Coordination selectivity in the mixed-ligand formation of copper(II)glycyl-DL-serine with some amino acids

Abstract

Binary and ternary complexes of Cu(II) with dipeptides containing amide carbonyl, alcoholic hydroxy and carboxy groups in the side chain have been studied. The peptide (X ligand) was glycyl-DL-sirene (gly-ser) and one of the amino acids, glycine, alanine, valine, threonine, serine, asparagine, tyrosine, aspartic and glutamic acids, was used as Y ligand. In the mixed-ligand complexes containing aspartic acid, it has been pointed out that the amino acid occupies one equatorial and one axial site in the coordination sphere of the copper(II) and at higher pH the carboxylate oxygen of the peptide glycyl-DL-serine is replaced by the deprotonated alcoholate (O −). Considerable attention has been paid in recent years to the coordination behaviour of dipeptides in their copper(II) binary and ternary complexes [1, 2]. The main interest is the influence of side chain donor groups on complex formation [3, 4]. Martin and co-workers [5] have established that in basic solution the side-chain amide of glycyl-asparagine undergoes hydrogen ionization, followed by amide nitrogen bonding to copper(II). No such effect was observed for the alcoholate group of glycyl-DL-serine. Glycyl-DL-serine, (glycyl-2-amino-3-hydroxy propionic acid), ▪ is of interest for complexation studies, as it contains five bonding sites, thus exhibiting a number of possibilities for the formation of complexes of different bonding types. On the basis of our previous studies [6] and in an attempt to clarify the nature of dipeptide–cation interaction, we have studied the mixed-ligand complexes of copper(II)dipeptidesamino acids [7–11], particularly the interaction of the aromatic group with metal ion glycyl-phenyl alanine complexes [7]. We undertook this work to study the effect of the hydroxy group on the complexation equilibria and the mode of binding of glycyl-serine in binary and ternary complex species.