Enantio-Selective Mechanism of the Poly-Proline Chiral Stationary Phase: a Molecular Dynamics Study

Abstract

Poly-proline-based chiral stationary phases are relatively new stationary phases and have shown to be competitive to other commercially available chiral stationary phases for high performance liquid chromatography (HPLC). The conformational studies, solvation properties and enantio-selective mechanism of this chiral stationary phase are the main focus of this thesis. Semi-flexible models are developed based on an extensive series of ab initio calculations for proline selectors from di- to hexa-proline and a series of six chiral analytes. Then molecular dynamics simulations are performed to study the solvation, conformational preferences at the interface, and the selectivity. The solvation and conformational preferences of poly-proline selectors at the interface are examined in a normal phase n-hexane/-2propanol and a reverse phase water/methanol solvent. We noticed a significant difference between conformational preferences of poly-proline chains in these solvents indicating the effect of solvent polarity and hydrogen bonding on the relative stabilities of poly-proline conformers. Solvent partitioning occurs at the interface and this creates a polarity gradient between the stationary phase and the bulk that encourages analyte docking at the interface. Hydrogen bonding to the poly-proline selectors is shown to be a function of solvent composition and poly-proline conformation at the interface. The selectivity of the poly-proline chains was studied by molecular dynamics simulations of chiral analytes docking into the interface. The selectivity factors for a set of enantiomers were predicted successfully. Enantio-resolution has been shown to mostly happen with hydrogen bonding to poly-proline carbonyl oxygens located close to the interface. Steric interactions and conformational flexibility of the analytes are also contributing factors for enantio-resolution.