Stability of Metal−Glutathione Complexes during Oxidation by Hydrogen Peroxide and Cu(II)-Catalysis

Thiol-containing ligands such as glutathione (GSH) are expected to degrade in the presence of oxygen; however, complexation by Hg, Ag, and other trace metals may protect free thiol functional groups (R−S^-) from oxidation, leading to persistence in surface water environments. In this study, the stability of GSH complexes with Hg²⁺, Ag⁺, and other metals including Cd²⁺, Zn²⁺, and Pb²⁺ was assessed during exposure to two potential environmental oxidants:  H₂O₂ and Cu²⁺. The results indicated that Hg(GSH)₂ and Ag(GSH) complexes were completely stable for at least 2 days in the presence of either H₂O₂ or Cu²⁺. In contrast, free GSH oxidized within minutes to hours. Complexation by Cd, Zn, and Pb slightly decreased or did not significantly affect the oxidation rate of GSH, depending upon the pH (tested between pH 6 and 9). Thermodynamic modeling of GSH speciation demonstrated that the observed oxidation rates were not consistent with predicted free GSH^3- concentration. These results indicated that Cd−, Zn−, and Pb−GSH complexes were susceptible to oxidation by a mechanism that differs from GSH^3- oxidation. In contrast, Hg− and Ag−GSH complexes were inert for days, suggesting that they are stable for relatively long periods in the oxic water column. These results demonstrate that coordination of Hg(II) and Ag(I) to thiol-containing ligands can potentially increase persistence and transport in surface waters.