Explanation of where and how enantioselective binding takes place on permethylated beta-cyclodextrin, a chiral stationary phase used in gas chromatography

A computational study was undertaken to discern where and how chiral alkanes, alcohols, and acetates enantioselectively bind to permethylated beta-cyclodextrin, the most commonly used chiral stationary phase in gas chromatography. We found that enantioselective binding data could be reproduced with standard molecular dynamics techniques if averages are taken over multiple trajectories of nanosecond simulation times each, while Metropolis Monte Carlo simulations using rigid body molecules are unable to reproduce chromatographic retention orders. Data extracted from the molecular simulations revealed the preferred binding site for small analytes to be the interior of the macrocycle, with rapid shuttling between the primary and secondary rims and low-energy excursions into and out of the host cavity. The dominant forces holding the host-guest complexes together are the short range dispersion forces. The enantiodiscriminating forces responsible for chiral recognition are also the short range van der Waals forces and these enantiodifferentiating forces are typically 1-2 orders of magnitude smaller than the binding forces. An assessment of the number of hydrogen bonds for the diastereomeric complexes is presented along with the locations of dominant hydrogen-bonding sites on the macrocycle. A comparison is made between analytes capable of intramolecular hydrogen bonding with those that can not. It is pointed out that the 3-point binding description of chiral discrimination can be used, but it loses its appeal at such high temperatures due to ill-defined structures.