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The amine buffer Tris slowly reduces tetranitromethane (TNM) to the nitroform anion in a non-accelerating reaction. The amine buffers HEPES and MOPS also (slowly) react with TNM but the dialkylaminoalkyl radicals formed from these two buffers undergo further reactions resulting in a rapid, accelerating, free radical chain process whereby the amine is oxidized and TNM reduced. The chemical functionality in any reaction component, not necessarily the buffer, required for this radical chain mechanism is >N-CH

The spontaneous and CO2-catalyzed decomposition of peroxynitrite yields HO• and -CO3• radicals, respectively, together with •NO2. The geminate HO•/•NO2 and -CO3•/•NO2 pairs undergo competitive in-cage collapse to nitrate and diffusive separation. Free HO• and -CO3• radicals react with H2O2 and, in the presence of O2, suitable alcohols or formate to produce superoxide, which is trapped by the •NO2 to form peroxynitrate. The formation of peroxynitrate may influence the rate of change in optical density at 302 nm, the wavelength normally employed to monitor peroxynitrite decay, leading to misleading kinetic traces. Tetranitromethane (TNM) was used as a colorimetric probe for superoxide to quantify the yield of free HO• (27−28%) and free -CO3• (32−33%). The yields of both of these free radicals are in excellent agreement with other recent estimates. Superoxide was also detected in some oxygenated aldehyde-catalyzed peroxynitrite decompositions both by peroxynitrate formation and by its reaction with TNM. Supe...

Product studies on the oxidation of two 1-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy acids and by tert-butyl hydroperoxide catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin in solution have been carried out. In single turnover experiments at pH 11.88, 9.30 and 6.93, dye bleaching by peroxy acids is a two-step process; an initial fast reaction, during which the oxoiron(IV) porphyrin is also formed, is followed by a slow phase involving the regeneration of the iron(III) porphyrin from the oxoiron(IV) porphyrin. The peroxy acid∶dye stoichiometry is 1∶1 for the initial fast dye bleaching and 2∶1 for the slow phase; the latter is equivalent to a 4∶1 stoichiometry of oxoiron(IV) porphyrin to dye. In multiple turnover reactions the dye bleaching occurs in two sequential oxidations by oxoiron(IV) porphyrin π-cation radical, each with a peroxy acid∶substrate stoichiometry of 1∶1.Evidence is presented to show that, in multi-turnover reactions at pH 11.88 where the substrate is the dye anion, the initial product arises from the hydroxylation of the 1-position of the naphthol ring which in the second step is oxidised further to give an azoxy compound. At pH 9.30, where the dyes exist predominantly as the hydrazone tautomers, the initial oxidation generates an azoxy compound which following a Wallach rearrangement is further oxidised on the naphthol ring. More extensive oxidation results in the cleavage of the N–N bond.The iron(III) porphyrin-catalysed dye oxidation with excess of tert-butyl hydroperoxide at pH 6.93, which occurs by hydrogen atom-abstraction, has a hydroperoxide∶dye stoichiometry approaching 2∶1 and gives the same azoxy product as obtained at pH 11.88 with the peroxyacids.Mechanisms to account for the observed products and stoichiometries are discussed.

The oxidation of 1-(substituted phenylazo)-2-hydroxynaphthalene-6-sulfonate dyes by oxoiron(IV) tetra(2,6-dichloro-3-sulfonatophenyl)porphyrin (OFeIVTDCSPP) in aqueous solution is first-order in the concentration of the dye and of the oxoiron(IV) species over the pH range 6.93–12.68. The pH dependence of the second-order rate constant, kobs, which shows a minimum between pH 8.5 and 9.5, can be simulated using two pH-dependent equilibria [OFeIVTDCSPP(OH2)/OFeIVTDCSPP(OH) and dye/dye anion] and the rate constants for three oxidations, dye by OFeIVTDCSPP(OH2) and OFeIVTDCSPP(OH) and dye anion by OFeIVTDCSPP(OH). The fourth combination, the oxidation of the dye anion by OFeIVTDCSPP(OH2), makes an insignificant contribution to the overall rate of reaction. The mechanisms of the three oxidation processes have been studied using Hammett correlations of substituent effects and by comparisons with the oxidations of isomeric 1-aryl-4-hydroxynaphthalenesulfonate dyes, of an O-methylated dye and of a deuterated 1-phenylazo-2-hydroxynaphthalene-6-sulfonate dye, bearing in mind that the dyes in aqueous solution exist as an equilibrium between azo and hydrazone tautomers.In strongly basic solution the dominant reaction is an electron-transfer oxidation between the dye anion and OFeIVTDCSPP(OH) whereas at neutral pH the major reaction is hydrogen atom-abstraction by OFeIVTDCSPP(OH2) from the azo tautomer of the dye. The mechanism at the pH–rate minimum, between OFeIVTDCSPP(OH) and dye, is less clear and alternatives are discussed.

The rate of formation of superoxide measured by its reduction of tetranitromethane (TNM) and by its reduction of ferric cytochrome c (Fe(III) cc) are in excellent agreement when the superoxide is generated from a simple chemical precursor. In contrast, the rate of formation of superoxide generated in the reaction of xanthine oxidase with acetaldehyde is much higher (up to a factor of 6) when measured with TNM and compared with Fe(III) cc. It is shown that Fe(III) cc measures superoxide that has diffused from the enzyme, and that TNM probably scavenges all the dioxygen that is reduced by one electron by the enzyme. The TNM traps enzyme-bound superoxide in competition with the second-electron transfer and proton transfer, which normally yield hydrogen peroxide. The proton transfer is probably rate determining, k(p) ≤ 3.8 x 103s-1. (C) 2000 Elsevier Science Inc.

The first method for generating (*)NO and O(2)(*)(-) at any known, constant ratio has been developed. Spermine NONOate and di(4-carboxybenzyl)hyponitrite decay with first-order kinetics and exactly equal rate constants (half-lives of 80 min) at 37 degrees C and pH 7.5 to give 200 and 40 mol % (*)NO and O(2)(*)(-), respectively. Tyrosine oxidation to dityrosine and 3-nitrotyrosine (the major and minor products under the conditions used in these experiments) has been studied (mainly in the presence of CO(2)) using various different ratios of the rates of formation of (*)NO and O(2)(*)(-). The (*)NO/O(2)(*)(-) = 1.0 product profiles are very similar to those of the products derived from equal amounts of (*)NO and O(2)(*)(-) generated at a (*)NO/O(2)(*)(-) ratio of 1.0 from SIN-1 but are very different from those derived from preformed peroxynitrite. All the experimental results can be explained in terms of free radical chemistry. The product profiles at all the (*)NO/O(2)(*)(-) ratios could be satisfactorily simulated provided an important group of reactions which lead to the consumption of dityrosine was included.

Publisher Summary The generally accepted mechanism for phenolic oxidations catalyzed by horseradish peroxidase (HRP), the most thoroughly studied peroxidase, involves an initial two-electron oxidation of the iron(III) resting state of the enzyme by the hydroperoxide (or hydrogen peroxide) to give compound I (HRP I), that is, subsequently reduced back to the iron(III) haem, in two one-electron steps, via compound II (HRP II). The phenoxyl radical intermediates react to give products that depend on their structure and may further oxidize on their own. This chapter discusses the results from the investigations of the mechanism of oxidation of 1-phenylazo-2-naphthol-6-sulfonates by oxoiron(IV) tetra(2,6- ichloro-3-sulfonatophenyl) porphyrin in aqueous solution. The research discussed in the chapter shows that, at pH 7.6, oxidation proceeds by hydrogen atom abstraction by the oxometallo(IV) species from the phenol rather than electron transfer from the phenolate anion. Preliminary studies with commercial azo dyes has revealed that these are also substrates for oxoiron(IV) porphyrins. The azo dyes used in the present experiment were 1-phenylazo-2-naphthol-6-sulfonate and seven derivatives with substituents in the meta or para positions of the phenyl ring. These were selected as representative 1-azo-2-naphthol sulfonate dyes and because the substituents on phenyl ring would allow a systematic study of the mechanism of their oxidation by metalloporphyrin-catalysed systems in aqueous solution. Dye 2 is commercially available (as Acid Orange 12) and is purified by recrystallization, while the others were prepared by standard diazonium ion/2-naphthol coupling reactions.

The complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with vinyl acetic acid and a selection of α-halo and α-hydroxycarboxylic acids, α-,β- and γ-ketoacids, β-dicarboxylic acids and polyacrylic acid have been photolysed (λ > 390 nm) in anaerobic aqueous solution. UV–VIS spectroscopy shows that the primary reaction is photoreduction of the iron(III) atom by the axial carboxylate ligand to give a solvent-caged carboxyl radical and iron(II) porphyrin.The rate of iron(II) formation is determined by the competitive reactions of the carboxyl radical in the solvent cage: recombination with the iron(II) porphyrin to regenerate the starting complex versus decarboxylation. EPR spin-trapping methods have been used to detect the carbon radicals arising from decarboxylation.Vinylacetic and α-hydroxycarboxylic acids lead to a rapid rate of iron(II) porphyrin formation; with the latter this is partly due to the one-electron reduction of the iron(III) porphyrin by the α-hydroxyalkyl radicals from the photo-decarboxylation. By contrast the build-up of iron(II) porphyrin from the photoreactions of the α-haloacids is slow and does not go to completion since the iron(III) porphyrin is regenerated by reaction of the halosubstrate with the iron(II) compound.Ketoacids show an unusual variety of reactions, depending on structure. The α-ketoacids give acyl and, by decarboxylation, alkyl radicals and the iron(II) porphyrin CO complex, whereas with β-ketoacids the photoreaction is catalytic, since, following decarboxylation of the carboxyl radical, the iron(III) porphyrin is rapidly regenerated by oxidation of the iron(II) porphyrin by the α-keto radical. γ-Ketoacids behave in a similar fashion to alkylcarboxylic acids.The photoreactions of β-dicarboxylic acids resemble those of their monocarboxylic analogues. With polyacrylic acid the rate of iron(II) formation is slow and this is attributed to the polyacid encouraging recombination of iron(II) porphyrin and carboxyl radical.