CHIRAL SEPARATION OF UNMODIFIED AMINO-ACIDS BY LIGAND-EXCHANGE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY USING COPPER(II) COMPLEXES OF L-AMINO-ACID AMIDES AS ADDITIVES TO THE ELUENT

Copper(II) complexes of L-amino acid amides added to the eluent in rev ersed-phase high-performance liquid chromatography are able to perform chiral discrimination of unmodified amino acids with high enantiosele ctivity. The mechanism is consistent with a ligand exchange between th e binary initial copper(II) complex and the enantiomers. Evidence is p rovided that the exchange of the ligand is actually occurring during t he chromatographic separation. The system involves a series of equilib ria of exchange in the aqueous solution, in the stationary phase and b etween the two phases. Enantioselectivity is essentially due to the ad sorption of the diastereomeric ternary species on the column, whereas the relative stabilities of the mixed complexes in the mobile phase se em to be negligible with respect to the overall discrimination process . The structural features of the initial copper complexes greatly affe ct the stereoselectivity of the process. The chromatographic parameter s (pH, selector concentration, eluent polarity, ionic strength) are ex amined.