Speciation of Cadmium - D-penicilamine, mercaptosuccinic acid and glutathione systems in NaNO3 ionic medium

Cadmium shows a high affinity towards most of the binding groups present in biologically active molecules. In particular, the thiol ligands present in the amino acid residues, are the main vectors through which transport and distribution of cadmium in the human body occur. In spite of the large number of investigations reported in the literature about the toxic effects of cadmium and its environmental impact, relatively few quantitative data are reported on the stability of species formed by the interaction between Cd2+ and S-donor ligands in aqueous solution. Therefore, it is difficult to define the speciation picture of this element in natural waters and biological fluids where thiol ligands are often naturally present. With the aim to contribute to the knowledge of cadmium(II) speciation in the presence of sulphur containing ligands, we report here the preliminary results of a study on the interaction of Cd2+ with 2-mercaptosuccinic (or thiomalic) acid (tma), penicillamine (psh) and glutathione (gsh) ligands. The complex species formation and the relative stability constants were determined by potentiometric (ISE-H+ and ISE-Cd2+) measurements in NaNO3 aqueous medium at I = 0.1 mol L-1 and t = 25 °C. ElectroSpray Ionization (ESI) mass spectrometry measurements confirmed the complexation model proposed on the basis of potenziometric results. For all the Cd2+- L (L = psh, tma, gsh) systems investigated, the calculations gave evidence for the formation of ML and ML2 complex species. In addition, some protonated M(L)Hi species were found, in particular for the Cd-gluthatione system. By using the stability data obtained for all the complex species formed, the sequestering capacity of the ligands considered here towards cadmium(II) ion, expressed as pL50 parameter [i.e. the –log(total ligand concentration) necessary to bind 50% of cadmium ion], was calculated at different pH values.