Mixed Ligand Copper(II) Complexes of α-Amino Acids with Ligand-Ligand Interactions

Synthetic and spectroscopic studies have been made on the ternary copper(II) complexes containing two optically active α-amino acids with charged groups in their side chains. Thus, for all combinations of ligands A and B, where A refers to aspartic or glutamic acid and B to arginine, lysine, or ornithine, the mixed ligand complexes, [Cu(l-A)(l-B)] and [Cu(d-A)(l-B)] except [Cu(l-aspartate)(l-ornithine)], were isolated as crystals. Infrared spectra of the isolated complexes suggested the presence of the geometrical isomers. By comparing the observed circular dichroism spectra of the ternary systems with the spectra estimated from those of the corresponding binary systems, the magnitude enhancements were detected for all the Gu(l-A)(l-B) systems, whereas no such enhancement was observed when one of the ligands was l-alanine or l-valine. The spectral behaviors as well as the solid complexes isolated have been interpreted as indicative of the existence of the intramolecular electrostatic bonding between the oppositely charged groups in the side chains of the ligands and their significance as an effective driving force leading to the ternary complex formation.