Methods for stabilizing and activating enzymes in ionic liquids—a review

Abstract

Ionic liquids (ILs) have evolved as a new type of non-aqueous solvents for biocatalysis, mainly due to their unique and tunable physical properties. A number of recent review papers have described a variety of enzymatic reactions conducted in IL solutions; on the other hand, it is important to systematically analyze methods that have been developed for stabilizing and activating enzymes in ILs. This review discusses the biocatalysis in ILs from two unique aspects (1) factors that impact the enzyme's activity and stability, (2) methods that have been adopted or developed to activate and/or stabilize enzymes in ionic media. Factors that may influence the catalytic performance of enzymes include IL polarity, hydrogen-bond basicity/anion nucleophilicity, IL network, ion kosmotropicity, viscosity, hydrophobicity, the enzyme dissolution, and surfactant effect. To improve the enzyme's activity and stability in ILs, major methods being explored include the enzyme immobilization (on solid support, sol-gel, or CLEA), physical or covalent attachment to PEG, rinsing with n-propanol methods (PREP and EPRP), water-in-IL microemulsions, IL coating, and the design of enzyme-compatible ionic solvents. It is exciting to notice that new ILs are being synthesized to be more compatible with enzymes. To utilize the full potential of ILs, it is necessary to further improve these methods for better enzyme compatibility. This is what has been accomplished in the field of biocatalysis in conventional organic solvents. c 2010 Society of Chemical Industry Keywords: ionic liquid; enzyme activity; enzyme stabilization; immobilization; biocatalysis NOTATION CationsBMIM + = 1-butyl-3-methylimidazolium C 2 OC 1 MIM + = 1-(2-methoxy)ethyl-3-methylimidazolium C 2 OHMIM + = 1-(2-hydroxy)ethyl-3-methylimidazolium EMIM + = 1-ethyl-3-methylimidazolium

Enzymatic Hydrolytic Resolution of Racemic Ibuprofen Ethyl Ester Using an Ionic Liquid as Cosolvent

“…Table 1 shows that the structure of the IL had a large impact on the degree of conversion (X = 2.3%-46.5%), enantiomeric excess of the product (eep = 1.4%-96.2%), and enantiomeric ratio (E = 1.0-134.7) of EST10. The catalytic efficiencies of the different anions were in the following order: [12,21]. Anions having lower hydrogen bond basicity and nucleophilicity are found to be enzyme-compatible, because low hydrogen bond basicity and nucleophilicity minimize their interference with the internal hydrogen bonds and interaction with the positively charged sites in the enzyme and hence reduce the tendency to change the conformation of the enzyme's structure [12,[21][22][23] …”

“…ILs containing structurally different anions could potentially interact with protein molecule in different ways, including hydrogen bonding, van der Waals, ionic and dipolar interactions. The hydrogen bond basicity and nucleophilicity of anions have been found to affect the enzyme activity and stability in IL-based solvent media [12,21]. Anions having lower hydrogen bond basicity and nucleophilicity are found to be enzyme-compatible, because low hydrogen bond basicity and nucleophilicity minimize their interference with the internal hydrogen bonds and interaction with the positively charged sites in the enzyme and hence reduce the tendency to change the conformation of the enzyme's structure [12,[21][22][23].…”

“…They are non-volatile, non-flammable, and have excellent chemical and thermal stability, which can provide enzymes with excellent levels of activity, stability, and stereoselectivity [10,11]. For enzyme catalysis, the activity and stability of enzymes are closely related to the physicalchemical properties of ILs (e.g., polarity, viscosity, hydrogen bond basicity, hydrophobicity and anion concentration) [12]. The use of enzymes, especially lipases, in ionic liquids, has demonstrated many advantages, such as high conversion rates, good enantioselectivity, better recoverability and recyclability.…”

“…Besides acting as a reaction media, ILs can also be used to pre-treat enzyme. There are many literature data on enhanced the activity and stability of enzyme by coating it with ILs which had been used for transesterification, esterification and hydrolysis reaction with higher enantioselecticity and yields [12,18].…”

Probing the specific ion effects of biocompatible hydrated choline ionic liquids on lactate oxidase biofunctionality in sensor applications

“…Several studies have revealed that the best stabilising conditions are normally provided by chaotropic cations and kosmotropic anions. [36][37][38][39] Choline is well known as a chaotropic cation for the stabilisation of proteins when coupled with a kosmotropic anion, such as DHP. 37 According to the Hofmeister anion series, DHP is considered a stabilising anion, while nitrate and chloride anions are normally considered as strong and slightly destabilising anions, respectively.…”