many experiments and clinical trials have focused on immune checkpoint blockers in cancer therapy, with some of these approved as valid therapies that are now in clinical use. [ 92 ] Inhibitory receptors targeted by those medical procedures are vital in T cell exhaustion and blocking them can reverse these changes. [ 93 ] | https://en.wikipedia.org/wiki/T_cell |
The T centre is a radiation damage centre in silicon composed of a carbon-carbon pair (C-C) sharing a substitutional site of the silicon lattice. Additionally, one of the substitutional carbon atoms is bonded with a hydrogen atom while the other carbon contains an unpaired electron in the ground state of a dangling bond . [ 1 ] [ 2 ] Much like the nitrogen-vacancy centres in diamond, the T centre contains spin-dependent optical transitions addressable through photoluminescence . These spin-dependent transitions, however, emit light within the technologically efficient telecommunication O-band. Consequentially, the T centre is an intriguing candidate for quantum information technologies with development of integrated quantum devices benefiting from techniques within the silicon photonic community.
The T centre is a radiation damage centre in silicon. It contains a substitutional carbon-carbon pair terminated by an additional hydrogen atom within the lattice. This structure also contains a dangling bond on the other substitutional carbon. [ citation needed ]
Historically, the structure of the T centre was uncovered using spectroscopic measurements. The presence of carbon as the main constituent within the lattice was hypothesized when a shift in the defect's zero phonon line (ZPL) was observed in samples enriched with 13 C. [ 3 ] Similarly, the presence of hydrogen was determined using a shift in the ZPL in a deuterium defused sample. [ 4 ] Splitting within the local vibration modes (LVM) introduced by the presence of 13 C from 2 lines into 4 subsequent lines suggested the presence of a second carbon atom. [ 2 ] The suggested formation mechanism is, therefore, the capture of an interstitial C-H pair onto a substitutional carbon [ 5 ] [ 6 ] with a dangling bond predicted by ab initio calculations [ 2 ] [ 7 ] [ 8 ]
External field perturbation measurements are used to determine axial symmetry and orientation of luminescent transitions. Stress-dependent spectral line studies have previously suggested that rhombic I ( C 2v ) symmetry is present within the defect.; [ 3 ] however, it was later shown to have monoclinic I ( C 1h ) symmetry. [ 2 ] [ footnotes 1 ] Consequentially, the defect is expected to have 24 orientations, which form 12 optically resolvable orientation pairs under a magnetic field. These have been studied using photoluminescence spectroscopy [ 1 ]
The current formation model for the T centre contains an interstitial carbon capturing a hydrogen atom before migrating to a substitutional site with another carbon during heat treatment between 350 and 600 °C. [ 1 ] [ 2 ] [ 9 ] T centres have been observed in silicon semiconductors grown using the float-zone and Czochralski (CZ) technique [ 2 ] as well as Silicon-On-Insulator devices. [ 9 ] They are produced by irradiating the sample followed by a thermal annealing process. It has been shown that both plasma etching [ 10 ] as well as irradiating the sample with either neutrons or electrons [ 2 ] may produce the desired radiation centre. Hydrogen may be introduced through water vapour or in its gaseous state, or it may be present within the sample. An excess of hydrogen may, however, fill the dangling bond and render the radiation damage center optically inert. [ 11 ] Alternatively, rather than irradiating the sample and treating it with a subsequent thermal annealing process, T centres may be developed using only a thermal treatment in carbon rich CZ grown silicon. [ 12 ]
The T centre's zero-phonon line photoluminescence feature is near 935 meV. This represents a transition from an unpaired electron in the ground state to a bound exciton within the first excited state. The 1.8 meV-split doublet is the result of two states within the same defect. [ citation needed ]
The inhomogeneous linewidth for this feature reduces in isotopically pure silicon-28. Natural silicon contains a mixture of various isotope masses resulting in variations in both the local band gap and binding energies. Without these variations introduced from neighbouring 29 Si nuclei, the linewidth reduces from 26.9(8) μ {\displaystyle \mu } eV to 0.25 μ {\displaystyle \mu } eV. [ 1 ]
The current accepted model of the T centre proposes an unpaired electron in the ground state and an additional bound exciton in the excited states labeled T and TX respectively. [ 1 ] [ 2 ] The two electrons in the excited state pair into a spin-0 singlet and the remaining unpaired spin-3/2 hole spin state is split into two Kramers doublets TX 0 and TX 1 by the internal stress of the defect. The TX centre is characterized as a pseudo-acceptor with effective mass-like states labeled N Γ {\displaystyle N\Gamma } K ± {\displaystyle \pm } for even and odd parity. N = 1 , 2 , . . . {\displaystyle N=1,2,...} represents the principal quantum number and K {\displaystyle K} indicates the symmetry group of the state. The TX ground state is, therefore, an acceptor-like fourfold degenerate 1 Γ {\displaystyle 1\Gamma } 8 + state.
Both the ground state electron and the first excited state hole are doubly degenerate and split under the Zeeman interaction when exposed to an external magnetic field. Due to the splitting of each state, each orientation subset of the T-centre allows for 4 optical transitions from the ground state to TX 0 . For the i t h {\displaystyle i^{th}} subset, the transitions are labeled { A , B , C , D } i {\displaystyle \{A,B,C,D\}_{i}} . Characterization of these transitions is essential for hyperpolarizing the electron into the different transitions for various state manipulation protocols. [ 1 ] Further hyperfine spin interactions between the electron and hydrogen are resolved under electron paramagnetic resonance or read using optically detected magnetic resonance signals.
For a centre composed of two 12 C constituents subject to an external magnetic field B 0 {\displaystyle \mathbf {B} _{0}} , the spin Hamiltonian for the ground state is given by
H T = μ B B 0 g E S + μ N g N B 0 I + h S A I {\displaystyle {\mathcal {H_{T}}}=\mu _{B}\mathbf {B} _{0}\mathbf {g_{E}} \mathbf {S} +\mu _{N}g_{N}\mathbf {B} _{0}\mathbf {I} +h\mathbf {SAI} }
This Hamiltonian describes the coupling between the unpaired electron and the hydrogen nucleus. The coefficient μ B {\displaystyle \mu _{B}} denotes the Bohr magneton . The electron spin vector and g-factor tensor are given by S {\displaystyle \mathbf {S} } and g E {\displaystyle g_{E}} . The g-factor tensor is approximately isotropic with g E = 2.005 ( 8 ) {\displaystyle g_{E}=2.005(8)} . The hydrogen nuclear spin vector is given by I {\displaystyle \mathbf {I} } . g N {\displaystyle g_{N}} represents the hydrogen nuclear spin g-factor, and μ N {\displaystyle \mu _{N}} is the nuclear spin magneton. The hyperfine tensor A {\displaystyle \mathbf {A} } is specific to each optically resolvable orientation subset.
Both the electron and nuclear spins can by hyperpolarized using a single optical radio frequency (RF) and a selectively resonant microwave frequency (MF). [ 1 ] Continuous-wave electron paramagnetic resonance can be used to depolarize or mix the electron spin state, and the optical transitions B i {\displaystyle B_{i}} and D i {\displaystyle D_{i}} are used for state preparation. Specifically, continuously driving the B i {\displaystyle B_{i}} transition excites the | ↓ E ⟩ {\displaystyle |\downarrow _{E}\rangle } electron into the | ↓ H ⟩ {\displaystyle |\downarrow _{H}\rangle } . The state is prepared in the | ↑ E ⟩ {\displaystyle |\uparrow _{E}\rangle } spin-up state following a subsequent decay through the spin-dependent A i {\displaystyle A_{i}} transition. Alternatively, driving the D i {\displaystyle D_{i}} transition hyperpolarizes the population to the spin-down state through the C i {\displaystyle C_{i}} transition.
The T 1 lifetimes for both the electron and nuclear spin state have been measured using nuclear magnetic resonance and have been shown to far exceed 16 seconds in 28 Si. [ 1 ] The averaged electron and nuclear Hahn-echo ( T 2 ) times are 2.1(1) ms and 0.28(1)s respectively. [ footnotes 2 ] A tighter lower bound for the nuclear coherence time was found by averaging the top 10% highest measurements per time, resulting in an average maximum magnitude nuclear coherence time of T 2 N m m = 1.1 ( 2 ) {\displaystyle T_{2N}^{mm}=1.1(2)} s. | https://en.wikipedia.org/wiki/T_centre |
The T helper cells ( T h cells ), also known as CD4 + cells or CD4-positive cells , are a type of T cell that play an important role in the adaptive immune system . They aid the activity of other immune cells by releasing cytokines . They are considered essential in B cell antibody class switching , breaking cross-tolerance in dendritic cells, in the activation and growth of cytotoxic T cells , and in maximizing bactericidal activity of phagocytes such as macrophages and neutrophils . CD4 + cells are mature T h cells that express the surface protein CD4. Genetic variation in regulatory elements expressed by CD4 + cells determines susceptibility to a broad class of autoimmune diseases . [ 1 ]
T h cells contain and release cytokines to aid other immune cells. Cytokines are small protein mediators that alter the behavior of target cells that express receptors for those cytokines. These cells help polarize the immune response depending on the nature of the immunological insult (for example; virus vs. extracellular bacterium vs. intracellular bacterium vs. helminth vs. fungus vs. protist). [ citation needed ]
Mature T h cells express the surface protein CD4 and are referred to as CD4 + T cells . CD4 + T cells are generally treated as having a pre-defined role as helper T cells within the immune system . For example, when an antigen-presenting cell displays a peptide antigen on MHC class II proteins, a CD4 + cell will aid those cells through a combination of cell to cell interactions (e.g. CD40 (protein) and CD40L ) and through cytokines . [ citation needed ]
T h cells are not a monolithic immunological entity because they are diverse in terms of function and their interaction with partner cells. In general, mature naive T cells are stimulated by professional antigen presenting cells to acquire an effector function. These are defined by the presence of a lineage-determining (or lineage-specifying) transcription factor (also called master regulator , though the term has been criticized for being too reductive). [ 2 ] The loss of function in a lineage specifying transcription factor results in the absence of the corresponding class of helper T cell which can be devastating for the health of the host. [ citation needed ]
Following development in the thymus , these cells (termed recent thymic emigrants (RTE)) egress from the thymus and home to secondary lymphoid organs (SLO; spleen and lymph nodes ). Of note, only a very small minority of T cells egresses from the thymus (estimates commonly range from 1–5% but some experts feel even this is generous). [ 3 ] Maturation of RTE in SLO results in the generation of mature naive T cells (naïve meaning they have never been exposed to the antigen that they are programmed to respond to), but naive T cells now lack or have downregulated (reduced) expression of the RTE-related surface markers, such as CD31 , PTK7 , Complement Receptor 1 and 2 ( CR1 , CR2 ) and the production of interleukin 8 (IL-8) . [ 4 ] [ 5 ] Like all T cells, they express the T cell receptor - CD3 complex. The T cell receptor (TCR) consists of both constant and variable regions. The variable region determines what antigen the T cell can respond to. CD4 + T cells have TCRs with an affinity for Class II MHC , and CD4 is involved in determining MHC affinity during maturation in the thymus . Class II MHC proteins are generally only found on the surface of professional antigen-presenting cells (APCs). Professional antigen-presenting cells are primarily dendritic cells , macrophages and B cells , although dendritic cells are the only cell group that expresses MHC Class II constitutively (at all times). Some APCs also bind native (or unprocessed) antigens to their surface, such as follicular dendritic cells (these are not the same type of cells as the dendritic cells of the immune system but rather have a non-hematopoietic origin, and in general lack MHC Class II, meaning they are not true professional antigen-presenting cells; however, follicular dendritic cells may acquire MHC Class II proteins via exosomes that become attached to them [ 6 ] ). T cells require antigens to be processed into short fragments which form linear epitopes on MHC Class II (in the case of helper T cells because they express CD4) or MHC class I (in the case of cytotoxic T cells which express CD8 ). MHC Class II binding pockets are flexible with respect to the length of the peptides they hold. Generally, there are 9 core amino acid residues with several flanking amino acids which form a length of about 12–16 amino acids total [ 7 ] but have been known to hold as many as 25 amino acids. [ 8 ] By comparison, MHC Class I proteins are usually 9-10 peptides long. [ 9 ] The activation of naive T cells is commonly explained in terms of the 3-signal model, elaborated upon below. [ 10 ]
During an immune response, professional antigen-presenting cells (APCs) endocytose antigens (typically bacteria or viruses), which undergo processing , then travel from the infection site to the lymph nodes . Typically, the APC responsible is a dendritic cell. If the antigen expresses appropriate molecular patterns (sometimes known as signal 0), it can induce maturation of the dendritic cell which results in enhanced expression of costimulatory molecules needed to activate T cells (see signal 2) [ 11 ] and MHC Class II. [ 12 ] Once at the lymph nodes, the APCs begin to present antigen peptides that are bound to Class II MHC, allowing CD4 + T cells that express the specific TCRs against the peptide/MHC complex to activate. [ citation needed ]
When a T h cell encounters and recognizes the antigen on an APC, the TCR - CD3 complex binds strongly to the peptide-MHC complex present on the surface of professional APCs. CD4 , a co-receptor of the TCR complex, also binds to a different section of the MHC molecule. It is estimated that approximately 50 of these interactions are required for the activation of a helper T cell and assemblies known as microclusters have been observed forming between the TCR-CD3-CD4 complexes of the T cell and the MHC Class II proteins of the dendritic cell at the zone of contact. When these all come together, the CD4 is able to recruit a kinase called Lck which phosphorylates immunoreceptor tyrosine-based activation motifs (ITAMs) present on the CD3 gamma, delta, epsilon, and zeta chains. The protein ZAP-70 can bind these phosphorylated ITAMs via its SH2 domain and then itself becomes phosphorylated, wherein it orchestrates the downstream signaling required for T cell activation. Lck activation is controlled by the opposing actions of CD45 and Csk . [ 13 ] CD45 activates Lck by dephosphorylating a tyrosine in its C-terminal tail, while Csk phosphorylates Lck at that site. The loss of CD45 produces a form of SCID because failure to activate Lck prevents appropriate T cell signaling. Memory T cells also make use of this pathway and have higher levels of Lck expressed and the function of Csk is inhibited in these cells. [ 14 ]
The binding of the antigen-MHC to the TCR complex and CD4 may also help the APC and the T h cell adhere during T h cell activation, but the integrin protein LFA-1 on the T cell and ICAM on the APC are the primary molecules of adhesion in this cell interaction. [ citation needed ]
It is unknown what role the relatively bulky extracellular region of CD45 plays during cell interactions, but CD45 has various isoforms that change in size depending on the T h cell's activation and maturation status. For example, CD45 shortens in length following T h activation (CD45RA + to CD45RO + ), but whether this change in length influences activation is unknown. It has been proposed that the larger CD45RA may decrease the accessibility of the T cell receptor for the antigen-MHC molecule, thereby necessitating an increase in the affinity (and specificity) of the T cell for activation. However, once the activation has occurred, CD45 shortens, allowing easier interactions and activation as an effector T helper cell. [ citation needed ]
Having received the first TCR/CD3 signal, the naïve T cell must activate a second independent biochemical pathway, known as Signal 2. This verification step is a protective measure to ensure that a T cell is responding to a foreign antigen. If this second signal is not present during initial antigen exposure, the T cell presumes that it is auto-reactive. This results in the cell becoming anergic (anergy is generated from the unprotected biochemical changes of Signal 1). Anergic cells will not respond to any antigen in the future, even if both signals are present later on. These cells are generally believed to circulate throughout the body with no value until they undergo apoptosis . [ 15 ]
The second signal involves an interaction between CD28 on the CD4 + T cell and the proteins CD80 (B7.1) or CD86 (B7.2) on the professional APCs. Both CD80 and CD86 activate the CD28 receptor. These proteins are also known as co-stimulatory molecules . [ citation needed ]
Although the verification stage is necessary for the activation of naïve helper T cells, the importance of this stage is best demonstrated during the similar activation mechanism of CD8 + cytotoxic T cells . As naïve CD8 + T cells have no true bias towards foreign sources, these T cells must rely on the activation of CD28 for confirmation that they recognize a foreign antigen (as CD80/CD86 is only expressed by active APC's). CD28 plays an important role in decreasing the risk of T cell auto-immunity against host antigens. [ citation needed ]
Once the naïve T cell has both pathways activated, the biochemical changes induced by Signal 1 are altered, allowing the cell to activate instead of undergoing anergy. The second signal is then obsolete; only the first signal is necessary for future activation. This is also true for memory T cells, which is one example of learned immunity . Faster responses occur upon reinfection because memory T cells have already undergone confirmation and can produce effector cells much sooner. [ citation needed ]
Once the two-signal activation is complete the T helper cell (T h ) then allows itself to proliferate . It achieves this by releasing a potent T cell growth factor called interleukin 2 (IL-2) which acts upon itself in an autocrine fashion. Activated T cells also produce the alpha sub-unit of the IL-2 receptor ( CD25 or IL-2R), enabling a fully functional receptor that can bind with IL-2, which in turn activates the T cell's proliferation pathways. [ citation needed ]
The autocrine or paracrine secretion of IL-2 can bind to that same T h cell or neighboring T h 's via the IL-2R thus driving proliferation and clonal expansion. The T h cells receiving both signals of activation and proliferation will then become T h 0 (T helper 0) cells that secrete IL-2, IL-4 and interferon gamma (IFN-γ). The T h 0 cells will then differentiate into T h 1 or T h 2 cells depending on cytokine environment. IFN-γ drives T h 1 cell production while IL-10 and IL-4 inhibit T h 1 cell production. Conversely, IL-4 drives T h 2 cell production and IFN-γ inhibits T h 2 cells. These cytokines are pleiotropic and carry out many other functions of the immune response. [ citation needed ]
In 1991, three groups reported discovering CD154, which is the molecular basis of T cell helper function. Seth Lederman at Columbia University generated a murine monoclonal antibody, 5c8 that inhibited contact-dependent T cell helper function in human cells which characterized the 32 kDa surface protein transiently expressed on CD4 + T cells. [ 16 ] Richard Armitage at Immunex cloned a cDNA encoding CD154 by screening an expression library with CD40-Ig. [ 17 ] Randolph Noelle at Dartmouth Medical School generated an antibody that bound a 39 kDa protein on murine T cells and inhibited helper function. [ 18 ]
Helper T cells are capable of influencing a variety of immune cells, and the T cell response generated (including the extracellular signals such as cytokines ) can be essential for a successful outcome from infection. In order to be effective, helper T cells must determine which cytokines will allow the immune system to be most useful or beneficial for the host. Understanding exactly how helper T cells respond to immune challenges is currently of major interest in immunology , because such knowledge may be very useful in the treatment of disease and in increasing the effectiveness of vaccination . [ citation needed ]
Proliferating helper T cells that develop into effector T cells differentiate into two major subtypes of cells known as T h 1 and T h 2 cells (also known as Type 1 and Type 2 helper T cells, respectively). [ citation needed ]
T h 1 helper cells lead to an increased cell-mediated response (primarily by macrophages and cytotoxic T cells ), [ 19 ] typically against intracellular bacteria and protozoa. They are triggered by the polarising cytokine IL-12 and their effector cytokines are IFN-γ and IL-2. The main effector cells of T h 1 immunity are macrophages as well as CD8 T cells, IgG B cells, and IFN-γ CD4 T cells. The key T h 1 transcription factors are STAT4 and T-bet. IFN-γ secreted by CD4 T cells can activate macrophages to phagocytose and digest intracellular bacteria and protozoa. In addition, IFN-γ can activate iNOS (inducible nitric oxide synthase) to produce nitric oxide free radicals to directly kill intracellular bacteria and protozoa. T h 1 overactivation against autoantigens will cause Type IV or delayed-type hypersensitivity reaction. Tuberculin reaction and Type 1 diabetes belong to this category of autoimmunity. [ 20 ]
T h 2 helper cells lead to a humoral immune response , [ 19 ] typically against extracellular parasites such as helminths . They are triggered by the polarising cytokines IL-4 and IL-2, and their effector cytokines are IL-4, IL-5, IL-9, IL-10, IL-13 and IL-25. The main effector cells are eosinophils, basophils, and mast cells as well as B cells, and IL-4/IL-5 CD4 T cells. The key T h 2 transcription factors are STAT6 and GATA3 . [ 21 ] IL-4 is the positive feedback cytokine for T h 2 cells differentiation. Besides, IL-4 stimulates B-cells to produce IgE antibodies, which in turn stimulate mast cells to release histamine , serotonin , and leukotriene to cause broncho-constriction, intestinal peristalsis, gastric fluid acidification to expel helminths. IL-5 from CD4 T cells will activate eosinophils to attack helminths. IL-10 suppresses T h 1 cells differentiation and function of dendritic cells. T h 2 overactivation against antigen will cause Type I hypersensitivity which is an allergic reaction mediated by IgE. Allergic rhinitis, atopic dermatitis, and asthma belong to this category of overactivation . [ 20 ] In addition to expressing different cytokines, T h 2 cells also differ from T h 1 cells in their cell surface glycans (oligosaccharides), which makes them less susceptible to some inducers of cell death. [ 22 ] [ 23 ]
While we know about the types of cytokine patterns helper T cells tend to produce, we understand less about how the patterns themselves are decided. Various evidence suggests that the type of APC presenting the antigen to the T cell has a major influence on its profile. Other evidence suggests that the concentration of antigen presented to the T cell during primary activation influences its choice. The presence of some cytokines (such as the ones mentioned above) will also influence the response that will eventually be generated, but our understanding is nowhere near complete.
T h 17 helper cells are a subset of T helper cells developmentally distinct from T h 1 and T h 2 lineages.
T h 17 cells produce interleukin 17 (IL-17), a pro-inflammatory substance, as well as interleukins 21 and 22 . [ 26 ] This means that T h 17 cells are especially good at fighting extracellular pathogens [ 26 ] and fungi, particularly during mucocutaneous immunity against Candida spp. [ 27 ]
THαβ helper cells provide the host immunity against viruses. Their differentiation is triggered by IFN α/β or IL-10 . Their key effector cytokine is IL-10. Their main effector cells are NK cells as well as CD8 T cells, IgG B cells, and IL-10 CD4 T cells. The key THαβ transcription factors are STAT1 and STAT3 as well as IRFs. IL-10 from CD4 T cells activate NK cells' ADCC to apoptose virus-infected cells and to induce host as well as viral DNA fragmentation. IFN alpha/beta can suppress transcription to avoid virus replication and transmission. Overactivation of THαβ against autoantigen will cause type 2 antibody-dependent cytotoxic hypersensitivity. Myasthenia gravis or Graves' disease belong to this category. [ 28 ]
The interactions between cytokines from the T h 1/T h 2 model can be more complicated in some animals. For example, the T h 2 cytokine IL-10 inhibits cytokine production of both T h subsets in humans. Human IL-10 (hIL-10) suppresses the proliferation and cytokine production of all T cells and the activity of macrophages, but continues to stimulate plasma cells , ensuring that antibody production still occurs. As such, hIL-10 is not believed to truly promote the T h 2 response in humans, but acts to prevent over-stimulation of helper T cells while still maximising the production of antibodies .There are also other types of T cells that can influence the expression and activation of helper T cells, such as natural regulatory T cells , along with less common cytokine profiles such as the T h 3 subset of helper T cells. Terms such as "regulatory" and "suppression" have become ambiguous after the discovery that helper CD4 + T cells are also capable of regulating (and suppressing) their own responses outside of dedicated regulatory T cells. [ citation needed ]
One major difference between regulatory T cells and effector T cells is that regulatory T cells typically serve to modulate and deactivate the immune response, while effector T cell groups usually begin with immune-promoting cytokines and then switch to inhibitory cytokines later in their life cycle. The latter is a feature of T h 3 cells, which transform into a regulatory subset after its initial activation and cytokine production. [ citation needed ]
Both regulatory T cells and T h 3 cells produce the cytokine transforming growth factor-beta (TGF-β) and IL-10. Both cytokines are inhibitory to helper T cells; TGF-β suppresses the activity of most of the immune system. There is evidence to suggest that TGF-β may not suppress activated Th2 cells as effectively as it might suppress naive cells, but it is not typically considered a Th2 cytokine. [ citation needed ]
The novel characterisation of another T helper subtype, T helper 17 cells (T h 17) [ 29 ] has cast further doubt on the basic T h 1/T h 2 model. These IL-17 producing cells were initially described as a pathogenic population implicated in autoimmunity but are now thought to have their own distinct effector and regulatory functions. Of note, some evidence suggest that functional plasticity is an intrinsic capacity of T helper cells. Indeed, a study in mice demonstrated that T h 17 cells transform into T h 1 cells in vivo . [ 30 ] A subsequent study furthermore showed that extensive T helper cell plasticity is also prominent in humans. [ 31 ]
Many of the cytokines in this article are also expressed by other immune cells (see individual cytokines for details), and it is becoming clear that while the original T h 1/T h 2 model is enlightening and gives insight into the functions of helper T cells, it is far too simple to define its entire role or actions. Some immunologists question the model completely, as some in vivo studies suggest that individual helper T cells usually do not match the specific cytokine profiles of the T h model, and many cells express cytokines from both profiles. [ 32 ] That said, the T h model has still played an important part in developing our understanding of the roles and behaviour of helper T cells and the cytokines they produce during an immune response. [ citation needed ]
Studies by Stockinger et al. revealed that another T helper subset may exist. Th9 cells are claimed to be an IL9 ( interleukin 9 )–producing T cell subset focused on defending helminth infections. [ 33 ]
Historically, memory T cells were thought to belong to either the effector or central memory subtypes, each with their own distinguishing set of cell surface markers. [ 34 ] Central memory T cells reside in the lymph nodes while effector memory T cells lack the C-C chemokine receptor type 7 (CCR7) and L-selectin (CD62L) receptors, which prevents them from trafficking to the lymph nodes.
Additional populations of memory T cells are now known to exist. These include tissue-resident memory T (Trm) cells and virtual memory T cells. [ 35 ] The single unifying theme for all memory T cell subtypes is that they are long-lived and can expand quickly to large numbers of effector T cells upon encountering their cognate antigen. By this mechanism they provide the immune system with "memory" against previously encountered pathogens.
Considering the diverse and important role helper T cells play in the immune system, it is not surprising that these cells often influence the immune response against disease. They also occasionally generate non-beneficial responses. Very rarely, the helper T cell response could lead to the death of the host. [ citation needed ]
The immune system must achieve a balance of sensitivity in order to respond to foreign antigens without responding to the antigens of the host itself. When the immune system responds to very low levels of antigen that it usually shouldn't respond to, a hypersensitivity response occurs. Hypersensitivity is believed to be the cause of allergy and some auto-immune disease .
Hypersensitivity reactions can be divided into four types:
Other cellular hypersensitivities include cytotoxic T cell mediated auto-immune disease , and a similar phenomenon; transplant rejection . Helper T cells are required to fuel the development of these diseases. In order to create sufficient auto-reactive killer T cells, interleukin-2 must be produced, and this is supplied by CD4 + T cells. CD4 + T cells can also stimulate cells such as natural killer cells and macrophages via cytokines such as interferon-gamma , encouraging these cytotoxic cells to kill host cells in certain circumstances.
The mechanism that killer T cells use during auto-immunity is almost identical to their response against viruses , and some viruses have been accused of causing auto-immune diseases such as Type 1 diabetes mellitus . Cellular auto-immune disease occurs because the host antigen recognition systems fail, and the immune system believes, by mistake, that a host antigen is foreign. As a result, the CD8 + T cells treat the host cell presenting that antigen as infected, and go on to destroy all host cells (or in the case of transplant rejection, transplant organ) that express that antigen.
Some of this section is a simplification. Many auto-immune diseases are more complex. A well-known example is rheumatoid arthritis , where both antibodies and immune cells are known to play a role in the pathology. Generally the immunology of most auto-immune diseases is not well understood.
Perhaps the best example of the importance of CD4 + T cells is demonstrated with human immunodeficiency virus (HIV) infection. HIV mainly targets lymphoid CD4 + T cells, but can infect other cells that express CD4 such as macrophages and dendritic cells (both groups express CD4 at low levels). [ citation needed ]
It has been proposed that during the non-symptomatic phase of HIV infection, the virus has a relatively low affinity towards T cells (and has a higher affinity for macrophages), resulting in a slow kill rate of CD4 + T cells by the immune system. [ citation needed ] This is initially compensated for via the production of new helper T cells from the thymus (originally from the bone marrow ). Once the virus becomes lymphotropic (or T-tropic) however, it begins to infect CD4 + T cells far more efficiently (likely due to a change in the co-receptors it binds to during infection), and the immune system is overwhelmed. Studies suggest that only ~5% of the lymphoid-derived CD4 T cells targeted by HIV are permissive and become productively infected with the virus. More than 95% of the CD4 T cells that die are resting and are unable to support productive infection. These cells undergo abortive infection with HIV. [ 36 ] Cell death is triggered when the host cell detects HIV foreign DNA intermediates and initiates a suicidal death pathway in an attempt to protect the host, leading to caspase-1 activation in inflammasomes , thus causing pyroptosis (a highly inflammatory form of programmed cell death). [ 37 ] [ 38 ] [ 39 ]
At this point chronic inflammation ensues, and functional CD4 + T cell levels begin to decrease, eventually to a point where the CD4 + T cell population is too small to recognize the full range of antigens that could potentially be detected. The depletion of CD4 T cells and the development of chronic inflammation are signature processes in HIV pathogenesis that propel progression to acquired immune deficiency syndrome (AIDS). CD4 T cell depleted to the cell count of less than 200cell/μL in blood during AIDS allows various pathogens to escape T cell recognition, thus allowing opportunistic infections that would normally elicit a helper T cell response to bypass the immune system. [ 40 ] While these complete bypass situations only occur when the helper T cell response is absolutely necessary for infection clearance, most infections increase in severity and/or duration because the immune system's helper T cells provide less efficient immune response.
Two components of the immune system are particularly affected in AIDS, due to its CD4 + T cell dependency:
If the patient does not respond to (or does not receive) HIV treatment they will succumb usually to either cancers or infections; the immune system finally reaches a point where it is no longer coordinated or stimulated enough to deal with the disease.
Inhibition of CD4 T-cell expansion during HIV infection may occur due to microbial translocation in an IL-10-dependent way. Triggering PD-1 expressed on activated monocytes by its ligand PD-L1, induces IL-10 production which inhibits CD4 T-cell function. [ 41 ]
In coronavirus disease 2019 (COVID-19) B cell , natural killer cell , and total lymphocyte counts decline, but both CD4 + and CD8 + cells decline to a far greater extent. [ 42 ] Indicating that SARS-Cov-2 attacks the CD4 + cells during infection. Low CD4 + predicted greater likelihood of intensive care unit admission, and CD4 + cell count was the only parameter that predicted length of time for viral RNA clearance. [ 42 ] Despite the reduced levels of CD4 + , COVID-19 patients with severe disease had higher levels of T h 1 CD4 + cells than patients with moderate disease. [ 43 ] It is noted that SARS-Cov-2 does reverse transcriptase like HIV against these cells. [ 43 ] | https://en.wikipedia.org/wiki/T_helper_cell |
T independent antigens elicit antibody production by B lymphocytes without T lymphocyte involvement. There are two distinct subgroups of TI antigens, different in mechanism of activating B lymphocytes: TI-1 antigen , which has an activity that can directly activate B cells and TI-2 antigen , which has highly repetitive structure and causes simultaneous cross-linking of specific B cell receptors (BCR) on B lymphocytes. The most commonly released isotype of antibodies in this type of immune reaction is low-affinity immunoglobulin M (IgM). [ 1 ]
For most protein antigens, the production of antibodies by B lymphocytes is dependent on stimulation of helper T cells . However bacterial polysaccharides and lipopolysaccharides , and some polymeric proteins, can stimulate B lymphocytes without involvement of helper T cells. The non-protein microbial antigens cannot stimulate classical T cell response by themselves, but they are able to elicit the production of antibodies, so that is why we call them T cell or thymus independent antigens . [ 2 ]
T independent antigens are divided into two classes by the mechanism of activating B cells.
TI-1 antigens have an intrinsic B cell activating activity, that can directly cause proliferation and differentiation of B lymphocytes without T cell stimulation and independently of their BCR specificity. TI-1 antigens activate B-cells via Toll like receptors , which are expressed in humans on the surface of B lymphocytes after BCR stimulation. TI-1 antigens are classified as B-cell mitogens , because they induce numerous cell divisions. In higher concentrations, TI-1 antigens bind to BCR and TLR of various clones of B lymphocytes, which leads to production of multiclonal antibodies. But when the concentration of TI-1 is lower, it can activate only B lymphocytes with specific binding of TI-1 on their BCR, and leads to production of monoclonal antibodies. [ 1 ] This part of immune response may be important in some early stages of infection by extracellular pathogens, because it is rapidly activated and does not require T cell help or clonal maturation and expansion. An example of TI-1 antigen is lipopolysaccharide (LPS) or bacterial DNA. [ 2 ]
The second group of TI antigens consists mainly of highly repetitive surface structures ( epitopes ) of encapsulated bacteria . They do not have an intrinsic B-cell activating activity. The activation of B lymphocytes is caused by cross-linking of a critical number of B cell receptors, which leads to accumulation of BCRs and cross activation of these receptors. It results in proliferation and differentiation of B lymphocytes and production of antibodies. TI-2 antigens can activate only mature B lymphocytes. Immature B cells are anergized, so they do not elicit any immune response. That may explain why children up to 5 years are not capable of producing effective antibodies against polysaccharide antigens, as the majority of their B cell population is immature. [ 2 ] Even though the response on TI antigens is not dependent on T lymphocytes, there are some cytokines, produced mainly by T lymphocytes and natural killer (NK) cells , necessary for eliciting reaction against these antigens. The most necessary are interleukin 2 (IL-2), interleukin 3 (IL-3) and interferon γ (IFN-γ). [ 1 ] Moreover, additional stimulation by dendritic cells (DC) and macrophages is required. [ 2 ] | https://en.wikipedia.org/wiki/T_independent_antigen_(TI) |
A T memory stem cell (T SCM ) is a type of long-lived memory T cell with the ability to reconstitute the full diversity of memory and effector T cell subpopulations as well as to maintain their own pool through self-renewal . First described in mice in 2009 [ 1 ] then in humans [ 2 ] represent a cell type that has reshaped the landscape of immunology and medicine because of their superior ability to self-renew and persist in the setting of cancer and infectious disease. Developmentally, T SCM are an intermediate subset between naïve (Tn) and central memory (Tcm) T cells, expressing both naïve T cells markers, such as CD45RA+ , CD45RO-, high levels of CD27 , CD28 , IL-7Rα (CD127), CD62L , and C-C chemokine receptor 7 ( CCR7 ), as well as markers of memory T cells, such as CD95 , CD122 (IL-2Rβ) , CXCR3 , LFA-1 . [ 3 ] [ 4 ] [ 5 ] These cells represent a small fraction of circulating T cells, approximately 2-3%. [ 3 ] Like naïve T cells , T SCM cells are found more abundantly in lymph nodes than in the spleen or bone marrow; but in contrast to naïve T cells, T SCM cells are clonally expanded. Similarly to memory T cells , T SCM are able to rapidly proliferate and secrete pro-inflammatory cytokines ( IFN-γ , IL-2 , and TNF-α ) in response to antigen re-exposure, but show higher proliferation potential compared with Tcm cells; their homeostatic turnover is also dependent on IL-7 and IL-15 .[ [ 4 ] ]
Longitudinal studies on T SCM dynamics in patients undergoing hematopoietic stem cell transplantation (HSCT) have shown that donor-derived T SCM cells were highly enriched early after HSCT, differentiated directly from Tn, and that Tn and T SCM cells (but not central memory or effector T cells) were able to reconstitute the entire heterogeneity of memory T cell subsets including T SCM cells. [ 6 ] Together with the transcriptome analysis of differentially expressed genes reflecting the relatedness of T SCM and Tn cells, these data were in sharp contrast to the existing hierarchical model of human T cell differentiation: naïve T cells (Tn) → effector T cells (Teff) → effector memory T cells → central memory T cells (Tcm)/.
After primary antigen exposure and elimination, antigen-specific T SCM preferentially survive among memory T cells and stably persist for a long term throughout the human lifespan. [ 7 ] Multiparametric flow cytometry and TCR sequencing studies showed that more than 30% of naïve T cells primed by antigen directly differentiate into T SCM cells. [ 3 ] Current observations allow to suggest that T SCM is a population which plays an essential role in maintaining a long-term memory in vivo. [ 4 ] Long-term studies on T cells in a cohort of patients vaccinated against yellow fever revealed that vaccine-induced CD8+ T SCM cells specific to yellow fever antigens were stably maintained for 25 years, capable of self-renewal ex vivo, and preserved surface markers and mRNA profiles closest to naïve T cells. [ 8 ] In another longitudinal study on leukaemia patients who had undergone HSCT , it was reported that genetically modified T SCM could be detected up to 14 years after infusion. [ 9 ] Complex analysis of T SCM dynamics under physiological conditions including stable isotope labeling, mathematical modeling, cross-sectional data from vaccinated individuals, and telomere length analysis revealed that there are at least 2 distinct T SCM subpopulations with different longevity and turnover rates: 1) short-lived, with an average half-life of 5 months, 2) long-lived, with a high degree of self-renewal and the half-life of approximately 9 years, which is consistent with the long-term maintenance of the recall response to antigen (8–15 years). [ 4 ]
Analysis of TCR β repertoire of T SCM and Tm revealed that T SCM have higher TCRβ diversity compared with Tm, that TCR sequences of T SCM were antigen-experienced and their composition differed with those of naïve T cells. It also revealed that in type I diabetes patients there was an enrichment of self-reactive clonotypes in T SCM rather than in Tm, suggesting that T SCM might serve as a pool of autoreactive T cells . [ 10 ]
Pathogen-specific T SCM cells have been identified in a number of studies of human acute and chronic infections caused by viruses, bacteria and parasites. The presence of T SCM might be essential for the control of persisting infections, in which effector T cells undergo exhaustion and need to be restored; this was supported by the evidence of a negative correlation between the severity of chronic viral ( HIV-1 ) and parasitic ( trypanosome ) infections and the frequency of circulated T SCM cells. [ 3 ]
T SCM are considered as a promising approach in immune cell therapy in cancers due to their high proliferation capacity, longevity and increased survival as well as more potent antitumor effects compared with Tcm and Tem in vivo. Studies on adoptive cell therapy in mouse melanoma model revealed a significant linear correlation between the differentiation status of infused T cells and the strength of tumor regression in the order T SCM >T CM > T EM ; T SCM infusion led to a more sustained reduction in tumor growth and correlated with a significant increase in overall survival of treated mice. Previous works on humans and mice also demonstrated that less differentiated T cells show greater proliferative capacity and ability to persist after cell transfer compared with their more differentiated counterparts; in humans, the ability of infused T cells to persist has been positively correlated with response to adoptive cell therapy. [ 5 ] [ 11 ]
However, the clinical exploitation of T SCM cells is impeded due to their paucity in the peripheral blood and due to the current lack of unified protocols for generating and maintaining T SCM in vitro for clinical manufacturing. Among current efficient strategies, there is a combination of IL-7 and IL-15 , which have been successfully used to generate tumor-redirected T SCM cells from naive cell precursors, with yielding cells having a gene signature of naturally occurring T SCM cells and enhanced proliferative capacity compared to other T cell subsets. This strategy can be particularly suitable for generating virus-specific T SCM cells for adoptive cell therapy to prevent or treat viral infections after transplantation or in other immunocompromised patients. Another strategy promoting the efficient generation of tumor-reactive T SCM cells relies on the activation of naïve-like T cells in the presence of IL-7, IL-21 and TWS119 which is an agonist of Wnt-β signaling. It has been found that CAR-modified T SCM cells generated this way are phenotypically, functionally and transcriptomically equivalent to naturally occurring T SCM cells; moreover, they had metabolic features which are specific for long-lived memory T cells, such as high spare respiratory capacity and low glycolytic metabolism (predominance of oxidative phosphorylation ). Such CAR-modified T cells can be redirected efficiently against required tumor antigens, and have been shown to generate durable anti-tumor responses. [ 3 ]
One of the hardest challenges in application of T cell therapies in treatment of solid tumors is the problem of CD8+ T cells exhaustion resulting from their repeated exposure to tumor antigens and immunosuppressive tumor microenvironment sending inhibitory signals through the cytokines and cell surface receptors. Exhausted T cells are characterized by the expression of large amounts of inhibitory molecules such as PD-1 , CTLA-4 , LAG3 , Tim-3 , CD244/2B4, CD160, and TIGIT; they do not respond to TCR stimulation and have reduced capacity to secrete anti-tumor cytokines such as IFN-γ and TNF-α. [ 12 ] On a transcriptional level, recent studies have found that transcription factors which play key role in T cells exhaustion include TCF-1, T-bet, Eomes , PRDM1, NFAT, NR4A, IRF4 and BATF. According to the current differentiation model of T cells exhaustion, T cells stepwise lose their “stemness” while acquiring “exhaustion”. Therefore, approaches that would avoid T cells exhaustion and would “reinvigorate” exhausted T cells have a potential to significantly improve the efficacy of cancer immunotherapies. [ 5 ]
Studies of the recent years revealed that TCF-1+ T cells, which represent early memory T cells including T SCM cells, play important roles in T cells persistence and efficacy in cancer immunotherapy. Flow cytometry analysis of tumor-infiltrating antigen-presenting cell (APC) populations in human kidney, prostate and bladder tumors revealed a significant correlation between the presence of dendritic cells (but not macrophages) and the number of TCF1 + stem-like CD8+ T cells in the tumor. [ 13 ] Subsequent immunofluorescence staining showed that TCF1 + stem-like T cells were found only in regions with high density of MHC II+ cells; in contrast, the TCF1- population of terminally exhausted CD8+ T cells was distributed across the tissue with no preference for APC dense zones. Expanded analysis of large sections of tumor tissues confirmed that tumors had many regions with dense APC zones, and TCF-1+ stem-like CD8 cells preferentially resided there. These data suggest that regions highly enriched with APC serve as an intratumoral niche for stem-like CD8+ T cells, which give rise to terminally differentiated T cells and thus sustain the anti-tumor immune response. Furthermore, immunofluorescence analysis of large regions of tumor tissue from 26 patients with kidney cancer revealed that patients with controlled disease had significantly more MHC-II dense regions where TCF1 + CD8 T cells resided; further stratification of patients showed that patients with low MHC-II + cell density in such regions experienced significantly impaired progression-free survival. A focused study of patients with stage III kidney cancer, around 50% of whom progress after surgery, revealed that there were >10-fold fewer immune niches in patients who progressed. [ 13 ]
Despite some variations depending on tumor type and therapy, most studies agree that tumor-infiltrating lymphocytes (TIL) in patients responding to checkpoint-blockade therapy, such as anti-PD1 therapy, contain more TCF1+ early memory T cells, while fewer T cells with exhausted phenotype compared with TILs in non-responders. A study performed on the preclinical model of colon cancer has shown that PD-1 blockade induced a shift from naïve-like to memory precursor-like subsets, which are maintained by the transcriptional regulator TCF-1. The effectiveness of CAR-T cell therapy in chronic lymphocytic leukemia has also been reported to depend on the number of early memory T cells and T cell exhaustion. [ 12 ] | https://en.wikipedia.org/wiki/T_memory_stem_cell |
Tantalum(V) iodide is the inorganic compound with the formula Ta 2 I 10 . Its name comes from the compound's empirical formula , TaI 5 . [ 2 ] It is a diamagnetic, black solid that hydrolyses readily. The compound adopts an edge-shared bioctahedral structure, which means that two TaI 5 units are joined by a pair of iodide bridges . There is no bond between the Ta centres. [ 3 ] Niobium(V) chloride , niobium(V) bromide , niobium(V) iodide , tantalum(V) chloride , and tantalum(V) bromide all share this structural motif.
Tantalum pentaiodide forms from the reaction of tantalum pentoxide with aluminium triiodide : [ 4 ] | https://en.wikipedia.org/wiki/Ta2I10 |
Tantalum pentoxide , also known as tantalum (V) oxide, is the inorganic compound with the formula Ta 2 O 5 . It is a white solid that is insoluble in all solvents but is attacked by strong bases and hydrofluoric acid . Ta 2 O 5 is an inert material with a high refractive index and low absorption (i.e. colourless), which makes it useful for coatings. [ 2 ] It is also extensively used in the production of capacitors , due to its high dielectric constant .
Tantalum occurs in the minerals tantalite and columbite (columbium being an archaic name for niobium), which occur in pegmatites , an igneous rock formation. Mixtures of columbite and tantalite are called coltan . Tantalum was discovered in Tantalite in 1802 by Anders Gustaf Ekeberg at Ytterby , Sweden, and Kimoto, Finland. The minerals microlite and pyrochlore contain approximately 70% and 10% Ta, respectively.
Tantalum ores often contain significant amounts of niobium , which is itself a valuable metal. As such, both metals are extracted so that they may be sold. The overall process is one of hydrometallurgy and begins with a leaching step; in which the ore is treated with hydrofluoric acid and sulfuric acid to produce water-soluble hydrogen fluorides , such as the heptafluorotantalate . This allows the metals to be separated from the various non-metallic impurities in the rock.
The tantalum and niobium hydrogenfluorides are then removed from the aqueous solution by liquid-liquid extraction using organic solvents , such as cyclohexanone or methyl isobutyl ketone . This step allows the simple removal of various metal impurities (e.g. iron and manganese) which remain in the aqueous phase in the form of fluorides . Separation of the tantalum and niobium is then achieved by pH adjustment. Niobium requires a higher level of acidity to remain soluble in the organic phase and can hence be selectively removed by extraction into less acidic water.
The pure tantalum hydrogen fluoride solution is then neutralised with aqueous ammonia to give hydrated tantalum oxide (Ta 2 O 5 (H 2 O) x ), which is calcinated to tantalum pentoxide (Ta 2 O 5 ) as described in these idealized equations: [ 3 ]
Natural pure tantalum oxide is known as the mineral tantite , although it is exceedingly rare. [ 4 ]
Tantalum oxide is frequently used in electronics, often in the form of thin films . For these applications it can be produced by MOCVD (or related techniques), which involves the hydrolysis of its volatile halides or alkoxides :
The crystal structure of tantalum pentoxide has been the matter of some debate. The bulk material is disordered , [ 5 ] being either amorphous or polycrystalline ; with single crystals being difficult to grow. As such Xray crystallography has largely been limited to powder diffraction , which provides less structural information.
At least 2 polymorphs are known to exist. A low temperature form, known as L- or β-Ta 2 O 5 , and the high temperature form known as H- or α-Ta 2 O 5 . The transition between these two forms is slow and reversible; taking place between 1000 and 1360 °C, with a mixture of structures existing at intermediate temperatures. [ 5 ] The structures of both polymorphs consist of chains built from octahedral TaO 6 and pentagonal bipyramidal TaO 7 polyhedra sharing opposite vertices; which are further joined by edge-sharing. [ 6 ] [ 7 ] The overall crystal system is orthorhombic in both cases, with the space group of β-Ta 2 O 5 being identified as Pna2 by single crystal X-ray diffraction. [ 8 ] [ 9 ]
A high pressure form ( Z -Ta 2 O 5 ) has also been reported, in which the Ta atoms adopt a 7 coordinate geometry to give a monoclinic structure (space group C2). [ 10 ]
Purely amorphous tantalum pentoxide has a similar local structure to the crystalline polymorphs, built from TaO 6 and TaO 7 polyhedra, while the molten liquid phase has a distinct structure based on lower coordination polyhedra, mainly TaO 5 and TaO 6 . [ 11 ]
The difficulty in forming material with a uniform structure has led to variations in its reported properties. Like many metal oxides Ta 2 O 5 is an insulator and its band gap has variously been reported as being between 3.8 and 5.3 eV, depending on the method of manufacture. [ 12 ] [ 13 ] [ 14 ] In general the more amorphous the material the greater its observed band gap.
These observed values are significantly higher than those predicted by computational chemistry (2.3 - 3.8 eV). [ 15 ] [ 16 ] [ 17 ]
Its dielectric constant is typically about 25 [ 18 ] although values of over 50 have been reported. [ 19 ] In general tantalum pentoxide is considered to be a high-k dielectric material.
Ta 2 O 5 does not react appreciably with either HCl or HBr, however it will dissolve in hydrofluoric acid , and reacts with potassium bifluoride and HF according to the following equation: [ 20 ] [ 21 ]
Ta 2 O 5 can be reduced to metallic Ta via the use of metallic reductants such as calcium and aluminium.
Owing to its high band gap and dielectric constant , tantalum pentoxide has found a variety of uses in electronics, particularly in tantalum capacitors . These are used in automotive electronics , cell phones, and pagers, electronic circuitry; thin-film components; and high-speed tools. In the 1990s, interest grew in the use of tantalum oxide as a high-k dielectric for DRAM capacitor applications. [ 22 ] [ 23 ]
It is used in on-chip metal-insulator-metal capacitors for high frequency CMOS integrated circuits. Tantalum oxide may have applications as the charge trapping layer for non-volatile memories . [ 24 ] [ 25 ] There are applications of tantalum oxide in resistive switching memories . [ 26 ]
Due to its high refractive index , Ta 2 O 5 has been utilized in the fabrication of the glass of photographic lenses . [ 2 ] [ 27 ] It can also be deposited as an optical coating with typical applications being antireflection and multilayer filter coatings in near UV to near infrared . [ 28 ]
Ta 2 O 5 has also been found to have a high nonlinear refractive index , [ 29 ] [ 30 ] on the order of three times that of silicon nitiride , which has led to interest in the utilization of Ta 2 O 5 in photonic integrated circuits . Ta 2 O 5 has been recently utilized as the material platform for the generation of supercontinuum [ 31 ] [ 32 ] and Kerr frequency combs [ 30 ] in waveguides and optical ring resonators . Through the addition of rare-earth dopants in the deposition process, Ta 2 O 5 waveguide lasers have been presented for a variety of applications, such as remote sensing and LiDAR . [ 33 ] [ 34 ] [ 35 ] | https://en.wikipedia.org/wiki/Ta2O5 |
Tantalum(V) bromide is the inorganic compound with the formula Ta 2 Br 10 . Its name comes from the compound's empirical formula , TaBr 5 . [ 2 ] It is a diamagnetic, orange solid that hydrolyses readily. The compound adopts an edge-shared bioctahedral structure, which means that two TaBr 5 units are joined by a pair of bromide bridges . There is no bond between the Ta centres. [ 3 ] Niobium(V) chloride , niobium(V) bromide , niobium(V) iodide , tantalum(V) chloride , and tantalum(V) iodide all share this structural motif.
The material is usually prepared by the reaction of bromine with tantalum metal (or tantalum carbide ) at elevated temperatures in a tube furnace . The bromides of the early metals are sometimes preferred to the chlorides because of the relative ease of handling liquid bromine vs gaseous chlorine . Like other molecular halides, it is soluble in nonpolar solvents such as carbon tetrachloride (1.465 g/100 mL at 30 °C), but it reacts with some solvents. [ 4 ]
It can also be produced from the more accessible oxide by metathesis using aluminium tribromide :
Carbothermal reduction of the oxide in the presence of bromine has also been employed, the byproduct being COBr 2 . [ 5 ] | https://en.wikipedia.org/wiki/TaBr5 |
Tantalum(III) chloride or tantalum trichloride is non-stoichiometric chemical compound with a range of composition from TaCl 2.9 to TaCl 3.1 [ 2 ] Anionic and neutral clusters containing Ta(III) chloride include [Ta 6 Cl 18 ] 4− and [Ta 6 Cl 14 ](H 2 O) 4 . [ 3 ]
Tantalum(III) chloride is formed by reducing tantalum(V) chloride with tantalum metal. this is done by heating tantalum(III) chloride to 305 °C, passing the vapour over tantalum foil at 600°, and condensing the trichloride at 365 °C. If the condensing region is kept at too high a temperature, then TaCl 2.5 deposits instead. [ 5 ]
The trichloride can also be prepared by thermal decomposition of TaCl 4 , with removal of volatile TaCl 5 . TaCl 5 can be vapourised leaving behind TaCl 3 . [ 6 ]
" Salt-free reduction " of a toluene solution of TaCl 5 with 1,4-disilyl-cyclohexadiene in the presence of ethylene produces a complex of TaCl 3 : [ 7 ]
Above 500 °C, TaCl 3 disproportionates further releasing TaCl 5 . [ 6 ] TaCl 3 is insoluble in room temperature water, or dilute acid, but dissolves in boiling water. A blue-green solution is formed. [ 6 ]
Tantalum(III) chloride can form complexes with some ligands as a monomer or dimer.
Complexes include Ta(=C-CMe 3 )(PMe 3 ) 2 Cl 3 , [TaCl 3 (P(CH 2 C 6 H 5 ) 3 THF] 2 μ-N 2 and [TaCl 3 THF 2 ] 2 μ-N 2 (dinitrogen complexes). [ 8 ]
As a dimer, complexes include Ta 2 Cl 6 (SC 4 H 8 ) 3 (SC 4 H 8 =tetrahydrothiophene). Ta 2 Cl 6 (SMe 2 ) 3 , Ta 2 Cl 6 (thiane) 3 and Ta 2 Cl 6 (thiolane) 3 have a double bond between the two tantalum atoms, and two bridging chlorides, and a bridging ligand. [ 7 ] | https://en.wikipedia.org/wiki/TaCl3 |
Tantalum(V) chloride , also known as tantalum pentachloride , is an inorganic compound with the formula TaCl 5 . It takes the form of a white powder, [ contradictory ] [ not verified in body ] and is commonly used as a starting material in tantalum chemistry. It readily hydrolyzes to form tantalum(V) oxytrichloride ( TaOCl 3 ) and eventually tantalum pentoxide ( Ta 2 O 5 ); [ not verified in body ] this requires that it be synthesised and manipulated under anhydrous conditions, using air-free techniques . [ not verified in body ]
TaCl 5 crystallizes in the monoclinic space group C 2/ m . [ 2 ] [ non-primary source needed ] The ten chlorine atoms define a pair of octahedra that share a common edge. The tantalum atoms occupy the centres of the octahedra and are joined by two chlorine bridging ligands . The dimeric structure is retained in non-complexing solvents and to a large extent in the molten state. In the vapour state, however, TaCl 5 is monomeric . This monomer adopts a trigonal bipyramidal structure , like that of PCl 5 . [ 3 ] [ page needed ]
The solubility of tantalum pentachloride increases slightly for the following series of aromatic hydrocarbons :
This is reflected in the deepening of colour of the solutions from pale yellow to orange. Tantalum pentachloride is less soluble in cyclohexane and carbon tetrachloride than in the aromatic hydrocarbons. Such solutions of tantalum pentachloride are also known to be poor conductors of electricity, indicating little ionization. TaCl 5 is purified by sublimation to give white needles. [ contradictory ]
Tantalum pentachloride can be prepared by reacting powdered metallic tantalum with chlorine gas at between 170 and 250 °C. This reaction can also be performed using HCl at 400 °C. [ 4 ] [ non-primary source needed ]
It can also be prepared by a reaction between tantalum pentoxide and thionyl chloride at 240 °C
Tantalum pentachloride is commercially available, however samples can be contaminated with tantalum(V) oxytrichloride ( TaOCl 3 ), formed by hydrolysis.
TaCl 5 is electrophilic and it behaves like a Friedel–Crafts catalyst , similar to AlCl 3 . It forms adducts with a variety of Lewis bases . [ 5 ] [ page needed ]
TaCl 5 forms stable complexes with ethers :
TaCl 5 also reacts with phosphorus pentachloride and phosphoryl chloride , the former as a chloride donor and the latter serves as a ligand, binding through the oxygen:
Tantalum pentachloride reacts with tertiary amines to give crystalline adducts.
Tantalum pentachloride reacts at room temperature with an excess of triphenylphosphine oxide to give oxychlorides:
The presumed initial formation of adducts between TaCl 5 and hydroxyl compounds such as alcohols , phenols and carboxylic acids is followed immediately by the elimination of hydrogen chloride and the formation of Ta–O bonds:
In the presence of ammonia as a HCl acceptor, all five chloride ligands are displaced with formation of tantalum(V) ethanolate Ta(OEt) 5 . Similarly TaCl 5 reacts with lithium methoxide in anhydrous methanol to form related methoxy derivatives:
Ammonia will displace most of the chloride ligands from TaCl 5 to give a cluster. Chloride is displaced more slowly by primary or secondary amines but the replacement of all five chloride centers by amido groups has been achieved by the use of lithium dialkylamides, as illustrated by the synthesis of pentakis(dimethylamido)tantalum :
With alcohols, the pentachloride reacts to give alkoxides . As shown for the preparation of tantalum(V) ethoxide , such reactions are often conducted in the presence of base:
Tantalum pentachloride is reduced by nitrogen heterocycles such as pyridine .
Reduction of tantalum(V) chloride gives anionic and neutral clusters including [Ta 6 Cl 18 ] 4− and [Ta 6 Cl 14 ](H 2 O) 4 . [ 7 ] [ full citation needed ] | https://en.wikipedia.org/wiki/TaCl5 |
Tantalum(V) fluoride is the inorganic compound with the formula Ta F 5 . It is one of the principal molecular compounds of tantalum . Characteristic of some other pentafluorides, the compound is volatile but exists as a tetramer in the solid state.
TaF 5 is prepared by treating tantalum metal with fluorine gas. [ 2 ]
NbF 5 is prepared similarly.
Solid and molten TaF 5 is tetrameric , consisting of four octahedral TaF 6 centers linked via bridging fluoride centers. Gaseous TaF 5 is monomeric and adopts the trigonal bipyramidal structure with D 3h symmetry . [ 3 ]
The tendency of TaF 5 to form clusters in the solid state indicates the Lewis acidity of the monomer. Indeed, TaF 5 reacts with fluoride sources to give the ions [TaF 6 ] − (hexafluorotantalate(V)), [TaF 7 ] 2− (heptafluorotantalate(V)), and [TaF 8 ] 3− (octafluorotantalate(V)). With neutral Lewis bases , such as diethyl ether , TaF 5 forms adducts .
TaF 5 is used in combination with HF as a catalyst for the alkylation of alkanes and alkenes and for the protonation of aromatic compounds. The TaF 5 –HF system is stable in reducing environments, unlike SbF 5 –HF . [ 4 ] In the presence of fluoride, tantalum pentafluoride forms the anions [TaF 8 ] 3− , [TaF 7 ] 2− , or [TaF 6 ] − , depending on the nature of the counterion and the concentration of HF. High concentrations of HF favor the hexafluoride by virtue of the formation of [HF 2 ] − : [ 5 ]
The salts M 3 TaF 8 have been crystallized. For M = K , the crystals consist of [TaF 7 ] 2− anions together with fluoride that does not coordinate to Ta(V), [ 6 ] and the salt is actually potassium fluoride heptafluorotantalate(V) (K + ) 3 [TaF 7 ] 2− F − . For M = Na , the crystals features [TaF 8 ] 3− , [ 7 ] and the salt is sodium octafluorotantalate(V) (Na + ) 3 [TaF 8 ] 3− .
In the Marignac process, Nb and Ta are separated by fractional crystallization of K 2 [TaF 7 ] from solutions of hydrofluoric acid . Under these conditions, niobium forms K 2 [NbOF 5 ] , which is more soluble than K 2 [TaF 7 ] . Reduction of K 2 [TaF 7 ] with sodium gives metallic Ta. [ 8 ] | https://en.wikipedia.org/wiki/TaF5 |
Tantalum(V) iodide is the inorganic compound with the formula Ta 2 I 10 . Its name comes from the compound's empirical formula , TaI 5 . [ 2 ] It is a diamagnetic, black solid that hydrolyses readily. The compound adopts an edge-shared bioctahedral structure, which means that two TaI 5 units are joined by a pair of iodide bridges . There is no bond between the Ta centres. [ 3 ] Niobium(V) chloride , niobium(V) bromide , niobium(V) iodide , tantalum(V) chloride , and tantalum(V) bromide all share this structural motif.
Tantalum pentaiodide forms from the reaction of tantalum pentoxide with aluminium triiodide : [ 4 ] | https://en.wikipedia.org/wiki/TaI5 |
Tantalum nitride (TaN) is a chemical compound , a nitride of tantalum . There are multiple phases of compounds, stoichimetrically from Ta 2 N to Ta 3 N 5 , including TaN.
As a thin film TaN find use as a diffusion barrier and insulating layer between copper interconnects in the back end of line of computer chips. Tantalum nitrides are also used in thin film resistors.
The tantalum - nitrogen system includes several states including a nitrogen solid solution in Tantalum, as well as several nitride phases, which can vary from expected stoichiometry due to lattice vacancies. [ 1 ] Annealing of nitrogen rich "TaN" can result in conversion to a two phase mixture of TaN and Ta 5 N 6 . [ 1 ]
Ta 5 N 6 is thought to be the more thermally stable compound - though it decomposes in vacuum at 2500 °C to Ta 2 N. [ 1 ] It was reported the decomposition in vacuum from Ta 3 N 5 via Ta 4 N 5 , Ta 5 N 6 , ε-TaN, to Ta 2 N. [ 2 ]
TaN is often prepared as thin films. Methods of depositing the films include RF-magnetron-reactive sputtering, [ 3 ] [ 4 ] Direct current (DC) sputtering , [ 5 ] Self-propagating high-temperature synthesis (SHS) via 'combustion' of tantalum powder in nitrogen, [ 1 ] low‐pressure metalorganic chemical vapor deposition (LP‐MOCVD), [ 6 ] ion beam assisted deposition (IBAD), [ 7 ] and by electron beam evaporation of tantalum in concert with high energy nitrogen ions. [ 8 ]
Depending on the relative amount of N 2 , the deposited film can vary from (fcc) TaN to (hexagonal) Ta 2 N as nitrogen decreases. [ 4 ] A variety of other phases have also been reported from deposition including bcc and hexagonal TaN; hexagonal Ta 5 N 6 ; tetragonal Ta 4 N 5 ; orthorhombic Ta 6 N 2.5 , Ta 4 N, or Ta 3 N 5 . [ 4 ] The electrical properties of TaN films vary from metallic conductor to insulator depending on the relative nitrogen ratio, with N rich films being more resistive. [ 9 ]
It is sometimes used in integrated circuit manufacture to create a diffusion barrier or "glue" layers between copper , or other conductive metals. In the case of BEOL processing (at c. 20 nm ), copper is first coated with tantalum, then with TaN using physical vapour deposition (PVD); this barrier coated copper is then coated with more copper by PVD, and infilled with electrolytically coated copper, before being mechanically processed (grind/polishing). [ 10 ]
It also has application in thin film resistors . [ 3 ] It has the advantage over nichrome resistors of forming a passivating oxide film which is resistant to moisture. [ 11 ] | https://en.wikipedia.org/wiki/TaN |
Tantalum(IV) sulfide is an inorganic compound with the formula Ta S 2 . It is a layered compound with three-coordinate sulfide centres and trigonal prismatic or octahedral metal centres. [ 2 ] It is structurally similar to molybdenum disulfide MoS 2 , and numerous other transition metal dichalcogenides . Tantalum disulfide has three polymorphs 1T-TaS2, 2H-TaS2, and 3R-TaS2, representing trigonal, hexagonal, and rhombohedral respectively.
The properties of the 1T-TaS 2 polytype have been described. [ 3 ] [ 4 ] [ 5 ] In common with many other transition metal dichalcogenide (TMD) compounds, which are metallic at high temperatures, it exhibits a series of charge-density-wave (CDW) phase transitions from 550 K to 50 K. It is unusual amongst them in showing a low-temperature insulating state below 200 K, which is believed to arise from electron correlations, similar to many oxides. The insulating state is commonly attributed to a Mott state. [ 6 ] It is also superconducting under pressure or upon doping, with a familiar dome-like phase diagram as a function of dopant, or substituted isovalent element concentration.
Metastability. 1T-TaS 2 is unique, not only amongst TMDs but also amongst 'quantum materials' in general, in showing a metastable metallic state at low temperatures. [ 7 ] Switching from the insulating to the metallic state can be achieved either optically or by the application of electrical pulses. The metallic state is persistent below ~20K, but its lifetime can be tuned by changing the temperature. The metastable state lifetime can also be tuned by strain. The electrically-induced switching between states is of current interest, because it can be used for ultrafast energy-efficient memory devices. [ 8 ]
Because of the frustrated triangular arrangement of localized electrons, the material is suspected of supporting some form of quantum spin liquid state. It has been the subject of numerous studies as a host for intercalation of electron donors. [ 9 ]
TaS 2 is prepared by reaction of powdered tantalum and sulfur at ~900 °C. [ 11 ] It is purified and crystallized by chemical vapor transport using iodine as the transporting agent: [ 12 ]
It can be easily cleaved and has a characteristic golden sheen. Upon extended exposure to air, the formation of an oxide layer causes darkening of the surface. Thin films can be prepared by chemical vapour deposition and molecular beam epitaxy.
Three major crystalline phases are known for TaS 2 : trigonal 1T with one S-Ta-S sheet per unit cell , hexagonal 2H with two S-Ta-S sheets, and rhombohedral 3R with three S-Ta-S sheets per cell; 4H and 6R phases are also observed, but less frequently. These polymorphs mostly differ by the relative arrangement of the S-Ta-S sheet rather than the sheet structure. [ 13 ]
2H-TaS 2 is a superconductor with the bulk transition temperature T C = 0.5 K, which increases to 2.2 K in flakes with a thickness of a few atomic layers. [ 11 ] The bulk T C value increases up to ~8 K at 10 GPa and then saturates with increasing pressure. [ 14 ] In contrast, 1T-TaS 2 starts superconducting only at ~2 GPa; as a function of pressure its T C quickly rises up to 5 K at ~4 GPa and then saturates. [ 6 ]
At ambient pressure and low temperatures 1T-TaS 2 is a Mott insulator . [ 6 ] Upon heating it changes to a Triclinic charge density wave (TCDW) state at T TCDW ~ 220 K, [ 15 ] [ 16 ] [ 17 ] to a nearly commensurate charge density wave (NCCDW) state at T NCCDW ~ 280 K, [ 2 ] to an incommensurate CDW (ICCDW) state at T ICCDW ~ 350 K, [ 2 ] and to a metallic state at T M ~ 600 K. [ 10 ]
In the CDW state the TaS 2 lattice deforms to create a periodic Star of David pattern. Application of (e.g. 50fs) optical laser pulses [ 7 ] or voltage pulses (~2–3 V) through electrodes [ 18 ] or in a scanning tunneling microscope (STM) to the CDW state causes it to drop electrical resistance and creates a "mosaic" or domain state consisting of nanometer-sized domains, where both the domains and their walls exhibit metallic conductivity. This mosaic structure is metastable and gradually disappears upon heating. [ 12 ] [ 19 ] [ 18 ]
Switching of the material to and from the "mosaic", or domain state, by optical or electrical pulses is used for "Charge configuration memory" (CCM) devices. The distinguishing feature of such devices is that they exhibit very efficient and fast non-thermal resistance switching at low temperatures. [ 8 ] Room temperature operation of a charge-density-wave oscillator and thermally-driven GHz modulation of the CDW state has been demonstrated. [ 20 ] [ 21 ] | https://en.wikipedia.org/wiki/TaS2 |
Tantalum telluride ( Ta Te 2 ) is a chemical compound of tantalum and tellurium . Tantalum also forms a tantalum rich telluride with the approximate formula Ta 1.6 Te that is unusual in that it forms dodecagonal chalcogenide quasicrystals , a formation that cannot occur in a normal crystal because it does not result in a periodic crystal lattice .
This inorganic compound –related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TaTe2 |
Tabernanthe iboga ( iboga ) is an evergreen rainforest shrub native to Central Africa. A member of the Apocynaceae family indigenous to Gabon , the Democratic Republic of Congo , and the Republic of Congo , it is cultivated across Central Africa for its medicinal and other effects.
In African traditional medicine and rituals , the yellowish root or bark is used to produce hallucinations and near-death outcomes, with some fatalities occurring. [ 3 ] [ 4 ] In high doses, ibogaine is considered to be toxic , and has caused serious comorbidities when used with opioids or prescription drugs . [ 3 ] [ 5 ] The United States Drug Enforcement Administration (DEA) lists ibogaine as a controlled substance of the Controlled Substances Act . [ 3 ]
T. iboga is native to tropical forests, preferring moist soil in partial shade. [ 3 ] It bears dark green, narrow leaves and clusters of tubular flowers on an erect and branching stem, with yellow-orange fruits resembling chili pepper . [ 3 ]
Normally growing to a height of 2 m, T. iboga may eventually grow into a small tree up to 10 m tall, given the right conditions. The flowers are yellowish-white or pink and followed by a fruit, orange at maturity, that may be either globose or fusiform . Its yellow-fleshed roots contain a number of indole alkaloids , most notably ibogaine , which is found in the highest concentration in the bark of the roots. The root material, bitter in taste, causes a degree of anaesthesia in the mouth as well as systemic numbness of the skin. [ 6 ]
Tabernanthe iboga was described by Henri Ernest Baillon and published in Bulletin Mensuel de la Société Linnéenne de Paris 1: 783 in the year 1889.
The genus name Tabernanthe is a compound of the Latin taberna , " tavern "/" hut "/" (market) stall " and Greek : ἄνθος ( anthos ) "flower" – giving a literal meaning of "tavern flower". On the other hand, it may equally well have been intended (by way of a type of botanical shorthand) to mean "having a flower resembling that of plants belonging to the genus Tabernaemontana " (q.v.). If the first conjecture is the correct one, the name could also have been intended to suggest that the plant is cultivated near huts, sold at market stalls or even that – like the beverages sold at a tavern – the plant is intoxicating, all of which alternatives would constitute apt descriptions of an oft-cultivated and popular psychoactive plant .
The specific name iboga comes from the Myene name for the plant, [ 7 ] which was also borrowed into a number of other regional languages with mild variation. [ 8 ]
The first (probable...and confused) reference to Iboga is that of Bowdich in chapter 13 of his "Mission from Cape Coast Castle to Ashantee..." of 1819
The Eroga, a favourite but violent medicine, is no doubt a fungus, for they describe it as growing on a tree called the Ocamboo, when decaying; they burn it first, and take as much as would lay on a shilling.
If this is indeed a reference to the drug derived from Tabernanthe iboga ( Eroga appears to be a variant form of the names iboga and eboka ) it is, of course, grossly in error in its assumption that iboga is not a plant but a fungus. Notable however is the observation of the potency of the drug – effective in small quantities. The description of the plant as growing on a tree is puzzling: Tabernanthe iboga does not usually grow as an epiphyte – if at all. [ 9 ] [ 10 ]
The ritual use of iboga in Africa was first reported by French and Belgian explorers in the 19th century, beginning with the work of French naval surgeon and explorer of Gabon Griffon du Bellay , [ 11 ] who identified it correctly as a shrub belonging to the Apocynaceae – as recorded in a short essay by Charles Eugène Aubry-Lecomte on the plant poisons of West Africa, published in the year 1864.
Parmi les plantes rares ou nouvelles rapportées par le docteur Griffon du Bellay, la famille des apocynées contient encore deux poisons; l'un, nommé iboga, n'est toxique qu'à hautes doses et a l'état frais. Pris en petit quantité, il est aphrodisiaque et stimulante du systeme nerveux; les guerriers et chasseurs en font grand usage pour se tenir éveillés dans les affûts de nuit; de même que pour le M'boundou , le principe actif réside dans la racine qu'on mâche comme la coca. [ Translation: Among the rare or new plants brought back by Dr. Griffon du Bellay, the plant family Apocynaceae contains two further poisons; the first of these, called Iboga , is only toxic in high doses and in the fresh state. Taken in small quantities, it is an aphrodisiac and stimulant of the (central) nervous system; warriors and hunters make considerable use of it in order to stay awake during their night vigils; as with the (plant) M'boundou , the active principle (of Iboga) resides in the root which is chewed like coca (leaf) ]. [ 12 ]
Indole alkaloids make up about 6% of the root chemical composition of iboga. [ 3 ] Alkaloids that are present in more than 1% in root bark are: [ 13 ] (in descending order)
18-Methoxycoronaridine , a synthetic derivative of ibogaine, also occurs naturally in this plant. [ 3 ] [ 4 ]
The Iboga tree is central to the Bwiti spiritual practices in West-Central Africa, mainly Gabon , Cameroon , and the Republic of the Congo , where the alkaloid-containing roots or bark are used in various ceremonies, sometimes to create a near-death experience. [ 3 ] [ 14 ] Iboga is taken in massive doses by initiates of this spiritual practice, and on a more regular basis is eaten in smaller doses in connection with rituals and tribal dances performed at night.
While in lower doses iboga has a stimulant effect and is used to maintain alertness while hunting, [ 15 ] [ 16 ] in moderate or high doses, iboga induces dream-like states with vivid visions and hallucinations . [ 3 ]
Anecdotal reports of self-treated opioid addicts indicated a reduced desire to sustain opiate abuse following iboga ingestion. [ 3 ] Since 1970, iboga has been legally prohibited in the United States following several fatalities. [ 3 ] [ 4 ] Iboga extracts, as well as the purified alkaloid ibogaine , have attracted attention because of their purported ability to reverse addiction to drugs such as alcohol and opiates . [ 3 ] Due to the cardiac safety risks of iboga, research is also considering iboga analogues. [ 17 ]
Ibogaine is classified as a schedule 1 controlled substance in the United States, and is not approved there for addiction treatment (or any other therapeutic use) because of its hallucinogenic and cardiovascular side effects, as well as the absence of safety and efficacy data in human subjects. [ 3 ] [ 5 ] In most other countries, it remains unregulated and unlicensed. [ 18 ]
Independent ibogaine treatment clinics have emerged in Mexico, Canada, the Netherlands , South Africa, and New Zealand, all operating in what has been described as a "legal gray area". [ 18 ] [ 19 ] Covert, illegal neighborhood clinics are also known to exist in the United States, despite active DEA surveillance. [ 16 ] Addiction specialists warn that the treatment of drug dependence with ibogaine in non-medical settings, without expert supervision and unaccompanied by appropriate psychosocial care, can be dangerous – and, in approximately one case in 300, potentially fatal. [ 3 ] [ 19 ]
Ibogaine may induce nausea, vomiting, tremors, and headaches. [ 3 ] When ibogaine is used chronically, manic episodes lasting for several days may occur, accompanied by insomnia , irritability, delusions, aggressive behavior, and thoughts of suicide, among other effects. [ 3 ] [ 20 ]
Iboga is outlawed or restricted in Belgium , Poland , Denmark , Croatia , France, [ 21 ] Sweden , and Switzerland . In the United States, ibogaine is classified by the Controlled Substances Act on the list of schedule I drugs , [ 3 ] although the plant itself remains unscheduled.
Non-profit organization Föreningen för hollistisk missbruksvård is trying to convince the Swedish government to start up clinical investigations of its anti-addictive properties, loosen up the prohibition law against ibogaine, and allow the creation of treatment facilities in Sweden. [ 22 ]
Exportation of iboga from Gabon is illegal since the passage of a 1994 cultural protection law. [ 23 ]
While little data is available on the exploitation and existing habitat of the iboga plant, the destructive effects of harvesting and slow growth could have already severely damaged the wild iboga population. [ 24 ] | https://en.wikipedia.org/wiki/Tabernanthe_iboga |
This is a table of Clebsch–Gordan coefficients used for adding angular momentum values in quantum mechanics . The overall sign of the coefficients for each set of constant j 1 {\displaystyle j_{1}} , j 2 {\displaystyle j_{2}} , j {\displaystyle j} is arbitrary to some degree and has been fixed according to the Condon–Shortley and Wigner sign convention as discussed by Baird and Biedenharn . [ 1 ] Tables with the same sign convention may be found in the Particle Data Group 's Review of Particle Properties [ 2 ] and in online tables. [ 3 ]
The Clebsch–Gordan coefficients are the solutions to
Explicitly:
The summation is extended over all integer k for which the argument of every factorial is nonnegative. [ 4 ]
For brevity, solutions with m < 0 and j 1 < j 2 are omitted. They may be calculated using the simple relations
and
The Clebsch–Gordan coefficients for j values less than or equal to 5/2 are given below. [ 5 ]
When j 2 = 0 , the Clebsch–Gordan coefficients are given by δ j , j 1 δ m , m 1 {\displaystyle \delta _{j,j_{1}}\delta _{m,m_{1}}} .
Algorithms to produce Clebsch–Gordan coefficients for higher values of j 1 {\displaystyle j_{1}} and j 2 {\displaystyle j_{2}} , or for the su(N) algebra instead of su(2), are known. [ 6 ] A web interface for tabulating SU(N) Clebsch–Gordan coefficients is readily available. | https://en.wikipedia.org/wiki/Table_of_Clebsch–Gordan_coefficients |
This is a compilation of published detonation velocities for various high explosive compounds. Detonation velocity is the speed with which the detonation shock wave travels through the explosive. It is a key, directly measurable indicator of explosive performance, but depends on density which must always be specified, and may be too low if the test charge diameter is not large enough. Especially for little studied explosives there may be divergent published values due to charge diameter issues. In liquid explosives, like nitroglycerin, there may be two detonation velocities, one much higher than the other. The detonation velocity values presented here are typically for the highest practical density which maximizes achievable detonation velocity. [ 1 ]
The velocity of detonation is an important indicator for overall energy and power of detonation, and in particular for the brisance or shattering effect of an explosive which is due to the detonation pressure. The pressure can be calculated using Chapman-Jouguet theory from the velocity and density. | https://en.wikipedia.org/wiki/Table_of_explosive_detonation_velocities |
The following table lists many specialized symbols commonly used in modern mathematics , ordered by their introduction date. The table can also be ordered alphabetically by clicking on the relevant header title. | https://en.wikipedia.org/wiki/Table_of_mathematical_symbols_by_introduction_date |
This is a table of permselectivity for different substances in the glomerulus of the kidney in renal filtration . | https://en.wikipedia.org/wiki/Table_of_permselectivity_for_different_substances |
The connected 3-regular ( cubic ) simple graphs are listed for small vertex numbers.
The number of connected simple cubic graphs on 4, 6, 8, 10, ... vertices is 1, 2, 5, 19, ... (sequence A002851 in the OEIS ). A classification according to edge connectivity is made as follows: the 1-connected and 2-connected graphs are defined as usual. This leaves the other graphs in the 3-connected class because each
3-regular graph can be split by cutting all edges adjacent to any of the vertices. To refine this definition in the light of the algebra of coupling of angular momenta (see below), a subdivision of the 3-connected graphs is helpful. We shall call
This declares the numbers 3 and 4 in the fourth column of the tables below.
Ball-and-stick models of the graphs in another column of the
table show the vertices and edges in the style of
images of molecular bonds.
Comments on the individual pictures contain girth , diameter , Wiener index , Estrada index and Kirchhoff index . Aut is the order of the Automorphism group of the graph.
A Hamiltonian circuit (where present) is indicated by enumerating vertices
along that path from 1 upwards.
(The positions of the vertices have been defined by minimizing a pair potential defined by the squared difference of the Euclidean and graph theoretic distance, placed in a Molfile , then rendered by Jmol .)
The LCF notation is a notation by Joshua Lederberg , Coxeter and Frucht , for the representation of cubic graphs that are Hamiltonian .
The two edges along the cycle adjacent to any of the vertices are not written down.
Let v be the vertices of the graph and describe the Hamiltonian circle along the p vertices by the edge sequence v 0 v 1 , v 1 v 2 , ...,v p−2 v p−1 , v p−1 v 0 . Halting at a vertex v i , there is one unique vertex v j at a distance d i joined by a chord with v i ,
The vector [d 0 , d 1 , ..., d p−1 ] of the p integers is a suitable, although not unique, representation of the cubic Hamiltonian graph. This is augmented by two additional rules:
Since the starting vertex of the path is of no importance, the numbers in the representation may be cyclically permuted. If a graph contains different Hamiltonian circuits, one may select one of these to accommodate the notation. The same graph may have different LCF notations, depending on precisely how the vertices are arranged.
Often the anti-palindromic representations with
are preferred (if they exist), and the redundant part is then replaced by a semicolon and a dash "; –". The LCF notation [5, −9, 7, −7, 9, −5] 4 , for example, and would at that stage be condensed to [5, −9, 7; –] 4 .
The LCF entries are absent above if the graph has no Hamiltonian cycle , which is rare (see Tait's conjecture ). In this case a list of edges between pairs of vertices labeled 0 to n−1 in the third column serves as an identifier.
Each 4-connected (in the above sense) simple cubic graph on 2 n vertices defines a class of quantum mechanical 3 n -j symbols. Roughly speaking, each vertex represents a 3-jm symbol , the graph is converted to a digraph by assigning signs to the angular momentum quantum numbers j , the vertices are labelled with a handedness representing the order of the three j (of the three edges) in the 3-jm symbol, and the graph represents a sum over the product of all these numbers assigned to the vertices.
There are 1 ( 6-j ), 1 ( 9-j ), 2 (12-j), 5 (15-j), 18 (18-j), 84 (21-j), 607 (24-j), 6100 (27-j), 78824 (30-j), 1195280 (33-j), 20297600 (36-j), 376940415 (39-j) etc. of these (sequence A175847 in the OEIS ).
If they are equivalent to certain vertex-induced binary trees (cutting one edge and finding a cut that splits the remaining graph into two trees), they are representations of recoupling coefficients, and are then also known as Yutsis graphs (sequence A111916 in the OEIS ). | https://en.wikipedia.org/wiki/Table_of_simple_cubic_graphs |
Soil biology is the study of microbial and faunal activity and ecology in soil . Soil life , soil biota , soil fauna , or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil profile, or at the soil- litter interface.
These organisms include earthworms , nematodes , protozoa , fungi , bacteria , different arthropods , as well as some reptiles (such as snakes ), and species of burrowing mammals like gophers , moles and prairie dogs . Soil biology plays a vital role in determining many soil characteristics. The decomposition of organic matter by soil organisms has an immense influence on soil fertility , plant growth , soil structure , and carbon storage . As a relatively new science, much remains unknown about soil biology and its effect on soil ecosystems .
The soil is home to a large proportion of the world's biodiversity . The links between soil organisms and soil functions are complex. The interconnectedness and complexity of this soil 'food web' means any appraisal of soil function must necessarily take into account interactions with the living communities that exist within the soil. We know that soil organisms break down organic matter , making nutrients available for uptake by plants and other organisms. The nutrients stored in the bodies of soil organisms prevent nutrient loss by leaching . Microbial exudates act to maintain soil structure , and earthworms are important in bioturbation . However, we find that we do not understand critical aspects about how these populations function and interact. The discovery of glomalin in 1995 indicates that we lack the knowledge to correctly answer some of the most basic questions about the biogeochemical cycle in soils. There is much work ahead to gain a better understanding of the ecological role of soil biological components in the biosphere .
In balanced soil, plants grow in an active and steady environment. The mineral content of the soil and its heartiful [ clarification needed ] structure are important for their well-being, but it is the life in the earth that powers its cycles and provides its fertility. Without the activities of soil organisms, organic materials would accumulate and litter the soil surface, and there would be no food for plants.
The soil biota includes:
Of these, bacteria and fungi play key roles in maintaining a healthy soil. They act as decomposers that break down organic materials to produce detritus and other breakdown products. Soil detritivores , like earthworms, ingest detritus and decompose it. Saprotrophs , well represented by fungi and bacteria, extract soluble nutrients from delitro.
The ants (macrofaunas) help by breaking down in the same way but they also provide the motion part as they move in their armies. Also the rodents, wood-eaters help the soil to be more absorbent.
Soil biology involves work in the following areas:
Complementary disciplinary approaches are necessarily utilized which involve molecular biology , genetics , ecophysiology , biogeography , ecology, soil processes, organic matter, nutrient dynamics [ 1 ] and landscape ecology .
Bacteria are single-cell organisms and the most numerous denizens of agriculture, with populations ranging from 100 million to 3 billion in a gram. They are capable of very rapid reproduction by binary fission (dividing into two) in favourable conditions. One bacterium is capable of producing 16 million more in just 24 hours. Most soil bacteria live close to plant roots and are often referred to as rhizobacteria. Bacteria live in soil water, including the film of moisture surrounding soil particles, and some are able to swim by means of flagella . The majority of the beneficial soil-dwelling bacteria need oxygen (and are thus termed aerobic bacteria), whilst those that do not require air are referred to as anaerobic , and tend to cause putrefaction of dead organic matter. Aerobic bacteria are most active in a soil that is moist (but not saturated, as this will deprive aerobic bacteria of the air that they require), and neutral soil pH , and where there is plenty of food ( carbohydrates and micronutrients from organic matter) available. Hostile conditions will not completely kill bacteria; rather, the bacteria will stop growing and get into a dormant stage, and those individuals with pro-adaptive mutations may compete better in the new conditions. Some Gram-positive bacteria produce spores in order to wait for more favourable circumstances, and Gram-negative bacteria get into a "nonculturable" stage. Bacteria are colonized by persistent viral agents ( bacteriophages ) that determine gene word order in bacterial host.
From the organic gardener's point of view, the important roles that bacteria play are:
Nitrification is a vital part of the nitrogen cycle , wherein certain bacteria (which manufacture their own carbohydrate supply without using the process of photosynthesis) are able to transform nitrogen in the form of ammonium , which is produced by the decomposition of proteins , into nitrates , which are available to growing plants, and once again converted to proteins.
In another part of the cycle, the process of nitrogen fixation constantly puts additional nitrogen into biological circulation. This is carried out by free-living nitrogen-fixing bacteria in the soil or water such as Azotobacter , or by those that live in close symbiosis with leguminous plants, such as rhizobia . These bacteria form colonies in nodules they create on the roots of peas , beans , and related species. These are able to convert nitrogen from the atmosphere into nitrogen-containing organic substances. [ 2 ]
While nitrogen fixation converts nitrogen from the atmosphere into organic compounds, a series of processes called denitrification returns an approximately equal amount of nitrogen to the atmosphere. Denitrifying bacteria tend to be anaerobes, or facultatively anaerobes (can alter between the oxygen dependent and oxygen independent types of metabolisms), including Achromobacter and Pseudomonas . The purification process caused by oxygen-free conditions converts nitrates and nitrites in soil into nitrogen gas or into gaseous compounds such as nitrous oxide or nitric oxide . In excess, denitrification can
lead to overall losses of available soil nitrogen and subsequent loss of soil fertility . However, fixed nitrogen may circulate many times between organisms and the soil
before denitrification returns it to the atmosphere. The diagram above illustrates the nitrogen cycle.
Actinomycetota are critical in the decomposition of organic matter and in humus formation. They specialize in breaking down cellulose and lignin along with the tough chitin found on the exoskeletons of insects. Their presence is responsible for the sweet "earthy" aroma associated with a good healthy soil. They require plenty of air and a pH between 6.0 and 7.5, but are more tolerant of dry conditions than most other bacteria and fungi. [ 3 ]
A gram of garden soil can contain around one million fungi , such as yeasts and moulds . Fungi have no chlorophyll , and are not able to photosynthesise . They cannot use atmospheric carbon dioxide as a source of carbon, therefore they are chemo-heterotrophic , meaning that, like animals , they require a chemical source of energy rather than being able to use light as an energy source, as well as organic substrates to get carbon for growth and development.
Many fungi are parasitic, often causing disease to their living host plant, although some have beneficial relationships with living plants, as illustrated below. In terms of soil and humus creation, the most important fungi tend to be saprotrophic ; that is, they live on dead or decaying organic matter, thus breaking it down and converting it to forms that are available to the higher plants. A succession of fungi species will colonise the dead matter, beginning with those that use sugars and starches, which are succeeded by those that are able to break down cellulose and lignins .
Fungi spread underground by sending long thin threads known as mycelium throughout the soil; these threads can be observed throughout many soils and compost heaps. From the mycelia the fungi is able to throw up its fruiting bodies, the visible part above the soil (e.g., mushrooms , toadstools , and puffballs ), which may contain millions of spores . When the fruiting body bursts, these spores are dispersed through the air to settle in
fresh environments, and are able to lie dormant for up to years until the right conditions for their activation arise or the right food is made available.
Those fungi that are able to live symbiotically with living plants, creating a relationship that is beneficial to both, are known as mycorrhizae (from myco meaning fungal and rhiza meaning root). Plant root hairs are invaded by the mycelia of the mycorrhiza, which lives partly in the soil and partly in the root, and may either cover the length of the root hair as a sheath or be concentrated around its tip. The mycorrhiza obtains the carbohydrates that it requires from the root, in return providing
the plant with nutrients including nitrogen and moisture. Later the plant roots will also absorb the mycelium into its own tissues.
Beneficial mycorrhizal associations are to be found in many of our edible and flowering crops. Shewell Cooper suggests that these include at least 80% of the Brassica and Solanum families (including tomatoes and potatoes ), as well as the majority of tree species, especially in forest and woodlands. Here the mycorrhizae create a fine underground mesh that extends greatly beyond the limits of the tree's roots, greatly increasing their feeding range and actually causing neighbouring trees to become physically interconnected. The benefits of mycorrhizal relations to their plant partners are not limited to nutrients, but can be essential for plant reproduction. In situations where little light is able to reach the forest floor, such as the North American pine forests, a young seedling cannot obtain sufficient light to photosynthesise for itself and will not grow properly in a sterile soil. But, if the ground is underlain by a mycorrhizal mat, then the developing seedling will throw down roots that can link with the fungal threads and through them obtain the nutrients it needs, often indirectly obtained from its parents or neighbouring trees.
David Attenborough points out the plant, fungi, animal relationship that creates a "three way harmonious trio" to be found in forest ecosystems , wherein the plant/fungi symbiosis is enhanced by animals such as the wild boar, deer, mice, or flying squirrel, which feed upon the fungi's fruiting bodies, including truffles, and cause their further spread ( Private Life Of Plants , 1995). A greater understanding of the complex relationships that pervade natural systems is one of the major justifications of the organic gardener , in refraining from the use of artificial chemicals and the damage these might cause. [ citation needed ]
Recent research has shown that arbuscular mycorrhizal fungi produce glomalin , a protein that binds soil particles and stores both carbon and nitrogen. These glomalin-related soil proteins are an important part of soil organic matter . [ 4 ]
Soil fauna affect soil formation and soil organic matter dynamically on many spatiotemporal scales. [ 5 ] Earthworms , ants and termites mix the soil as they burrow, significantly affecting soil formation. Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through and out of their bodies. By aerating and stirring the soil, and by increasing the stability of soil aggregates, these organisms help to assure the ready infiltration of water. These organisms in the soil also help improve pH levels.
Ants and termites are often referred to as "Soil engineers" because, when they create their nests, there are several chemical and physical changes made to the soil. Among these changes are increasing the presence of the most essential elements like carbon, nitrogen, and phosphorus—elements needed for plant growth. [ 6 ] They also can gather soil particles from differing depths of soil and deposit them in other places, leading to the mixing of soil so it is richer with nutrients and other elements.
The soil is also important to many mammals. Gophers , moles, prairie dogs, and other burrowing animals rely on this soil for protection and food. The animals even give back to the soil as their burrowing allows more rain, snow and water from ice to enter the soil instead of creating erosion. [ 7 ]
This table includes some familiar types of soil life of soil life, [ 8 ] coherent with prevalent taxonomy as used in the linked Wikipedia articles. | https://en.wikipedia.org/wiki/Table_of_soil_life |
The values below are standard apparent reduction potentials (E°') for electro-biochemical half-reactions measured at 25 °C, 1 atmosphere and a pH of 7 in aqueous solution . [ 1 ] [ 2 ]
The actual physiological potential depends on the ratio of the reduced ( Red ) and oxidized ( Ox ) forms according to the Nernst equation and the thermal voltage .
When an oxidizer ( Ox ) accepts a number z of electrons ( e − ) to be converted in its reduced form ( Red ), the half-reaction is expressed as:
The reaction quotient ( Q r ) is the ratio of the chemical activity ( a i ) of the reduced form (the reductant, a Red ) to the activity of the oxidized form (the oxidant, a ox ). It is equal to the ratio of their concentrations ( C i ) only if the system is sufficiently diluted and the activity coefficients ( γ i ) are close to unity ( a i = γ i C i ):
The Nernst equation is a function of Q r and can be written as follows:
E red = E red ⊖ − R T z F ln Q r = E red ⊖ − R T z F ln a Red a Ox . {\displaystyle E_{\text{red}}=E_{\text{red}}^{\ominus }-{\frac {RT}{zF}}\ln Q_{r}=E_{\text{red}}^{\ominus }-{\frac {RT}{zF}}\ln {\frac {a_{\text{Red}}}{a_{\text{Ox}}}}.}
At chemical equilibrium , the reaction quotient Q r of the product activity ( a Red ) by the reagent activity ( a Ox ) is equal to the equilibrium constant ( K ) of the half-reaction and in the absence of driving force ( ΔG = 0 ) the potential ( E red ) also becomes nul.
The numerically simplified form of the Nernst equation is expressed as:
Where E red ⊖ {\displaystyle E_{\text{red}}^{\ominus }} is the standard reduction potential of the half-reaction expressed versus the standard reduction potential of hydrogen. For standard conditions in electrochemistry (T = 25 °C, P = 1 atm and all concentrations being fixed at 1 mol/L, or 1 M) the standard reduction potential of hydrogen E red H+ ⊖ {\displaystyle E_{\text{red H+}}^{\ominus }} is fixed at zero by convention as it serves of reference. The standard hydrogen electrode (SHE), with [ H + ] = 1 M works thus at a pH = 0.
At pH = 7, when [ H + ] = 10 −7 M, the reduction potential E red {\displaystyle E_{\text{red}}} of H + differs from zero because it depends on pH.
Solving the Nernst equation for the half-reaction of reduction of two protons into hydrogen gas gives:
In biochemistry and in biological fluids, at pH = 7, it is thus important to note that the reduction potential of the protons ( H + ) into hydrogen gas H 2 is no longer zero as with the standard hydrogen electrode (SHE) at 1 M H + (pH = 0) in classical electrochemistry, but that E red = − 0.414 V {\displaystyle E_{\text{red}}=-0.414\mathrm {V} } versus the standard hydrogen electrode (SHE). [ 2 ]
The same also applies for the reduction potential of oxygen:
For O 2 , E red ⊖ {\displaystyle E_{\text{red}}^{\ominus }} = 1.229 V, so, applying the Nernst equation for pH = 7 gives:
For obtaining the values of the reduction potential at pH = 7 for the redox reactions relevant for biological systems, the same kind of conversion exercise is done using the corresponding Nernst equation expressed as a function of pH.
The conversion is simple, but care must be taken not to inadvertently mix reduction potential converted at pH = 7 with other data directly taken from tables referring to SHE (pH = 0).
The E h {\displaystyle E_{h}} and pH of a solution are related by the Nernst equation as commonly represented by a Pourbaix diagram ( E h {\displaystyle E_{h}} – pH plot) . For a half cell equation, conventionally written as a reduction reaction ( i.e. , electrons accepted by an oxidant on the left side):
The half-cell standard reduction potential E red ⊖ {\displaystyle E_{\text{red}}^{\ominus }} is given by
where Δ G ⊖ {\displaystyle \Delta G^{\ominus }} is the standard Gibbs free energy change, z is the number of electrons involved, and F is Faraday's constant . The Nernst equation relates pH and E h {\displaystyle E_{h}} :
where curly braces { } indicate activities , and exponents are shown in the conventional manner. This equation is the equation of a straight line for E h {\displaystyle E_{h}} as a function of pH with a slope of − 0.05916 ( h z ) {\displaystyle -0.05916\,\left({\frac {h}{z}}\right)} volt (pH has no units).
This equation predicts lower E h {\displaystyle E_{h}} at higher pH values. This is observed for the reduction of O 2 into H 2 O, or OH − , and for reduction of H + into H 2 .
To obtain the reduction potential as a function of the measured concentrations of the redox-active species in solution, it is necessary to express the activities as a function of the concentrations.
Given that the chemical activity denoted here by { } is the product of the activity coefficient γ by the concentration denoted by [ ]: a i = γ i ·C i , here expressed as {X} = γ x [X] and {X} x = ( γ x ) x [X] x and replacing the logarithm of a product by the sum of the logarithms ( i.e. , log (a·b) = log a + log b), the log of the reaction quotient ( Q r {\displaystyle Q_{r}} ) (without {H + } already isolated apart in the last term as h pH) expressed here above with activities { } becomes:
It allows to reorganize the Nernst equation as:
Where E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} is the formal standard potential independent of pH including the activity coefficients.
Combining E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} directly with the last term depending on pH gives:
For a pH = 7:
So,
It is therefore important to know to what exact definition does refer the value of a reduction potential for a given biochemical redox process reported at pH = 7, and to correctly understand the relationship used.
Is it simply:
This requires thus to dispose of a clear definition of the considered reduction potential, and of a sufficiently detailed description of the conditions in which it is valid, along with a complete expression of the corresponding Nernst equation. Were also the reported values only derived from thermodynamic calculations, or determined from experimental measurements and under what specific conditions? Without being able to correctly answering these questions, mixing data from different sources without appropriate conversion can lead to errors and confusion.
The formal standard reduction potential E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} can be defined as the measured reduction potential E red {\displaystyle E_{\text{red}}} of the half-reaction at unity concentration ratio of the oxidized and reduced species ( i.e. , when C red / C ox = 1) under given conditions. [ 3 ]
Indeed:
as, E red = E red ⊖ {\displaystyle E_{\text{red}}=E_{\text{red}}^{\ominus }} , when a red a ox = 1 {\displaystyle {\frac {a_{\text{red}}}{a_{\text{ox}}}}=1} ,
because ln 1 = 0 {\displaystyle \ln {1}=0} , and that the term γ red γ ox {\displaystyle {\frac {\gamma _{\text{red}}}{\gamma _{\text{ox}}}}} is included in E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} .
The formal reduction potential makes possible to more simply work with molar or molal concentrations in place of activities . Because molar and molal concentrations were once referred as formal concentrations , it could explain the origin of the adjective formal in the expression formal potential. [ citation needed ]
The formal potential is thus the reversible potential of an electrode at equilibrium immersed in a solution where reactants and products are at unit concentration. [ 4 ] If any small incremental change of potential causes a change in the direction of the reaction, i.e. from reduction to oxidation or vice versa , the system is close to equilibrium, reversible and is at its formal potential. When the formal potential is measured under standard conditions ( i.e. the activity of each dissolved species is 1 mol/L, T = 298.15 K = 25 °C = 77 °F, P gas = 1 bar) it becomes de facto a standard potential. [ 5 ] According to Brown and Swift (1949), "A formal potential is defined as the potential of a half-cell, measured against the standard hydrogen electrode , when the total concentration of each oxidation state is one formal". [ 6 ]
The activity coefficients γ r e d {\displaystyle \gamma _{red}} and γ o x {\displaystyle \gamma _{ox}} are included in the formal potential E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} , and because they depend on experimental conditions such as temperature, ionic strength , and pH , E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} cannot be referred as an immuable standard potential but needs to be systematically determined for each specific set of experimental conditions. [ 5 ]
Formal reduction potentials are applied to simplify results interpretations and calculations of a considered system. Their relationship with the standard reduction potentials must be clearly expressed to avoid any confusion.
The main factor affecting the formal (or apparent) reduction potentials E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} in biochemical or biological processes is the pH. To determine approximate values of formal reduction potentials, neglecting in a first approach changes in activity coefficients due to ionic strength, the Nernst equation has to be applied taking care to first express the relationship as a function of pH. The second factor to be considered are the values of the concentrations taken into account in the Nernst equation. To define a formal reduction potential for a biochemical reaction, the pH value, the concentrations values and the hypotheses made on the activity coefficients must always be clearly indicated. When using, or comparing, several formal (or apparent) reduction potentials they must also be internally consistent.
Problems may occur when mixing different sources of data using different conventions or approximations ( i.e. , with different underlying hypotheses). When working at the frontier between inorganic and biological processes (e.g., when comparing abiotic and biotic processes in geochemistry when microbial activity could also be at work in the system), care must be taken not to inadvertently directly mix standard reduction potentials ( E red ⊖ {\displaystyle E_{\text{red}}^{\ominus }} versus SHE, pH = 0) with formal (or apparent) reduction potentials ( E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} at pH = 7). Definitions must be clearly expressed and carefully controlled, especially if the sources of data are different and arise from different fields (e.g., picking and directly mixing data from classical electrochemistry textbooks ( E red ⊖ {\displaystyle E_{\text{red}}^{\ominus }} versus SHE, pH = 0) and microbiology textbooks ( E red ⊖ ′ {\displaystyle E_{\text{red}}^{\ominus '}} at pH = 7) without paying attention to the conventions on which they are based).
For example, in a two electrons couple like NAD + : NADH the reduction potential becomes ~ 30 mV (or more exactly, 59.16 mV/2 = 29.6 mV) more positive for every power of ten increase in the ratio of the oxidised to the reduced form. | https://en.wikipedia.org/wiki/Table_of_standard_reduction_potentials_for_half-reactions_important_in_biochemistry |
Common thermodynamic equations and quantities in thermodynamics , using mathematical notation , are as follows:
Many of the definitions below are also used in the thermodynamics of chemical reactions .
τ = k B ( ∂ U / ∂ S ) N {\displaystyle \tau =k_{\text{B}}\left(\partial U/\partial S\right)_{N}} 1 / τ = 1 / k B ( ∂ S / ∂ U ) N {\displaystyle 1/\tau =1/k_{\text{B}}\left(\partial S/\partial U\right)_{N}}
S = − ( ∂ F / ∂ T ) V , N {\displaystyle S=-\left(\partial F/\partial T\right)_{V,N}} , S = − ( ∂ G / ∂ T ) P , N {\displaystyle S=-\left(\partial G/\partial T\right)_{P,N}}
P = − ( ∂ U / ∂ V ) S , N {\displaystyle P=-\left(\partial U/\partial V\right)_{S,N}}
μ i = ( ∂ F / ∂ N i ) T , V {\displaystyle \mu _{i}=\left(\partial F/\partial N_{i}\right)_{T,V}} , where F {\displaystyle F} is not proportional to N {\displaystyle N} because μ i {\displaystyle \mu _{i}} depends on pressure. μ i = ( ∂ G / ∂ N i ) T , P {\displaystyle \mu _{i}=\left(\partial G/\partial N_{i}\right)_{T,P}} , where G {\displaystyle G} is proportional to N {\displaystyle N} (as long as the molar ratio composition of the system remains the same) because μ i {\displaystyle \mu _{i}} depends only on temperature and pressure and composition. μ i / τ = − 1 / k B ( ∂ S / ∂ N i ) U , V {\displaystyle \mu _{i}/\tau =-1/k_{\text{B}}\left(\partial S/\partial N_{i}\right)_{U,V}}
The equations in this article are classified by subject.
For an ideal gas p 1 V 1 γ = p 2 V 2 γ {\displaystyle p_{1}V_{1}^{\gamma }=p_{2}V_{2}^{\gamma }} T 1 V 1 γ − 1 = T 2 V 2 γ − 1 {\displaystyle T_{1}V_{1}^{\gamma -1}=T_{2}V_{2}^{\gamma -1}} p 1 1 − γ T 1 γ = p 2 1 − γ T 2 γ {\displaystyle p_{1}^{1-\gamma }T_{1}^{\gamma }=p_{2}^{1-\gamma }T_{2}^{\gamma }}
For an ideal gas W = k T N ln ( V 2 / V 1 ) {\displaystyle W=kTN\ln(V_{2}/V_{1})} W = n R T ln ( V 2 / V 1 ) {\displaystyle W=nRT\ln(V_{2}/V_{1})}
W = p Δ V , Q = Δ U + p δ V {\displaystyle W=p\Delta V,\quad Q=\Delta U+p\delta V}
W = 0 , Q = Δ U {\displaystyle W=0,\quad Q=\Delta U}
W = ∫ V 1 V 2 p d V {\displaystyle W=\int _{V_{1}}^{V_{2}}p\mathrm {d} V}
Net work done in cyclic processes W = ∮ c y c l e p d V = ∮ c y c l e Δ Q {\displaystyle W=\oint _{\mathrm {cycle} }p\mathrm {d} V=\oint _{\mathrm {cycle} }\Delta Q}
p 1 V 1 p 2 V 2 = n 1 T 1 n 2 T 2 = N 1 T 1 N 2 T 2 {\displaystyle {\frac {p_{1}V_{1}}{p_{2}V_{2}}}={\frac {n_{1}T_{1}}{n_{2}T_{2}}}={\frac {N_{1}T_{1}}{N_{2}T_{2}}}}
− n R T ln P 1 P 2 {\displaystyle -nRT\ln {\frac {P_{1}}{P_{2}}}\;}
C p = 7 2 n R {\displaystyle C_{p}={\frac {7}{2}}nR} (for diatomic ideal gas)
C V = 5 2 n R {\displaystyle C_{V}={\frac {5}{2}}nR\;} (for diatomic ideal gas)
Below are useful results from the Maxwell–Boltzmann distribution for an ideal gas, and the implications of the Entropy quantity. The distribution is valid for atoms or molecules constituting ideal gases.
K 2 is the modified Bessel function of the second kind.
P ( v ) = 4 π ( m 2 π k B T ) 3 / 2 v 2 e − m v 2 / 2 k B T {\displaystyle P\left(v\right)=4\pi \left({\frac {m}{2\pi k_{\text{B}}T}}\right)^{3/2}v^{2}e^{-mv^{2}/2k_{\text{B}}T}}
Relativistic speeds (Maxwell–Jüttner distribution) f ( p ) = 1 4 π m 3 c 3 θ K 2 ( 1 / θ ) e − γ ( p ) / θ {\displaystyle f(p)={\frac {1}{4\pi m^{3}c^{3}\theta K_{2}(1/\theta )}}e^{-\gamma (p)/\theta }}
where: P i = 1 / Ω {\displaystyle P_{i}=1/\Omega }
Δ S = k B N ln V 2 V 1 + N C V ln T 2 T 1 {\displaystyle \Delta S=k_{\text{B}}N\ln {\frac {V_{2}}{V_{1}}}+NC_{V}\ln {\frac {T_{2}}{T_{1}}}}
⟨ E k ⟩ = 1 2 k T {\displaystyle \langle E_{\mathrm {k} }\rangle ={\frac {1}{2}}kT}
Internal energy U = d f ⟨ E k ⟩ = d f 2 k T {\displaystyle U=d_{\text{f}}\langle E_{\mathrm {k} }\rangle ={\frac {d_{\text{f}}}{2}}kT}
Corollaries of the non-relativistic Maxwell–Boltzmann distribution are below.
For quasi-static and reversible processes, the first law of thermodynamics is:
where δQ is the heat supplied to the system and δW is the work done by the system.
The following energies are called the thermodynamic potentials ,
and the corresponding fundamental thermodynamic relations or "master equations" [ 2 ] are:
The four most common Maxwell's relations are:
( ∂ T ∂ P ) S = + ( ∂ V ∂ S ) P = ∂ 2 H ∂ S ∂ P {\displaystyle \left({\frac {\partial T}{\partial P}}\right)_{S}=+\left({\frac {\partial V}{\partial S}}\right)_{P}={\frac {\partial ^{2}H}{\partial S\partial P}}}
+ ( ∂ S ∂ V ) T = ( ∂ P ∂ T ) V = − ∂ 2 F ∂ T ∂ V {\displaystyle +\left({\frac {\partial S}{\partial V}}\right)_{T}=\left({\frac {\partial P}{\partial T}}\right)_{V}=-{\frac {\partial ^{2}F}{\partial T\partial V}}}
− ( ∂ S ∂ P ) T = ( ∂ V ∂ T ) P = ∂ 2 G ∂ T ∂ P {\displaystyle -\left({\frac {\partial S}{\partial P}}\right)_{T}=\left({\frac {\partial V}{\partial T}}\right)_{P}={\frac {\partial ^{2}G}{\partial T\partial P}}}
More relations include the following.
Other differential equations are:
where N is number of particles, h is that Planck constant , I is moment of inertia , and Z is the partition function , in various forms:
Since
Since
(where δW rev is the work done by the system),
λ n e t = ∑ j λ j {\displaystyle \lambda _{\mathrm {net} }=\sum _{j}\lambda _{j}}
Parallel 1 λ n e t = ∑ j ( 1 λ j ) {\displaystyle {\frac {1}{\lambda }}_{\mathrm {net} }=\sum _{j}\left({\frac {1}{\lambda }}_{j}\right)}
η = | W Q H | {\displaystyle \eta =\left|{\frac {W}{Q_{\text{H}}}}\right|}
Carnot engine efficiency: η c = 1 − | Q L Q H | = 1 − T L T H {\displaystyle \eta _{\text{c}}=1-\left|{\frac {Q_{\text{L}}}{Q_{\text{H}}}}\right|=1-{\frac {T_{\text{L}}}{T_{\text{H}}}}}
K = | Q L W | {\displaystyle K=\left|{\frac {Q_{\text{L}}}{W}}\right|}
Carnot refrigeration performance K C = | Q L | | Q H | − | Q L | = T L T H − T L {\displaystyle K_{\text{C}}={\frac {|Q_{\text{L}}|}{|Q_{\text{H}}|-|Q_{\text{L}}|}}={\frac {T_{\text{L}}}{T_{\text{H}}-T_{\text{L}}}}} | https://en.wikipedia.org/wiki/Table_of_thermodynamic_equations |
This is a table of volume of distribution (V d ) for various medication . For comparison, those with a V d L/kg body weight of less than 0.2 are mainly distributed in blood plasma , 0.2-0.7 mostly in the extracellular fluid and those with more than 0.7 are distributed throughout total body water. | https://en.wikipedia.org/wiki/Table_of_volume_of_distribution_for_drugs |
David Bierens de Haan (3 May 1822, in Amsterdam – 12 August 1895, in Leiden ) was a Dutch mathematician and historian of science .
Bierens de Haan was a son of the rich merchant Abraham Pieterszoon de Haan (1795–1880) and Catharina Jacoba Bierens (1797–1835). In 1843 he completed a study in the exact sciences and received his PhD from the University of Leiden in 1847 under Gideon Janus Verdam (1802–1866) for the work De Lemniscata Bernouillana . After this he became a teacher of physics and mathematics at a gymnasium in Deventer . In 1852 he married Johanna Catharina Justina IJssel de Schepper (1827–1906) in Deventer.
In 1856 he became member of the Royal Netherlands Academy of Arts and Sciences . [ 1 ] Since 1866 he was professor of mathematics at Leiden University . Since 1888 he was co-editor of the works of Christiaan Huygens and in 1892 edited the Algebra of Willem Smaasen (1820–1850).
He had a large library on mathematics, the history of science and pedagogy, which currently resides at the Leiden University Library .
His most important contribution to mathematics consist of the issuing of a large table of integrals (Nouvelles) tables d'intégrales définies in 1858 (and 1867). His doctoral students include Pieter Hendrik Schoute . | https://en.wikipedia.org/wiki/Tables_d'intégrales_définies |
A tablet (also known as a pill ) is a pharmaceutical oral dosage form ( oral solid dosage , or OSD) or solid unit dosage form. Tablets may be defined as the solid unit dosage form of medication with suitable excipients . It comprises a mixture of active substances and excipients, usually in powder form, that are pressed or compacted into a solid dose. The main advantages of tablets are that they ensure a consistent dose of medicine that is easy to consume.
Tablets are prepared either by moulding or by compression . The excipients can include diluents , binders or granulating agents, glidants (flow aids) and lubricants to ensure efficient tabletting; disintegrants to promote tablet break-up in the digestive tract; sweeteners or flavours to enhance taste; and pigments to make the tablets visually attractive or aid in visual identification of an unknown tablet. A polymer coating is often applied to make the tablet smoother and easier to swallow, to control the release rate of the active ingredient, to make it more resistant to the environment (extending its shelf life), or to enhance the tablet's appearance.
Medicinal tablets were originally made in the shape of a disk of whatever colour their components determined, but are now made in many shapes and colours to help distinguish different medicines. Tablets are often imprinted with symbols, letters, and numbers, which allow them to be identified, or a groove to allow splitting by hand. Sizes of tablets to be swallowed range from a few millimetres to about a centimetre.
The compressed tablet is the most commonly seen dosage form in use today. About two-thirds of all prescriptions are dispensed as solid dosage forms, and half of these are compressed tablets. A tablet can be formulated to deliver an accurate dosage to a specific site in the body; it is usually taken orally, but can be administered sublingually , buccally , rectally or intravaginally . The tablet is just one of the many forms that an oral drug can take such as syrups , elixirs , suspensions , and emulsions .
Pills are thought to date back to around 1500 BC. [ 1 ] Earlier medical recipes, such as those from 4000 BC, were for liquid preparations rather than solids. [ 1 ] The first references to pills were found on papyruses in ancient Egypt and contained bread dough, honey, or grease. Medicinal ingredients, such as plant powders or spices, were mixed in and formed by hand to make little balls, or pills.
In ancient Greece, such medicines were known as katapotia ("something to be swallowed"), and the Roman scholar Pliny, who lived from 23 to 79 AD, first gave a name to what we now call pills, calling them pilula . [ 1 ]
Pills have always been difficult to swallow, and efforts have been made to make them go down easier. In medieval times, people coated pills with slippery plant substances. Another approach, used as recently as the 19th century, was to gild them in gold and silver, although this often meant that they would pass through the digestive tract with no effect. [ 1 ] In the 1800s, sugar coating and gelatin coating were invented, as were gelatin capsules . [ 1 ]
In 1843, the British painter and inventor William Brockedon was granted a patent for a machine capable of "Shaping Pills, Lozenges, and Black Lead by Pressure in Dies". The device was capable of compressing powder into a tablet without the use of an adhesive. [ 2 ]
A pill was originally defined as a small, round, solid pharmaceutical oral dosage form of medication. The word's etymology reflects the historical concepts of grinding the ingredients with a mortar and pestle and rolling the resultant paste or dough into lumps to be dried. Today, in its strict sense, the word pill still refers specifically to tablets (including caplets) rather than capsules (which were invented much later), but because a simple hypernym is needed to intuitively cover all such oral dosage forms, the broad sense of the word pill is also widely used and includes both tablets and capsules — colloquially, any solid oral form of medication falls into the "pill" category (see pill § Usage notes ).
An early example of a pill comes from ancient Rome. They were made of zinc carbonates, hydrozincite and smithsonite . The pills were used for sore eyes and were found aboard a Roman ship that wrecked in 140 BC. However, these tablets were meant to be pressed on the eyes, not swallowed. [ 3 ] [ 4 ]
A caplet is a smooth, coated, oval-shaped medicinal tablet in the general shape of a capsule . Many caplets have an indentation running down the middle, so they may be split in half more easily. [ 5 ] Consumers have viewed capsules as the most effective way to take medication ever since they first appeared. For this reason, producers of drugs such as OTC analgesics wanting to emphasize the strength of their product developed the "caplet", a portmanteau [ 6 ] of capsule-shaped tablet , [ 7 ] [ 8 ] in order to tie this positive association to more efficiently produced tablet pills as well as being an easier-to-swallow shape than the usual disk-shaped tablet.
An orally disintegrating tablet or orodispersible tablet (ODT), is a drug dosage form available for a limited range of over-the-counter (OTC) and prescription medications.
In the tablet-pressing process, it is important that all ingredients be fairly dry, powdered or granular, somewhat uniform in particle size, and freely flowing. Mixed particle sized powders may separate during the manufacturing process due to differing particle densities. This can result in tablets with non-uniform concentrations of drug or active pharmaceutical ingredient (API), resulting in uneven dosage between tablets, but granulation should prevent this.
Some APIs may be compressed into tablets as pure substances, but this is rarely the case; most formulations include pharmacologically inactive ingredients ( excipients ):
Tablets can be made in virtually any shape, although the requirements of patients and tableting machines mean that most are round, oval, or capsule-shaped. More unusual shapes have been manufactured, but patients find these harder to swallow, and they are more vulnerable to chipping or manufacturing problems.
Tablet diameter and shape are determined by the machine tooling used to produce them; a die plus an upper and a lower punch are required. This is called a station of tooling. The amount of tablet material and the placement of the punches in relation to one another during compression determine the thickness. Once this is done, we can measure the corresponding pressure applied during compression. The shorter the distance between the punches, the greater the pressure applied during compression, and sometimes the harder the tablet. Tablets need to be hard enough that they do not break up in the bottle, yet friable enough that they disintegrate in the gastric tract.
Tablets need to be strong enough to resist the stresses of packaging, shipping, and handling by the pharmacist and patient. The mechanical strength of tablets is assessed using a combination of simple failure and erosion tests, and more sophisticated engineering tests. The simpler tests are often used for quality control purposes, whereas the more complex tests are used during the design of the formulation and manufacturing process in the research and development phase. Standards for tablet properties are published in the various international pharmacopeias (USP/NF, EP, JP, etc.). The hardness of tablets is the principal measure of mechanical strength. Hardness is tested using a tablet hardness tester . The units for hardness have evolved since the 1930s but are commonly measured in kilograms per square centimetre. Models of testers include the Monsanto (or Stokes) Hardness Tester from 1930, the Pfizer Hardness Tester from 1950, the Strong Cob Hardness Tester and the Heberlain (or Schleeniger) Hardness Tester.
Lubricants prevent ingredients from clumping together and from sticking to the tablet punches or capsule filling machine. Lubricants also ensure that tablet formation and ejection can occur with low friction between the solid and die wall, as well as between granules, which helps in uniform filling of the die.
Common minerals like talc or silica, and fats, e.g. vegetable stearin, magnesium stearate or stearic acid are the most frequently used lubricants in tablets or hard gelatin capsules. [ 9 ]
In the tablet pressing process, the appropriate amount of active ingredient must be in each tablet. Hence, all the ingredients should be well mixed. If a sufficiently homogenous mix of the components cannot be obtained with simple blending processes, the ingredients must be granulated prior to compression to assure an even distribution of the active compound in the final tablet. Two basic techniques are used to granulate powders for compression into tablets: wet granulation and dry granulation. Powders that can be mixed well do not require granulation and can be compressed into tablets through direct compression ("DC"). Direct compression is desirable as it is quicker. There is less processing, equipment, labor, and energy consumption. However, DC is difficult when a formulation has a high content of poorly compressible active ingredients.
Wet granulation is a process of using a liquid binder to lightly agglomerate the powder mixture. The amount of liquid has to be properly controlled, as over-wetting will cause the granules to be too hard and under-wetting will cause them to be too soft and friable. Aqueous solutions have the advantage of being safer to deal with than solvent-based systems but may not be suitable for drugs which are degraded by hydrolysis.
Low shear wet granulation processes use very simple mixing equipment, and can take a considerable time to achieve a uniformly mixed state. High shear wet granulation processes use equipment that mixes the powder and liquid at a very fast rate, and thus speeds up the manufacturing process. Fluid bed granulation is a multiple-step wet granulation process performed in the same vessel to pre-heat, granulate, and dry the powders. It is used because it allows close control of the granulation process.
Dry granulation processes create granules by light compaction of the powder blend under low pressures. The compacts so-formed are broken up gently to produce granules (agglomerates). This process is often used when the product to be granulated is sensitive to moisture and heat. Dry granulation can be conducted on a tablet press using slugging tooling or on a roll press called a roller compactor. Dry granulation equipment offers a wide range of pressures to attain proper densification and granule formation. Dry granulation is simpler than wet granulation, therefore the cost is reduced. However, dry granulation often produces a higher percentage of fine granules, which can compromise the quality or create yield problems for the tablet. Dry granulation requires drugs or excipients with cohesive properties, and a 'dry binder' may need to be added to the formulation to facilitate the formation of granules.
Hot melt extrusion is utilized in pharmaceutical solid oral dose processing to enable delivery of drugs with poor solubility and bioavailability . Hot melt extrusion has been shown to molecularly disperse poorly soluble drugs in a polymer carrier increasing dissolution rates and bioavailability. The process involves the application of heat, pressure and agitation to mix materials together and 'extrude' them through a die. Twin-screw high shear extruders blend materials and simultaneously break up particles. The extruded particles can then be blended and compressed into tablets or filled into capsules. [ 10 ]
After granulation, a final lubrication step is used to ensure that the tableting blend does not stick to the equipment during the tableting process. This usually involves low shear blending of the granules with a powdered lubricant, such as magnesium stearate or stearic acid .
Whatever process is used to make the tableting blend, the process of making a tablet by powder compaction is very similar. First, the powder is filled into the die from above. The mass of powder is determined by the position of the lower punch in the die, the cross-sectional area of the die, and the powder density. At this stage, adjustments to the tablet weight are normally made by repositioning the lower punch. After die filling, the upper punch is lowered into the die and the powder is uniaxially compressed to a porosity of between 5 and 20%. The compression can take place in one or two stages (main compression, and, sometimes, pre-compression or tamping) and for commercial production occurs very fast (500–50 ms per tablet). Finally, the upper punch is pulled up and out of the die (decompression), and the tablet is ejected from the die by lifting the lower punch until its upper surface is flush with the top face of the die. This process is repeated for each tablet.
Common problems encountered during tablet manufacturing operations include:
Consequently, permanent consistency checks are required during the manufacturing process. [ 11 ]
Tablet formulations are designed and tested using a laboratory machine called a Tablet Compaction Simulator or Powder Compaction Simulator. This is a computer controlled device that can measure the punch positions, punch pressures, friction forces, die wall pressures, and sometimes the tablet internal temperature during the compaction event. Numerous experiments with small quantities of different mixtures can be performed to optimise a formulation. Mathematically corrected punch motions can be programmed to simulate any type and model of production tablet press. Initial quantities of active pharmaceutical ingredients are very expensive to produce, and using a Compaction Simulator reduces the amount of powder required for product development.
Tablet presses , also called tableting machines, range from small, inexpensive bench-top models that make one tablet at a time (single-station presses), with only around a half-ton pressure, to large, computerized, industrial models (multi-station rotary presses) that can make hundreds of thousands to millions of tablets an hour with much greater pressure. The tablet press is an essential piece of machinery for any pharmaceutical and nutraceutical manufacturer. Tablet presses must allow the operator to adjust the position of the lower and upper punches accurately, so that the tablet weight, thickness and density/hardness can each be controlled. This is achieved using a series of cams, rollers, or tracks that act on the tablet tooling (punches). Mechanical systems are also incorporated for die filling, and for ejecting and removing the tablets from the press after compression. Pharmaceutical tablet presses are required to be easy to clean and quick to reconfigure with different tooling, because they are usually used to manufacture many different products.
There are two main standards of tablet tooling used in pharmaceutical industry: American standard TSM and European standard EU. TSM and EU configurations are similar to each other but cannot be interchanged. [ 12 ]
Modern tablet presses reach output volumes of up to 1,700,000 tablets per hour. These huge volumes require frequent in-process quality control for the tablet weight, thickness and hardness. Due to efforts to reduce rejects rates and machine down-time, automated tablet testing devices are used on-line with the tablet press or off-line in the IPC-labs.
Many tablets today are coated after being pressed. Although sugar-coating was popular in the past, the process has many drawbacks. Modern tablet coatings [ 13 ] are polymer and polysaccharide based, with plasticizers and pigments included. Tablet coatings must be stable and strong enough to survive the handling of the tablet, must not make tablets stick together during the coating process, and must follow the fine contours of embossed characters or logos on tablets. Coatings are necessary for tablets that have an unpleasant taste, and a smoother finish makes large tablets easier to swallow. Tablet coatings are also useful to extend the shelf-life of components that are sensitive to moisture or oxidation. Special coatings (for example with pearlescent effects) can enhance brand recognition.
If the active ingredient of a tablet is sensitive to acid, or is irritant to the stomach lining, an enteric coating can be used, which is resistant to stomach acid, and dissolves in the less acidic area of the intestines. Enteric coatings are also used for medicines that can be negatively affected by taking a long time to reach the small intestine , where they are absorbed. Coatings are often chosen to control the rate of dissolution of the drug in the gastrointestinal tract. Some drugs are absorbed better in certain parts of the digestive system. If this part is the stomach, a coating is selected that dissolves quickly and easily in acid. If the rate of absorption is best in the large intestine or colon, a coating is used that is acid resistant and dissolves slowly to ensure that the tablet reaches that point before dispersing. To measure the disintegration time of the tablet coating and the tablet core, automatic disintegration testers are used which are able to determine the complete disintegration process of a tablet by measuring the rest height of the thickness with every upward stroke of the disintegration tester basket.
There are two types of coating machines used in the pharmaceutical industry: coating pans and automatic coaters. [ 14 ] Coating pans are used mostly to sugar coat pellets. Automatic coaters are used for all kinds of coatings; they can be equipped with a remote control panel, a dehumidifier, and dust collectors. An explosion-proof design is required for applying coatings that contain alcohol.
It is sometimes necessary to split tablets into halves or quarters. Tablets are easier to break accurately if scored, but there are devices called pill-splitters which cut unscored and scored tablets. Tablets with special coatings (for example, enteric coatings or controlled-release coatings) should not be broken before use, as this exposes the tablet core to the digestive juices, circumventing the intended delayed-release effect. | https://en.wikipedia.org/wiki/Tablet_(pharmacy) |
Tablet hardness testing is a laboratory technique used by the pharmaceutical industry to determine the breaking point and structural integrity of a tablet and find out how it changes "under conditions of storage, transportation, packaging and handling before usage" [ 1 ] The breaking point of a tablet is based on its shape. [ 2 ] It is similar to friability testing, [ 1 ] but they are not the same thing.
Tablet hardness testers first appeared in the 1930s. [ 3 ] In the 1950s, the Strong-Cobb tester was introduced. It was patented by Robert Albrecht on July 21, 1953. [ 4 ] and used an air pump. The tablet breaking force was based on arbitrary units referred to as Strong-Cobbs. [ 3 ] The new one gave readings that were inconsistent to those given by the older testers. [ 3 ] Later, electro-mechanical testing machines were introduced. They often include mechanisms like motor drives, and the ability to send measurements to a computer or printer. [ 3 ]
There are 2 main processes to test tablet hardness: compression testing and 3 point bend testing. For compression testing, the analyst generally aligns the tablet in a repeatable way, [ 2 ] and the tablet is squeezed between a fixed and a moving jaw. The first machines continually applied force with a spring and screw thread until the tablet started to break. [ 3 ] When the tablet fractured, the hardness was read with a sliding scale. [ 3 ]
There are several devices used to perform this task:
According to the International System of Units , the units of measurement of tablet hardness mostly follow standards used in materials testing. | https://en.wikipedia.org/wiki/Tablet_hardness_testing |
A tablet press is a mechanical device that compresses powder into tablets of uniform size and weight. A tablet press can be used to manufacture tablets of a wide variety of materials, including pharmaceuticals , nutraceuticals , cleaning products, industrial pellets and cosmetics . To form a tablet, the granulated powder material must be metered into a cavity formed by two punches and a die , and then the punches must be pressed together with great force to fuse the material together. [ 1 ]
A tablet is formed by the combined pressing action of two punches and a die. In the first step of a typical operation, the bottom punch is lowered in the die creating a cavity into which the granulated feedstock is fed. The exact depth of the lower punch can be precisely controlled to meter the amount of powder that fills the cavity. The excess is scraped from the top of the die, and the lower punch is drawn down and temporarily covered to prevent spillage. Then, the upper punch is brought down into contact with the powder as the cover is removed. The force of compression is delivered by high pressure compression rolls which fuse the granulated material together into a hard tablet. After compression, the lower punch is raised to eject the tablet. [ 2 ]
Tablet tooling design is critical to ensuring a robust tablet compression process. Considerations when designing pharmaceutical tablet compression tool design include tooling set, head flat, top head angle, top head radius, head back angle, and punch shank. As well as ensuring a single dose of drug, the tablet tooling is also critical in ensuring the size, shape, embossing and other physical characteristics of the tablet that are required for identification. [ 3 ]
There are 2 types of tablet presses: single-punch and rotary tablet presses. Most high-speed tablet presses take the form of a rotating turret that holds any number of punches. As they rotate around the turret, the punches come into contact with cams which control the punch's vertical position. Punches and dies are usually custom made for each application, and can be made in a wide variety of sizes, shapes, and can be customized with manufacturer codes and scoring lines to make tablets easier to break. Depending on tablet size, shape, material, and press configuration, a typical modern press can produce from 250,000 to over 1,700,000 tablets an hour. [ 4 ] | https://en.wikipedia.org/wiki/Tablet_press |
The Tac-Promoter (abbreviated as Ptac ), or tac vector is a synthetically produced DNA promoter , produced from the combination of promoters from the trp and lac operons. [ 1 ] It is commonly used for protein production in Escherichia coli . [ 2 ]
Two hybrid promoters functional in Escherichia coli were constructed. These hybrid promoters, tacI and tacII, were derived from sequences of the trp and the lac UV5 promoters. In the first hybrid promoter (tacI), the DNA upstream of position –20 with respect to the transcriptional start site was derived from the trp promoter. The DNA downstream of position –20 was derived from the lac UV5 promoter. In the second hybrid promoter (tacII), the DNA upstream of position –11 at the Hpa I site within the Pribnow box was derived from the trp promoter. The DNA downstream of position –11 is a 46-base-pair synthetic DNA fragment that specifies part of the hybrid Pribnow box and the entire lac operator. It also specifies a Shine–Dalgarno sequence flanked by two unique restriction sites (portable Shine–Dalgarno sequence).
The tacI and the tacII promoters respectively direct transcription approximately 11 and 7 times more efficiently than the derepressed parental lac UV5 promoter and approximately 3 and 2 times more efficiently than the trp promoter in the absence of the trp repressor. Both hybrid promoters can be repressed by the lac repressor and both can be derepressed with isopropyl-beta-D-thiogalactoside. Consequently, these hybrid promoters are useful for the controlled expression of foreign genes at high levels in E. coli. In contrast to the trp and the lac UV5 promoters, the tacI promoter has not only a consensus –35 sequence but also a consensus Pribnow box sequence. This may explain the higher efficiency of this hybrid promoter with respect to either one of the parental promoters. [ 1 ]
The tac promoter is used to control and increase the expression levels of a target gene and is used in the over-expression of recombinant proteins . The tac promoter is named after the two promoters which comprise its sequence: the ' t rp' and the 'l ac ' promoters.
Bacterial promoters consist of two parts, the '–35' region and the '–10' region (the Pribnow box ). These two regions bind the sigma factor of RNA polymerase , which then initiates transcription of the downstream gene. The tac promoter consists of the '–35' region of the trp promoter and the '–10' region of the lac promoter (and differs from a related trc promoter by 1 bp [ 3 ] ). The tac promoter is, therefore, inducible by IPTG (Isopropyl β- D -1-thiogalactopyranoside), whilst also allowing higher maximum gene expression than either the lac or trp promoters. This makes it suitable for high-efficiency protein production of a recombinant protein. [ 1 ] The strong repression of expression in the 'off' state is important since foreign proteins can be toxic to the host cell.
The tac promoter finds various applications. The tac promoter/operator (dubbed PTAC ) is one of the most widely used expression systems. Ptac is a strong hybrid promoter composed of the –35 region of the trp promoter and the –10 region of the lacUV5 promoter/operator. The expression of PTAC is repressed by the lacI protein. The lacI q allele is a promoter mutation that increases the intracellular concentration of LacI repressor, resulting in the strong repression of PTAC. The addition of the inducer IPTG inactivates the LacI repressor. Thus, the amount of expression from PTAC is proportional to the concentration of IPTG added: low concentrations of IPTG result in relatively low expression from PTAC and high concentrations of IPTG result in high expression from PTAC. By varying the IPTG concentration the amount of gene product cloned downstream from PTAC can be varied over several orders of magnitude. [ 4 ] [ better source needed ] For example, the PTAC system is used for fusion protein expression within the PMAL-C2X expression. [ 5 ] [ better source needed ] | https://en.wikipedia.org/wiki/Tac-Promoter |
The tach(ometer) timer is an instrument used in aviation to accumulate the total number of revolutions performed by the engine. The unit of measure is equivalent to the number of hours of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 RPM then the timer runs in real time when the engine is running at 2400 RPM, half speed while the engine is run at 1200 RPM (a fast idle for some aviation engines) or at 5/6ths real time at 2000 RPM (a slow cruise speed). [ 1 ] The tach timer integrates over time the instantaneous rotation speed displayed by the tachometer . The displayed number is incremented by one if the engine is run at its reference speed for one hour. The quantity recorded is referred to as tach(ometer) hours. If the reference rotation speed is 2400 RPM then the tach timer records
The tach timer is usually used to schedule engine maintenance, although it is just an approximation of "Time in service" which is used to time and schedule aircraft maintenance. Time in service is defined in 14 CFR 1.1 [ 2 ] as the actual time in the air, whereas tach time measures engine revolutions, which would still count time on the ground while the engine is idling (at a lower rate). It can also be used as a basis for charging for aircraft rental as opposed to charging for elapsed time. This encourages the renter to properly warm the engine before takeoff and not to run the engine continuously at maximum speed. | https://en.wikipedia.org/wiki/Tach_timer |
Tacheometry ( / ˌ t æ k i ˈ ɒ m ɪ t r i / ; from Greek for "quick measure") is a system of rapid surveying , by which the horizontal and vertical positions of points on the Earth's surface relative to one another are determined using a tacheometer (a form of theodolite ). It is used without a chain or tape for distance measurement and without a separate levelling instrument for relative height measurements.
Instead of the pole normally employed to mark a point, a staff similar to a level staff is used in tacheometry. This is marked with heights from the base or foot, and is graduated according to the form of tacheometer in use. [ 1 ]
The ordinary methods of surveying with a theodolite, chain, and levelling instrument are fairly satisfactory when the ground is relatively clear of obstructions and not very precipitous, but it becomes extremely cumbersome when the ground is covered with bush , or broken up by ravines . Chain measurements then become slow and liable to considerable error; the levelling, too, is carried on at great disadvantage in point of speed, though without serious loss of accuracy. These difficulties led to the introduction of tacheometry. [ 1 ]
In western countries, tacheometry is primarily of historical interest in surveying, as professional measurement nowadays is usually carried out using total stations and recorded using data collectors. Location positions are also determined using GNSS . Traditional methods and instruments are still in use in many areas of the world and by users who are not primarily surveyors.
The horizontal distance S is inferred from the vertical angle subtended between two well-defined points on the staff and the known distance 2 L between them. Alternatively, also by readings of the staff indicated by two fixed stadia wires in the diaphragm ( reticle ) of the telescope. The difference of height Δh is computed from the angle of depression z or angle of elevation α of a fixed point on the staff and the horizontal distance S already obtained.
The azimuth angle is determined as normally. Thus, all the measurements requisite to locate a point both vertically and horizontally with reference to the point where the tacheometer is centred are determined by an observer at the instrument without any assistance beyond that of a person to hold the level staff. [ 1 ]
Other forms of tacheometry in surveying include the use of a level staff known as a stadia rod with a theodolite or plane-table alidade . [ 2 ] These use stadia marks on the instrument's reticle to measure the distance between two points on the stadia rod (the stadia interval). This is converted to distance from the instrument to the stadia rod by multiplying the stadia interval by the stadia interval factor . If the stadia rod is not at the same elevation as the instrument, the value must be corrected for the angle of elevation between the instrument and the rod.
The formula most widely used for finding the distances is:
Here, s {\displaystyle s} is the stadia interval (top intercept minus bottom intercept); k {\displaystyle k} and c {\displaystyle c} are multiplicative and additive constants. Generally, the instrument is made so that k = 100 {\displaystyle k=100} and c = 0 {\displaystyle c=0} exactly, to simplify calculations.
Another device used in tacheometry to measure distance between the measuring station and a desired point is the subtense bar . [ 2 ] This is a rigid rod, usually of a material insensitive to change in temperature such as invar , of fixed length (typically 2 metres (6.6 ft)). The subtense bar is mounted on a tripod over the station to which the distance is desired. It is brought to level, and a small telescope on the bar enables the bar to be oriented perpendicular to the line of sight to the angle measuring station. Since the subtense bar is always 2m. The formula for the subtense bar is:
A theodolite is used to measure the horizontal angle between indicators on the two ends of the subtense bar. The distance from the telescope to the subtense bar is the height of an isosceles triangle formed with the theodolite at the upper vertex and the subtense bar length at its base, determined by trigonometry .
A tachymeter or tacheometer is a type of theodolite used for rapid measurements and in modern form determines, electronically or electro-optically , the distance to target. The principles of action are similar to those of rangefinders . | https://en.wikipedia.org/wiki/Tacheometry |
A tachograph is a device fitted to a vehicle that automatically records its speed and distance, together with the driver's activity selected from a choice of modes. The drive mode is activated automatically when the vehicle is in motion, and modern tachograph heads usually default to the other work mode upon coming to rest. The rest and availability modes can be manually selected by the driver whilst stationary.
A tachograph system comprises a sender unit mounted to the vehicle gearbox, the tachograph head and a recording medium. Tachograph heads are of either analogue or digital types. All relevant vehicles manufactured in the EU since 1 May 2006 must be fitted with digital tachograph heads. The recording medium for analogue heads are wax coated paper discs, and for digital heads there are two recording mediums: internal memory (which can be read out with one of a variety of download devices into a so-called .ddd file) and digital driver cards containing a microchip with flash memory. Digital driver cards store data in a format that can later also be read out as a .ddd file. These files – both those read from internal memory with a download device, and those read from the driver cards – can be imported into tachograph analysis/archival software.
Drivers and their employers are legally required [ where? ] to accurately record their activities, retain the records (files from internal memory and from driver cards must both be retained) and produce them on demand to transport authorities who are in charge of enforcing regulations governing drivers' working hours . [ citation needed ]
They are also used in the maritime world, for example through the Central Commission for Navigation on the Rhine .
The tachograph was originally introduced for the railways so that companies could better document irregularities. The inventor was Max Maria von Weber , a civil servant, engineer and author. The Daniel Tachometer has been known in the railway industry since 1844. [ 1 ] The Hasler Event recorder was introduced in the 1920s. [ citation needed ]
For reasons of public safety, many jurisdictions have limits on the working hours of drivers of certain vehicles, such as buses and trucks. A tachograph can be used to monitor this and ensure that appropriate breaks are taken.
The Verkehrs-Sicherungs-Gesetz (German Traffic Safety Law) of 19 December 1952, made tachographs mandatory in Germany for all commercial vehicles weighing over 7.5 tonnes. [ citation needed ] Since 23 March and 23 December 1953, all new commercial vehicles and buses must be equipped with the device per law Straßenverkehrs-Zulassungs-Ordnung § 57a. [ citation needed ]
Tachographs are mandatory for vehicles allowed to carry a total weight of over 3.5 tonnes and vehicles built to carry at least 9 passengers, if the vehicle is used for commercial purposes. They are used to review the driving and rest time of drivers during reviews by traffic standards organizations or accident investigation. A driver must carry the tachograph records with him for all days of the current week and the last day of the previous week that he drove. Companies must keep the records for 1 year. In Germany, § 16 of the work time regulations lengthens this time to 2 years if the records will be used as proof of work time. [ citation needed ]
In Russia, buses with more than 8 seats and trucks with a permitted gross weight of more than 3.5 tons (Order of the Ministry of Transport of Russia dated 13 February 2013, No. 36) must be equipped with tachographs in order to ensure safe conditions. [ 2 ] A tachograph is mandatory for vehicles of class M2, M3, N2, N3. [ 3 ] Vehicles must be equipped with on-board monitoring devices in accordance with regulatory documents. [ 4 ]
From 1 April 2014, in the absence of a tachograph, fines will be applied to the following types of transport: - buses with more than 8 seats; - vehicles used to transport dangerous goods ; — According to the order of the Ministry of Transport dated 17 December 2013 No. 470, Moscow "On Amendments to the Order of the Ministry of Transport of the Russian Federation dated 13 February 2013 No. 36", penalties will be applied to freight vehicles with a gross weight of over 15 tons engaged in intercity transportation starting 1 July 2014, penalties will be applied to freight vehicles with a gross weight of over 12 tons engaged in intercity transportation starting 1 September 2014, and penalties will be applied to freight vehicles with a gross weight of over 3.5 tons engaged in intercity transportation starting 1 April 2015.
Russian tachographs use a cryptographic information protection tool (CIPF). Requirements for the use of the CIPF unit as part of tachographs are established by Order of the Ministry of Transport of Russia No. 36 dated 13.02.2013.
Digital tachographs appeared in Russia on 16 June 2010 (digital tachographs appeared before the specified date). Starting from 16 June 2010, the Russian Federation was supposed to implement the European digital tachography system on its territory for international transportation, but by that date the digital tachography infrastructure was not ready, as was the case in a number of other countries, as a result the first AETR digital tachograph cards were issued at the end of 2010 in accordance with the international agreement of the countries that joined the AETR. Before that date, no later than 3 months, all interested parties (drivers, inspectors, mechanics, administrators of motor transport enterprises) must be provided with special cards.
Unlike analogue devices, the disks of which were easily counterfeited, this device cannot be opened or the recorded information changed in any way. All information is recorded on an individual driver card and transmitted to the central computer of the motor transport enterprise.
Since 23 January 2012, the tachograph in Russia has become mandatory for installation and use when transporting passengers and dangerous goods. This is evidenced by the Technical Regulations on the Safety of Wheeled Vehicles in the latest edition. And since 1 April 2013, the presence of a digital tachograph has become mandatory for all wheeled transport owned by legal entities and individual entrepreneurs (including GAZelles and taxis), which is in operation on the territory of Russia, as reported in Federal Law No. 78-FZ adopted on 14 June 2012 (Article 1 and Article 12).
In accordance with Order No. 36 dated 13.02.13 of the Ministry of Transport of Russia , the following categories and types of vehicles issued for circulation and in operation on the territory of the Russian Federation are equipped with tachographs:
On 16 December 2015, the Order of the Ministry of Transport of Russia No. 348 was published, which amends order No. 273. Order No. 348 changes the deadline for equipping vehicles of categories N2, N3, M2, M3 with digital tachographs. Now vehicles must be equipped with digital tachographs by 1 July 2016. [ 5 ]
EEC regulation 3821/85 on recording equipment in road transport [ 6 ] from 20 December 1985 made tachographs mandatory throughout the EEC as of 29 September 1986. (Regulation 1463/70 amended by regulation 2828/77 made tachographs mandatory by 1 July 1979, reference to these regulations can be found in Regulation 3821/85).
A "European arrangement in regard to the work of driving personnel engaged in international traffic" (AETR, from French Accord Européen sur les Transports Routiers ) became effective on 31 July 1985.
Regulation 561/2006/EC of the European Union, adopted on 11 April 2007, [ 7 ] specified the driving and rest times of professional drivers. These time periods can be checked by the employers, police and other authorities with the help of the tachograph. Regulation (EU) 165/2014 of the European Parliament and of the Council on tachographs in road transport, issued on 4 February 2014, repealed Regulation 3821/85 and amended Regulation 561/2006. [ 8 ] The 2014 regulation introduced the specification for a so-called "smart" tachograph. [ 9 ] A "smart" tachograph records the position of a vehicle automatically via a global navigation satellite system at the places where the daily working period begins and ends, and at every three hour interval of accumulated driving. [ 9 ]
Most tachographs produced prior to 1 May 2006 were of the analogue type. Later analogue tachograph head models are of a modular design, enabling the head to fit into a standard DIN slot in the vehicle dashboard. This would enable a relatively easy upgrade to the forthcoming digital models, which were manufactured to the same physical dimensions.
The analogue tachograph head uses styli to trace lines on a wax coated paper disc that rotates throughout the day, where one rotation encompasses a 24-hour period. If the disc is left in the head over 24 hours, a second trace will be written onto the first, and so on until the disc is removed. It is an infringement of EU Regulation 561/2006 to use a disc for a period longer than it is designed for. Multiple overlapping traces may still be deciphered in the speed and distance fields, but it is far more difficult for the activity field where one trace can easily be obliterated by another. Analogue tachograph heads provide no indication to the driver of the need to change the disc.
Analogue data is retrieved visually, and can be assisted by manual analysis tools. Analogue discs can also be electronically scanned and analysed by computer, although this analogue to digital conversion process still requires human expert interpretation for best results, due to imperfections in the source disc such as dirt and scratch marks in the wax surface that can be incorrectly read as trace marks.
The analogue chart must be EU type approved. The country of type approval can be found on the rear of the chart, i.e. a mark of E11 would indicate the chart to have been approved in the UK for use in the EU. The chart is manufactured out of heavyweight paper with a black printed face that is thinly coated with a white wax, upon which is printed a number of features. The surface can be scratched or rubbed to reveal the black paper underneath. This enables the traces to be made without the use of ink. The chart features a pear-shaped aperture in the centre, ensuring it is perfectly aligned upon insertion into the tachograph head. There is no facility to prevent it being inserted back to front, where the styli would be prevented from making contact with the wax surface.
The centrefield is used by the driver to store certain handwritten information. This includes the drivers name, the date(s) the disc refers to, the start and end odometer readings and the registration mark of the vehicle.
Three traces are made in the wax surface by the head. These traces are either made by three separate styli or a single multipurpose stylus.
The trace closest to the centrefield is the distance trace . The stylus moves up and down with distance travelled, producing a zig-zag pattern, often referred to as a 'V' trace. A complete deflection is created every 5 kilometres, and therefore each completed 'V' represents 10 kilometres travelled. By counting the zig-zags, the total distance travelled can be calculated and compared against the stated odometer readings in the centrefield. By comparing the end position of the trace for a particular day against the start position for the following day, it can be seen if the vehicle has moved in the intermediate period.
The trace in the central area is the mode trace . The driver's activity is displayed in this area, and is always displayed as either drive , other work , availability or rest . Earlier tachograph heads displayed the mode as a thin line in one of four concentric tracks within the activity band. These heads are known as manual heads as the activity was manually selected using the mode switch. Automatic heads succeeded manual heads, and differ from them in two main areas. Firstly, the automatic head will always display the drive mode when the vehicle is in motion, regardless of the setting of the mode switch. For this reason, the drive mode is no longer available to be selected by the mode switch. Secondly, the activity is displayed on the chart as a sequence of block traces of differing thickness. The rest mode appears as a thin line, availability as a slightly thicker line, other work as slightly thicker again and the drive trace being the thickest.
The trace closest to the outer edge is the speed trace . The disc is preprinted with a speed scale and the stylus produces a mark corresponding with the speed of the vehicle at any given time. It is important that the maximum speed (Vmax) specification of the chart matches that of the tachograph head for the speed to be correctly recorded. It can be expected that a high speed trace will correlate with a tightly spaced zig-zag pattern within the distance trace.
The disc is preprinted with a 24-hour scale that completes the outer circumference.
The rear face of the chart is printed with a grid that enables the driver to make handwritten additions or amendments to the information on the front.
Apart from enforcing regulations, tachographs are often used in Germany to investigate and punish speeding. This practice was approved by the German high regional court in the 1990s. Also, after an accident, the discs are often examined with a microscope to discover the events that took place at a collision site. [ citation needed ]
Tachographs can be tampered with in various ways, such as slightly twisting the marker, blocking the path of the arm with a piece of rubber or foam, short-circuiting the unit for short periods, intentionally preventing the detection of gear movement with a magnet, or interrupting the (older analogue) tachograph's power supply with a blown fuse to stop operation completely thus recording no information whatsoever. There is also "forgetting to insert" the chart when beginning duty. Unauthorized changing of the discs (and then discarding one of the two, so that some activities are "forgotten") is well known throughout Europe. "Ghosting" is another common trick when false driver information is entered onto a second chart to give the appearance that there is a second driver present in the cab for long-distance runs that cannot be completed within a single driver's daily driving period.
Digital tachographs make tampering much more difficult by sending signals in an encrypted manner. EU regulation 1360/2002 makes digital tachographs mandatory for all vehicles described in the above section Regulations and manufactured after 1 August 2005. Digital tachographs have been required as of 1 May 2006 for all new vehicles for which EWG regulation VO(EWG)3820/85 applies, as was published in the official newsletter of the European Union L102 from 11 April 2006.
Digital tachographs have been implemented in Mexico since 1994, but this is not a federal regulation. The last implementations developed in Mexico have GPS capabilities such as mapping, altitude and location-activated video triggering. | https://en.wikipedia.org/wiki/Tachograph |
A tachometer ( revolution-counter , tach , rev-counter , RPM gauge ) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. [ 1 ] The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common.
The word comes from Ancient Greek τάχος (táchos) ' speed ' and μέτρον (métron) ' measure ' . Essentially the words tachometer and speedometer have identical meaning: a device that measures speed. It is by arbitrary convention that in the automotive world one is used for engine revolutions and the other for vehicle speed. In formal engineering nomenclature, more precise terms [ which? ] are used to distinguish the two.
The first tachometer was described by Bryan Donkin in a paper to the Royal Society of Arts in 1810 for which he was awarded the Gold medal of the society. This consisted of a bowl of mercury constructed in such a way that centrifugal force caused the level in a central tube to fall when it rotated and brought down the level in a narrower tube above filled with coloured spirit. The bowl was connected to the machinery to be measured by pulleys. [ 2 ]
The first mechanical tachometers were based on measuring the centrifugal force , similar to the operation of a centrifugal governor . The inventor is assumed to be the German engineer Dietrich Uhlhorn ; he used it for measuring the speed of machines in 1817. [ 3 ] Since 1840, it has been used to measure the speed of locomotives .
Tachometers or revolution counters on cars, aircraft, and other vehicles show the rate of rotation of the engine's crankshaft , and typically have markings indicating a safe range of rotation speeds. This can assist the driver in selecting appropriate throttle and gear settings for the driving conditions. Prolonged use at high speeds may cause inadequate lubrication , overheating (exceeding capability of the cooling system), exceeding speed capability of sub-parts of the engine (for example spring retracted valves) thus causing excessive wear or permanent damage or failure of engines. On analogue tachometers, speeds above maximum safe operating speed are typically indicated by an area of the gauge marked in red, giving rise to the expression of " redlining " an engine — revving the engine up to the maximum safe limit. Most modern cars typically have a revolution limiter which electronically limits engine speed to prevent damage. Diesel engines with traditional mechanical injector systems have an integral governor which prevents over-speeding the engine, so the tachometers in vehicles and machinery fitted with such engines sometimes lack a redline.
In vehicles such as tractors and trucks, the tachometer often has other markings, usually a green arc showing the speed range in which the engine produces maximum torque , which is of prime interest to operators of such vehicles. Tractors fitted with a power take-off (PTO) system have tachometers showing the engine speed needed to rotate the PTO at the standardized speed required by most PTO-driven implements. In many countries, tractors are required to have a speedometer for use on a road. To save fitting a second dial, the vehicle's tachometer is often marked with a second scale in units of speed. This scale is only accurate in a certain gear, but since many tractors only have one gear that is practical for use on-road, this is sufficient. Tractors with multiple 'road gears' often have tachometers with more than one speed scale. Aircraft tachometers have a green arc showing the engine's designed cruising speed range.
In older vehicles, the tachometer is driven by the RMS voltage waves from the low tension (LT contact breaker ) side of the ignition coil , [ 4 ] while on others (and nearly all diesel engines , which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output. This is from a special connection called an "AC tap" which is a connection to one of the stator's coil output, before the rectifier. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft ) exist - usually on simple diesel-engined machinery with basic or no electrical systems. On recent EMS found on modern vehicles, the signal for the tachometer is usually generated from an ECU which derives the information from either the crankshaft or camshaft speed sensor.
Tachometers are used to estimate traffic speed and volume (flow). A vehicle is equipped with the sensor and conducts "tach runs" which record the traffic data. These data are a substitute or complement to loop detector data. To get statistically significant results requires a high number of runs, and bias is introduced by the time of day, day of week, and the season. However, because of the expense, spacing (a lower density of loop detectors diminishes data accuracy), and relatively low reliability of loop detectors (often 30% or more are out of service at any given time), tach runs remain a common practice.
Speed sensing devices, termed variously "wheel impulse generators" (WIG), pulse generators, speed probes, or tachometers are used extensively in rail vehicles. Common types include opto-isolator slotted disk sensors [ 5 ] and Hall effect sensors .
Hall effect sensors typically use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets, or it may be a toothed wheel. The teeth on the wheel vary the flux density of a magnet inside the sensor head. The probe is mounted with its head a precise distance from the target wheel and detects the teeth or magnets passing its face. One problem with this system is that the necessary air gap between the target wheel and the sensor allows ferrous dust from the vehicle's underframe to build up on the probe or target, inhibiting its function.
Opto-isolator sensors are completely encased to prevent ingress from the outside environment. The only exposed parts are a sealed plug connector and a drive fork, which is attached to a slotted disk internally through a bearing and seal. The slotted disk is typically sandwiched between two circuit boards containing a photo- diode , photo- transistor , amplifier, and filtering circuits which produce a square wave pulse train output customized to the customer's voltage and pulses per revolution requirements. These types of sensors typically provide 2 to 8 independent channels of output that can be sampled by other systems in the vehicle such as automatic train control systems and propulsion/braking controllers.
The sensors mounted around the circumference of the disk provide quadrature encoded outputs and thus allow the vehicle's computer to determine the direction of rotation of the wheel. This is a legal requirement in Switzerland to prevent rollback when starting from standstill. Strictly, such devices are not tachometers since they do not provide a direct reading of the rotational speed of the disk. The speed has to be derived externally by counting the number of pulses in a time period. It is difficult to prove conclusively that the vehicle is stationary, other than by waiting a certain time to ensure that no further pulses occur. This is one reason why there is often a time delay between the train stopping, as perceived by a passenger, and the doors being released. Slotted-disk devices are typical sensors used in odometer systems for rail vehicles, such as are required for train protection systems — notably the European Train Control System .
As well as speed sensing, these probes are often used to calculate distance travelled by multiplying wheel rotations by wheel circumference.
They can be used to automatically calibrate wheel diameter by comparing the number of rotations of each axle against a master wheel that has been measured manually. Since all wheels travel the same distance, the diameter of each wheel is proportional to its number of rotations compared to the master wheel. This calibration must be done while coasting at a fixed speed to eliminate the possibility of wheel slip/slide introducing errors into the calculation. Automatic calibration of this type is used to generate more accurate traction and braking signals, and to improve wheel slip detection.
A weakness of systems that rely on wheel rotation for tachometry and odometry is that the train wheels and the rails are very smooth and the friction between them is low, leading to high error rates if the wheels slip or slide. To compensate for this, secondary odometry inputs employ Doppler radar units beneath the train to measure speed independently.
In analogue audio recording , a tachometer is a device that measures the speed of audiotape as it passes across the head. On most audio tape recorders the tachometer (or simply "tach") is a relatively large spindle near the ERP head stack , isolated from the feed and take-up spindles by tension idlers.
On many recorders the tachometer spindle is connected by an axle to a rotating magnet that induces a changing magnetic field upon a Hall effect transistor . Other systems connect the spindle to a stroboscope , which alternates light and dark upon a photodiode .
The tape recorder's drive electronics use signals from the tachometer to ensure that the tape is played at the proper speed. The signal is compared to a reference signal (either a quartz crystal or alternating current from the mains ). The comparison of the two frequencies drives the speed of the tape transport. When the tach signal and the reference signal match, the tape transport is said to be "at speed." (To this day on film sets, the director calls "Roll sound!" and the sound man replies "Sound speed!" This is a vestige of the days when recording devices required several seconds to reach a regulated speed.)
Having perfectly regulated tape speed is important because the human ear is very sensitive to changes in pitch, particularly sudden ones, and without a self-regulating system to control the speed of tape across the head, the pitch could drift several percent. This effect is called a wow -and- flutter , and a modern, tachometer-regulated cassette deck has a wow-and-flutter of 0.07%.
Tachometers are acceptable for high-fidelity sound playback, but not for recording in synchronization with a movie camera . For such purposes, special recorders that record pilottone must be used.
Tachometer signals can be used to synchronize several tape machines together, but only if in addition to the tach signal, a directional signal is transmitted, to tell slave machines in which direction the master is moving. | https://en.wikipedia.org/wiki/Tachometer |
Tachyhydrite is an unstable mineral, a hydrous chloride of calcium and magnesium with formula: Ca Mg 2 Cl 6 ·12 H 2 O . It is a rare component of marine evaporite salt deposits. Upon exposure to moist air it rapidly deliquesces and dissolves.
It forms a colorless to yellow trigonal crystal with a vitreous luster. It is soft with a Mohs hardness of 2 and has a low specific gravity of 1.66. It has good cleavage in three directions and typically occurs in crystalline masses.
It was first described in 1856 for an occurrence in Stassfurt , Saxony , Germany . Its name is from the Greek for quick water , in reference to its ready deliquescence. [ 4 ]
According to a patent filed years ago by a Halliburton researcher, high strength hydrochloric acid treatment of magnesium containing carbonates creates tachyhydrite, which will seal the rock pores and inhibit oil flow unless washed with a light acid or water before and after the treatment. [ 5 ]
This article about a specific halide mineral is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tachyhydrite |
There are three known mammalian tachykinin receptors termed NK 1 , NK 2 and NK 3 . All are members of the 7 transmembrane G-protein coupled receptor family and induce the activation of phospholipase C , producing inositol triphosphate (so called G q -coupled).
Inhibitors of NK-1, known as NK-1 receptor antagonists , can be used as antiemetic agents , such as the drug aprepitant . [ 1 ]
The genes and receptor ligands are as follows: [ 2 ]
( Hökfelt et al., 2001; Page, 2004; Pennefather et al., 2004; Maggi, 2000)
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tachykinin_receptor |
Tacit Networks, Inc. is an I.T. company based in South Plainfield, New Jersey . [ 1 ] It was founded in 2000.
Their product lines are:
On January 30, 2004, Tacit Networks acquired the assets of AttachStor . [ 2 ] The AttachStor technology provided the basis for the email acceleration feature in the iShared product.
On December 30, 2005, Tacit Networks acquired the assets of Mobiliti and integrated the Mobiliti product line into its portfolio. [ citation needed ]
On May 15, 2006, Packeteer acquired Tacit Networks and integrated the iShared and Mobiliti product lines into the Packeteer portfolio. [ 3 ]
This article related to a computer company is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tacit_Networks |
Tacit collusion is a collusion between competitors who do not explicitly exchange information but achieve an agreement about coordination of conduct. [ 1 ] There are two types of tacit collusion: concerted action and conscious parallelism . [ 2 ] [ 3 ] In a concerted action also known as concerted activity, [ 4 ] competitors exchange some information without reaching any explicit agreement, while conscious parallelism implies no communication. [ 1 ] [ 5 ] In both types of tacit collusion, competitors agree to play a certain strategy without explicitly saying so . It is also called oligopolistic price coordination [ 6 ] or tacit parallelism . [ 7 ]
A dataset of gasoline prices of BP , Caltex , Woolworths , Coles , and Gull from Perth gathered in the years 2001 to 2015 was used to show by statistical analysis the tacit collusion between these retailers. [ 8 ] BP emerged as a price leader and influenced the behavior of the competitors. As result, the timing of price jumps became coordinated and the margins started to grow in 2010.
In competition law , some sources use conscious parallelism as a synonym to tacit collusion in order to describe pricing strategies among competitors in an oligopoly that occurs without an actual agreement [ 9 ] or at least without any evidence of an actual agreement between the players. [ 10 ] In result, one competitor will take the lead in raising or lowering prices. The others will then follow suit, raising or lowering their prices by the same amount, with the understanding that greater profits result.
This practice can be harmful to consumers who, if the market power of the firm is used, can be forced to pay monopoly prices for goods that should be selling for only a little more than the cost of production. Nevertheless, it is very hard to prosecute because it may occur without any collusion between the competitors. Courts have held that no violation of the antitrust laws occurs where firms independently raise or lower prices, but that a violation can be shown when plus factors occur, such as firms being motivated to collude and taking actions against their own economic self-interests. [ 11 ] [ 12 ] This procedure of the courts is sometimes called as setting of a conspiracy theory . [ 13 ]
Oligopolists usually try not to engage in price cutting, excessive advertising or other forms of competition. Thus, there may be unwritten rules of collusive behavior such as price leadership . Price leadership is the form of a tacit collusion, whereby firms orient at the price set by a leader. [ 14 ] A price leader will then emerge and set the general industry price, with other firms following suit. For example, see the case of British Salt Limited and New Cheshire Salt Works Limited. [ 15 ]
Classical economic theory holds that Pareto efficiency is attained at a price equal to the incremental cost of producing additional units. Monopolies are able to extract optimum revenue by offering fewer units at a higher cost. An oligopoly where each firm acts independently tends toward equilibrium at the ideal, but such covert cooperation as price leadership tends toward higher profitability for all, though it is an unstable arrangement.
There exist two types of price leadership. [ 14 ] In dominant firm price leadership , the price leader is the biggest firm. In barometric firm price leadership , the most reliable firm emerges as the best barometer of market conditions, or the firm could be the one with the lowest costs of production, leading other firms to follow suit. Although this firm might not be dominating the industry, its prices are believed to reflect market conditions which are the most satisfactory, as the firm would most likely be a good forecaster of economic changes.
In repeated auctions , bidders might participate in a tacit collusion to keep bids low. [ 16 ] A profitable collusion is possible, if the number of bidders is finite and the identity of the winner is publicly observable. It can be very difficult or even impossible for the seller to detect such collusion from the distribution of bids only. In case of spectrum auctions , some sources claim that a tacit collusion is easily upset: [ 17 ]
"It requires that all the bidders reach an implicit agreement about who should get what. With thirty diverse bidders unable to communicate about strategy except through their bids, forming such unanimous agreement is difficult at best."
Nevertheless, Federal Communications Commission (FCC) experimented with precautions for spectrum auctions like restricting visibility of bids , limiting the number of bids and anonymous bidding. [ 18 ] So called click-box bidding used by governmental agencies in spectrum auctions restricts the number of valid bids and offers them as a list to a bidder to choose from. [ 19 ] Click-box bidding was invented in 1997 by FCC to prevent bidders from signalling bidding information by embedding it into digits of the bids. [ 20 ] Economic theory predicts a higher difficulty for tacit collusions due to those precautions. [ 18 ] In general, transparency in auctions always increases the risk of a tacit collusion. [ 21 ]
Once the competitors are able to use algorithms to determine prices, a tacit collusion between them imposes a much higher danger. [ 22 ] E-commerce is one of the major premises for algorithmic tacit collusion . [ 23 ] Complex pricing algorithms are essential for the development of e-commerce. [ 23 ] European Commissioner Margrethe Vestager mentioned an early example of algorithmic tacit collusion in her speech on "Algorithms and Collusion" on 16 March 2017, described as follows: [ 24 ]
"A few years ago, two companies were selling a textbook called The Making of a Fly. One of those sellers used an algorithm which essentially matched its rival’s price. That rival had an algorithm which always set a price 27% higher than the first. The result was that prices kept spiralling upwards, until finally someone noticed what was going on, and adjusted the price manually. By that time, the book was selling – or rather, not selling – for 23 million dollars a copy."
An OECD Competition Committee Roundtable "Algorithms and Collusion" took place in June 2017 in order to address the risk of possible anti-competitive behaviour by algorithms. [ 25 ]
It is important to distinguish between simple algorithms intentionally programmed to raise price according to the competitors and more sophisticated self-learning AI algorithms with more general goals. Self-learning AI algorithms might form a tacit collusion without the knowledge of their human programmers as result of the task to determine optimal prices in any market situation. [ 22 ] [ 26 ]
Tacit collusion is best understood in the context of a duopoly and the concept of game theory (namely, Nash equilibrium ). Let's take an example of two firms A and B, who both play an advertising game over an indefinite number of periods (effectively saying 'infinitely many'). Both of the firms' payoffs are contingent upon their own action, but more importantly the action of their competitor. They can choose to stay at the current level of advertising or choose a more aggressive advertising strategy . If either firm chooses low advertising while the other chooses high, then the low-advertising firm will suffer a great loss in market share while the other experiences a boost. If they both choose high advertising, then neither firms' market share will increase but their advertising costs will increase, thus lowering their profits. If they both choose to stay at the normal level of advertising, then sales will remain constant without the added advertising expense. Thus, both firms will experience a greater payoff if they both choose normal advertising (this set of actions is unstable, as both are tempted to defect to higher advertising to increase payoffs). A payoff matrix is presented with numbers given:
Notice that Nash's equilibrium is set at both firms choosing an aggressive advertising strategy. This is to protect themselves against lost sales. This game is an example of a prisoner's dilemma .
In general, if the payoffs for colluding (normal, normal) are greater than the payoffs for cheating (aggressive, aggressive), then the two firms will want to collude (tacitly). Although this collusive arrangement is not an equilibrium in the one-shot game above, repeating the game allows the firms to sustain collusion over long time periods. This can be achieved, for example if each firm's strategy is to undertake normal advertising so long as its rival does likewise, and to pursue aggressive advertising forever as soon as its rival has used an aggressive advertising campaign at least once (see: grim trigger ) (this threat is credible since symmetric use of aggressive advertising is a Nash equilibrium of each stage of the game). Each firm must then weigh the short term gain of $30 from 'cheating' against the long term loss of $35 in all future periods that comes as part of its punishment. Provided that firms care enough about the future, collusion is an equilibrium of this repeated game.
To be more precise, suppose that firms have a discount factor δ {\displaystyle \delta } . The discounted value of the cost to cheating and being punished indefinitely are
The firms therefore prefer not to cheat (so that collusion is an equilibrium) if | https://en.wikipedia.org/wiki/Tacit_collusion |
The Tactical Airborne Reconnaissance Pod System ( TARPS ) was a large and sophisticated camera pod carried by the Grumman F-14 Tomcat . [ N 1 ] It contains three camera bays with different type cameras which are pointed down at passing terrain. It was originally designed to provide an interim aerial reconnaissance capability until a dedicated F/A-18 Hornet reconnaissance version could be fielded. TARPS was pressed into service upon arrival in the fleet in 1981, and remained in use up to the end of Tomcat service in 2006.
The pod itself is 17 feet (5.2 m) long, and weighs 1,850 lb (840 kg). and is carried on the starboard side of the tunnel between the engine nacelles. The F-14A and F-14B Tomcats had to be specially modified to carry the TARPS pod which involved routing of control wiring from the rear cockpit and environmental control system (ECS) connections to the pod. Standard allowance was at least three TARPS aircraft per designated squadron (only one per airwing ). All F-14Ds were modified to be TARPS capable, which allowed greater flexibility in scheduling aircraft and conducting maintenance. A control panel is fitted to the rear cockpit and the RIO has total control over pod operation except for a pilot controlled button that can activate cameras as selected by the RIO (but seldom used).
Each of the camera bays was designed to carry different cameras for specific tasks on reconnaissance missions. The forward bay held a 150 mm (6") focal length serial frame camera (KS-87) on a two position rotating mount which could direct the camera's view straight down or be moved to a 45° angle for a forward oblique view. The second bay or middle bay of the TARPS pod originally held the 230 mm (9") focal length KA-99 panoramic camera which rotated from horizon to horizon and could be used for side oblique photography. Each image in the wide field of view position produced a 91 cm (36") negative. The KA-99 could carry up to 2,000 feet (610 m) of film that could be exhausted if not managed carefully by the RIO. The third camera bay held an infrared line scanner camera used for night missions or daylight mission traces. All TARPS cameras were monitored by a device called a CIPDU in the tail cone section of the pod that provided camera status to maintenance personnel and during flight provided aircraft position data onto the camera imagery for intel analysis. An electrical umbilical cord connected the pod to the control panel that was positioned on the left side of the rear cockpit. A hose from the ECS from the F-14 cooled/heated the internals of the pod in flight and kept the appropriate humidity levels constant. In 1987 VF-111 was the first squadron to deploy with a KS-153 camera system in bay two. The KS-153 used a 610 mm (24") lens and was used for stand-off photography in the Persian Gulf. During Operation Desert Shield the KS-153 was used to monitor the no fly zones in Iraq.
Tomcat TARPS squadrons were staffed with Navy photographer's mates and Avionics Technicians that maintained the cameras and worked with the carrier to process the imagery. TARPS squadrons also included an extra Intelligence officer and Intelligence Specialists to help plan TARPS missions and exploit the imagery afterwards. The TARPS shop maintained the cameras and removed or loaded the pod when and if needed. Wet film processing was conducted in a processing room connected to the ship's Intelligence Center (CVIC) where the Intelligence Specialists has a dedicated space with a light table for analyzing the hundreds of feet of film and exploiting the data.
The TARPS pod provided capability for the Tomcat to conduct a variety of reconnaissance tasking including:
Although TARPS was originally planned to be an interim solution, combat experience with VF-32 over Lebanon in 1983 resulted in upgrades to the TARPS camera suite and to the aircraft survivability. Since the KA-99 camera was designed for low-medium altitude missions, the Tomcats were forced to fly as low as 10,000 feet (3,000 m) over active anti-aircraft artillery (AAA) and surface to air missile (SAM) sites in the Bekaa Valley , again by VF-32, resulting in 6th Fleet requesting higher altitude cameras such as had been available in the dedicated reconnaissance platforms such as the RA-5C, RF-8 and RF-4. As a result, the first set of four KA-93 910 mm (36") focal length Long Range Optic (LOROP) cameras were shipped to Naval Air Station Oceana in the spring of 1984 for deployment with the next Tomcat TARPS squadron. VF-102 conducted an operational evaluation (OPEVAL) of the cameras enroute to the MED in expectation of flying them over Lebanon, but the crisis had cooled down by then. The cameras then became forward deployed assets and cross-decked between TARPS squadrons. Later, KS-153 LOROP cameras were also procured and also used as forward deployed assets. The KS-87 camera bay was eventually upgraded with a digital sensor so that imagery could be captured onto a PCMCIA Type II card for debrief, but could also be transmitted as desired by the RIO.
The TARPS mission first exposed the Tomcat to the AAA and SAM threat on a routine basis and spurred upgrades not only to the cameras, but to the aircraft itself. The existing Radar Homing and Warning (RHAW) gear, the ALR-45/50, was vintage Vietnam era and could not keep up with the latest threats of the SA-5 and SA-6 missiles, both present in several threat countries in the Mediterranean. As such, TARPS Tomcats were provided with an Expanded Chaff Adapter (ECA) rail that provided 120 extra expendable rounds and another rail that mounted an ALQ-167 "Bullwinkle" jammer. Eventually, the F-14B arrived with the improved ALR-67 RHAW gear capable of keeping pace with the latest threats. Prior to that, some Tomcat squadrons used modified "Fuzz-buster" automotive police radar detectors mounted ad hoc on the pilot's glare shield to detect threats not handled by the ALR-45/50. [ 2 ]
TARPS was immediately impressed into the Cold War and used for surveillance of Soviet ships at sea and in their anchorages sometimes from over 1,000 miles (1,600 km) distant from patrolling aircraft carriers in the classic cat and mouse tactics of that era.
TARPS resulted in Tomcats being put in harm's way shortly after it was introduced to the fleet in 1981. VF-102 Tomcats had been inadvertently been fired on by AAA and a single SA-2 SAM over Somalia in April 1983 while conducting peacetime mapping prior to a major exercise. A few months later VF-32 conducted TARPS missions in support of the invasion of Grenada and went on to join VF-143 and VF-31 in flying missions in the Eastern Med where three carriers had gathered to respond to the crisis in Lebanon. Thus, TARPS was responsible for the Tomcat's first sustained combat baptism of fire when the crisis in Lebanon heated up in 1983 requiring daily overflights over hostile AAA and SAMs. During operation El Dorado Canyon in 1986, Libya launched SCUD missiles at a US outpost on an island in the Mediterranean and VF-102 flew TARPS to ascertain if there had been any damage.
Initially, TARPS was not a priority on the air tasking order during Desert Shield/Storm due to availability of strategic assets like the U-2/TR-1 and plentiful USAF RF-4 units. However, once Desert Storm started, the demand for realtime intel overwhelmed the other assets and TARPS missions were called upon to meet the demand. Immediately, it became obvious that Tomcats were favored for in country missions over the RF-4 as they required no escort and needed less fuel pre- and post-mission, which was a real concern at the time. TARPS continued to be utilized post Desert Storm and training was modified to take into account medium altitude tactics such as were flown in Desert Storm. Prior to that, the majority of TARPS missions training missions were low altitude overland and over water navigation and imagery. Only mapping was flown at medium altitudes. TARPS was used routinely in Operation Southern Watch over Iraq and called upon in Bosnia in 1995 and then again over Kosovo in 1999. The advent of LANTIRN into Tomcat operations provided a useful complement to TARPS. Since both systems need the same real estate in the rear cockpit for sensor operation control panels, they cannot be mounted on the aircraft at the same time, but they can be flown in formation yielding the best of both systems.
TARPS was used in the United States in 1993 when areas of the Mississippi River flooded. The Federal Emergency Management Agency (FEMA) requested TARPS flights be taken over the area to determine which locations were hardest hit. TARPS has also been used for hurricane damage assessment. TARPS was also used to assess damages following the Waco siege in 1993, as well as damage to the Alfred P. Murrah Federal Building following the Oklahoma City bombing . In addition, TARPS equipped F-14s were used for DEA intel missions for anti-drug operations in the early 1990s. [ citation needed ] | https://en.wikipedia.org/wiki/Tactical_Airborne_Reconnaissance_Pod_System |
A tactical air navigation system , commonly referred to by the acronym TACAN , is a navigation system initially designed for naval aircraft to acquire moving landing platforms (i.e., ships) and later expanded for use by other military aircraft. It provides the user with bearing and distance (slant-range or hypotenuse) to a ground or ship-borne station. It is, from an end-user perspective, a more accurate version of the VOR / DME system that provides bearing and range information for civil aviation . The DME portion of the TACAN system is available for civil use; at VORTAC facilities where a VOR is combined with a TACAN, civil aircraft can receive VOR/DME readings. Aircraft equipped with TACAN avionics can use this system for enroute navigation as well as non-precision approaches to landing fields. However, a TACAN-only equipped aircraft cannot receive bearing information from a VOR-only station.
The TACAN navigation system is an evolution of radio transponder navigation systems that date back to the British Oboe system of World War II . In the United States, many companies were involved with the development of TACAN for military aircraft. Hoffman Laboratories Div. of the Hoffman Electronics Corp.–Military Products Division [ 1 ] (now NavCom Defense Electronics) [ 2 ] was a leader in developing the present TACAN system in the US starting in the late 1950s.
TACAN in general can be described as the military version of the VOR/DME system, though despite providing similar information as its civilian counterpart, its method of operation is significantly different. It operates in the UHF frequency band 962-1213 MHz , utilizing a pulse-pair transponder system not dissimilar to that of secondary surveillance RADAR . Interrogating aircraft transmit in the 1024-1150 MHz band, split into 1 MHz channels numbered 1-126; the responding station (ground, ship, or another aircraft) is 63 MHz (63 channels) above or below the originating frequency, depending on the channel and mode of operation selected. Spacing between pulses in an individual pulse-pair is also determined by TACAN operating mode.
Range information is functionally identical to the method provided by civilian DME: pairs of 3.5 microsecond (μs) pulses (measured edge-to-edge at 50% modulation strength) from an aircraft are repeated by the station being interrogated, using the round-trip time to calculate slant-range distance. Randomized spacing between interrogation pulse-pairs allows the interrogator to separate its own signal from that of other aircraft, enabling multiple users to access the ranging function without mutual interference. A fixed-round trip delay time (dependent on system mode) is added to each pulse-pair when being retransmitted by its station. The interrogator will generate up to 150 pulse-pairs per second when first acquiring a station in range in "search" mode, then drop down to ≈30 per second when acquired in "track" mode. Memory circuits in the ranging function enable a track to be quickly reestablished when ranging pulses are temporarily suppressed by other TACAN functions (see below).
Bearing information is derived from amplitude modulation (AM) of the responding station's pulse-pair signals, the AM signal being generated via physical rotation of a station's directional antenna or electronic steering of the same signal using an antenna array. Two AM signals are generated: a fundamental AM signal at 15 Hz, and an auxiliary AM signal (implemented using fixed signal reflectors in rotating-antenna installations) at 135 Hz, the ninth harmonic of the fundamental signal. These correspond to a "coarse" and "fine" bearing signal, the latter improving the accuracy of the former. The time is compared between the point of peak positive signal strength with a reference train or "burst" of pulse-pairs of specific repetition rate and duration, timed to transmit at a specific point in the signal's sweep; these replace all other pulse types when transmitted. The civilian VOR system differs from TACAN in utilizing a single continuous-wave 30 Hz modulation signal, using the phase difference between a fixed-phase and variable phase (rotating) component to derive bearing info.
TACAN stations transmit pulse-pairs at a composite rate of 3600 pairs/second: 900 of which are bearing reference bursts, and the other 2700 being composed of ranging and identification pulses. When insufficient interrogation pulses from aircraft are present, the station will use a squitter circuit to inject additional randomized pulse-pairs to maintain the desired pulse rate. This ensures that sufficient signal is present to support demodulating bearing signals.
TACAN stations are identified by Morse code . The transmitting station periodically replaces the randomized ranging pulse-pairs with regularly spaced pairs that de-modulate to a 1350 Hz tone, keying a three-letter identification code at approximately 6-7 wpm every 40 seconds. Ranging and squitter pulses are permitted during the gaps between dots and dashes. There is no capability for voice transmission in a TACAN-only system.
There are two basic channel configurations available: X (the original implementation) and Y (added in the 1960s to expand available channels and reduce mutual interference between closely-spaced stations). These configurations differ in pulse-pair width, fixed receiver response delay, and polarity of frequency offset from the interrogation channel. TACAN interrogators can operate in four modes: receive (for bearing/identification only), transmit/receive (for bearing, range, and ID), and air-to-air versions of the previous two.
The typical TACAN onboard user panel has control switches for setting the channel (corresponding to the desired station's assigned frequency), the operation mode for either transmit/receive (T/R, to get both bearing and range) or receive only (REC, to get bearing but not range). Capability was later upgraded to include an air-to-air mode (A/A), where two airborne users can get relative slant-range information depending on specific installations, [ 3 ] though an air-to-air mode allows distance to be established between transmitters/receivers.
When initially deployed, TACAN was intended to provide a bearing accuracy of ±0.22°, based on the main bearing signal's own accuracy of ±2° and the corrections applied by the ninth-harmonic auxiliary bearing signal. [ 5 ] Theoretically a TACAN should provide a 9-fold increase in accuracy compared to a VOR, but operational use has shown only an approximate 3-fold increase. [ 6 ]
Operational accuracy of the 135 Hz azimuth component is ±1° or ±63 m at 3.75 km. [ 7 ]
Manufacturers of TACAN sets mention the ability to track stations out to 400NM, though these systems will cap their instrumented range signals at approximately 200NM. [ 8 ] Per official FAA service volume information, reliable TACAN/DME reception can be guaranteed out to 130NM below 45,000 feet above the surface for a high-altitude certified unit. [ 9 ]
On the first Space Shuttle flight, Capcom Joseph P. Allen reported up to the crew that their TACANs had locked onto the Channel 111X signals at St. Petersburg , FL at a range of 250 miles.
Because the azimuth and range units are combined in one system it provides for simpler installation. Less space is required than a VOR because a VOR requires a large counterpoise and a fairly complex phased antenna system. A TACAN system theoretically might be placed on a building, a large truck, an airplane or a ship, and be operational in a short period of time. An airborne TACAN receiver can be used in air-to-air mode to provide the approximate distance between two coordinating aircraft by selecting channels with 63 channels of separation (e.g., aircraft #1 sets channel 29 into its TACAN and aircraft #2 sets channel 92 into its TACAN.). It does not provide relative bearing.
For military usage a primary drawback is lack of the ability to control emissions ( EMCON ) and stealth. Naval TACAN operations are designed so an aircraft can find the ship and land. Since there is no encryption, an enemy can use the range and bearing provided to attack a ship equipped with a TACAN. Some TACANs have the ability to employ a "Demand Only" mode: only transmitting when interrogated by an aircraft on-channel. It is likely that TACAN will be replaced with a differential GPS system similar to the Local Area Augmentation System called JPALS. The Joint Precision Approach and Landing System has a low probability of intercept to prevent enemy detection and an aircraft carrier version can be used for autoland operations.
Some systems used in the United States modulate the transmitted signal by using a 900 RPM rotating antenna. This antenna is fairly large and must rotate 24 hours a day, possibly causing reliability issues. Modern systems have antennas that use electronic rotation (instead of mechanical rotation), hence no moving parts. | https://en.wikipedia.org/wiki/Tactical_air_navigation_system |
The tactical diameter of a ship is the distance:
The ratio of the tactical diameter divided by the ship's length between perpendiculars gives a dimensionless parameter which can be used to compare ships maneuverability.
This naval article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tactical_diameter |
Tacticity (from Greek : τακτικός , romanized : taktikos , "relating to arrangement or order") is the relative stereochemistry of adjacent chiral centers within a macromolecule . [ 1 ] [ better source needed ] The practical significance of tacticity rests on the effects on the physical properties of the polymer . [ not verified in body ] The regularity of the macromolecular structure influences the degree to which it has rigid, crystalline long range order or flexible, amorphous long range disorder. [ not verified in body ] Precise knowledge of tacticity of a polymer also helps understanding at what temperature a polymer melts , how soluble it is in a solvent , [ not verified in body ] as well as its mechanical properties. [ not verified in body ]
A tactic macromolecule in the IUPAC definition is a macromolecule in which essentially all the configurational (repeating) units are identical. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in a tetrahedral molecular geometry , the zigzag backbone is in the paper plane with the substituents either sticking out of the paper or retreating into the paper; [ excessive detail? ] , this projection is called the Natta projection after Giulio Natta . [ not verified in body ] Tacticity is particularly significant in vinyl polymers of the type - H 2 C-CH(R)- , where each repeating unit contains a substituent R attached to one side of the polymer backbone . The arrangement of these substituents can follow a regular pattern- appearing on the same side as the previous one, on the opposite side, or in a random configuration relative to the preceding unit. Monotactic macromolecules have one stereoisomeric atom per repeat unit, [ not verified in body ] ditactic to n-tactic macromolecules have more than one stereoisomeric atom per unit. [ not verified in body ]
The orderliness of the succession of configurational repeating units in the main chain of a regular macromolecule , a regular oligomer molecule, a regular block, or a regular chain. [ 2 ]
Two adjacent structural units in a polymer molecule constitute a diad . Diads overlap: each structural unit is considered part of two diads, one diad with each neighbor. If a diad consists of two identically oriented units, the diad is called an m diad (formerly meso diad , as in a meso compound , now proscribed [ 3 ] ). If a diad consists of units oriented in opposition, the diad is called an r diad (formerly racemo diad , as in a racemic compound, now proscribed [ 3 ] ). In the case of vinyl polymer molecules, an m diad is one in which the substituents are oriented on the same side of the polymer backbone; in the Natta projection, they both point into the plane or both point out of the plane.
The stereochemistry of macromolecules can be defined even more precisely with the introduction of triads. An isotactic triad ( mm ) is made up of two adjacent m diads, a syndiotactic triad (also spelled syndyotactic [ 4 ] ) ( rr ) consists of two adjacent r diads , and a heterotactic triad ( rm ) is composed of an r diad adjacent to an m diad . The mass fraction of isotactic ( mm ) triads is a common quantitative measure of tacticity.
When the stereochemistry of a macromolecule is considered to be a Bernoulli process , the triad composition can be calculated from the probability P m of a diad being m type . For example, when this probability is 0.25 then the probability of finding:
with a total probability of 1. Similar relationships with diads exist for tetrads. [ 5 ] : 357
The definition of tetrads and pentads introduce further sophistication and precision to defining tacticity, especially when information on long-range ordering is desirable. [ citation needed ] Tacticity measurements obtained by carbon-13 NMR are typically expressed in terms of the relative abundance of various pentads within the polymer molecule, e.g. mmmm , mrrm . [ according to whom? ]
The primary convention for expressing tacticity is in terms of the relative weight fraction of triad or higher-order components, as described above. An alternative expression for tacticity is the average length of m and r sequences within the polymer molecule. The average m-sequence length may be approximated from the relative abundance of pentads as follows: [ 6 ]
M S L = m m m m + 3 2 m r r r + 2 r m m r + 1 2 r m r m + 1 2 r m r r 1 2 m m m r + r m m r + 1 2 r m r m + 1 2 r m r r {\displaystyle MSL={\frac {mmmm+{\tfrac {3}{2}}mrrr+2rmmr+{\tfrac {1}{2}}rmrm+{\tfrac {1}{2}}rmrr}{{\tfrac {1}{2}}mmmr+rmmr+{\tfrac {1}{2}}rmrm+{\tfrac {1}{2}}rmrr}}}
Isotactic polymers are composed of isotactic macromolecules (IUPAC definition). [ 7 ] In isotactic macromolecules, all the substituents are located on the same side of the macromolecular backbone. An isotactic macromolecule consists of 100% m diads , though IUPAC also allows the term for macromolecules with at least 95% m diads if that looser usage is explained. [ 3 ] Polypropylene formed by Ziegler–Natta catalysis is an example of an isotactic polymer. [ 8 ] Isotactic polymers are usually semicrystalline and often form a helix configuration. [ citation needed ]
In syndiotactic or syntactic macromolecules the substituents have alternate positions along the chain. The macromolecule comprises 100% r diads , though IUPAC also allows the term for macromolecules with at least 95% r diads if that looser usage is explained. Syndiotactic polystyrene , made by metallocene catalysis polymerization , is crystalline with aa melting point of 161 °C. Gutta percha is also an example syndiotactic polymer. [ 9 ]
In atactic macromolecules the substituents are placed randomly along the chain. The percentage of m diads is understood to be between 45 and 55% unless otherwise specified, but it could be any value other than 0 or 100% if that usage is clarified. [ 3 ] With the aid of spectroscopic techniques such as NMR , it is possible to pinpoint the composition of a polymer in terms of the percentages for each triad. [ 10 ] [ better source needed ]
Polymers that are formed by free-radical mechanisms, such as polyvinyl chloride are usually atactic. [ citation needed ] Due to their random nature atactic polymers are usually amorphous . [ citation needed ] In hemi-isotactic macromolecules every other repeat unit has a random substituent. [ citation needed ]
Atactic polymers such as polystyrene ( PS ) are technologically very important. [ citation needed ] It is possible to obtain syndiotactic polystyrene using a Kaminsky catalyst , [ 11 ] but most industrial polystyrene produced is atactic. [ citation needed ] The two materials have very different properties because the irregular structure of the atactic version makes it impossible for the polymer chains to stack in a regular fashion: whereas syndiotactic PS is a semicrystalline material, the more common atactic version cannot crystallize and forms a glass instead. [ citation needed ] This example is quite general in that many polymers of economic importance are atactic glass formers. [ citation needed ]
In eutactic macromolecules, substituents may occupy any specific (but potentially complex) sequence of positions along the chain. [ citation needed ] Isotactic and syndiotactic polymers are instances of the more general class of eutactic polymers, which also includes heterogeneous macromolecules in which the sequence consists of substituents of different kinds (for example, the side-chains in proteins and the bases in nucleic acids). [ citation needed ]
In vinyl polymers , the complete configuration can be further described by defining polymer head/tail configuration. In a regular macromolecule, monomer units are normally linked in a head to tail configuration such that β-substituents are located on alternating carbon atoms. However, it is possible for defects to form where substituents are placed on adjacent carbon atoms, producing a head/head tail/tail configuration, such as by recombination of two growing radical chains , or by direct head-head addition if steric effects are weak enough, such as in polyvinylidene fluoride . [ 12 ]
Tacticity may be measured directly using proton or carbon-13 NMR . This technique enables quantification of the tacticity distribution by comparison of peak areas or integral ranges corresponding to known diads (r, m), triads (mm, rm+mr, rr) and/or higher order n -ads, depending on spectral resolution. In cases of limited resolution, stochastic methods such as Bernoullian or Markovian analysis may also be used to fit the distribution and predict higher n -ads and calculate the isotacticity of the polymer to the desired level. [ 13 ]
Other techniques sensitive to tacticity include x-ray powder diffraction , secondary ion mass spectrometry (SIMS), [ 14 ] vibrational spectroscopy (FTIR) [ 15 ] and especially two-dimensional techniques. [ 16 ] Tacticity may also be inferred by measuring another physical property, such as melting temperature, when the relationship between tacticity and that property is well-established. [ 17 ] | https://en.wikipedia.org/wiki/Tacticity |
Tactile graphics , including tactile pictures , tactile diagrams , tactile maps , and tactile graphs , are images that use raised surfaces so that a visually impaired person can feel them. They are used to convey non-textual information such as maps, paintings, graphs and diagrams.
Tactile graphics can be seen as a subset of accessible images . Images can be made accessible to the visually impaired in various ways, such as verbal description, sound, or haptic (tactual) feedback.
One of the most common uses for tactile graphics is the production of tactile maps.
The types and forms of tactile maps began with the oldest and most rudimentary or a mixed media format. This tactile map is produced by simply attaching objects to a substrate to represent different items or symbols. More recent tactile maps are produced by computers through different means such as an ink-jet printers.
Thermoform is one of the most common methods of producing tactile maps. This process is also known as vacuum forming . Thermoform maps or plans are created from a process where a sheet of plastic is heated and vacuumed on top of a model or master. The master can be made from many substances, although certain materials are more durable than others. Since this process involves creating a mold, it is somewhat time-consuming.
Swell paper has a special coating of heat-reactive chemicals. Microcapsules of alcohol implanted in the paper fracture when exposed to heat and make the surface of the paper inflate. Placing black ink on the paper prior to a heat process provides control over the raised surface areas. This type of map is not as robust as the Thermoform map, but can be produced with less effort and expense.
Modified Braille embossers can also be used to produce tactile paper maps. [ 1 ]
Ink-jet tactile maps are made by layering a specially designed ink. Each layer is cured by UV irradiation before the next layer is added. This technology is an offshoot of other industries, such as circuit board manufacturing and biomedical applications. [ 2 ]
The substrate for tactile maps is a very important attribute, since different materials can enhance or reduce legibility and durability. Several types of substrates can be used to produce a tactile map. These include rough and smooth plastic, rough and smooth paper, microcapsule paper, Braillon, and aluminum. Many factors should be considered when choosing a substrate ; these include but are not limited to function, durability, and portability. [ 3 ]
Tactile map variables: Just as Jacques Bertin retinal variables help determine how visual maps are produced; tactile maps have a formula as well. Although researchers have not standardized tactile map variables, these nine are usually included depending on the substrate : vibration, flutter, pressure, temperature, size, shape, texture/grain, orientation, and elevation. [ 4 ]
Typical tactile elevations: Thermoformed maps usually have an elevation of at least 1mm. Swell Paper averages 0.5 mm and braille embossers have a range from 0.25–1 mm. Ink-jet printers can be controlled to vary elevation as needed. A (2009) study conducted by Sandra Jehoel tested various height levels and estimated that preferred tactile elevations fall between 40 and 80 micrometres depending on the substrate background, shape of the object and smoothness of lines. Symbols such as a triangle, square and a circle should have a minimum base line length of 6.4, 5.0 and 5.5 mm respectively for proper recognition. [ 5 ]
Audio tactile maps or graphic tablets are interactive devices. Electronic tactile talking touch pad instruments use Macromedia Flash software with audio files to convey information to the blind or visually impaired user. As the user's finger engages a feature or symbol a recording provides information about the object, symbol or area. For example, the sound of splashing water can be used for areas such as rivers or oceans. This format has great potential for transmitting information over the Internet which can be downloaded to a computer or hand-held device. [ 6 ]
A great deal of hardware already exists that can be used by the blind or visually impaired to interact with computer screen graphics. A vibrating mouse or other force feedback devices can be adapted to turn any visual software generated map into a hybrid tactile map. The interactive signal to a device can be varied when crossing a boundary or symbol. [ 7 ]
High resolution refreshable braille displays containing 1,500 to 12,000 pixels are already available in market. Graphic braille display available in the market is DV-2 (from KGS [ 8 ] ) with 1,536 pixels, Hyperbraille [ 9 ] with 7,200 pixels and TACTISPLAY Table/Walk (from Tactisplay Corp. [ 10 ] ) with 2,400/12,000 pixels respectively. TACTISPLAY table [ 11 ] has total 12,000 pixels arranged in 120x100.
Zoom maps are a recently developed tactile map. These maps are designed specifically for those who can read braille and have had no previous interaction with tactile maps. The term zoom is comparable to a zoom-able visual raster internet map. A country is divided into regions on the first map then the next zoomed map will have a breakdown of the regions and so forth until a city level is reached. These successive maps rely on a dependable texture as the map zoom progresses. This produces a familiarity as one zooms from the proceeding map. This is achieved in many instances with line orientation, area and consistent shape. The Braille text on the map is placed next to a rectangular textured legend for area identification. [ 12 ] | https://en.wikipedia.org/wiki/Tactile_graphic |
A tactile transducer or " bass shaker " is a device which is made on the principle that low bass frequencies can be felt as well as heard. They can be compared with a common loudspeaker , just that the diaphragm is missing. Instead, another object is used as a diaphragm. A shaker transmits low-frequency vibrations into various surfaces so that they can be felt by people. This is called tactile sound. Tactile transducers may augment or in some cases substitute for a subwoofer . One benefit of tactile transducers is they produce little or no noise, if properly installed, as compared with a subwoofer speaker enclosure.
A bass-shaker is meant to be firmly attached to some surface such as a seat, couch or floor. The shaker houses a small weight which is driven by a voice coil similar to those found in dynamic loudspeakers. The voice-coil is driven by a low-frequency audio signal from an amplifier ; common shakers typically handle 25 to 50 watts of amplifier power. The voice coil exerts force on both the weight and the body of the shaker, with the latter forces being transmitted into the mounting surface. Tactile transducers may be used in a home theater , a gaming chair or controller , a commercial movie theater , or for special effects in an arcade game , amusement park ride or other application.
Related to bass shakers are a newer type of transducer referred to as linear actuators . These piston-like electromagnetic devices transmit motion in a direct fashion by lifting home theater seating in the vertical plane rather than transferring vibrations (by mounting within a seat, platform or floor). This technology is said to transmit a high-fidelity sound-motion augmentation, whereas "Shakers" may require heavy equalization and/or multiple units to approach a realistic effect.
There are other products which employ hydraulic (long-throw) linear actuators and outboard motion processors for home applications as popularized in "virtual reality" rides. These products differ radically from tactile transducers in that they require the manual composition and synchronized playback of motion signals, in addition to the standard soundtrack that the motion is meant to accompany.
Various designs for tactile transducers have been presented since the 1960s, most of which fall under the "shaker" category. Shakers create a vigorous vibration by moving a mass (usually a magnet) which is bolted to a final mass (like a chair or couch). A simple example of this is the vibration available on a common cellphone. Another way of producing tactile sound uses "linear actuators", which move furniture (usually up and down), rather than shaking it. The main advantage of linear actuators is that they deliver actual motion (ground excursion), not just vibration.
In the 2010s, tactile sound transducers have evolved to include higher frequencies and produce higher fidelity. The human tactile frequency range is from 1 Hz, very low frequency such as earthquakes, up to 5 kHz in some hearing impaired individuals. For most individuals 2 to 3 kHz is the upper threshold for tactile reception. These 2010s-era devices must have higher resolution than previous "shakers" to produce these frequencies. Most humans have tactile resolution to 2 Hz which is the smallest change in frequency that can be perceived. The primary use for this extended bandwidth is to reproduce the vibratory signature for musical instruments such as violins, guitars, the human voice or sound effects in movies (for example, the speeders in Star Wars ). Also higher frequencies may be used to augment hearing through bone conduction, a consideration for people who have compromised their hearing from exposure to loud music.
Tactile sound is often used to increase the realism of an artificial environment. For example, mounting a tactile sound transducer in a chair or couch in a home cinema or video game setup can give more of a sense of "being there". For such use, the transducer is often connected to the LFE channel of an A/V receiver. Tactile sound is often used in combination with a subwoofer so that low frequencies can be both felt and heard. To facilitate broadband tactile sound, all channels are summed to provide a full range signal to the transducer amplifier. Graphic equalizers can also be used to further modify the effect.
For musical performance, drummers will often use a tactile sound transducer mounted on their drum stool so they can "feel" themselves playing, rather than using a more conventional stage monitor . The size and power of a stage monitor required to adequately reproduce low frequency drum sounds would be expensive and hard to transport, while a tactile sound transducer can be rather small and require much less power to get the job done. As well, sound engineers may prefer a tactile transducer over a loud, powerful subwoofer monitor cabinet, because a monitor speaker may produce a lot of stage volume.
Nicolas Collins describes several tactile transducers, including some wide-range drivers, which are able to transmit a broader frequency spectrum. [ 1 ] The composer David Tudor used tactile transducers in his work Rainforest (1968). He used "Rolen-Star" wide-range drivers to create all kinds of different loudspeaker sculptures. The tactile transducers are attached to large objects such as metal buckets and bring these objects in vibration. The vibrations of these sculptures are then picked up by contact microphones and amplified through a common loudspeaker system. [ 2 ] More recent examples can be found in the work of Sabrina Schroeder, [ 3 ] who places tactile transducers on bass drums. Lynn Pook attaches small tactile transducers on the bodies of the audience members [ 4 ] and Carola Bauckholt used in her piece Doppelbelichtung for violin 12 so-called violin loudspeakers. These consists of violins hanging from the ceiling, each with a small tactile transducer attached to them. [ 5 ] | https://en.wikipedia.org/wiki/Tactile_transducer |
Tactoids are liquid crystal microdomains nucleated in isotropic phases, which can be distinguished as spherical or spindle-shaped birefringent microdroplets under polarized light microscopy . Tactoids are a transition state between isotropic and macroscopic liquid crystalline phases. The first observation of tactoids was made by Zocher in 1925, when he studied the nematic phase formed in vanadium pentoxide sols . [ 1 ] After that, tactoids have been found in the phase transition processes in many lyotropic liquid crystalline substances, such as tobacco mosaic virus , [ 2 ] polypeptides , [ 3 ] and cellulose nanocrystals . [ 4 ]
It has been shown that filamin causes actin to condense into tactoids. [ 5 ] The filamentous phage Pf4 generates a tactoid shell around host P. aeruginosa cells that confer antibiotic resistance. [ 6 ]
This material -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tactoid |
Tadashi Nakayama or Tadasi Nakayama ( 中山 正 , Nakayama Tadashi , July 26, 1912, Tokyo Prefecture – June 5, 1964, Nagoya ) was a mathematician who made important contributions to representation theory .
He received his degrees from Tokyo University and Osaka University and held permanent positions at Osaka University and Nagoya University . He had visiting positions at Princeton University , Illinois University , and Hamburg University . Nakayama's lemma , Nakayama algebras , Nakayama's conjecture and Murnaghan–Nakayama rule are named after him.
This article about a Japanese scientist is a stub . You can help Wikipedia by expanding it .
This article about an Asian mathematician is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tadashi_Nakayama_(mathematician) |
tado° GmbH is a German technology company headquartered in Munich (Germany) and a manufacturer of home thermostats and air conditioning controls.
tado° was founded in Munich by Christian Deilmann, Johannes Schwarz and Valentin Sawadski in September 2011. [ 1 ] The company name is derived from the Japanese salutation "tadaima" ("I'm back home") and "okaeri" ("Welcome back") since geofencing was the first key feature of the tado° system in 2011. [ 2 ] The company's first product, tado° Heating, was launched in November 2012 in Germany, Austria, and Switzerland, and in 2013 in the United Kingdom. [ 3 ]
In July 2014, tado° closed a €10 million ($13.6m) funding round from, among others, Target Partners and Shortcut Ventures. [ 4 ]
The company's second product, tado° Smart AC Control, for intelligent cooling, was launched in Europe and the USA in June 2015. The launch event took place in New York City. [ 5 ]
Besides reducing energy consumption and increasing savings, the thermostat considers the residents' overall comfort. [ 6 ] tado° detects the absence or presence of residents via their smartphones, and then adjusts the heating automatically. Weather forecasts and the heating properties of a home such as insulation are taken into account. [ 7 ]
One challenge for tado° is the heterogeneous heating landscape in Europe and the integration and support of their control interfaces. By 2014, over 5,000 different heating systems can be controlled by tado° Heating. [ 8 ] [ 9 ] [ 10 ]
In June 2015, tado° launched its second product, the Smart AC Control. It is not a thermostat. It turns any air conditioning unit into a smarter device. [ 11 ] tado° Smart AC Control uses a smartphone's location to be able to determine whether the resident is at home and automatically turns the AC on or off. The air conditioner can also be manually operated via the Smart AC Control using the tado° app. [ 12 ]
In August 2017, tado° announced the integration of Google Assistant , allowing customers who both own a tado° system and a Google Assistant device to control their Tado system via voice commands. [ 13 ] For example, available voice commands are "What's the humidity in the bedroom?" or "Lower the bedroom temperature by 2°C". [ 14 ] | https://en.wikipedia.org/wiki/Tado° |
The Tafel equation is an equation in electrochemical kinetics relating the rate of an electrochemical reaction to the overpotential . [ 1 ] The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification. The equation is named after Swiss chemist Julius Tafel .
It describes how the electrical current through an electrode depends on the voltage difference between the electrode and the bulk electrolyte for a simple, unimolecular redox reaction. [ 2 ] [ circular reference ]
Where an electrochemical reaction occurs in two half reactions on separate electrodes , the Tafel equation is applied to each electrode separately. On a single electrode the Tafel equation can be stated as:
where
A verification plus further explanation for this equation can be found here. [ 6 ] The Tafel equation is an approximation of the Butler–Volmer equation in the case of | η | > 0.1 V {\displaystyle |\eta |>0.1V} .
"[ The Tafel equation ] assumes that the concentrations at the electrode are practically equal to the concentrations in the bulk electrolyte, allowing the current to be expressed as a function of potential only. In other words, it assumes that the electrode mass transfer rate is much greater than the reaction rate, and that the reaction is dominated by the slower chemical reaction rate ". [ 7 ] [ circular reference ]
Also, at a given electrode the Tafel equation assumes that the reverse half reaction rate is negligible compared to the forward reaction rate.
The exchange current is the current at equilibrium, i.e. the rate at which oxidized and reduced species transfer electrons with the electrode. In other words, the exchange current density is the rate of reaction at the reversible potential (when the overpotential is zero by definition). At the reversible potential, the reaction is in equilibrium meaning that the forward and reverse reactions progress at the same rates. This rate is the exchange current density.
The Tafel slope is measured experimentally. It can, however, be shown theoretically that when the dominant reaction mechanism involves the transfer of a single electron that λ k B T e < A {\displaystyle {\frac {\lambda k_{\text{B}}T}{e}}<A}
where A is defined as
where
In a more general case,
The following derivation of the extended Butler–Volmer equation is adapted from that of Bard and Faulkner and Newman and Thomas-Alyea. [ 8 ] [ circular reference ] [ ... ] the current is expressed as a function not only of potential (as in the simple version), but of the given concentrations as well. The mass-transfer rate may be relatively small, but its only effect on the chemical reaction is through the altered (given) concentrations. In effect, the concentrations are a function of the potential as well. [ 7 ]
The Tafel equation can be also written as:
where
As seen in equation ( 1 ), η = ± A ⋅ log 10 ( i i 0 ) {\displaystyle \eta =\pm A\cdot \log _{10}\left({\frac {i}{i_{0}}}\right)} η = ± A ⋅ ln ( i i 0 ) ln ( 10 ) , {\displaystyle \eta =\pm A\cdot {\frac {\ln \left({\frac {i}{i_{0}}}\right)}{\ln(10)}},} so: i = i 0 exp ( ± ln ( 10 ) η A ) {\displaystyle i=i_{0}\exp \left(\pm {\frac {\ln(10)\eta }{A}}\right)} i = i 0 exp ( ± α e η k T ) , {\displaystyle i=i_{0}\exp \left(\pm \alpha e{\frac {\eta }{kT}}\right),} as seen in equation ( 2 ) and because λ = ln ( 10 ) {\displaystyle \lambda =\ln(10)} . i = i 0 exp ( ± α F η R T ) {\displaystyle i=i_{0}\exp \left(\pm \alpha F{\frac {\eta }{RT}}\right)} because e k = e / N a k / N a = F R {\displaystyle {\frac {e}{k}}={\frac {e/Na}{k/Na}}={\frac {F}{R}}} [ 9 ] [ circular reference ] [ 10 ] [ circular reference ] due to the electrode mass transfer i 0 = n k F C {\displaystyle i_{0}=nkFC} [ 11 ] [ circular reference ] , which finally yields equation ( 3 ).
An other equation is applicable at low values of polarization | η | ≃ 0 V {\displaystyle |\eta |\simeq 0V} . In such case, the dependence of current on polarization is usually linear (not logarithmic):
This linear region is called polarization resistance due to its formal similarity to Ohm's law .
The pace at which corrosion develops is determined by the kinetics of the reactions involved, hence the electrical double layer is critical.
Applying an overpotential to an electrode causes the reaction to move in one direction, away from equilibrium. Tafel's law determines the new rate, and as long as the reaction kinetics are under control, the overpotential is proportional to the log of the corrosion current. [ 12 ] | https://en.wikipedia.org/wiki/Tafel_equation |
A chip log , also called common log , [ 1 ] ship log , or just log , is a navigation tool mariners use to estimate the speed of a vessel through water. The word knot , to mean nautical mile per hour , derives from this measurement method.
All nautical instruments that measure the speed of a ship through water are known as logs. [ 2 ] This nomenclature dates back to the days of sail, when sailors attached a piece of lumber (a "log" of wood) to a rope knotted at regular intervals off the stern of a ship. Sailors counted the number of knots that passed through their hands in a given time to determine the ship's speed. Today, sailors and aircraft pilots still express speed in knots.
A chip log consists of a wooden board attached to a line (the log-line ). The log-line has a number of knots at uniform intervals. The log-line is wound on a reel so the user can easily pay it out .
Over time, log construction standardized. The shape is a quarter circle , or quadrant with a radius of 5 inches (130 mm) or 6 inches (150 mm), [ 1 ] and 0.5 inches (13 mm) thick. [ 1 ] The log-line attaches to the board with a bridle of three lines that connect to the quadrant's vertex and the two ends of its arc. To ensure the log submerges and orients correctly in the water, the bottom of the log is weighted with lead . [ 1 ] This provides more resistance in the water, and a more accurate and repeatable reading. The bridle attaches in such a way that a strong tug on the log-line makes one or two of the bridle's lines release, enabling a sailor to retrieve the log.
A navigator who needed to know the speed of the vessel had a sailor drop the log over the ship's stern. The log acted as a drogue , remaining roughly in place while the vessel moved away. The sailor let the log-line run out for a fixed time while counting the knots that passed over. The length of log-line passing (the number of knots) determined the reading.
The first known device that measured speed is often claimed to be the Dutchman's log. This invention is attributed to the Portuguese Bartolomeu Crescêncio , who designed it in the end of the 15th century or in the beginning of the 16th century. [ 3 ] A sailor threw a floating object overboard and used a sandglass to measure the time it took to pass between two points on deck. The first reference to a Dutchman's log is in 1623—later than the ship log. [ 4 ] The Dutchman's log could be used with a brass tobacco box, rectangular with rounded ends. This box had tables on it to convert log timing to speed. [ 5 ] [ 6 ]
Mariners have used the log for a long time. The first known description of the device in print is in A Regiment for the Sea by William Bourne , in 1574. Bourne devised a half-minute sandglass for timing. [ 7 ] At the time, a mile was reckoned as 5,000 feet, so in 30 seconds at one mile per hour, a ship would travel about 42 feet:
Initially, the log-line was not knotted and sailors measured the length directly on the line. With the introduction of the nautical mile as a standard unit of measure at sea in the 15th century, they began to mark the line at equal intervals proportional to the nautical mile and to the time interval used for measurement. Initially, the markings were simply knots in the line. Later, sailors worked knotted cords into the log-line. Many ships used knots spaced 8 fathoms (48 feet or 14.6 meters ) apart, while other ships used the 7-fathom prescription. [ 8 ] The time interval needs to be adjusted according to the distance between knots. Substituting 6,000 feet for 1 mile, the above formula yields 28.8 seconds for a distance of 8 fathoms. In fact, 28-second and 14-second glasses used to be common among navigation equipment. [ 9 ]
Use of a log did not give an exact speed measure. The sailor had to incorporate a number of considerations:
Frequent measurements helped mitigate some of these inaccuracies by averaging out individual errors, and experienced navigators could determine their speed through the water with a fair degree of accuracy. Because a log measures the speed through the water, some errors—especially the effect of currents, the movement of the water itself—could not be corrected. Navigators relied on position fixes to correct for these errors. Modern navigation tools, such as GPS, report speed over ground, and in general do not give the same result as a log when currents are present. | https://en.wikipedia.org/wiki/Taffrail_log |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.