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 (CH2R2), bis-(imidazol-2-yl)methylamine (R2CHNH2) and 3,3-[bis(imidazol-2-yl)]propionic acid (R2CHCH2CO2H) and peptides in which the bis(imidazolyl) groups are coupled at the C-terminus, MeCO-Pro-Leu-Gly-NHCHR2 and ButOCO-Pro-His-Gly-NHCHR2, or the N-terminus, R2CHCH2CO-lle-Ala-Gly-OEt and R2CHCH2CO-lle-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 nitrogens are the main metal binding sites. Tridentate co-ordination of R2CHNH2 was concluded to exist in the equimolar solutions of copper(II) and R2CHNH2, which results in the formation of a dinuclear mixed-hydroxo complex with imidazole bridging. Deprotonation of the co-ordinated water molecules was also observed around the physiological pH range in the zinc(II)-R2CHNH2complex. The involvement of the side-chain imidazole residues of the peptides ButOCO-Pro-His-Gly-NHCHR2 and especially R2CHCH2CO-lle-His-Gly-OEt in co-ordination has also been demonstrated.