Molecular Modeling Study of Chiral Separation and Recognition Mechanism of beta-Adrenergic Antagonists by Capillary Electrophoresis

Chiral separations of five beta-adrenergic antagonists (propranolol, esmolol, atenolol, metoprolol, and bisoprolol) were studied by capillary electrophoresis using six cyclodextrins (CDs) as the chiral selectors. Carboxymethylated-beta-cyclodextrin (CM-beta-CD) exhibited a higher enantioselectivity power compared to the other tested CDs. The influences of the concentration of CM-beta-CD, buffer pH, buffer concentration, temperature, and applied voltage were investigated. The good chiral separation of five beta-adrenergic antagonists was achieved using 50 mM Tris buffer at pH 4.0 containing 8 mM CM-beta-CD with an applied voltage of 24 kV at 20 degrees C. In order to understand possible chiral recognition mechanisms of these racemates with CM-beta-CD, host-guest binding procedures of CM-beta-CD and these racemates were studied using the molecular docking software Autodock. The binding free energy was calculated using the Autodock semi-empirical binding free energy function. The results showed that the phenyl or naphthyl ring inserted in the hydrophobic cavity of CM-beta-CD and the side chain was found to point out of the cyclodextrin rim. Hydrogen bonding between CM-beta-CD and these racemates played an important role in the process of enantionseparation and a model of the hydrogen bonding interaction positions was constructed. The difference in hydrogen bonding formed with the -OH next to the chiral center of the analytes may help to increase chiral discrimination and gave rise to a bigger separation factor. In addition, the longer side chain in the hydrophobic phenyl ring of the enantiomer was not beneficial for enantioseparation and the chiral selectivity factor was found to correspond to the difference in binding free energy.