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

The complex formation between copper(II) and the tetrapeptide L-histidyl-gl ycyl-L-histidyl-glycine (HL) has been studied in aqueous solution in the pH range 2-10.5, by potentiometric and spectroscopic methods (visible, CD, EP R, H-1 NMR and Raman scattering). Between pH 3 and 6 the species [CuHL](2+) and [CuH-1L] have been detected. The former complex co-ordinates through t wo nitrogen and two oxygen atoms in the equatorial plane while, for the lat ter, the spectroscopic data (particularly Raman spectra) suggests an unusua l structure with the two imidazole rings and two peptide NH in the plane. T he species at pH 7, [CuH-2L](-), is involved in the co-ordination by the am ino group, the His(1) imidazole and two peptide nitrogens, while, for the c omplex [CuH-3L](2-), which seems to be predominant in alkaline medium, a th ird deprotonated backbone NH, replaces the imidazole nucleus. (C) 1999 Else vier Science B.V. All rights reserved.