Effects of mixed-ligand complex formation on deprotonation of amide groups in acid amides and peptides

Abstract

The stability constants of the parent complexes of the A ligands glycinamide, glycylglycinamide, and N-acetylhistidine with copper(II), and of the mixed complexes formed with the B ligands glycine, 2,3-diaminopropionic acid, tiron, histamine, L-histidine, and 2,2′-bipyridyl, have been determined from pH-metric measurements. It has been found that glycinamide forms mixed-ligand complexes of composition [CuAB] and [CuABH_–1](charges omitted), and most favourably with histamine, L-histidine, or 2,2′-bipyridyl. In the glycylglycinamide complexes of type [CuABH_–1] the B ligand is co-ordinated to a considerable extent via two equatorial sites, while the bonding in the complex [CuABH_–2], similarly to the other dipeptides, is mainly axial–equatorial. In the parent complexes of copper(II) with N-acetylhistidine and with N-acetylhistamine, deprotonation and co-ordination of the amide group could not be detected. It has been found, however, that the presence of B ligands containing an aromatic N donor permits deprotonation of the amide groups in both N-acetylhistidine and N-acetylhistamine.