An ESR study of the copper(II)-glycyl- l-histidine system in aqueous solution by the simultaneous analysis of multi-component spectra. Formation constants and coordination modes

Abstract

A series of multi-component ESR spectra of fluid aqueous solutions containing glycyl- l-histidine and copper(II) at 1:1 and 20:1 concentration ratios, taken in a circulating system at various pH, have been analysed simultaneously. The isotropic ESR parameters (g-factors, hyperfine coupling constants for the 63Cu and 65Cu isotopes and maximum four non-equivalent 14N nuclei, and relaxation parameters) and the formation constants of the various complexes were optimized. In the acidic region, the new species [CuLH 2] 3+, has been shown and the formation of the pH-metrically identified species [CuLH] 2+, [CuL] + and [CuLH −1] has been supported. The coordination in the first three complexes is reminiscent of that in simple dipeptides: it is likely to take place at first by the carboxylate O, then by the amino N and peptide O, and then by the amino N, deprotonated peptide N and carboxylate O atoms in equatorial positions, respectively, while the imidazole ring remains protonated. In the species [CuLH −1], the non-protonated imidazole nitrogen displaces the carboxylate oxygen from the equatorial coordination. In the alkaline region of the equimolar solutions the spectra could be described consistently in terms of the formation of four species: three of them, the complex [Cu 2L 2H −3] − and the two coordination isomers of the complex [CuLH −2] − are ESR-active, while the tetramer [Cu 4L 4H −8] 4− is ESR-inactive. For the complex [CuLH −2] −, one of the isomers is the species [CuLH −1(OH)] −, while the other one has the imidazole ring deprotonated. At ligand excess the region pH 7–10 is dominated by the pH-metrically identified complexes [CuL 2] and [CuL 2H −1] −. In these species the first ligand is bound equatorially by the amino, deprotonated peptide and imidazole nitrogen atoms. The second dipeptide is co-ordinated either by the amino nitrogen in equatorial and the peptide oxygen in axial position (4N isomer), or vice versa (3N isomer) in both bis complexes.