Effect of oligomer chain length and substituent configuration on the enantioselectivity of a maltooligosaccharide chiral stationary phrase for HPLC

Abstract

The effect of naphthylethylcarbamate (NEC) substituent configuration on the enantioselectivity of maltooligosaccharide-based chiral stationary phases (CSPs) was investigated by preparing bonded phases derivatized with (R)-, (S)-, and racemic l-(l-naphthyl)ethyl isocyanate. Various 3,5-dinitrobenzoyl derivatized amines and amino acid esters, as well as 3,S-dinitrophenylcarbamoylated alcohols, were utilized as analytes. In most instances, retention was found to correlate with the surface concentration of the bonded ligands. The best enantioselectivity typically occurred on the S-NEC CSP. Elution order of the enantiomers provided an indication of the importance of carbamate substituent configuration in chiral recognition. The chiral carbohydrates were also found to contribute to the chiral selectivity of the CSP. The effect of mobile phase and derivatizing agent on the chromatographic behavior of the NEC CSPs was also evaluated. An isocyanate derived from (S)-6-methoxy-α-methyl-2-naphthaleneacetic acid, via an azide intermediate, was evaluated as a derivatizing agent and found to produce a CSP with comparable selectivity and superior resolution relative to the NEC CSPs. In order to gain insight into the nature of the bonded maltooligosaccharide mixture, derivatized bonded phases were prepared using maltotriose, maltopentaose, and maltoheptaose as the carbohydrate ligand. Surface concentrations of the bonded glucose oligomers were found to decrease as the carbon chain length of the oligomer increased. In general, retention, selectivity, and resolution diminished as the oligomer chain lengthened. Comparison of the selectivities of the derivatized glucose oligomer CSPs to those of a derivatized maltooligosaccharide CSP provided a measure of the relative effectiveness of the maltooligosaccharide mixture as a chiral selector.