Biocatalysis in Ionic Liquid Media

Ionic liquids have drawn increasing interest as biotransformation media (21,270). In contrast to polar organic solvents, ionic liquids of higher polarity do not deactivate enzymes. Instead, in a number of investigations, high polarity in ionic liquids appeared to even lead to improved enzymatic performance (271). Furthermore, good solubility of polar substrates in ionic liquids can create a favorable situation for the biotransformations. 

In general, hydrophobic ionic liquids, [BMIM]Tf2N, [BMIMJPFe, and [OMIM]PFg, provide environments that are less disruptive of the structures of biocatalysts than molecular solvents of similar polarity, methanol or DMSO (220). 

An ionic liquid can be used as a pure solvent or as a co-solvent. An enzyme-ionic liquid system can be operated in a single phase or in multiple phases. Although most research has focused on enzymatic catalysis in ionic liquids, application to whole cell systems has also been reported (272). Besides searches for an alternative non-volatile and polar media with reduced water and orgamc solvents for biocatalysis, significant attention has been paid to the dispersion of enzymes and microorganisms in ionic liquids so that repeated use of the expensive biocatalysts can be realized. Another incentive for biocatalysis in ionic liquid media is to take advantage of the tunability of the solvent properties of the ionic liquids to achieve improved catalytic performance. Because biocatalysts are applied predominantly at lower temperatures (occasionally exceeding 100°C), thermal stability limitations of ionic liquids are typically not a concern. Instead, the solvent properties are most critical to the performance of biocatalysts. 

Some general trends have started to appear correlating the properties of some ionic liquids to performance in biocatalysis (273), but conflicting observations have also been reported. The inconsistency of the performance of some systems could arise from the presence of impurities in the ionic liquids (or possibly in the substrates). 

In a recent review, some positive attributes of ionic liquids in biocatalysis were discussed 273). An example was given, which compares the enzymatic performance of Pseudomonas cepacia lipase (PCL)-catalyzed reactions as a function of the solvent polarity in both organic and ionic solvents, as shown in Fig. 17. The PCL shows no activity in organic solvents in the polarity range of the ionic liquids, but it is active in the ionic liquids. 

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