Voltammetric Analysis of Phytochelatin Complexation in Ternary Metal Mixtures Supported by Multivariate Analysis and ESI-MS

The competitive binding of Cu2+, Cd2+ and Pb2+ in ternary mixtures by the phytochelatins PC2 and PC5 (PCn; (?Glu-Cys)n-Gly, n=2 and 5) are examined by voltammetry, which allows one to follow the displacements of the voltammetric signals induced by the competitive binding among the metal ions towards PC2 or PC5 complexation, and direct injection positive-mode Electrospray Ionization Mass Spectrometry (ESI-MS), in order to obtain the stoichiometries of the complexes. Voltammetric data are analyzed by Gaussian Peak Adjustment (GPA), which is a recently developed multivariate analysis method for nonlinear electrochemical data. Different complexes have been detected or deduced their presence, depending on the experimental conditions. Ternary complexes CuCdPCn and CdPbPCn were detected for both PC2 and PC5, while the ternary CuPbPCn complex was only detected for PC2. Some of the complexes have been only detected by ESI-MS because in some cases voltammetry data could not be totally resolved, even by using GPA. The quaternary CdPbCuPCn complex has been detected for PC5, but for PC2 data are not so conclusive. In summary, the signal evolution for mixed CdCuPCn complexes is quite different. These observations could be a reflection of an antagonistic effect for the case of PC2 and a synergetic one for PC5.