Kinetic studies on oxidations of leucomethylene blue and leucothionine by iron (III) in aqueous solution

The reaction kinetics and mechanism of the oxidations of leucomethylene blue (MBH) and leucothionine (TH) by iron (III) sulfate in aqueous solution were studied spectrophotometrically by the stopped-flow technique. MBH and TH, which were freshly prepared by photoreduction of methylene blue and thionine, respectively, with ascorbic acid were used in the kinetic measurements. The pseudo-first-order rate constants (k_obsd) show kinetic saturation at high initial concentrations of iron (III) sulfate for MBH and TH. It was found that the reciprocal of k_obsd increases linearly with increase in the reciprocal of [Fe³⁺]₀. A broad absorption band was observed on mixing MBH and Fe³⁺ solutions at low temperatures, and this was attributed to a charge-transfer complex between MBH and Fe³⁺. The effects of Fe²⁺ ion and L-(+)-ascorbic acid on the rates of oxidation were also investigated. A small kinetic isotope effect on the oxidation rate for MBH was observed. The results can be explained by a general mechanism with stepwise electron-proton-electron transfers with the formation of a complex between reactants.