Cadmium II Complexes of Amino Acids and Peptides

2 Complexes of Amino Acids with Non-coordinating Side Chains. 279 

3 Complexes of Amino Acids Containing Sulfiff Donor Atoms. 282 

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A considerable number of NQR studies have been made on zinc and cadmium complexes, for example, the 35C1, 81Br and 127I NQR spectra of a number of CdX2-polyether complexes have been reported and indicate that the compounds are dimeric in solution, with symmetrical halogen bridges between the metal atoms.69 A number of amino acids and peptide complexes of cadmium(II) have been investigated by 14N NQR spectroscopy.70... 

The results reported in the literature and summarized in the previous sections reveal that cadmium(II) can form complexes with all natural amino acids and peptides. The thermodynamic stabilities of the cadmium(II) complexes are, however, relatively low as compared to those of the corresponding copper(II) and nickel(II) complexes. On the other hand, these data are comparable to those of the essential zinc(II) and the small differences are dependent on the effects of various side chain residues. These effects together with the structural characteristics of the cadmium(II) complexes can be evaluated separately for amino acids and peptides. 

In the case of the common peptides the comparison of the stability constants of zinc(II) and cadmium(II) complexes reveals the same trends as reported for the amino acids Favored complexation with zinc(II) in almost all cases, except for the thiolate ligands. The binding of several side chain residues may enhance the stability of cadmium(II) complexes but the extent of this stabihzation is relatively low. This is true even for the aspartyl or histidyl residues which are generally considered as the most strongly coordinating side chains in the corresponding... 

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