Edward T. Borish

Hydroxyterephthalate as a Fluorescent Probe for Hydroxyl Radicals: Application to Hair Melanin

The known photoreactions of melanin include production of melanin free radicals and oxygen consumption. While qualitative descriptions of the intermediates and products of these processes have been published, no quantitative procedures have been reported that allow the convenient measurement of the products of oxygen reduction. We have used complementary fluorescence and electron spin resonance techniques to study free radical production from extracted hair melanin irradiated at wavelengths above 320 nm. As a comparison, sepia was also studied. Irradiation of aerated suspensions of melanin in the presence of terephthalic acid dianion (TA) ‡ gives rise to the characteristic fluorescence spectrum of the 2‐hydroxyterephthalate ion (HTA, λeλ= 315 nm, λem= 425 nm). Production of HTA has been studied as a function of time (at constant light flux) and in the presence of other substrates including hydroxyl radical sources and scavengers. The use of TA as a fluorescent probe can be conveniently adapted to other systems.

The kinetics and stoicheiometry of silver(III) reduction by the octacyano-complexes of molybdenum(IV) and tungsten(IV)

The reactions of [Ag(OH)4]– with [Mo(CN)8]4– and [W(CN)8]4– involve an initial one-electron reduction of AgIII to AgII with second-order rate constants of 80 and 2.9 × 104 dm3 mol–1 s–1(25 °C, I= 1.2 mol dm–3), respectively. The tungstate reaction is quantitative with a stoicheiometry of 1 : 1. The reaction with [Mo(CN)8]4–, however, does not go to completion. There is apparently an equilibrium of oxidation states. From the value of the equilibrium constant a formal potential of 0.87 V ([OH–]= 1.2 mol dm–3, 25 °C) is estimated for the reduction of [Ag(OH)4]– to dimeric AgII. The rate of the initial one-electron redox step is strongly correlated with EM/M+ for the series of reductants [Fe(CN)6]4–, MnO42–, [W(CN)8]4–, and [Mo(CN)8]4–. The results are interpreted in terms of a general mechanism which involves silver(II) polymerisation following the initial redox step.

Kinetics and mechanism of hydrogen peroxide oxidation by silver(III) in aqueous alkaline media

The oxidation of hydrogen peroxide by AgIII in strongly alkaline media has been studied by stopped-flow spectrophotometry in the temperature range 6–45 °C at a total ionic strength of 1.2 mol dm–3(NaClO4). The reaction is first order in [AgIII], [HO2–], and [OH–] with a third-order rate constant of (4.2 ± 0.6)× 105 dm6 mol–2 s–1 at 25 °C. The form of the rate law and the activation parameters (ΔH‡= 25 ± 5kJ mol–1, ΔS‡=–113 ± 5 J K–1 mol–1) suggest the formation of a five-co-ordinate intermediate of the form [Ag(OH)4O2]3–. Although kinetic results do not distinguish between a oneor two-electron transfer, a change in stoicheiometry from Δ[AgIII]/Δ[HO2–]= 1 at high [HO2–] to 2 at excess [AgIII] leads to the conclusion that a bivalent silver intermediate is produced.