Formazan anion

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 alkylation of formazan anions (166) with methyl iodide yields the A-methylformazan (202) which cannot be prepared by direct methods (Scheme 31).4 However, formazans that contain hydroxyaryl (203) or heterocyclic groups (205) alkylate preferentially on these groups to yield the alkylation products 204349 and 206,346-348 respectively (Eqs. 24,25). Although formazans are resistant to acylation,334,350 351 it can be accom-... 

Tables 15 and 16 show the absorption maxima of some metal complexes of benzothiazolyl-substituted formazans 230 and 231.283 The wavelengths are metal ion dependent, making formazans useful reagents for the identification of specific metal ions or the simultaneous determination of two ions. The wavelengths are much longer than those of the formazan anion (Table 14). The general trend for electron-rich substituents is toward a larger shift this is to be expected as it tends to enhance the aromatic character of the ring and increase the covalent character of the metal — nitrogen bond. The sharpness of the absorption band has been attributed to coordination to the heterocyclic nitrogen as in 232.57S...

An appropriate ion-specific electrode was found to provide a convenient, precise and relatively inexpensive method for potentiometry of copper(II) ion in copper-complex azo or formazan dyes. Copper(II) ion in copper phthalocyanine dyes can be quantified after anion exchange. Twelve commercial premetallised dyes evaluated using this technique contained copper(II) ion concentrations in the range 0.007 to 0.2%. Thus many copper-complex direct or reactive dyes are likely to contribute low but possibly significant amounts of ionic copper to textile dyeing effluents [52]. 

Electrochemical oxidation of formazans is a particularly advantageous preparative route to tetrazolium salts, which can be performed by controlled-potential or constant-current electrolysis. Tetrazolium salts with widely differing anions can be prepared by merely using a supporting electrolyte carrying the desired anion. The two-electron oxidative cyclization of forma-zan to tetrazolium salt may occur through an ECPE(d) mechanism. 

Treatment of l-phenylhydrazino-3-phenylazo-2-cyclohexene with a strong base also produces a blue coloration. The amphoteric character of this compound and of other formazan vinylogs has been ascribed502 to the formation of such resonance-stabilized anions as 278a,b. The absorption spectrum of this anion resembles the spectrum of the resonance-stabilized cation of 2-oxo-l,3-bis(phcnylhydrazones). indicating some amphoteric character in these compounds (Scheme 65). 

The addition of NBT, which is known to interact with the superoxide anion and is reduced to formazan, led to the concentration-dependent inhibition of NOS activity, with an apparent K of 3-4 p.M (Mittal et al., 1993). The inhibitory effect of NBT was compared with N -monomethyl-L-arginine (NMA), a structural analog of L-arginine. Inclusion of NMA or NBT at 100 fjiM led to the complete inhibition of L-arginine-dependent stimulation of guanylate cyclase activity without any effect on the basal cGMP produc-... 

The ability of a test compound to induce differentiation of HL-60 cells (derived from a patient with acute promyelocytic leukaemia) is also an important assay of retinoid activity [91-93]. HL-60 cells do not produce superoxide anions upon stimulation by agents like TPA. Differentiated HL-60 cells, however, do produce these anions upon similar stimulation. The presence of these anions can be monitored because of their ability to reduce the yellow nitroblue tetrazolium (NBT) to the water insoluble blue-black formazan. Determination of the concentration (ED50) which results in differentiation of 50% of the cells is the measure of the anti-APL activity of the retinoid. Morphological maturation of the cells can also be checked. 

T. Poiger, S.D. Richardson and G.L. Baughman, Analysis of anionic metallized azo and formazan dyes by capiUary electrophoresis-mass spectrometry, J. Chromatogr. A, 886, 259-270, 2000. 

The phospholipase Aj inhibitor dibromoaceto-phenone, the anti-inflammatory steroid fluocino-lone acetonide or the lipoxygenase inhibitor nordi-hydroguiaretic acid just prior to intraperitoneal injection of 100 ng 12-0-tetradecanoylphorbol-13-acetate into unmanipulated CD-I female mice resulted in a dose-dependent decrease in the number of peritoneal exudate cells producing superoxide anion radical as assessed by the reduction of nitro-blue tetrazolium, while the cycloxygenase inhibitor indomethacin had no effect on the number of formazan-positive peritoneal exudate cells caused by PMA treatment (Czerniecki and Witz 1989). 

Superoxide radicals are produced by a xanthine-xanthine oxidase system. Xanthine is able to generate 02 and H2O2 by using xanthine oxidase as a substrate. The superoxide radicals are able to reduce the yellow colored nitro triazolium blue (NTE) to the blue formazan, which is used for monitoring the reaction. The superoxide anions are measured spectrophotometricaUy. This test method was developed to explore the reaction between antioxidants and 02. So the inhibition of the superoxide reductase is a mark for the ability of antioxidative activity. 

The Ni + cation shows little discrimination between ligands in propylene carbon-ate. 2 Kinetic parameters have been communicated for reactions of Ni-+ and Co + with ammonia in aqueous methanol and for Ni + with ethylenediamine or 2,2 -bipyridyl in aqueous dimethylformamide, and with 2,2 -bipyridyl derivatives in aqueous ethanol. The kinetics of reaction of Fe + with benzoylacetanilides have also been studied in aqueous ethanol the kinetic pattern for reaction of Fe + with thiocyanate in aqueous carboxylic acids is complicated by parallel complex formation with solvent-derived carboxylate. Copper(ii)-formazan (2) chelate complex formation has been shown to take place in discrete steps in aqueous ethanol. Carboxy-late anion effects on the copper(ii)-tetraphenylporphine reaction have been studied in dimethylformamide. ... 

Dithionite-anion exchange resin Formazans from tetrazolium salts... 

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