A kinetic study of the Ru(III)-catalysed oxidation of glutathione by Methylene Blue in acidic medium

Glutathione (GSH, RSH) and Methylene Blue (MB) interact in a mole ratio of 2: 1 in acidic medium forming the corresponding disulfide and the leuco base. The reaction shows zero-order kinetics in MB and first-order kinetics in GSH, i.e. the rate is not affected on varying [MB]. The rate shows a fractional dependence on [H+] while there is no influence of variation of ionic strength on the rate of reaction. The rate constant increases linearly on increasing [Ru(III)]. The zero-order rate constant remains unaffected on varying the time of equilibration of the catalyst with the ingredients of the reaction system. The rate is not affected on addition of the disulfide, but addition of the leuco base slightly retards the rate. Activation parameters have been evaluated and a plausible reaction scheme has been proposed by presuming the interaction of transient Ru(II) with protonated MB as the rate limiting step. The kinetic results indicate that the glutamyl tripeptide moiety of GSH inhibits the reactivity of cysteine - SH towards MB in the presence of Ru(III).