Circular Dichroism and Voltammetry, Assisted by Multivariate Curve Resolution, and Mass Spectrometry of the Competitive Metal Binding by Phytochelatin PC5

Abstract

The competitive binding of Cd(2+) and Pb(2+) by the phytochelatin (γGlu-Cys)5-Gly (PC5) has been examined by several analytical techniques. Positive-mode electrospray ionization mass spectrometry (ESI-MS) is used to determine the stoichiometries of the complexes, while voltammetric and spectroscopic data are analyzed by multivariate curve resolution with alternating least-squares (MCR-ALS) and some recently developed chemometric tools, which allows one to follow the displacement induced by Cd(2+) or Pb(2+) in the binding of Pb(2+) or Cd(2+), respectively, by PC5, and to obtain a complete overview of the processes involved. Differential pulse polarography (DPP) signals of these systems present, besides overlapping peaks due to free metal ion and metal complexes, interference of mercury anodic signals. Circular dichroism (CD) spectroscopy is a useful complementary technique due to the absence of these types of signals and its selective response to different species. A tentative complexation/electrochemical model is proposed for Cd(2+) and Pb(2+) competition toward PC5 complexation. The formation of the previously unreported ternary CdPb(PC5) complex was suggested by DPP and CD spectroscopy experiments and confirmed by ESI-MS.