Spectroscopic and potentiometric study of copper(II) complexes with l-histidyl-glycyl-l-histidyl-glycine in aqueous solution

Abstract The complex formation between copper(II) and the tetrapeptide l -histidyl-glycyl- l -histidyl-glycine (HL) has been studied in aqueous solution in the pH range 2–10.5, by potentiometric and spectroscopic methods (visible, CD, EPR, 1 H NMR and Raman scattering). Between pH 3 and 6 the species [CuHL] 2+ and [CuH −1 L] have been detected. The former complex co-ordinates through two nitrogen and two oxygen atoms in the equatorial plane while, for the latter, the spectroscopic data (particularly Raman spectra) suggests an unusual structure with the two imidazole rings and two peptide NH in the plane. The species at pH 7, [CuH −2 L] − , is involved in the co-ordination by the amino group, the His 1 imidazole and two peptide nitrogens, while, for the complex [CuH −3 L] 2− , which seems to be predominant in alkaline medium, a third deprotonated backbone NH, replaces the imidazole nucleus.