ESR study of the copper(II)–glycylglycine equilibrium system in fluid aqueous solution. Computer analysis of overlapping multispecies spectra

Abstract

Various predominant and minor complexes of glycylglycine with the copper(II) ion were studied in fluid aqueous solution by ESR spectroscopy. The computer analysis of a series of overlapping spectra was carried out including at most four independent species. The spectra were built as a hyperfine structure of copper isotopes and a variable number of non-equivalent nitrogen atoms. The isotropic ESR parameters and relative concentrations of different species were optimized giving a good spectral fit. The variations of the relative concentrations of the complexes are consistent with those calculated from formation constants obtained by pH-metric and spectrophotometric studies, and the ESR parameters of each species determined from different spectra agree well. For the complex [CuL2H−1]− two kinds of coordination exist. In both isomers one of the glycylglycine ligands is coordinated equatorially through its amino nitrogen, deprotonated peptide nitrogen and carboxylate oxygen atoms. For the major isomer the amino group of the second ligand occupies an axial position, while the fourth equatorial donor atom is the peptide oxygen. For the minor isomer the donor atoms of the second ligand are interchanged. For the 1 : 1 complexes also the above terdentate equatorial coordination of the ligand exists, except for [CuL]+. Some features of the metal–ligand bonds and geometric distortions for the various species are discussed. Copyright © 1999 John Wiley & Sons, Ltd.