Nickel(), zinc() and cadmium() complexes of hexapeptides containing separate histidyl and cysteinyl binding sites

Nickel( ii ), zinc( ii ) and cadmium( ii ) complexes of two N-terminally free hexapeptides AAHAAC and AHAAAC containing separate histidyl and cysteinyl residues have been studied using potentiometric and various spectroscopic (UV-Vis, CD, 1 H-NMR) techniques. Both peptides have outstanding metal binding ability but the speciation of the systems and the binding sites of peptides reveal a significant specificity. In the nickel( ii )-AAHAAC system the amino terminus is the primary nickel( ii ) binding site in the form of the (NH 2 ,N − ,N − ,N im ) chelate. However, the C-terminal thiolate function can bind another nickel( ii ) ion with the involvement of amide nitrogens in metal binding. Zinc( ii ) and cadmium( ii )ions are not able to promote deprotonation and coordination of the peptide amide groups of AAHAAC and only mononuclear complexes are formed, in which imidazole-N and thiolate-S − donors are the primary metal binding sites. In the case of AHAAAC both nickel( ii ) and zinc( ii ) ions can induce deprotonation and coordination of the first amide bond in the sequence. This results in the enhanced stability of the corresponding species, [MH −1 L], containing a tridentate (NH 2 ,N − ,N im ) binding mode at the amino terminus supported by a macrochelate from the distant thiolate group. Hexapeptides containing separate histidyl and cysteinyl residues have outstanding metal binding ability but the binding sites of peptides reveal a significant specificity.