POTENTIOMETRIC AND SPECTROSCOPIC STUDIES ON COPPER(II) AND ZINC(II) COMPLEXES OF PEPTIDES CONTAINING BIS(IMIDAZOLYL) LIGANDS

Copper- and zinc-(ii) complexes of various bis(imidazolyl) ligands have been studied by potentiometric, visible and EPR Spectroscopic methods. The ligands included bis(imidazol-2-yl)methane (CH(2)R(2)), bis(imidazol-2-yl)methylamine (R(2)CHNH(2)) and 3.3-[bis(imidazol-2-yl)]propionic acid (R(2)CHCH(2)CO(2)H) and peptides in which the bis(imidazolyl) groups are coupled at the C-terminu, MeCO-Pro-Leu-Gly-NHCHR(2) and Bu(t)OCO-Pro-His-Gly-NHCHR(2) or the N-terminus, R(2)CHCH(2)CO-lle-Ala-GIy-OEt and R(2)CHCH(2)CO-Ile-His-Gly-OEt (where R = imidazol-2-yl). The data revealed that stable mono- and bis-(ligand) complexes are formed with all ligands and the imidazole nitrogensare the main metal binding sites. Tridentate co-ordination of R(2)CHNH(2) was concluded to exist in the equimolar solutions of copper(II) and R(2)CHNH(2), which results in the formation of a dinuclear mixed-0hydroxo-complex with imidazole bridging. Deprotonation of the co-ordinated water molecules was also observed around the physiological pH range in the zinc(II)-R(2)CHNH(2) complex. The involvement of the side-chain imidazble residues of the peptides Bu(t)OCO-Pro-His-Gly-NHCHR2 and especially R(2)CHCH(2)CO-Ile-His-GIy-OEt in co-ordination has also been demonstrated.