Mechanism of the Electrochemical Reaction of Glutathione at a Mercury Electrode

Abstract

The adsorptive and voltammetric characteristics of reduced glutathione (GSH) and oxidized glutathione (GSSG) have been investigated by cyclic voltammetry at a hanging mercury drop electrode in Na2HPO4+NaH2PO4 (pH 7). GSH has three pairs of peaks. The reaction scheme of GSH can be written as follows: For the first pair of peaks 2GSH(soln) – 2H+ 2e– ⇌ GSSG(ads) (1) For the second pair of peaks GSH(soln) + Hg – H+ – e– ⇌ GSHg(I)(ads) (2) For the third pair of peaks GSHg(I)(ads) – e– ⇌ GSHg(II)(ads) (3) GSHg(II)(ads) + GSH ⇌ (GS)2Hg(II)(ads) + H+ (4) Besides, when the concentration of GSH is higher, another reduction peak of GSH appears at the cathodic branch of the cyclic voltammogram. It results from reduction of GSSG (the product of oxidation of GSH through pass Equation 1), which forms a compact film on the surface of the electrode. The reaction scheme may be represented as: GSSG (compact film) + 2H+ + 2e– ⇌ 2GSH(soln) (5) GSSG has only a reduction peak at the cathodic branch of the cyclic voltammogram at the first scan. The reaction scheme of this peak is GSSG(ads) + 2H+ + 2e– ⇌ 2GSH(soln) (6) The anodic branch of the cyclic voltammogram of GSSG at the first scan and its cyclic voltammograms at the second scan and following scans are the same as those of GSH. Therefore, the reaction scheme of oxidation peaks of GSSG at the anodic branch of its cyclic voltammogram is the same as the Equations 1 to 4.