Electron spin resonance study of copper(II) complexes of X-glycine and glycyl-X type dipeptides, and related tripeptides. Variation of co-ordination modes with ligand excess and pH in fluid and frozen aqueous solutions

Abstract

Co-ordination modes for the various copper(II) complexes of glycine(Gly)-containing di- and tri-peptides (HL) with non-co-ordinating side-chains have been investigated. The e.s.r. spectra of predominant species at 1 : 1, 2 : 1, and 50 : 1 ligand : metal concentration ratios in the region pH ≈ 6–13 have been recorded in fluid and frozen aqueous solutions, and evaluated by computer simulation. The energies of the d–d electronic transitions have been determined by Gaussian analysis of the visible absorption spectra. Molecular-orbital coefficients characteristic of metal–ligand bonds for the various 1 : 1 and 1 : 2 complexes have been calculated assuming effective D_4h symmetry. At ligand excess in alkaline solution, the temperature strongly affects the chemical equilibria: low temperature promotes the formation of 1 : 2 complexes: [Cu(LH_–1)L]^– at pH ≈ 9, and [Cu(LH_–1)₂]^2– at pH ≈ 13 in the case of X-Gly type dipeptides. In the predominant isomers of these complexes one of the dipeptide molecules is co-ordinated equatorially through its amino nitrogen, deprotonated peptide nitrogen, and carboxylate oxygen atoms. The amino group of the other dipeptide occupies an axial position, while the fourth equatorial donor atom is either the peptide oxygen (pH ≈ 9) or the deprotonated peptide nitrogen (pH ≈ 13) of the second ligand. In the latter case, axial co-ordination of the second carboxylate group is also likely. Competition can be observed between the σ and π bonds in the equatorial plane on the one hand, and between the σ bonds of different symmetries on the other hand. The influence of the co-ordination modes, the type of ligand, and the temperature on the covalent character of the metal–ligand bonds is discussed.