Recent Development of Enzymatic Reaction Systems Using Ionic Liquids

The use of ionic liquids (ILs) to replace organic or aqueous solvents in biocatalysis processes has recently gained much attention and great progress has been accomplished in this area. This paper reviews our recent developments in the application of ionic liquids as solvents in enzymatic reactions. (1) The lipase recyclable use system has been accomplished using an ionic liquid solvent system. However, the reaction rate was gradually dropped with reputation of the process due to inhibitory action of acetaldehyde oligomer accumulated in the IL when vinyl acetate was used as acyl donor. This problem was solved when the reaction was carried out under reduced pressure conditions using methyl phenylthioacetate as acyl donor; there was no drop in the reaction rate despite five repetitions of the process in the [bmim] [PF6] solvent system. We next attempted to evaluate the imidazolium salts again to see if they were suitable for recycling use of the enzyme using vinyl acetate as acyl donor. 1-Butyl-2,3-dimethylimidazolium tetrafluoroborate ([bdmim] [BF4]) was thus found to be an excellent solvent to realize a perfect lipase-recycling system: no accumulation of an acetaldehyde oligomer was observed in this solvent system and it was possible to use the lipase repeatedly ten times while still maintaining perfect enantio selectivity and high reactivity. (2) Lipase PS from Burkholderia cepacia is one of the most widely used enzymes applicable for various substrates, however, the activity in non-aqueous media is reduced and it has been reported that the enantioselectivity is significantly dependent on the solvent system. A remarkable acceleration was accomplished by the alkylPEG sulfate imidazolium salt (IL 1) coating with lipase PS in an i-Pr2O solvent system while maintaining excellent enantioselectivity: ca. 500- to 1,000-fold acceleration was recorded for some substrates with excellent enantioselectivity. Furthermore, we discovered that (R)-pyrrolidine-substituted imidazolium cetyl-PEG 10-sulfate (D-ProMe) derived from D-proline worked as more excellent activating agent of lipase PS. (3) It is generally recognized that lipase-catalyzed reaction in an ionic liquid solvent proceeded slower than those in many conventional organic solvents. To overcome this problem, attempts have been made to evolve ionic liquids appropriate for lipase-catalyzed reaction. Very rapid lipase PS-catalyzed transesterification of secondary alcohols was achieved using 2-methoxyethyl (tri-n-butyl)phosphonium bis(trifluoromethanesulfonyl)imide (TU-P1) as solvent and afforded the first example of a reaction rate superior to that in diisopropyl ether.