Mechanism of the electrochemical reaction of glutathione at a mercury electrode

The adsorptive and voltammetric characteristics of reduced glutathione (GSH ) and oxidized glutathione (GSSG) have been investigated by cyclic voltamme try at a hanging mercury drop electrode in Na2HPO4 + NaH2PO4 (pH 7). GSH ha s three pairs of peaks. The reaction scheme of GSH can be written as follow s: For the first pair of peaks 2GSH(soln)-2H(+)-2e(-) reversible arrow GSSG(ad s) (1) For the second pair of peaks GSH(soln) + Hg-H+ -e(-) reversible arrow GSHg(I)(ads) (2) For the third pair of peaks GSHg(I)(ads) - e(-) reversible arrow GSHg(II)(ads) (3) GSHg(II)(ads) + GSH reversible arrow (GS)(2)Hg(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 Equati on 1), which forms a compact film on the surface of the electrode. The reac tion scheme may be represented as: GSSG (compact film) + 2H(+) + 2e(-) reversible arrow 2GSH(soln) (5) GSSG has only a reduction peak at the cathodic branch of the cyclic voltamm ogram at the first scan, The reaction scheme of this peak is GSSG(ads) + 2H (+) + 2e(-) reversible arrow 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 sam e as those of GSH. Therefore, the reaction scheme of oxidation peaks of GSS G at the anodic branch of its cyclic voltammogram is the same as the Equati ons 1 to 4.