Theoretical study on chiral recognition mechanism of methyl mandelate enantiomers on permethylated β-cyclodextrin.

Host-guest interactions of permethylated β-cyclodextrin (PM-β-CD) with methyl mandelate enantiomers ((R/S)-MMA) were simulated using semiempirical PM3 and ONIOM (B3LYP/6-31G(d):PM3) method. The chiral recognition mechanism of (R/S)-MMA enantiomers on PM-β-CD was investigated. The binding energies for all orientations considered in this research are reported. The most stable geometry structures of the two complexes are different. The benzene ring of (R)-MMA locates horizontally approximately on the wider edge of the PM-β-CD cavity, but the aromatic ring of (S)-MMA is deeply included into the hydrophobic cavity. Furthermore, the results of NBO analysis show that the main driving forces in the inclusion process of PM-β-CD with (R/S)-MMA are hydrogen bonding interaction, dipole-dipole interaction, charge-transfer and hydrophobic interaction. The stabilization energy of the (R)-MMA/PM-β-CD complex is lower than that of the (S)-MMA/PM-β-CD complex. Moreover, the chiral carbon in MMA of (R/S)-MMA/PM-β-CD complexes are close to the C2 and C3 in the glucose unit. The chiral recognition mechanism is thus closely related to the chiral environment provided by C2 and C3 in the glucose unit and the degree of (R/S)-MMA and PM-β-CD inclusion.