Oxidation of Methionine by Colloidal MnO2 in Aqueous and Micellar Media: A Kinetic Study

The oxidation of methionine by freshly prepared colloidal manganese dioxide in aqueous as well as micellar media was studied spectrophotometrically at 35°C. The reaction between methionine and MnO₂ in both media exhibits 1:1 stoichiometry (methionine:MnO₂). The oxidation reaction is first order with regard to the MnO₂ concentration, but is fractional-order in the methionine concentration and HClO₄ concentrations. A catalytic effect of nonionic surfactant TX-100 on the rate of oxidation was observed and reaction rate was found to be proportional to {k′ + k″ [TX-100]}, where k′ and k″ are the rate constants in absence and presence of surfactant, respectively. The use of surfactant micelles is highlighted as, in favorable cases; the micelles help the redox reactions by bringing the reactants in a close proximity through hydrogen bonding. The oxidation reaction in aqueous and micellar media is shown to proceed via methionine–MnO₂ and methionine–MnO₂–TX-100 complexes, respectively, which decomposes slowly in a rate determining step to give methionine sulfoxide as the product. A suitable mechanism is proposed for these observations.