Increased stability of an esterase from Bacillus stearothermophilus in ionic liquids as compared to organic solvents

Three different ionic liquids, 1-butyl-3-methyl imidazolium bis[(trifluoromethyl)sulfonyl]amide [BMIM][BTA], 1-butyl-3-methyl imidazolium hexafluorophosphate [BMIM][PF6] and 1-butyl-3-methyl imidazolium tetrafluoroborate [BMIM][BF4] were used as reaction media for the transesterification of 1-phenylethanol catalysed by esterases from Bacillus subtilis and Bacillus stearothermophilus. No transesterification activity could be detected in the ionic liquids when the lyophilised powder of the esterases was used as biocatalysts. By immobilising the esterases onto Celite it was possible to obtain activity in the ionic liquids. The specific activity and enantioselectivity for the Celite-immobilised enzyme was compared to conventional organic solvents (n-hexane, MTBE and vinyl acetate). Highest specific activity was obtained in n-hexane for both enzymes while the specific activity was similar in MTBE, vinyl acetate and in the ionic liquids. The enantioselectivity (E ∼ 7.0 and 3.0 for B. subtilis and B. stearothermophilus esterase, respectively) was however independent of the solvent. The kinetic resolution was also performed with two lipases, Candida antarctica lipase B and Pseudomonas sp. lipase (PS-D). Also for these two enzymes, the enantioselectivity was not affected by the solvent and high enantioselectivity E>100 was obtained in all solvents investigated. ability of the esterase from B. stearothermophilus at 40 °C was considerably increased in the ionic liquids [BMIM][BF4] and [BMIM][PF6] as compared to n-hexane and MTBE. A half-life of >240 h was obtained in [BMIM][PF6], which was >30- and >3-fold higher as compared to n-hexane and MTBE.