Infrared Spectroscopy (IR) and Molecular Simulations of Polymeric Sorbent and Its Enantioselective Interactions with Benzoin Enantiomers

Derivatized amylose or cellulose polymeric sorbents show substantial interactions and enantioselectivities (S) for a variety of solutes. These are important for effective chromatographic separations. In this presentation we focus on understanding the substantial enantioselectivity observed for benzoin (B) enantiomers with Amylose Tris(S)-α-methylbenzylcarbamate, or AS. Retention factors (k_R and k_S) and enantioselectivities (S ≡ k_R/k_S) were measured for various isopropanol/n-hexane compositions of the mobile phase, and with pure n-hexane, for which k_R= 106, k_S= 49.6, for the two enantiomers, and S= 2.13. IR (infrared) spectra showed evidence of substantial hydrogen or H-bond interactions in the pure polymer, and additional H-bond interactions between AS and B.

DFT (Density Functional Theory) simulations (6-311+g(d,p) basis set, B3LYP lever of theory) were used to model the chain-chain interactions and chain-benzoin interactions. They were also used to predict fairly well the shifts in the IR wavenumbers caused by the H-bonds. Then MD (Molecular Dynamics) simulations, using the cvff (Consistent Valence Force Field), from the Material Studio software, were used to model a single 12-mer helical polymer chain. The predicted polymer structure shows a range of H-bonding strengths which are comparable to the ones inferred from IR spectral analysis. MD simulations predict the existence of various potentially enantioselective cavities, two of which are sufficiently large to accommodate a benzoin molecule. Then “docking” studies with MD or MC (Monte Carlo) simulations were done to probe AS-B interactions. Even though these simulations do not account for inter-polymer interactions, they predict a substantial enantioselectivity for one cavity. This enantioselectivity is due to two H-bonds, of the kind (AS) CO … HO (R-benzoin) and (AS) NH … OC (R-benzoin), and two π- π interactions for R-benzoin , and one H-bond, (AS) CO … HO (S-benzoin), and one π- π interaction for S-benzoin. The effect of absorbed n-hexane on the structure of the polymer was predicted to be negligible. Thus, these simulations can account qualitatively of the observed molecular recognition of the benzoin enantiomers.