CHIRAL DISCRIMINATION BY LIGAND-EXCHANGE CHROMATOGRAPHY - A COMPARISON BETWEEN PHENYLALANINAMIDE-BASED STATIONARY AND MOBILE PHASES

The copper(II) complexes of two new diastereomeric ligands, N-2-(R)- a nd N-2-(S)-2'-hydroxypropyl-(S)-phenylalaninamide [R,S)-1 and (S,S)-1] , have been used as additives to the eluent in high-performance liquid chromatography (HPLC) reversed phase for the chiral separation of DNS -amino acids. The aim was that of comparing the separation process obt ained by the chiral eluent with that obtained by an analogous bonded s tationary phase containing (S)-phenylalaninamide, previously studied [ CSP-(S)-Phe-NH2]. The affinity of the ternary complexes for the C-18 c olumn was determined by adsorption experiments in HPLC. It was shown t hat the two systems (chiral eluent, chiral stationary phase) work acco rding to different mechanisms. Ternary complex formation in solution w as studied by fluorescence spectroscopy. It was shown that chiral sepa ration with the Cu(II) complexes added to the eluent was determined by the relative affinities of the ternary complexes for the column-stati onary phase rather than by their stabilities in solution. With CSP-(S) -Phe-NH2 the separation is accounted for by the relative stabilities o f the ternary complexes, which depends mainly on the ''allowed'' geome try of the complex and on the steric repulsion of the amino acid side chain with the spacer. (C) 1996 Wiley-Liss, Inc.