An ESR study of co-ordination modes in copper(II) complexes of l-serine in aqueous solution at ligand excess above pH 7

Abstract

Co-ordination modes for the bis complexes of l-serine with copper(II) ion have been studied in fluid aqueous solution by ESR spectroscopy. The computer analysis of a series of overlapping spectra taken in neutral and alkaline solutions was carried out including one to four independent species. For the component spectra a hyperfine splitting by isotopes 63Cu and 65Cu and two equivalent nitrogen atoms were taken into consideration. The isotropic ESR parameters and relative concentrations of different species were optimized. The distribution of copper(II) amongst different species is in accordance with literature formation constants obtained by pH-metric and spectrophotometric studies. For both the [CuL 2] and [CuL 2H −1] − complexes two kinds of co-ordination exist. The spectra of smaller copper and nitrogen hyperfine coupling constants, which give the major contribution to the experimental curves, are assigned to the cis isomer, while the minor species is the trans isomer of higher hyperfine coupling constants. For the complex [CuL 2H −2] 2− only one mode of co-ordination exists with cis arrangement of the equatorial N and O donor atoms, respectively. The values of ESR parameters indicate the equatorial binding of deprotonated OH groups of side chains and a geometric distortion of the cis complexes.