Chiral discrimination by ligand-exchange chromatography: a comparison between phenylalaninamide-based stationary and mobile phases

The copper(II) complexes of two new diastereomeric ligands, N2-(R)- and N2-(S)-2'-hydroxypropyl-(S)-phenylalaninamide [(R,S)-1 and (S,S)-1], were used as additives to the eluent in HPLC reversed phase for the chiral sepn. of DNS-amino acids. The aim was that of comparing the sepn. process obtained by the chiral eluent with that obtained by an analogous bonded stationary phase contg. (S)-phenylalaninamide, previously studied [CSP-(S)-Phe-NH2]. The affinity of the ternary complexes for the C18 column was detd. by adsorption expts. in HPLC. The two systems (chiral eluent, chiral stationary phase) work according to different mechanisms. Ternary complex formation in soln. was studied by fluorescence spectroscopy. Chiral sepn. with the Cu(II) complexes added to the eluent was detd. by the relative affinities of the ternary complexes for the column-stationary phase rather than by their stabilities in soln. With CSP-(S)-Phe-NH2 the sepn. is accounted for by the relative stabilities of the ternary complexes, which depends mainly on the allowed geometry of the complex and on the steric repulsion of the amino acid side chain with the spacer.