Characteristics of lipase-catalyzed esterification in reverse micellar media.

Abstract

Lipase-catalyzed esterification can be operated at ambient conditions and yield good product selectivity. Being carried in reverse micellar media which have low water contents, these reversible reactions favor the esterification direction instead of the normal hydrolysis catalyzed by lipases. Furthermore, the highly dynamic reverse micelles greatly lower the mass transfer limitation of the reactions between hydrophobic and hydrophilic reactants. The objective of this work focuses on the study of the interfacial phenomena and the enzymatic reaction kinetics and thermodynamics in reverse micellar media. Gas chromatography (GC) was employed to analyze the compositions of reaction solutions and dynamic light scattering (DLS) was used to measure the particle size of reverse micelles. The phase diagrams generated for different types of alcohol have revealed that the hydrophobic long-chained 1-decanol and lauric acid resulted in the rigidity of the reverse micelles and decreased the water solubilization. This could be due to their penetration into the surfactant Aerosol-OT tail groups. The amphiphilic short-chained 1-butanol and 1-propanol resulted in fluid reverse micellar surfaces at lower alcohol concentrations and rigid surfaces at higher alcohol concentrations. This may be due to their penetration into the surfactant head groups first and then into the surfactant tail groups. The hydrophilic 1,2-propanediol and 2,3-butanediol solubilized with water in reverse micelles competitively at lower water concentrations and cooperatively at higher water concentrations. A proposed enzyme kinetic model has successfully predicted the reaction mechanism of lauric acid with 1,3-propanediol in reverse micellar media. However, experiments conducted showed a likelihood of substrate inhibition at higher lauric acid concentrations ($>$70 mM) and a strong bonding effect among l,3-propanediol, water, and AOT head groups. The R. delemar lipase showed a satisfactory enantioselectivity (90% enantiomeric excess) to 1-positioned monoester through the esterification of 1,3-butanediol with lauric acid in reverse micellar media. The C. cylindracea lipase did show an enantioselectivity to the reactions of 1,3-butanediol, 2-butanol, and 2-octanol, but the enantiomeric excess was not very significant at the reaction conditions employed. An appropriate selection of the reaction conditions could enhance the enantioselectivity.