ROLE OF ALKYL AND ARYL SUBSTITUENTS IN CHIRAL LIGAND-EXCHANGE CHROMATOGRAPHY OF AMINO-ACIDS - STUDY USING POROUS GRAPHITIC CARBON-COATED WITH N-SUBSTITUTED-L-PROLINE SELECTORS

Five chiral stationary phases were prepared by coating the surface of porous graphitic carbon with a series of N-substituted L-proline chira l selectors. The N-substituents served as anchor molecules for immobil ization of chiral selectors on the support material, The effect of the alkyl (C-7, C-9, C-12) and aryl (naphthylmethyl, anthrylmethyl) ancho r molecules on retention and enantioselectivity were studied using thi rty-six amino acid enantiomers as probe compounds. The surface concent rations of the chiral selectors, determined using the breakthrough met hod, were found to be in the range 0.39-0.68 mu molm(-2). The coated P GC phases ail showed appreciable enantioselectivity for both non-polar and acidic amino acids with the naphthylmethyl-L-proline showing the greatest overall values. The N-substituents were shown to have strong influence on retention and enantioselectivity. An increase in the chai n length of the alkyl N-substituents resulted in improved enantioselec tivity whereas the retention decreased. However, with the aryl N-subst ituents, both the retention and enantioselectivity decreased with an i ncrease in the ring number of the aryl substituent. The retention orde r for the majority of the amino acids examined was the same on all the columns, that is D>L. These behaviours were interpreted in terms of t he involvement of the N-substituent in intramolecular hydrophobic inte ractions responsible for the observed chiral recognition. (C) 1997 Els evier Science B.V.