(437c) Mechanistic Studies of Chiral Recognition of Solutes by Amylose Tris[(S)-á-Methylbenzylcarbamate]

This amylose-based polymeric sorbent has the ability to separate a variety of chiral enantiomers (R and S), such as ethyl lactate, methyl mandelate, pantolactone, and benzoin.* These molecules contain OH and C=O functional groups, which may interact with the C=O and NH groups of the sorbent side chains via hydrogen (H-) bonding interactions. From the observed retention factors k_R and k_S and enantioselectivities (S= k_R/ k_S), as determined with High Performance Liquid Chromatography (HPLC), it was inferred that the molecular recognition of these molecules mechanisms were quite similar. Infrared spectroscopy (IR) data for polymer-solute and Density Functional Theory (DFT) simulations of the interactions of these solutes with the side chains of the polymer led to the following hypothesis for the chiral recognition mechanism. A strong H-bond forms between the solute OH groups and the sorbent C=O groups for both enantiomers as the primary interaction “anchor” point. Then a second H-bond forms between the solute C=O groups and the sorbent NH groups only for the R-enantiomer of the above four solutes. The S-enantiomer is prevented sterically from forming such a bond. Molecular Dynamics (MD) simulations were done for a 12-mer polymer model to determine the polymer structure, intrapolymer H-bonding, free functional groups, available for binding with the solute, and the geometry of the potentially enantioselective cavities. Monte Carlo (MC) and MD “docking” simulations were done to investigate the interactions of the benzoin enantiomers with the polymer. MD simulations were also done for the other solutes. The simulations are generally consistent with the above hypothesis, and they reveal the specific cavities which may allow enantioselective interactions. The results showed that S is lower as the solute molecular flexibility, as defined from the molecular geometric torsion angles distribution, increases. The results are useful in predicting how S varies with the solute molecular structure, and for selecting sorbents for specific chiral separations.

* H.-W. Tsui, J.N. Willing, R.B. Kasat, N.-H.L. Wang, E.I. Franses, J. Phys. Chem. B, 2011, 115, 12785-12800.