Uncatalyzed and copper(II) catalyzed oxidation of glutathione by Co(III)2 bound superoxide complex

In acid media, glutathione (GSH) is oxidised by metal bound superoxo complex, [(NH3)5Co(III)(μ-O2)Co(III)(NH3)5]5+ (1) to GSSG. Complex 1 is reduced to its corresponding peroxo complex, [(NH3)5Co(III)(μ-O2)Co(III)(NH3)5]4+ (2) which readily decomposes in acid media to Co(III), NH4+ and O2. The oxidation of GSH is profoundly catalyzed by the presence of Cu2+ ion. The observed rate constant ko was found to be proportional to [GSH]2 for both uncatalyzed and catalyzed reaction and for the latter ko was also proportional to [Cu]T2 ([Cu]T is the analytical concentration of Cu2+). The rate for uncatalyzed reaction decreases for with increasing ionic strength (I) of the reaction media whereas, ko for catalyzed reaction is invariant of ionic strength (I). Both for the uncatalyzed and catalyzed reaction, ko is proportional to [H+]−3. Under the reaction condition employed, it has been seen that GSH reduces Cu(II) to Cu(I) and forms either mononuclear Cu(I)–GSH or dinuclear (Cu(I)–GSH)2 complex with Cu(I) with a 1:1 composition. Suitable mechanistic pathways for the uncatalyzed and catalyzed reactions through the intermediate formation of mononuclear or dinuclear Cu–GSH complex have been suggested. The activation energy for the uncatalyzed and catalyzed reaction has been calculated as 97.1 ± 5.7 and 29.9 ± 1.2 kJ M−1 respectively.