Studies on transition-metal–peptide complexes. Part 7. Copper(II) complexes of dipeptides containing L-histidine

Abstract

The copper(II) complexes of three dipeptides (HA) containing L-histidine residues [L-carnosine, L-histidylglycine (HisGly), and glycyl-L-histidine (GlyHis)] have been studied by pH-metric, spectrophotometric, and, in part, ¹H n.m.r. and calorimetric methods. With L-carnosine and HisGly, the formation of a dimeric complex [Cu₂A₂H_–2] was found, in which the co-ordination of copper(II) is glycylglycine-like, while the fourth co-ordination site is occupied by the imidazole N³ nitrogen atom, forming a bridge between two copper(II) ions. An excess of HisGly suppresses the deprotonation of the peptide linkage and the complex [CuA₂] is formed with histidine-like co-ordination. It is found that deprotonation of the peptide group is the easiest in the copper(II) complexes of GlyHis. The effect of co-ordination on deprotonation of the pyrrole-NH group is discussed.