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

Abstract

Five chiral stationary phases were prepared by coating the surface of porous graphitic carbon with a series of N-substituted l-proline chiral selectors. The N-substituents served as anchor molecules for immobilization of chiral selectors on the support material. The effect of the alkyl (C 7, C 9, C 12) and aryl (naphthylmethyl, anthrylmethyl) anchor molecules on retention and enantioselectivity were studied using thirty-six amino acid enantiomers as probe compounds. The surface concentrations of the chiral selectors, determined using the breakthrough method, were found to be in the range 0.39–0.68 μmol m −2. The coated PGC phases all 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 chain length of the alkyl N-substituents resulted in improved enantioselectivity whereas the retention decreased. However, with the aryl N-substituents, both the retention and enantioselectivity decreased with an increase in the ring number of the aryl substituent. The retention order 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 the involvement of the N-substituent in intramolecular hydrophobic interactions responsible for the observed chiral recognition.