Studies of Interactions of an Amylose-Based Sorbent with Various Solvents for Chiral Separation Applications
Rahul B. Kasat, Yury Zvinevich, Hugh W. Hillhouse, Kendall T. Thomson, Nien-Hwa Linda Wang, and Elias I. Franses. School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907-2100
Polysaccharides based chiral stationary phases can successfully resolve almost 95% of the racemic compounds (e.g. chiral drugs, pesticides, etc.) and are used commercially for analytical and preparative scale chiral separations. The recognition mechanism of these phases is yet not clearly understood. One of the important steps in understanding this mechanism in chiral chromatography is establishing the effects of various organic solvents on the structures of these biopolymers. In the present study, the sorbent-solvent interactions for amylose tris(3, 5-dimethylphenylcarbamate) (ADMPC) with five commonly used polar and non-polar solvents are studied using attenuated total reflection infrared spectroscopy (ATR-IR) of thin sorbent films, X-ray diffraction (XRD) of thin films, 13C cross polarization/magic angle spinning (CP/MAS) and MAS solid state NMR of polymer-coated silica beads (commercially termed “Chiralpak AD”), and DFT modeling. The polymer forms helical rods with various nm-sized cavities formed between the polymer side-chains and rods. The changes in the H-bonding states of the C=O and NH groups of the polymer upon absorption of each of the five solvents at 25 ºC are determined with ATR-IR. The IR wavenumbers, the H-bonding interaction energies, and the H-bonding distances of the polymer side-chains with each of the solvent molecules are predicted using the DFT/B3LYP/6-311+g(d,p) level of theory. The changes in the polymer crystallinity upon absorption of each solvent are characterized with XRD. The changes in the polymer crystallinity and the H-bonding states of C=O groups are also probed with 13C CP/MAS solid-state NMR. The changes in the polymer side-chain mobility are detected using 13C MAS solid-state NMR. The H-bonding states of the polymer change upon absorption of each polar solvent and usually result in an increase in the polymer crystallinity and the side-chain mobility. The polymer rods are reorganized upon solvent absorption, and the distance between the rods increases with the increase in the solvent molecular size. These results may find significant applications in the solvent selection in chiral chromatography of enantiomers and have implications for understanding the selectivity, retention times, and sometimes observed changes in the elution order.