Abstract
The oxidation of methionine by freshly prepared colloidal manganese dioxide in aqueous as well as micellar media was studied spectrophotometrically at 35 degrees 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.