(403d) Characterization of Polysaccharide Based Chiral Sorbents Using Infrared Spectroscopy, X-Ray Diffraction, 13C Solid State NMR, and Density Functional Theory

Polysaccharide based chiral sorbents are used widely in preparative scale chiral separations. The chiral recognition mechanisms of these sorbents are not yet completely elucidated. These polymers form helical rods with intra- and inter-rod hydrogen bonds (H-bonds). There are various nm-sized cavities formed between the polymer side-chains and rods. In the present work, we study the effects of the molecular structures of their backbone and side-chain on the molecular environments in the chiral cavities of three important sorbents, cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC), amylose tris(3,5-dimethylphenylcarbamate) (ADMPC), and amylose tris((S)-á-methylbenzylcarbamate) (ASMBC), using attenuated total reflection infrared spectroscopy (ATR-IR), X-ray diffraction (XRD), 13C cross polarization/magic angle spinning (CP/MAS) and MAS solid state NMR, and DFT modeling [1]. ATR-IR is used to determine how the H-bonding states of the C=O and NH groups of the polymer depend on of the backbone and side-chain. The changes in the polymer crystallinity are characterized with XRD. The changes in the polymer helicity and molecular mobility for polymer-coated silica beads (commercially called Chiralcel OD, Chirapak AD, and Chiralpak AS) are probed with 13C CP/MAS and MAS solid-state NMR. The IR wavenumbers and the NMR chemical shifts for the polymer backbone monomers, dimers, and the side-chains are predicted using the DFT/B3LYP/6-311+g(d,p) level of theory. It is concluded that the molecular environments of the C=O, NH, and phenyl groups show significant differences in intramolecular and intermolecular interactions and in the nanostructures of the chiral cavities of these biopolymers. These results have implications for understanding how the molecular environments of chiral cavities of these polymers affect their molecular recognition mechanisms.

[1] Kasat, R. B.; Wang, N. H. L.; Franses, E. I. Biomacromolecules, 8, 2007, 1676-1685.