Stabilization of alpha-chymotrypsin by ionic liquids in transesterification reactions

Five different ionic liquids, based on dialkylimidazolium and quaternary am monium cations associated with perfluorinated and bis (trifluoromethyl) sul fonyl amide anions, were used as reaction media to synthesize N-acetyl-L-ty rosine propyl ester by transesterification with alpha -chymotrypsin at 2% ( v/v) water content at 50 degreesC. The synthetic activity was reduced by th e increase in alkyl chains length of cations and by increases in anion size , which was related to the decrease in polarity. Incubation of the enzyme ( with and without substrate) in ionic liquids exhibited first-order deactiva tion kinetics at 50 degreesC, allowing determination of deactivation rate c onstants and half-life times (1-3 h). Ionic liquids showed a clear relative stabilization effect on the enzyme, which was improved by increased chain length of the alkyl substituents on the imidazolium ring cations and the an ion size. This effect was 10-times enhanced by the presence of substrate, F or example, 1-butyl-3-methylimidazolium hexafluorophosphate increased the a lpha -chymotrypsin half-life by 200 times in the presence of substrate with respect to the 1-propanol medium. These results show that ionic liquids ar e excellent enzyme-stabilizing agents and reaction media for clean biocatal ysis in non-conventional conditions. (C) 2001 John Wiley & Sons, Inc.