Tetrazolium salts, reduction

Tetrahydrocannabinol (= THC)-11-carboxylic acid 290 Tetrahydrocortiso 221 Tetrahydrocortisone 221 Tetrahydrosteroids 222 Tetrazolium salts, reduction 61 Thalidomide 45 -, hydrolysis products 45 Thiabendazole 307, 308 Thiamine 235, 236, 397 Thickening agents 179 Thin-layer chromatography, advantage 5 -, numbers of publications per year 6 Thiobarbiturates 45,66 Thiocarbamide derivatives 322 Thiocarbamides, N -ary I-N -benzenesulfo-nyl- 248,249 Thiochrome 395... 

Measures tetrazolium salt reduction to gauge cell viability... 

Tetrazolium iodide, l,3-dialkyl-5-phenyl-synthesis, 5, 815 Tetrazolium salts electrochemical reduction, 5, 73 hydrogen exchange, 5, 70 Tetrazolium salts, 2,3-diphenyl-5-oxide... 

Solutions of tetrazolium salts, e.g., 53, have been reported to both become colored and bleached under the influence of both UV and visible light. Several workers have attributed this phenomenon to photoreduction to the corresponding formazan (51) and the formation of a fluorescent colorless compound (152) through photooxidation.240- 243 The reduction of 152 under UV or blue light to the intense green radical structure (153) has also been reported (Scheme 21).244 A one-electron reduction product (154) is proposed as a short-lived intermediate in the photoreduction.245... 

Electrochemical methods have been used extensively to elucidate the mechanism of reduction of tetrazolium salts. In aprotic media, the first step is a reversible one-electron reduction to the radical 154 as confirmed by ESR spectroscopy.256,266 As shown in Scheme 26, this radical can then disproportionate to the tetrazolium salt and the formazan anion (166) or take up another electron to the formazan dianion (167). The formation of the dianion through a direct reduction or through the intermediate tetrazolyl anion (168) has also been proposed.272-28 1,294 In aqueous solutions, where protonation/deprotonation equilibria contribute to the complexity of the reduction process, the reduction potentials are pH dependent and a one-electron wave is seldom observed. 

The reduction potentials, measured by scanning voltammetry, are substitution dependent.282 As shown in Table 12 for a series of triaryl tetrazoliums (169), the reduction becomes easier when electron-withdrawing substituents are present. This is in agreement with the polarographic data on a series of benzothiazolyl tetrazolium salts (170).283 With bis tetrazolium... 

As discussed in Section 7.4.2.5, the reduction of tetrazolium salts to formazans often results in further reduction products. As seen in Scheme 24, reduction of formazans with ammonium sulfide leads to the hydrazidine 161. The reduction can proceed further eliminating an arylamine, yielding an amidrazone, e.g., 162.364 By contrast, alcoholic hydrogen sulfide attacks... 

A few electrochemical reductions of formazans to hydrazidine have been reported.370,372 Ho wever, as discussed in Section 7.4.2.6, electrochemical techniques have been widely used to study the redox chemistry of tetrazolium salts and formazans. Opinions about the reversibility of the electron transfer step, the number of electrons involved, and the identity of the rate-determining step differ widely.369- 371,656 The electrochemical oxidation of some novel formazans, e.g., 215 produces the dicationic species 216 (Eq. 28). The mechanism is not clearly understood.372,373... 

The reduction of tetrazolium salts by NADH is greatly accelerated by electron transfer agents (ETAs) such as phenazine methosulfate (PMS 233) or its derivatives.451-454 Other classes of ETAs such as quinones.455,456 ferricinium,457 phenothiazine,458 the viologens,459 acridiniums,460 and phe-nazinium or quinoxalinium salts461 as well as the enzyme diaphorase462 have been used. 

Tetrazolium salts can be reduced nonselectively by many endogenous reductants such as thiol-containing proteins, as well as exogenous ones such as ascorbic acid.521 -523 This can lead to serious interferences and several measures have been described to reduce or eliminate their effect.524,525,650... 

Formazans and their metal complexes are used as textile dyes by direct application. The in situ reduction of tetrazolium salts has not been used to introduce the dyes to their substrates. Treatment with triaryl mono and bis tetrazolium salts followed by a reducing agent such as ascorbic acid or thioglycolic acid has been claimed as a method of introducing formazans as permanent hair dyes.641 There are some references to their use as therapeutic agents.642 544... 

It is remarkable that more than a century after the discovery of tetrazoliums, they continue to have wide applications. In particular, the reductive ring opening reaction leading to formazans continues to receive a lot of attention. In 1993, there were 316 citations to tetrazolium salts in Chemical Abstracts, 23 of which were to their reduction. Yet, the same reaction that makes them a unique class of reduction indicator leuco dyes, may be a part of a broader phenomenon. 

It would seem likely that tetrazolium salts are only one class of a wider variety of redox indicators acting through reductive ring opening, and that many more examples will be discovered in the future that may open some new directions to the chemistry of tetrazolium salts. 

The alkaline phosphatase substrates form precipitates based on either reduction of tetrazolium salts or on the production of colored diazo compounds. Substrates Vector Red, Vector Black, Vector Blue, and BCIP/NBT available from Vector Laboratories produce reaction products which are red, black, blue and purple/blue... 

A colorimetric version of the method uses a tetrazolium salt. The oxidation of the NADH is coupled to the reduction of 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide by a diaphorase (EC 1.6.4.3) and a deep blue formazan develops. The absorbance is read at a wavelength between 540 and 600 nm. This is more sensitive than the ultraviolet method but is more prone to interference, especially from any reducing agents present in the sample, which will result in a positive error. 

Phytochemical reduction of tetrazolium salts has been observed by Kuhn and Jerschel. If 2,3,5-triphenyl tetrazolium chloride is added to fermenting yeast the corresponding formazan is formed. Like most formazyl compounds it has a red color. 

Trisubstituted tetrazolium salts (57) are the oxidation products of formazans. They have been used to produce tetrazolinyl radicals by controlled reduction <73AG(E)455>. 7V-Alkyltetrazolium... 

Growth conditions in deep-well microtiter plates were optimized with respect to optimal expression of active enzymes (Fig. 2.2.1.1). The best results were obtained with an expression time of 20 h at 37 °C (Fig. 2.2.1.1, lanes 7-9). Subsequently, E. coli cells were enzymatically disrupted by lysozyme treatment, and the carboligase activity was monitored by a modified tetrazolium salt color assay [16], This color assay is based on the reduction of the 2,3,5-triphenyltetrazolium chloride (TTC) 13 to the corresponding formazan 15, which has an intense red color (Fig. 2.2.1.2A). Before screening ofa BFD variant library, substrates and products were tested in the color assay. Neither substrate, benzaldehyde 4 nor dimethoxy-acetaldehyde 8, reduced TTC 13 however, the product 2-hydroxy-3,3-dimethoxy-propiophenone 10 already caused color formation at low concentrations of 2.5-10 mM (Fig. 2.2.1.2B). Benzoin 12 as the product also gave a color change at a similar concentration (data not shown). 

The electrochemistry of tetrazolium salts (220) in aqueous media was discussed in Part I. In a strictly aprotic medium 220 gives a reversible one-electron reduction to a radical with a lifetime longer than the time scale of CV.352 In moist acetonitrile the reaction becomes a two-electron reduction similar to that in aqueous, basic media a formazan (221) is formed... 

The 1,5-diphenyl and 1,3,5-triphenyl derivatives were discovered almost simultaneously by H. von Pechmann [38] and E. Bamberger [39] in 1892. Interest in those compounds was renewed in 1939 when G. Lakon [40] and R. Kuhn and D. Jerchel [41] found that colorless tetrazolium salts were very sensitive to biological reduction processes and were converted to deeply colored formazans ... 

Bioindicators. Water-soluble, colorless tetrazolium salts can be reduced to water-insoluble, deeply colored formazans. The reduction of tetrazolium salts in plant tissue at pH 7.2 was first demonstrated in 1941 [77], Tetrazolium salts have since been used in biochemistry, cytochemistry, and histochemistry because of the great sensitivity of this reaction. They can be used to detect biological redox systems in blood serum, in living cells, tissues, tumors, and bacteria. Tetrazolium Blue [167429-81-7] (48) is a particularly sensitive reagent. 

T. Shimamura, A. Takamori, H. Ukeda, S. Nagata, and M. Sawamura, Relationship between reduction of tetrazolium salt XTT and DNA strand breakage with aminosugars, J. Agric. Food Chem., 2000, 48, 1204-1209. 

MTT assay is a standard colorimetric assay used to determine cytotoxicity of potential medicinal agents and other toxic materials. It is based on the reduction of the tetrazolium salt MTT by viable cells. A mitochondrial dehydrogenase enzyme is able to cleave the tetrazolium rings of the pale yellow MTT and form dark purple formazan crystals, which are largely impermeable to cell membranes resulting in the accumulation of these crystals within healthy cells. Solubilization of the cells by the addition of a detergent results in the liberation of crystals, which are solubilized. The metabolic activity of cells is directly proportional to the concentration of the created formazan product (22), whose color is quantified in a colorimetric assay. 

Pagliacci, M. et al. 1993. Genistein inhibits tumour cell growth in vitro but enhances mitochondrial reduction of tetrazolium salts a further pitfall in the use of the MTT assay for evaluating cell growth and survival. Eur. J. Cancer 29, 1573-1577. 

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