Stereoselective formation of ternary copper(II) complexes of (S)-amino-acid amides and (R)- or (S)-histidine and (R)- or (S)-tyrosine in aqueous solution

Formation constants of ternary complexes of Cu-II with (S)-amino-acid amides ((S)-phenylalaninamide, (S)-prolinamide, and (S)-tryptophanamide) and (R)- or (S)-histidine and (R)- or (S)-tyrosine were determined potentiometrically in aqueous solution. Significant stereoselectivity was presented by all three amides towards histidine, the diastereoisomeric complexes with 'heterochiral' ligands being more stable than those with 'homochiral' ligands (see Table 3). The stereoselectivity observed with (S)-phenylalaninamide and (S)-tryptophanamide may be explained on the basis of hydrophobic slacking interactions between 1H-imidazole and the aromatic side chain, favoured by the terdentate behaviour of histidine (see Fig. 2), whereas repulsive effects seem to be prevalent with (S)-prolinamide. Only (S)-prolinamide and (S)-phenylalaninamide show appreciable stereoselectivity with tyrosine, which is bidentate, probably on account of repulsive interactions. The present results on the stability of ternary complexes in solution allow to draw some conclusions on the mechanism of chiral discrimination performed by Cu-II complexes of (S)-amino-acid amides added to the mobile phase in HPLC (reversed phase).