Enhancement of immobilized protease catalyzed dipeptide synthesis by the presence of insoluble protonated nucleophile

Stabilization of alpha-chymotrypsin by ionic liquids in transesterification reactions

Five different ionic liquids, based on dialkylimidazolium and quaternary ammonium cations associated with perfluorinated and bis (trifluoromethyl) sulfonyl amide anions, were used as reaction media to synthesize N-acetyl-L-tyrosine propyl ester by transesterification with alpha-chymotrypsin at 2% (v/v) water content at 50 degrees C. The synthetic activity was reduced by the 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 deactivation kinetics at 50 degrees C, allowing determination of deactivation rate constants 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 anion size. This effect was 10-times enhanced by the presence of substrate. For example, 1-butyl-3-methylimidazolium hexafluorophosphate increased the alpha-chymotrypsin half-life by 200 times in the presence of substrate with respect to the 1-propanol medium. These results show that ionic liquids are excellent enzyme-stabilizing agents and reaction media for clean biocatalysis in non-conventional conditions.

Biocatalysis in low-water media: understanding effects of reaction conditions

The key role played by counter-ions with enzymes in low-water systems has become better appreciated with, for example, large effects on enantioselectivity. In low-dielectric media, counter-ions will associate strongly with charges in the protein or its substrates. Studies of temperature dependence have shown that hard-to-model entropies have a significant effect on behaviour, including enantioselectivity. Evidence has been presented that the supramolecular organisation of enzyme molecules can have important effects on behaviour, for example collapse of microstructure in cross-linked crystals.