Ligand-receptor binding increments in enantioselective liquid chromatography

Abstract

A set of N-derivatized amino acids were separated into enantiomers on a tert-butylcarbamoylated quinine-based chiral stationary phase (CSP). Quantitative structure-property relationship (QSPR) studies were then employed to investigate the retention behavior and factors responsible for enantioselectivity. Computations were performed using a general linear model and a Free-Wilson matrix with indicator variables as structural descriptors. The approach allowed calculations of retention increments for first and second eluted enantiomers as well as group contributions to enantioselectivity. The results demonstrated that the additivity principle of group contributions was obeyed for the majority of solutes in the data set. Only a few basic amino acids (Arg, His) needed to be removed as they did not fit to such a linear model leading to outliers. The model was carefully validated and then utilized to investigate retention and enantioselectivity contributions of different protection groups and individual amino acid residues. It turned out that primarily protection groups were driving retention and enantioselectivity. In contrast, the contribution of amino acid residues to enantioselectivity was only significant for secondary amino acids, α-methylated amino acids, aspartic acid and a few sterically bulky aliphatic amino acid residues (Tle, Ile, allo-Ile). Amongst them only the latter group contributed positively to enantioselectivity while the other residues mentioned reduced enantioselectivity significantly. This type of QSPR model may be valuable to analyze retention/selectivity data of closely related congeneric compound series, is illustrative and straightforward to implement. It is thus valuable for interpretation of retention mechanisms, while its utility for prediction of retention and enantioselectivity data is limited to compounds made up of groups included in the solute set used for deriving the increments.

Keywords: Chiral separation; Chiral stationary phase; Free-Wilson analysis; N-Derivatized amino acids; Quantitative structure–property relationships; Quinine carbamate.