Effects of backbone and side chain on the molecular environments of chiral cavities in polysaccharide-based biopolymers.

The effects of the backbone and side chain on the molecular environments in the chiral cavities of three commercially important polysaccharide-based chiral sorbents--cellulose tris(3,5-dimethylphenylcarbamate) (CDMPC), amylose tris(3,5-dimethylphenylcarbamate) (ADMPC), and amylose tris[(S)-alpha-methylbenzylcarbamate] (ASMBC)--are studied by 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 density functional theory (DFT) modeling. These sorbents are used widely in preparative-scale chiral separations. ATR-IR is used to determine how the H-bonding states of the C=O and NH groups of the polymer depend on 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 and dimers and the side chains are predicted at 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.