Studies on Kinetics and Mechanism of Oxidation of GSH by cis-(diaqua)-bis-(ethylenediamine) Cobalt(III) Ion

Abstract

The kinetics of GSH oxidation to GSSG by cis-(diaqua)-bis-(ethylenediamine) cobalt(III) perchlorate was studied spectophotometrically under pseudo-first order condition using 10³[Co(III)] = 5 mol dm^-3, 2.5 \(\le\) 10²[GSH] \(\le\) 10.00 mol dm^-3, 3.5 \(\le\) pH \(\le\) 5.0, 318K \(\le\) T \(\le\) 333K at a fixed ionic strength I = 0.3 mol dm^-3 (NaClO₄). The disappearance of [Co (III)] at 500 nm with time showed first-order kinetic trend. The first order dependence on [GSH] and pH was observed. Temperature dependence of the second order rate constant k'₂ = k_obs/[GSH] were analyzed for the Co(III)-OH₂³⁺(k₁) and Co(III)-OH²⁺(k₂) reactivities. An outer-sphere complex formation between the Co(III) and GSH followed by one-electron transfer from GSH to Co(III) resulting the formation of Co(II) and GSH radical. The GSH radical under- goes fast dimerisation to GSSG. Activation parameters were calculated. These values favor the electron transfer reaction. Negative value of activation entropy corresponds to ordered transition state.