Ether-functionalized ionic liquids for nonaqueous biocatalysis: Effect of different cation cores

Applications of Ionic Liquids in Whole-Cell and Isolated Enzyme Biocatalysis

Ionic liquids have unique chemical properties that have fascinated scientists in many fields. The effects of adding ionic liquids to biocatalysts are many and varied. The uses of ionic liquids in biocatalysis include improved separations and phase behaviour, reduction in toxicity, and stabilization of protein structures. As the ionic liquid state of the art has progressed, concepts of what can be achieved in biocatalysis using ionic liquids have evolved and more beneficial effects have been discovered. In this review ionic liquids for whole-cell and isolated enzyme biocatalysis will be discussed with an emphasis on the latest developments, and a look to the future.

What do we learn from enzyme behaviors in organic solvents? - Structural functionalization of ionic liquids for enzyme activation and stabilization

Enzyme activity in nonaqueous media (e.g. conventional organic solvents) is typically lower than in water by several orders of magnitude. There is a rising interest of developing new nonaqueous solvent systems that are more "water-like" and more biocompatible. Therefore, we need to learn from the current state of nonaqueous biocatalysis to overcome its bottleneck and provide guidance for new solvent design. This review firstly focuses on the discussion of how organic solvent properties (such as polarity and hydrophobicity) influence the enzyme activity and stability, and how these properties impact the enzyme's conformation and dynamics. While hydrophobic organic solvents usually lead to the maintenance of enzyme activity, solvents carrying functional groups like hydroxys and ethers (including crown ethers and cyclodextrins) can lead to enzyme activation. Ionic liquids (ILs) are designable solvents that can conveniently incorporate these functional groups. Therefore, we systematically survey these ether- and/or hydroxy-functionalized ILs, and find most of them are highly compatible with enzymes leading to high activity and stability. In particular, ILs carrying both ether and tert-alcohol groups are among the most enzyme-activating solvents. Future direction is to learn from enzyme behaviors in both water and nonaqueous media to design biocompatible "water-like" solvents.

"Water-like" Dual-Functionalized Ionic Liquids for Enzyme Activation

By mimicking the water structure to improve the enzyme activity, we designed imidazolium (Im)-based ionic liquids (ILs) functionalized with both ether and tert-alcohol groups (e.g., [CH3(OCH2CH2) n -Im-t-BuOH][Tf2N]). This unique combination of the "water-like" structure enabled very high transesterification (synthetic) activities for immobilized lipase B from Candida antarctica, which are up to 2-4 folds higher than nonfunctionalized "classical" ionic liquids (such as [BMIM][Tf2N]) and up to 40-100% higher than diisopropyl ether and tert-butanol. Fluorescence emission spectra confirmed the general protein structural preservation in these tailored ionic solvents. In addition, functionalized ILs showed high thermal stabilities, which are comparable with diisopropyl ether but much higher than tert-butanol.