Cadmium: From Toxicity to Essentiality

Imre Sóvágó, Katalin Várnagy

Cadmium(II) ions form complexes with all natural amino acids and peptides. The thermodynamic stabilities of the cadmium(II) complexes of the most common amino acids and peptides are generally lower than those of the corresponding zinc(II) complexes, except the complexes of thiolate ligands. The coordination geometry of the cadmium(II) amino acid complexes is generally octahedral with the involvement of the amino and carboxylate groups in metal binding. In the case of simple peptides, both octahedral and tetrahedral complexes can be formed depending on the steric conditions. The terminal amino group and the subsequent carbonyl-O atom are the primary binding sites and there is no example for cadmium(II)-induced peptide amide deprotonation and coordination. The various hydrophobic and polar side chains do not have a significant impact on the structural and thermodynamic parameters of cadmium(II) complexes of amino acids and peptides. β-carboxylate function of aspartic acid and imidazole-N donors of histidyl residues slightly enhance the thermodynamic stability of cadmium(II)-peptide complexes. The most remarkable effects of side chains are, however, connected to the involvement of thiolate residues in cadmium(II) binding. Stability constants of the cadmium(II) complexes of both L-cysteine and its peptides and related ligands are significantly higher than those of the zinc(II) complexes. Thiolate donor functions can be bridging ligands too, resulting in the formation of polynuclear cadmium(II) complexes.