Copper(II) complexes of carnosine, glycylglycine, and glycylglycine-imidazole mixtures

Abstract

A comparative spectroscopic study of the copper(II) complexes of carnosine, glycylglycine and glycylglycine-imidazole mixtures was undertaken to gain a better understanding of the solution structure of copper(II)-carnosine. Potentiometric titration data were obtained and correlated with the visible absorption spectra, water proton relaxation rates, and low temperature electron paramagnetic resonance spectra. Changes in the visible spectra and water relaxation rates with increasing pH reflect the displacement of solvent oxygen ligands from the coordination sphere of copper(II) by the ligating groups found on the dipeptides. Estimates of the number of solvent molecules in rapid exchange with the copper(II) for each dipeptide were obtained over the pH range 4 to 8. In addition, a weighted average of the metal-proton distances between copper(II) and bound solvent molecules in the axial and equitorial positions was obtained. From these measurements and EPR data, a monomeric solution structure for copper(II)-carnosine is proposed. This structure has a β-amino nitrogen, deprotonated peptide nitrogen, α-carboxyl oxygen, and a water molecule in the equitorial plane of copper(II), and an imidazole nitrogen as the fifth ligand in the axial position.