The coenzyme nicotinamide adenine dinucleotide (NADH) undergoes facile electron transfer reaction with vanadium (V) substituted Keggin-type heteropolyanions (HPA) [PV^VW₁₁O₄₀]^4- (PV₁) and [PV$^V_2$W₁₀O₄₀]^5- (PV₂) 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^IVW₁₁O₄₀]^5- and [PV^IVV^VW₁₀O₄₀]^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 PV₂ in phosphate buffer of pH = 6 has been determined spectrophotometrically.

Rates of electron transfer reaction of thioglycolic acid with vanadium(V) substituted Keggintype heteropolyanion, [PV^VV^VW₁₀O₄₀]^5-, in acetate-acetic acid buffers have been measured spectrophotometrically at 25°C. The order of the reaction with respect to substrate and oxidant is unity. The reaction shows simple second order kinetics at constant pH. The rate of the reaction increases with increase of pH of the medium. The mono-anion HSCH₂COO^- and di-anion ^-SCH₂COO^- are found to be the reactive species. Rate constants for mono-anion and di-anion are evaluated from rate law derived from the mechanism. By applying Rehm-Weller relationship, self exchange rate constant for the ^-SCH₂COO^-/S$^{\bullet}$ CH₂COO^- couple was evaluated as $3.3 \times 10^3$ dm³ mol^-1 s^-1 at 25°C.