Enantioselective cholesteric liquid crystalline membranes characterized using nonchiral HPLC with circular dichroism detection

Abstract

Novel porous membranes for optical resolution consisting of cholesteric liquid crystalline (LC) phases (mixtures of 4-pentyl-4′-cyanobiphenyl – 5CB – and cholesteryl oleyl carbonate – COC) impregnated in porous cellulose nitrate supports were prepared, and their permeability and selectivity for 1-phenylethanol were studied. The 5CB/COC membranes exhibited 1-phenylethanol enantiomer selectivities between 1.02 (isotropic phase) and 1.27 (cholesteric phase), and increased permeability and stability of operation compared to other sorption enantioselective membranes. Significantly decreased activation energies for diffusion were observed at temperatures above the cholesteric–isotropic phase transition, indicating reduced interactions between diffusing molecules and the LC-coated pore walls in the absence of an ordered helical LC phase. Higher activation energy for permeation was observed for S-1-phenylethanol than R-1-phenylethanol, consistent with increased interactions between the S-enantiomer and the cholesteric LC phase. Membranes produced with the undoped nematic phase of 5CB exhibited higher permeabilities than the 5CB/COC membranes and no chiral selectivity. The feasibility of using circular dichroism (CD) detection in combination with non-chiral HPLC separation (non-chiral HPLC-CD) to monitor the concentrations and ratios of enantiomers transported through enantioselective membranes was demonstrated. Non-chiral HPLC-CD has previously been used as an assay for drug purity and to monitor enantioselective syntheses but has not previously been reported for the characterization of enantioselective separations. The non-chiral HPLC-CD system was able to determine the compositions of 1-phenylethanol enantiomers on the receiving side of the permeation cell with detection limits of 0.07 mg/ml for the CD signal and 0.005 mg/ml for the UV signal.