(523f) IR Spectroscopy and Molecular Simulation Studies of Interactions of a Polymeric Sorbent with Chiral Enantiomers
The amylose tris(S)-α-methylbenzylcarbamate (ASMBC) polymer is a widely used polymer for chiral separations. Among several chiral molecules tested, benzoin racemate mixtures can be well separated with ASMBC, with 10 vol. % isopropanol-hexane mobile phase at 25℃. The retention factors were 6.07 (R-benzoin) and 2.16 (S-benzoin), respectively. The enantioselectivity of 2.81 is substantial. Benzoin contains one asymmetric carbon, one carbonyl C=O group, one hydroxyl OH group, and two phenyl groups. The polymer side chains contain one amide group (O=C-NH), one phenyl group and one methyl group. To elucidate the molecular recognition of the benzoin enantiomers, the IR spectra of the polymer and polymer-benzoin mixtures were obtained. DFT simulations (DFT/B3LYP/ 6-311G+d,p level of theory) were done to predict the structure, energy, and the IR spectra of cis-benzoin and trans-benzoin conformations. The first one is the most stable and shows an intra-molecular hydrogen bond (H-bond) in non-polar environments. DFT simulations were also done to predict the structure and IR spectra of ASMBC side chain, two side chains interacting with each other, and one or two side chains interacting with R- or S- benzoin. The results were used to interpret changes in the IR spectra of the polymer and of benzoin, and give a molecular picture of their interactions. Then, molecular mechanics and molecular dynamics (MM/MD) simulations of benzoin enantiomers ?docking in?, or interacting with, polymer nano-scale cavities were done, to predict the complex interactions which are responsible for the chiral recognition. The MD simulations predict various types of H-bond strengths in the polymer amide groups. Some of these bonds may break, and new H-bonds may form with each of the benzoin enantiomers. The results indicate that the R-enantiomer has stronger H-bonding interactions with the polymer. Moreover, the phenyl groups of each benzoin enantiomer have a different molecular microenvironment, which reflects their different interactions. The results are tested in terms of the three-point attachment model of chiral discrimination.