Enantioseparation and Chiral Recognition of -Cyclohexylmandelic Acid and Methyl -Cyclohexylmandelate on Hydroxypropyl- -Cyclodextrin as Chiral Selector: HPLC and Molecular Modeling

Enantioseparations of (R/S)-α-cyclohexylmandelic acid [(R/S)-CHMA] and methyl (R/S)-α-cyclohexylmandelate [(R/S)-MCHMA] were performed on an achiral column (ODS) with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) as a chiral mobile phase additive. The influences of chromatographic conditions on the retention behavior of (R/S)-CHMA and (R/S)-MCHMA were studied in detail. Meanwhile, the thermodynamics parameters of enantioseparations for (R/S)-CHMA and (R/S)-MCHMA were determined to discuss driven power in the enantioseparation process. The inclusion complexation of HP-β-CD with each enantiomer for (R/S)-CHMA and (R/S)-MCHMA was simulated by molecular docking to understand the chiral recognition mechanism of (R/S)-CHMA and (R/S)-MCHMA on HP-β-CD. The results showed that the chiral recognition ability of enantiometers of (R/S)-CHMA and (R/S)-MCHMA on HP-β-CD is better than α-CD, β-CD, γ-CD and DM-β-CD. Under the selected chromatographic conditions, baseline separations of enantiomers of (R/S)-CHMA and (R/S)-MCHMA were achieved. It is proved that the stoichiometry for (R/S)-CHMA-HP-β-CD and (R/S)-MCHMA-HP-β-CD complexes is 1:1. However, the results of thermodynamics parameters analysis and molecular modeling show that the enantioseparations of CHMA and MCHMA on HP-β-CD are enthalpy-driven processes and the primary driving forces responsible for chiral recognition are hydrophobic forces, dipole-dipole interaction, charge-transfer and hydrophobic interaction.