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

•Glutathione (GSH) is oxidized by the cobalt bound superoxide complex.•Observed rate for the uncatalyzed reaction is proportional to [GSH]2/[H+]3.•Cu2+ forms complex with GSH and catalyzes the reaction.•For catalyzed reaction, observed rate is proportional to {[GSH][Cu]T}2/[H+]3.•The key catalyst is a mononuclear or dinuclear, 1:1 complex of Cu2+ with GSH.

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.

Graphical abstractGlutathione (GSH) reduces and mineralize the highly reactive metal bound superoxo radical complex 1 to Co2+, NH4+ and O2 and itself is oxidised to GSSG through uncatalyzed and Cu2+ catalyzed pathway. The observed reaction rate not only proportional to the [GSH]2 for both uncatalyzed and catalyzed reaction but for the later, rate is also proportional to square of analytical copper concentration ([Cu]T).Figure optionsDownload full-size imageDownload as PowerPoint slide