Potentiometric, calorimetric and spectroscopic study of complexation between copper(II), nickel(II), and cobalt(II) and L,L-dipeptides containing weakly or non-co-ordinating side chains

Abstract

Complexation of Cu^II, Ni^II, and Co^II with four L,L-dipeptides containing weakly or non co-ordinating side chains (Phe-Leu, Leu-Phe, Phe-Met, and Met-Phe) was studied by potentiometric, calorimetric, and spectroscopic measurements. The results show that the stability of the complexes may be influenced by different factors according to the chemical composition of the species formed in aqueous solution. For species [MH_–1A](A denotes conjugate base form of the ligand) an increase in stability is observed with respect to glycylglycine or to dipeptides containing one non-glycine residue, and this effect is attributed to the hydrophobic interactions between the non-co-ordinating side chains. Another stabilising effect for this species is observed with a C-terminal Phe residue. This phenomenon is attributed to the interaction between the metal ion and the aromatic ring. The enthalpy of this non-covalent effect is evaluated as –9.5 kJ mol^–1. This stabilising effect is not found with N-terminal Phe residues. Comparison of the complex formation constants for these copper(II)–dipeptide systems shows a marked difference in the interactions between the benzyl ring and the alkyl chain and between the benzyl ring and the S-methyl alkyl group. Spectroscopic measurements (e.s.r. and u.v.-visible) suggest the presence of a CuN₂O₂ chromophore in [CuH_–1A] species. The ease of complex formation is in the order Cu^II > Ni^II > Co^II. Cobalt(II) does not deprotonate the peptide below pH 8. For a given species (for example [MA]), the complexes with the three transition metals appear to adopt a common structure.