Voltammetric Methods for Speciation of Plant Metallothioneins

Cd, Cu and Zn complexes of two synthetically prepared peptides, (γ-Glu-Cys)2-Gly and (γ-Glu-Cys)3-Gly, were studied as models of plant metallothioneins by several voltammetric methods. Reduction peak of the complex in which the stechiometric ratio of molar concentrations of Cd(II) and peptide corresponds to coordination of one Cd(II) by four sulfur atoms (Ep = −814 mV to −910 mV, DPV) is suitable for speciation. CSV after adsorption of Cu(I) complex on HMDE is also convenient, and two peaks (Ep = −817 mV to −850 mV and Ep = −940 mV to −953 mV) are attributed to plant MT with Cu(I). Cd and Cu complexes can be distinguished by different dependences for adsorptive accumulation of complexes on HMDE. In both cases the presence of zinc ions interferes with voltammetric speciation. Results of voltammetric studies were applied to the analysis of algal extracts, prepared from alga Chlorella, which was grown in cadmium enriched medium. Successful voltammetric identification of plant metallothionein in this type of material has been performed, in agreement with results from AAS and UV spectroscopy.