Reactivity of some sulphur- and non-sulphur-containing amino acids towards water soluble colloidal MnO2. A kinetic study

Abstract

Kinetic data for the oxidation of glutathione (reduced, GSH), cysteine, glycine and glutamic acid by colloidal manganese dioxide, (MnO2) n are reported. Colloidal MnO2, oxidized glutathione to disulphide (glutathione, oxidized), was reduced to manganese (II). Glycine and glutamic acid (structural units of glutathione) are not oxidized by colloidal MnO2, but the other structural unit, cysteine, is also oxidized by the same oxidant under similar experimental conditions. This is interpreted in terms of the rate-determining colloidal MnO2-S bonded intermediate. The reactivity of GSH towards colloidal MnO2 is very much higher than cysteine. Kinetics of oxidation of GSH and cysteine by colloidal MnO2 were performed spectrophotometrically as a function of [GSH], [cysteine], colloidal [(MnO2) n ], [HClO4], temperature and trapping agents sodium fluoride and manganese (II) (reduction product of colloidal MnO2). The purpose of this work was to study the role of –NH2, –COOH, –SH groups present in the carbon chain of the above amino acids. It was found that the reactivity of –SH group is higher than –NH2 and –COOH groups. The mechanisms, involving a colloidal MnO2 complex with GSH and cysteine, are proposed. The complexes decompose in a rate-determining step, leading to the formation of free radical and manganese (III), which is also an intermediate. The dimerization of radicals takes place in a subsequent fast step to yield the products.