Chiral membranes prepared by ionic interactions between sulfobutylether-β-cyclodextrin and anion-exchange membranes

Separation of enantiomers is of high interest mainly in pharmaceutical industry because of the stereomorphic nature of these compounds and their stereoselective biochemical effects in human body. Membrane processes might potentially be a promising, easily scalable technology to help produce safer and more effective enantiomerically pure drugs. However, the preparation of enantioselective membranes is usually very complicated and expensive, which also prevents them from being applied on an industrial scale. This work demonstrates a simple method for preparation of chiral membranes utilizing ionic interactions between a negatively charged chiral selector and a positively charged membrane. Anion exchange membranes (heterogeneous RALEX AMHPES and homogeneous TailorMem Hollex ADL 911 NR) were functionalized with a commercially available polyanionic chiral selector sulfobutylether-beta-cyclodextrin (SBE-beta-CD). The amount of the attached selector was 4.9 mu mol/ cm-2 and 0.68 mu mol/cm-2 for the RALEX and TailorMem membranes, respectively, and it was located both on the membranes' surface and in their inner structure. Separation properties of the modified TailorMem membrane were tested in diffusion cells for separation of N-Boc-tryptophan as a model analyte. Various experimental conditions were studied, including feed concentration, temperature and composition of the solvent (consisting of varied ratio of methanol and aqueous formic acid solution at pH 2.1). The highest enantiomeric excess in permeate was detected for the lowest temperature of 8 degrees C, the lowest amount of methanol in the solvent (20 vol %) and the lowest feed concentration of 0.1 mg/ml. The separation mechanism can be described as sorption- selective with N- Boc-L-Trp being preferentially adsorbed, and the sorption decreasing with increasing methanol content in the solvent. The membrane can be regenerated by analyte desorption into pure methanol to restore its enantioselective function. FTIR and EDX analyses confirm the stability of the membrane's modification after the experiment and regeneration.