Biotransformation ionic liquid

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

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 lipase-catalyzed reactions in an IL solvent system have now been established and several examples of biotransformation in this novel reaction medium have also been reported. Recent developments in the application of ILs as solvents in enzymatic reactions are reviewed. 

VARIOUS BIOTRANSFORMATIONS IN AN IONIC LIQUID SOLVENT SYSTEM... 

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. 

Biphasic systems consisting of ionic liquids and supercritical CO2 showed dramatic enhancement in the operational stability of both free and immobilized Candida antarctica lipase B (CALB) in the catalyzed kinetic resolution of rac- -phenylethanol with vinyl propionate at 10 MPa and temperatures between 120 and 150°C (Scheme 30) 275). Hydrophobic ionic liquids, [EMIM]Tf2N or [BMIM]Tf2N, were shown to be essential for the stability of the enzyme in the biotransformation. Notwithstanding the extreme conditions, both the free and isolated enzymes were able specifically to catalyze the synthesis of (J )-l-phenylethyl propionate. The maximum enantiomeric excess needed for satisfactory product purity (ee >99.9%) was maintained. The (S)-l-phenylethanol reactant was not esterified. The authors suggested that the ionic liquids provide protection against enzyme denaturation by CO2 and heat. When the free enzyme was used, [EMIM]Tf2N appeared to be the best ionic liquid to protect the enzyme, which... 

Although [BMIM]BF4 has been evaluated as an isolation medium for lipase-catalyzed biotransformations, the general experience with it has not been favorable, relative to that with other ionic liquids, such as [BMIM]PF6. However, excellent performance was recently reported for [BDMIM]BF4 when it was used to host Candida antarctica (Novozym 435) for the enantioselective transesterification of 5-phenyl-l-penten-3-ol (+ and -) with vinyl acetate. The working hypothesis was that the oligomerization of acetaldehyde may be caused by the C2 proton of the [BMIM] ion because of the unfavorable acidity of this group (226). In contrast, the cation in [BDMIM]BF4 lacks this acidity. 

Promising developments of ionic liquids for biocatalysis reflect their enhanced thermal and operational stabilities, sometimes combined with high regio- or enantioselectivities. Ionic liquids are particularly attractive media for certain biotransformations of highly polar substrates, which cannot be performed in water owing to equilibrium limitations 297). 

Fig. 24.1 Chemical structure of common ionic liquids in biotransformation 1-butyl-3-methyl-imidazolimn tetrafluoroborate and l-butyl-3-methyl-imidazolium hexafluoroborate...

The enzyme may be dissolved in a mixed aqueous-ionic liquid medium, which may be mono- or biphasic or it could be suspended or dissolved in an ionic liquid, with little or no water present. Alternatively, whole cells could be suspended in an ionic liquid, in the presence or absence of a water phase. Mixed aqueous-organic media are often used in biotransformations to increase the solubility of hydrophobic reactants and products. Similarly, mixed aqueous-ionic liquid media have been used for a variety of biotransformations, but in most cases there is no clear advantage over water-miscible organic solvents such as tert-butanol. 

We conclude that hydrophobic ionic liquids are promising and attractive replacements for molecular solvents in whole-cell biotransformations. 

The application of ILs is most surprising and intriguing in the area of biotransformations. Many enzymes are cataly tically active in ILs or in aqueous biphasic ionic liquid systems. Lipases maintain their activity in anhydrous IL media, and their enantioselectivity and operational stability are often better than in traditional media. See Table 12.5. 

The activity of microorganisms in organic solvents has been observed to have useful applications in the synthesis of organic materials. Using an ionic liquid solvent system of ionic liquid and water, Cull and coworkers have reported the a two-phase biotransformation with cells of Rhodococcus R312 [3]. Extensive research of Candida antarctica-cat lyzed reactions has also been carried out for catalyzing alcoholysis, ammoniolysis, and perhydrolysis reactions in ionic liquids ([bmim][BF4] or [bmimllPFe]), Scheme 10.1. 

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