Metal complexes of peptides containing monodentate or chelating imidazole nitrogen donors: Factors influencing the coordination of amide groups and imidazole side chains

Transition metal complexes of peptides containing monodentate or chelating imidazole side chains have been studied by the combined application of potentiometric and spectroscopic techniques. The results obtained on the complexes of peptides containing C-terminal histidyl residues (Gly 3 His, Gly 4 His and Gly 5 His) provided clear evidence that both amino and imidazole functions are effective metal binding sites. The formation of various macrochelates were described via the coordination of both termini, but the major species were characterised by 4N-coordination starting from the N-termini. The coordination chemistry of a series of peptide molecules containing bis(imidazolyl) agents revealed that the donor functions of the peptide backbone cannot compete with chelation of the bis(imidazolyl) residue. However, the presence of terminal amino group promotes amide coordination, while imidazole residues act as additional donor sites or bridging ligands.