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 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 (R_ex) in solutions of these complexes follow the kinetic equation R_ex=(k₁+k₂[L^–])[Cu(LH_–1) L^–]. Increasing the size of the side chain of the dipeptides leads to a lower R_ex, and a good correlation between log k₂ and the steric constants E_s^o of the side-chain substituents is observed. The high values for k₁ obtained indicate a considerable trans effect of the deprotonated peptide group.