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Kinetic studies on the oxidation of hydroquinone and catechol by the heteropoly 10-tungstodivanadophosphate anion, [PV V V V W 10 O 40 ] 5-, have been carried out in aqueous acidic medium at 25°C by UV-visible spectrophotometry. The oxidation of hydroquinone shows simple second-order kinetics overall, with first-order dependence of the rate on both [oxidant] and [hydroquinone] at constant [H ? ]. For catechol oxidation, the order of the reaction with respect to [oxidant] is unity, while the order with respect to [catechol] is variable; this reaction shows Michaelis-Menten-type kinetics at constant [H ? ]. The rate of the reaction is insensitive to [H ?] in the pH range 1.2-1.7. Rate retardation for deuterated hydroquinone and catechol (C 6 H 4 (OD) 2 ) in D 2 O indicates breaking of the -OH bond in the rate-limiting step. Based on the observed kinetic isotope effect and calculated ground-state free energy change (DG 0 ) values, a hydrogen atom transfer mechanism is suggested for the reaction; i.e., in the ratelimiting step, one electron and one proton are transferred from the reductant to the oxidant in a concerted manner. Rates of oxidation of hydroquinone by this oxidant in neat acetonitrile at 25°C have also been measured. By applying the Marcus equation, the self-exchange rate constant of the oxidant (V V V IV OH/H Á ) in acetonitrile has been evaluated.

The coenzyme nicotinamide adenine dinucleotide (NADH) undergoes facile electron transfer reaction with vanadium (V) substituted Keggin-type heteropolyanions (HPA) [PV V W 11 O 40 ] 4-(PV 1 ) and [PV V 2 W 10 O 40 ] 5-(PV 2 ) in aqueous phosphate buffer of pH 6 at ambient temperature. Electrochemical and optical studies show that the stoichiometry of the reaction is 1 : 2 (NADH : HPA). EPR and optical studies show that HPA act as one electron acceptor and the products of electron transfer reactions are one electron reduced heteropoly blues (HPB), viz. [PV IV W 11 O 40 ] 5-and [PV IV V V W 10 O 40 ] 6-.Oxygraph measurements show that there is no uptake of molecular oxygen during the course of reaction. The reaction proceeds through multi-step electron-proton-electron transfer mechanism, with rate limiting initial one electron transfer from NADH to HPA by outer sphere electron transfer process. Bimolecular rate constant for electron transfer reaction between NADH and PV 2 in phosphate buffer of pH = 6 has been determined spectrophotometrically.

Glutathione (GSH) undergoes facile electron transfer with vanadium(V)-substituted Keggin-type heteropolyoxometalates, ½PV V W 11 O 40 4À (HPA1) and ½PV V -V V W 10 O 40 5À (HPA2). The kinetics of these reactions have been investigated in phthalate buffers spectrophotometrically at 25°C in aqueous medium. One mole of HPA1 consumes one mole of GSH and the product is the oneelectron reduced heteropoly blue, ½PV IV W 11 O 40 5À . But in the GSH-HPA2 reaction, one mole of HPA2 consumes two moles of GSH and gives the two-electron reduced heteropoly blue ½PV IV V IV W 10 O 40 7À . Both reactions show overall third-order kinetics. At constant pH, the order with respect to both [HPA] species is one and order with respect to [GSH] is two. At constant [GSH], the rate shows inverse dependence on [H ? ], suggesting participation of the deprotonated thiol group of GSH in the reaction. A suitable mechanism has been proposed and a rate law for the title reaction is derived. The antimicrobial activities of HPA1, HPA2 and ½PV V V V V V W 9 O 40 6À (HPA3) against MRSA were tested in vitro in combination with vancomycin and penicillin G. The HPAs sensitize MRSA towards penicillin G.

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