Redox kinetic measurements of glutathione at the mercury electrode by means of square-wave voltammetry. The role of copper, cadmium and zinc ions

The electrode reaction of glutathione (GSH) at the hanging mercury drop electrode is studied by means of square-wave voltammetry (SWV). At potentials more positive than -0.350 V (vs. Ag/AgCl (3 mol/l KCl)) the oxidation of the mercury electrode in the presence of GSH leads to creation of a sparingly soluble mercury–GSH complex that deposits onto the electrode surface. Under cathodic potential scan, the deposited complex acts as a reducible reactant, giving raise to a well-defined cathodic stripping reversible SW voltammetric response. The electrode reaction can be described by the scheme: Hg(SG)2(s)+e-+2H(aq)+=Hg(l)+2GSH(aq). Thus, the electrode reaction provides information on both thermodynamics and kinetics of the chemical interactions of GSH with mercury. An experimental methodology for measuring the kinetics of the electrode reactions, based on the property known as “quasireversible maximum”, is developed. The standard redox rate constant is 5.09, 5.75 and 5.22 cm s-1 in a phosphate buffer at pH 5.6, 7.0 and 8.5, respectively, with a precision of ±10%. The high rate of the electrode reaction reflects the strong affinity of GSH towards chemical interaction with mercury. The electrode reaction is particularly sensitive to the presence of heavy metal ions such as Cu2+, Cd2+, and Zn2+. The rate of the electrode reaction decreases significantly in the presence of these ions due to simultaneous interactions of GSH with the respective ion and mercury. [Copyright &y& Elsevier]