Transition metal complexes of short multihistidine peptides.

Nickel(ii), cobalt(ii) and cadmium(ii) complexes of terminally protected multihistidine peptides including Ac-HGH-OH, Ac-HGH-NHMe, Ac-HHGH-OH, Ac-HAHVH-NH(2), Ac-HVHGH-NH(2), Ac-HGHVH-NH(2) and Ac-(His-Sar)(n)-His-NH(2) (n = 1, 2 or 3) were studied by potentiometric, UV-Vis, CD and (1)H NMR spectroscopic techniques. It was found that the complexes in which the histidine imidazole nitrogens coordinate with ML stoichiometry are the main species in the physiological pH-range in all cases. The stability of these complexes is determined by the number of bound imidazole rings, the presence of the carboxylate group and the quality of the metal ion centre. The larger the number of coordinated imidazole-N donor atoms, the higher the stability of the complex. The stability constants of the ML complexes follow the Ni(ii) > Co(ii) approximately Cd(ii) order. Cobalt(ii) and cadmium(ii) are not, but nickel(ii) is able to promote the deprotonation and the coordination of amide nitrogens and NiH(-2)L and NiH(-3)L (Ni(2)H(-4)L) species predominate in basic solutions. For the pentapeptides with the exception of the sarcosine containing ligand the presence of coordination isomers is supported by spectroscopic methods. These data reveal that the favoured isomers are coordinated on the C-termini, but the ratio of isomers depends on the sequence of peptides.