Separations of Chiral Pharmaceuticals: Direct Experimental Probing and Molecular Modeling for Elucidating the Molecular Recognition Mechanism of Amylose tris(3,5- dimethylphenylcarbamate)

摘要

The interactions of a commercially important polysaccharide-based chiral stationary phase, amylose tris(3,5-dimethylphenylcarbamate) (ADMPC) with two chiral solutes, norephedrine or 2-amino-1-phenyl-1-propanol (PPA) and 1-phenyl-1-propanol (PP), are studied in detail using HPLC, attenuated total reflection infrared spectroscopy (ATR-IR), X-ray diffraction (XRD) and molecular simulations. The HPLC capacity factors in hexane/IPA (90/10, v/v) at 25 °C of enantiomers of PPA and PP vary significantly with this sorbent, and the selectivities of PPA vs. PP with ADMPC are 2.4 and 1.0, respectively. The changes observed in the wavenumbers and the intensities of the amide I and amide II bands of ADMPC polymer in the ATR-IR spectra upon absorption of each enantiomer are stereo-specific and correlate with the HPLC selectivities. The IR wavenumbers, and the H-bonding interaction energies of the polymer side chain with each enantiomer in different configurations, predicted using the density functional theory (DFT/B3LYP/6-311+g(d,p) level of theory), are in agreement with the IR and HPLC results. XRD results show that the polymer crystallinity changes significantly upon absorption of each enantiomer. The helical pitch and the inter-rod packing for this polymer inferred from the XRD results are used in molecular dynamics (MD) simulations. The elution orders predicted for PPA and PP using MD simulations agree with the chromatography results. Based on IR, DFT, and MD simulations, it is concluded that the main binding sites of this polymer are the C=O, NH, and Ph groups of its side chain. The H-bonding strength of the C=O and NH groups of the polymer increases significantly upon absorption of each enantiomer and results in an increase in the polymer crystallinity. The significant selectivity observed in ADMPC for PPA is due to three simultaneous interactions of the polymer with –PPA vs. two interactions with +PPA and PP.