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 hav e been studied by potentiometric, visible and EPR Spectroscopic method s. The ligands included bis(imidazol-2-yl)methane (CH(2)R(2)), bis(imi dazol-2-yl)methylamine (R(2)CHNH(2)) and 3.3-[bis(imidazol-2-yl)]propi onic 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 B u(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). T he data revealed that stable mono- and bis-(ligand) complexes are form ed with all ligands and the imidazole nitrogensare the main metal bind ing sites. Tridentate co-ordination of R(2)CHNH(2) was concluded to ex ist in the equimolar solutions of copper(II) and R(2)CHNH(2), which re sults in the formation of a dinuclear mixed-0hydroxo-complex with imid azole bridging. Deprotonation of the co-ordinated water molecules was also observed around the physiological pH range in the zinc(II)-R(2)CH NH(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.