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

Cu(II) complexes of L-amino acid amides added to the eluent in reversed-phase HPLC are able to perform chiral discrimination of unmodified amino acids with high enantioselectivity. The mechanism is consistent with a ligand exchange between the binary initial Cu(II) complex and the enantiomers. Evidence is provided that the exchange of the ligand is actually occurring during the chromatog. sepn. The system involves equil. of exchange in the aq. soln., in the stationary phase and between the 2 phases. Enantioselectivity is essentially due to the adsorption of the diastereomeric ternary species on the column, whereas the relative stabilities of the mixed complexes in the mobile phase seem to be negligible with respect to the overall discrimination process. The structural features of the initial Cu complexes greatly affect the stereoselectivity of the process. The chromatog. parameters (pH, selector concn., eluent polarity, ionic strength) are examd.