Influence of amino acid moiety accessibility on the chiral recognition of cyclodextrin–amino acid mixed selectors in enantioselective gas chromatography

Abstract

Original mixed selectors were synthesized by coupling a single l-valine diamide moiety on permethylated β-cyclodextrin. The structures of the new selectors were designed to limit the interactions between the l-valine derivative and cyclodextrin by removing the amino acid moiety from the cyclodextrin cavity by means of an amide linkage on mono-6-amino permethylated β-CD or the insertion of a carboxymethyl group. The accessibility of the amino acid group moiety was thus facilitated. The new mixed selectors exhibited better enantioselectivity than Chirasil-l-Val for half (selector based on mono-6-amino permethylated β-CD) or more (selector with the carboxymethyl group) of the 41 amino acid derivatives. Molecular modeling confirmed that these results could be attributed to an increase in the distance between the chiral center of the amino acid and the cyclodextrin cavity allowing better access of the amino acid moiety. These new mixed chiral selectors demonstrated a novel enantioselective capability with the successful separation of more than 90 racemic mixtures among the 105 chiral compounds tested. These mixed selectors exhibited enhanced enantioselectivity in comparison to binary selectors previously described with respect to both enantiomer resolution and the number of separated chiral compounds. Moreover, an improvement of the enantioseparation factors compared to the corresponding ‘parent phases’ for the amino acid derivatives was observed in many cases. These mixed selectors should therefore be considered some of the most versatile selectors for chiral gas chromatography.