Complexation of Heavy Metals by Phytochelatins: Voltammetric Study of the Binding of Cd2+ and Zn2+ Ions by the Phytochelatin (γ-Glu-Cys)3Gly Assisted by Multivariate Curve Resolution

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Abstract
The complexation of Cd2+, Zn2+, and both together with the phytochelatin (γ-Glu-Cys)3Gly is studied by differential pulse polarography, and data are analyzed by multivariate curve resolution by alternating least squares (MCR-ALS). MCR-ALS yields the respective unitary voltammograms and concentration profiles of the resolved components, which contain information on the relative stabilities and stoichiometries of the formed complexes. The analysis of these results shows, for the Cd2+/(γ-Glu-Cys)3Gly system, the presence of different kinds of bound Cd2+. For the Zn2+/(γ-Glu-Cys)3Gly system, the poor definition of the reduction signals of the complexes prevents a clear discrimination among differently bound Zn2+ ions. A tentative complexation/electrochemical model is proposed for when both metal ions, Cd2+ and Zn2+, compete toward complexation, and some of the corresponding equilibrium constants are estimated.