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

Abstract

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 KNO<inf>3</inf>). The complexes [CuL]⁺, [Cu(HL)]²⁺, [Cu(LH<inf>-1</inf>)], [Cu(LH<inf>-1</inf>)(OH)]^-, [Cu(LH<inf>-1</inf>)L]^-, [Cu(LH<inf>-1</inf>)<inf>2</inf>]^2-, and [Cu<inf>2</inf>(LH<inf>-1</inf>)<inf>2</inf>(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<inf>-1</inf>)<inf>2</inf>]^2- are bound to the metal ion in a bidentate mode through the terminal amino nitrogen and the deprotonated peptide nitrogen. The species [Cu<inf>2</inf>(LH<inf>-1</inf>)<inf>2</inf>(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<inf>-1</inf>)L]^- with five aliphatic dipeptides has been investigated by n.m.r. relaxation of water protons. The ligand-exchange rates (R<inf>ex</inf>) in solutions of these complexes follow the kinetic equation R<inf>ex</inf> = (k<inf>1</inf> + k<inf>2</inf>[L^-])[Cu(LH<inf>-1</inf>)L^-]. Increasing the size of the side chain of the dipeptides leads to a lower R<inf>ex</inf>, and a good correlation between log k<inf>2</inf> and the steric constants E<inf>s</inf>⁰ of the side-chain substituents is observed. The high values for k<inf>1</inf> obtained indicate a considerable trans effect of the deprotonated peptide group.