Studies on transition-metal–peptide complexes. Part 8. Parent and mixed-ligand complexes of histidine-containing dipeptides

Abstract

The glycyl-L-histidine (GlyHis), L-histidylglycine (HisGly), and L-carnosine (ligands HA) parent complexes of cobalt(II), nickel(II), and zinc(II) were studied by pH-metry, spectrophotometry, and in part ¹³C n.m.r. spectroscopy. All three metal ions were found to promote deprotonation of the peptide amide group of GlyHis. HisGly co-ordinates ‘histidine-like’ to the metal ions, i.e. without deprotonation of the peptide amide group. With carnosine, only nickel(II) induces deprotonation of this group. Studies were also made of mixed-ligand systems involving Gly, GlyGly, His, and occasionally 2,2′-bipyridyl as second ligand B, including those containing copper(II). Gly and GlyGly do not hinder the co-ordination of GlyHis via three nitrogens, and mixed-ligand complexes are therefore not present in detectable concentration. With His as ligand B, although mixed-ligand complexes are formed at comparable GlyHis and His concentrations, in the presence of an excess of His the parent complex [M(HisO)₂] predominates. A mixed-ligand complex is obtained in significant amount with 2,2′-bipyridyl as ligand B. Appreciable formation of mixed-ligand complexes also occurs in the nickel(II)–carnosine–ligand B systems. For cobalt(II), however, practically only the parent complexes of ligands B are formed. The cobalt(II) complexes of all three dipeptides examined are able to take up molecular oxygen reversibly, the oxygen being released partially or almost completely. For GlyHis it is highly likely that the active complex is [Co(AH_–1)], while for HisGly and carnosine the presence of the bis complexes is presumed necessary for oxygen uptake.