Biotransformations in Ionic Liquid Media

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

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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. 

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. 

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. 

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). 

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. 

As evident from Fig. 8.4, an increase in the selectivity has been observed in IL/ scCOj biphasic systems media (>99.5%) with respect to scCO assayed alone (95%). These results could be explained by the use of water-immiscible ILs which have a specific ability to reduce water activity in the enzyme microenvironment. The synthetic activity of the immobilized lipase in IL/scCO biphasic systems is lower than that in scCO assayed alone. Similar results were found by Mori et al. [40] in IL/ hexane biphasic systems. These authors reported that the enzymatic membranes prepared by simple adsorption of CaLB onto the surface were more reactive than membranes prepared with ILs. As can be observed in Fig. 8.4, the initial reaction rate in the assayed IL/scCO biphasic systems increased in the following sequence [bdimim ][PF ]<[bmim ][PFg ]<[bmim ][NTfj ]<[omim ] [PF ], which was practically in agreement with flie activity sequence reported by these authors using free Candida antarctica lipase B in homogeneous ionic liquid systems ([bmim ] [PF ]<[bdmim+][PFg ]<[bmim+][NTfj ]<[omim ][PF ]), with the exception of [bmim [PF ] and [bdimim+][PFg ]. These results were explained taking into account that biotransformation occurs within the ionic liquid phase, so substrates have to be transported from scCOj to the ionic liquid phase. The mechanism of substrate transport between the ionic liquid and the supercritical carbon dioxide could be by three consecutive steps diffusion of the substrates through the diffusion... 

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