Chiral Chromatography of amino acids on porous graphitic carbon coated with a series of N-substituted L-phenylalanine selectors - Effect of the anchor molecule on enantioselectivity

Six chiral stationary phases were prepared by coating the surface of porous graphitic carbon (PGC) with a series of N-substituted L-phenylalanine 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 (methoxybenzyl, naphthylmethyl, anthrylmethyl) anchor molecules on retention and enantioselectivity was studied using 36 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 0.55-1.26 mu mol/m(2). The coated PGC phases all showed appreciable enantioselectivity for both non-polar and acidic amino acids, but basic amino acids were predominantly unretained. The order of retention of a pair of amino acid enantiomers was L>D on the alkyl-L-phenylalanine phases but a reversed retention order (D>L) was observed on the aryl-substituted L-phenylalanine phases. The chromatographic properties of each of the chiral stationary phases were compared in terms of retention order, enantioselectivity and diastereomeric complex stability. The reversal of the elution order on these two types of phases is discussed in relation to competitive intramolecular interactions involved in transient diasteriomeric complexes.