Composition, stability, and lability of copper(II) dipeptide complexes

Complex formation of copper(II) with glycylglycine (HL) has been studied spectrophotometrically over a wide range of ligand concentration (0.005-1.0 mol dm-3) and pH (0.5-13) in aqueous potassium nitrate (1.0 mol dm-3 KNO3). The complexes [CuL]+, [Cu(HL)]2+, [Cu(LH-1)], [Cu(LH-1)(OH)]-, [Cu(LH-1)L]-, [Cu(LH-1)2]2-, and [Cu2(LH-1)2(OH)]- were found and their formation constants determined. The ligand in [Cu(HL)]2+ is co-ordinated through the carboxylate group, and the terminal amino group remains protonated. Such a co-ordination mode is more favourable for depeptides as compared to amino acids. Both of the ligands in [Cu(LH-1)2]2- are bound to the metal ion in a bidentate mode through the terminal amino nitrogen and the deprotonated peptide nitrogen. The species [Cu2(LH-1)2(OH·)]- has an intense absorption band in the near-u.v. region which indicates that the OH- group is bridging. The lability of complexes [Cu(LH-1)L]- with five aliphatic dipeptides has been investigated by n.m.r. relaxation of water protons. The ligand-exchange rates (Rex) in solutions of these complexes follow the kinetic equation Rex = (k1 + k2[L-])[Cu(LH-1)L-]. Increasing the size of the side chain of the dipeptides leads to a lower Rex, and a good correlation between log k2 and the steric constants Es0 of the side-chain substituents is observed. The high values for k1 obtained indicate a considerable trans effect of the deprotonated peptide group