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Elucidating Chiral Discrimination Mechanisms In Polysaccharide-Based Sorbents Using Experimental Probing and Molecular Modeling

Rahul Kasat, Nien-Hwa Linda Wang, and Elias Franses. Purdue University, West Lafayette, IN 47906

The concept of “chirality” is known for almost two centuries. Chiral recognition plays an important role in peptide biosynthesis, amino acid synthesis, drug design, separations, etc. Separations of enantiomers of chiral molecules are important in pharmaceuticals, agrochemicals, and fine chemicals industries. One of the widely used models for explaining the chiral discrimination, the “three point attachment model” was proposed in 1930s for proteins and enzymes. Polysaccharide-based sorbents or biopolymers are used widely in chiral separations because of their versatility in separating wide classes of chiral molecules. The chiral recognition mechanisms of these biopolymers are difficult to determine because of their complex structures. The lack of fundamental understanding of the chiral recognition mechanisms for these systems results in empirical, time consuming, and expensive selection of sorbents in chromatographic enantioseparations.

In this research, the chiral recognition mechanisms for certain important but structurally complex polysaccharides-based sorbents are elucidated [1-4]. The sorbent-solute-solvent interactions are investigated by systematically studying first the sorbent in the dry state [2],and then sorbent-solvent [3], sorbent-non chiral solute [4] and sorbent-chiral solute interactions [1]. A combination of molecular modeling tools, molecular dynamics (MD), and density functional theory (DFT), and experimental techniques, high performance liquid chromatography (HPLC), attenuated total reflection-infrared spectroscopy (ATR-IR), 13C cross polarization magic angle spinning (CP/MAS), and MAS solid state NMR, and X-ray diffraction (XRD) are used.

HPLC is used to measure the retention factors and enantioselectivities [1]. ATR-IR spectra show that the H-bonding states of the key binding sites of the polymer, C=O, and NH, can be different in different sorbents [2]. The DFT-predicted IR results for single polymer side chains help in understanding the shifts in the IR wavenumbers and the subtle changes observed in the intensities in the IR spectra of the dry sorbents [2]. The sorbent H-bonding sites are affected significantly upon absorption of solvents or solutes, providing insights into sorbent-solute and sorbent-solvent interactions [1-4]. The DFT results are consistent with the observed IR shifts and provide measures of the strengths of different interactions (hydrogen bonding and phenyl-phenyl) in different sorbent-solute and sorbent-solvent configurations [1, 3, 4]. The DFT simulations also predict side-chain conformations for different sorbents [2]. From the experimental and the DFT-predicted NMR chemical shifts of the C-1 carbon in the polymer backbone, it is inferred that the backbone glycosidic bond conformations and helicities are similar for amylose-based sorbents and are different from those of cellulose-based sorbents [2]. Solid state NMR studies show that the polymer side chain groups are more mobile than the polymer backbone groups [2, 3]. The XRD spectra of the dry sorbents and of the sorbents upon absorption of solvents and solutes provide structural information on the polymer helical pitch and packing arrangements of these polymers [1-3]. The polymer backbone conformations, as inferred from NMR, the polymer helical pitch, as observed in XRD, and the polymer side chain conformations from DFT are used to help build the polymer molecular models used for the sorbent-solute MD studies [1-4].

A novel but simple functional group approach is proposed. It allows probing the complex sorbent-chiral solute interactions. The retention times of complex chiral solutes (molecules with 3 or 4 functional groups) are compared with those of simpler non-chiral or chiral solutes (molecules with 2 or 1 functional groups). The results have allowed us to quantify the number and the nature of the sorbent-chiral solute interactions for the first time in these systems. The systematic variations in the structures of the complex chiral solutes have helped us identify for these systems the minimum number of interactions required for the chiral recognition and resulting in a significant enantioselectivity. The elution orders predicted for the enantiomers of the chiral solutes studied using sorbent-solute MD simulations are consistent with the HPLC results. The results support the hypothesis that for a significant enantioselectivity a combination of at least three different interactions are required among three types: H-bonding, phenyl-phenyl, or steric hindrance. Variations in the number and the type of the interactions and in the spatial arrangements of the functional groups in chiral solutes result in significantly different retention factors and enantioselectivities.

In summary, molecular modeling, guided by detailed experimental results, provides valuable insights into molecular details not easily accessible by experiments, and helps elucidate chiral recognition mechanisms.

1. R. B. Kasat, N.-H. Linda Wang, and E. I. Franses, Journal of Chromatography A, 1190, 2008, 110-119.

2. R. B. Kasat, N.-H. Linda Wang, and E. I. Franses, Biomacromolecules, 8, 2007, 1676-1685.

3. R. B. Kasat, Y. Zvinevich, H. W. Hillhouse, K. T. Thomson, N.-H. Linda Wang, and E. I. Franses, Journal of Physical Chemistry B, 110, 2006, 14114-14122.

4. R. B. Kasat, C. Y. Chin, K. T. Thomson, E. I. Franses, and N.-H. Linda Wang, Adsorption, 12, 2006, 405-416.