Complexation of cadmium by the C-terminal hexapeptide Lys-Cys-Thr-Cys-Cys-Ala from mouse metallothionein: study by differential pulse polarography and circular dichroism spectroscopy with multivariate curve resolution analysis Original Research Article

The cadmium-binding properties of the C-terminal hexapeptide of mouse metallothionein I, Lys-Cys-Thr-Cys-Cys-Ala, were studied by circular dichroism spectroscopy (CD), differential pulse polarography (DPP) and 113Cd-nuclear magnetic resonance (NMR). The structure of the multiple cadmium binding sites could not be determined by 113Cd-NMR because of the insolubility of the Cd–peptide samples at the high concentrations required for NMR. Therefore, alternative approaches were used: CD and DPP. The data were analyzed using a multivariate curve resolution (MCR) approach, based on factor analysis techniques, which allows the identification of the signal corresponding to different metal ions bound in different chemical environments. The CD study confirmed that the binding of Cd2+ induces important conformational changes in the structure of the peptidic complex, including the formation of a binuclear cluster. The DPP results obtained at various Cd2+-to-peptide concentration ratios and pH values, under conditions where electrode adsorption is low, if not negligible, indicated the formation of different Cd2+–peptide complexes, and a scheme for the electrochemical reduction of the complexed Cd2+ ions is proposed. These results show that the application of MCR to complex data, such as those from DPP, allows to reach valuable information which is not possible to be obtained by univariate approaches.