An ESR study of co-ordination modes in copper(II) complexes of 1-serine inaqueous solution at ligand excess above pH 7

Co-ordination modes for the bis complexes of 1-serine with copper(II) ion h ave been studied in fluid aqueous solution by ESR spectroscopy. The compute r analysis of a series of overlapping spectra taken in neutral and alkaline solutions was carried out including one to four independent species. For t he component spectra a hyperfine splitting by isotopes Cu-63 and Cu-65 and two equivalent nitrogen atoms were taken into consideration. The isotropic ESR parameters and relative concentrations of different species were optimi zed. The distribution of copper(II) amongst different species is in accorda nce with literature formation constants obtained by pH-metric and spectroph otometric studies. For both the [CuL2] and [CuL2H-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 c urves, are assigned to the cis isomer, while the minor species is the trans isomer of higher hyperfine coupling constants. For the complex [CuL2H-2](2 -) only one mode of co-ordination exists with cis arrangement of the equato rial N and O donor atoms, respectively. The values of ESR parameters indica te the equatorial binding of deprotonated OH groups of side chains and a ge ometric distortion of the cis complexes. (C) 1999 Elsevier Science Ltd. All rights reserved.