Stereoselective Formation of Ternary Copper(II) Complexes of (S)-amino-acid amides and (R)- or (S)-amino acids in aqueous solution

Formation constants of ternary complexes of Cu^II with (S)-amino-acid amides (phenylalaninamide, prolinamide, and tryptophanamide) and (R)- or (S)-amino acids (valine, phenylalanine, proline, and tryptophan) were determined potentiometrically at 25° and I = 0.1M (KC1). Significant stereoselectivity was found for the systems (S)-tryptophanamide/proline, (S)-prolinamide/tryptophan, and (S)-phenylalaninamide/proline, the diastereoisomeric complexes with ‘homochiral’ (SS) being more stable than with ‘heterochiral’ (SR) ligands. The stereoselectivity observed may be explained on the basis of repulsive interactions between the ligand side-chain residues. The present data on the stability of mixed complexes in solution allow to draw some conclusions on the mechanism of chiral discrimination of amino acids in HPLC (reversed-phase) using Cu^II complexes of (S)-amino-acid amides as selectors for ligand-exchange chromatography (LEC).