LIPASE-CATALYZED ENANTIOSELECTIVE ESTERIFICATIONS USING DIFFERENT MICROEMULSION-BASED GELS

Chiral esters with high optical purity have been synthesized at 298.2 K from racemic 2-octanol and alkanoic acids using the commercial lipas es from Chromobacterium viscosum (CV) or Candida sp. (SP 525) immobili zed in microemulsion-based gelatin gels. The microemulsions consisted of water and alkanes stabilized by the anionic surfactant sodium 1,4-b is(2-ethylhexyl) sulfosuccinate (AOT) and the naturally occurring zwit terionic surfactant soybean lecithin, respectively. The enzymes were s olubilized both in water-in-oil (W/O) microemulsions and in microemuls ions with a bicontinuous structure. Different microstructures of the g els were chosen since the enzyme may undergo conformational changes in different environments resulting in different catalytic efficiencies toward competing substrates. Therefore, it is of great fundamental int erest to know the phase behaviour and the microstructures of the used microemulsion systems. Phase diagrams were determined at 298.2 K for t he systems water-hexane-AOT and ethanol/water (1:1)-hexadecane-soybean lecithin. The former system exhibited a large one-phase W/O microemul sion region, while in the latter a small one-phase region with biconti nuous structure was present. The kinetic enantiomeric ratios (E-values ), as determined from enantiomeric excess (e.e.) values at a conversio n below 0.5, were higher both in the W/O microemulsion as well as in t he bicontinuous microemulsion using the SP 525 lipase, than using the CV lipase. On the other hand, the conversions were higher using gels b ased on W/O microemulsions (AOT stabilized) than using gels based on m icroemulsions with a bicontinuous structure (lecithin stabilized).