Potentiometric and spectroscopic studies on the copper(II) and zinc(II) complexes of bis(imidazol-2-yl) derivatives of tripeptides

Abstract

Copper(II) and zinc(II) complexes of tripeptides containing chelating bis(imidazol-2-yl)methyl agents (BIM) at the C-termini (GlyIleGly-BIMA, AlePheGly-BIMA and GlyGlyHis-BIMA) were studied by potentiometric, UV–Vis, EPR, MALDI-MS and 1H NMR techniques. The imidazole nitrogen donor atoms were described as the primary metal binding sites forming protonated mono- and bis(ligand) complexes in the acidic pH range. The formation of stable mononuclear ML and ML 2 complexes was detected following the deprotonation of the terminal amino group. In these complexes the ligands are bound to the metal ions at least tridentately, resulting in macrochelates. In the copper(II) containing system the deprotonation and coordination of amide nitrogens take place at higher pH. For GlyIleGly-BIMA, polynuclear species exist in basic solution with participation of both binding sites of the molecules. For GlyGlyHis-BIMA the “GlyGlyHis” like coordination was able to replace the bis(imidazol-2-yl)methyl residue in the coordination sphere of the copper(II) ion, and the [CuH −2L] complex was predominant above pH 9. The formation of stable ML and ML 2 complexes in the zinc(II)-GlyGlyHis-BIMA system hinders the hydrolysis of the metal ion, but the deprotonation and coordination of the amide nitrogen was not detected.