Enzymes in Ionic Liquids

The seminal work of Klibanov in the early 1980s [46,47] made it clear that enzymes can be used in hydrophobic organic solvents, although at the price of a severely reduced reaction rate [48, 49]. Indeed, many Upases, as well as some proteases and acylases, are so stable that they maintain their activity even in anhydrous organic solvents. This forms the basis for their successful application in non-hydrolytic reactions, such as the (enantioselective) acylation of alcohols and amines, which now are major industrial applications [50]. 

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. 

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The report from Sheldon and co-workers was the second publication demonstrating the potential use of enzymes in ionic liquids and the first one for lipases (Entry 13) [43]. They compared the reactivity of Candida antarctica lipase in ionic liquids such as [BMIM][PFg] and [BMIM][BF4] with that in conventional organic solvents. In all cases the reaction rates were similar for all of the reactions investigated alcoholysis, ammoniolysis, and per hydrolysis. 

Although the use of enzymes in ionic liquids has been explored, the enzymatic carbon-carbon bond forming the reactions in ionic liquids has not been studied in detail. Since aldol reactions catalyzed by the aldolase antibody 38C2 in buffer... 

Zhao H (2010) Methods for stabilizing and activating enzymes in ionic liquids. J Chem Technol Biotechnol 85 891-907... 

To sum up, enzymes in ionic liquids could maintain their activity over a much longer period than in molecular organic solvents. This stabilization has been explained on the basis of the interaction of the ionic liquid ions as well as higher viscosity of ionic liquids with respect to conventional organic solvent, which could cause slower migration of protein domains from the active conformation into the inactive one. [33]. 

Abstract This chapter discusses the potential usefulness of ionic liquids with respect to biocatalysis by illustrating the stability and activity of enzymes in ionic liquids in the presence or absence of water. Ionic liquids are a class of coulombic fluids composed of organic cations like alkyl-substituted imidazolium, pyrrolidin-ium, and tetraalkylammonium ions and anions such as halides, tetrafluoroborates, hexafluorophosphates, tosylates, etc. The possibility of tunable solvent properties by alternation of cations and anions has made ionic liquids attractive to study biocatalysis which warrants an understanding of enzyme stability and activity in ionic liquids. This chapter systematically outlines the recent studies on the stability of enzymes and their reactivity toward a wide range of catalytic reactions. A careful approach has been taken toward analysis of relationship between stabil-ity/activity of enzymes versus chaotropic/kosmotropic nature of cations and anions of ionic liquids. 

Methods of Stabilizing Proteins/Enzymes in Ionic Liquids... 

Hemandez-Femandez FJ, de los Rios AP, Tomas-Alonso F et al (2009) Stability of hydrolase enzymes in ionic liquids. Can J Chem Eng 87 910-914... 

One of the novel properties exhibited by enzymes in ionic liquids, as compared to aqueous solution, is their altered enzyme specificity, such as enhanced regioselecti-vity and enantioselectivity. Examples of increased regiosdectivity in Hpase[Pg.651]

The often increased enantioselectivity of enzymes in ionic liquids was also explored by Kamal and Chouhan for the synthesis of enantiomerically pure 1,2-diols in ionic liquids. The kinetic resolution of the did was catalyzed by immobilized lipase... 

The incubation of enzymes in ionic liquids can also lead to increased reactivity. Sheldon showed that CaLB could be heated as a suspension in [bmim][PF6] before use. Higher transesterification activity was observed than for the untreated enzyme, both when the enzyme was free (SP 525, 120% activity after 20 h incubation)... 

Klahn, M., G. S. Lim, and P. Wu. 2011. How Ion Properties Determine the Stability of a Lipase Enzyme in Ionic Liquids A Molecular Dynamics Study. Physical Chemistry Chemical Physics 13 (41) 18647-18660. 

Feher, E. Major, B. Belafi-Bak6, K Gubicza, L. (2007). On the background of enhanced stability and reusabdity of enzymes in ionic liquids. Biochem. Soc. T., 35, 6, 1624-1627... 

Moniruzzaman, M. Kamiya, N. Goto, M. (2010). Activation and stabilization of enzymes in ionic liquids. Org, Biomol. Chem., 8,13, 2887-2899 Nagaoka, T. Banskota, A.H. Tezuka, Y. Midorikawa, K. Matsushige, K. Kadota, S. (2003). Caffeic acid phenethyl ester (cape) analogues Potent nitric oxide inhibitors from the netherlands propolis. Biol Pharm. Bull, 26,4,487-491 Nara, S.J. Harjani, J.R. Salunkhe, M.M. (2002). Lipase-catalysed transesterification in ionic liquids and organic solvents A comparative study. Tetrahedron Lett., 43, 16, 2979-2982... 

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