Lipase-Catalyzed Enantioselective Esterification of (±)-Menthol in Novel Ionic Liquids

Abstract:

Six novel symmetrical alkylimidazolium hexafluorophosphate ionic liquids(ILs) were designed and synthesized in this study. The enantioselective esterification of(±)-menthol with propionic anhydride catalyzed by Canadida rugosa lipase was used as a model reaction to investigate the catalytic characterization of the lipase in different medium. The results indicated that the activity and selectivity of the lipase in the ionic liquid 1，3-di(n-octyl)imidazolium hexafluorophosphate([DnOIM][PF6]) is obviously higher than that in other ILs and hexane. Therefore，[DnOIM][PF6] was chosen as the medium for the reaction. By investigating various factors on the conversion of(±)-menthol，the optimal conditions were determined as reaction temperature of 30 ℃，70 mg lipase，3.0 mL ionic liquid，menthol and propionic anhydride with the molar ratio of 1∶1，and reaction performed for 20 h. Under this optimal condition，the conversion of(±)-menthol and enantiomeric excess of(-)-menthyl propionate was up to 48.1% and 98.1%，respectively. The stability of the lipase in IL was 4.3-fold than that in hexane. Furthermore，the activity of IL and the lipase were not substantial diminution after recycled 7 times. Meanwhile，fluorescence spectroscopy and circular dichroism results showed that the lipase in IL had bigger exposure levels of amino acid residues and excellent secondary structure stability.