Immobilization of Enzymes for Use in Ionic Liquids

Pedro Lozano, Teresa de Diego, and José L. Iborra

Summary

The use of ionic liquids (ILs) as nonconventional reaction media in enzyme catalyzation has gained increasing attention for developing green chemical processes because of the physical and chemical characteristics of ILs. Some have been shown to be by far the best nonaqueous media for enzyme-catalyzed reactions, because the enzymes display a high level of activity and stereoselectivity in synthesizing many different substrates, as well as by their excellent ability to stabilize enzymes toward re-use. The different operational strategies used for enzymatic processes in ILs are analyzed. Protocols to synthesize different fine chemical products, such as aspartame, butyl butyrate, R-2-pentyl propionate, N-acetyllactosamine, and polyester are described in detail, including analytical techniques. A detailed description of the procedure for recovering and cleaning of ILs to be re-used is also included, as well as notes containing special cautions.

Key Words: Biotransformations; enzyme stability; ionic liquids; green chemistry; lipases; nonconventional media.

1. Introduction

Enzymatic reactions based on ionic liquids (ILs) are promising alternatives to organic solvents in the biotechnological research area known as "biocatalysis in nonconventional media." Organic solvents are usually volatile liquids that evaporate into the atmosphere with detrimental effects on the environment and human health. Therefore, it is necessary to develop new alternatives for their enzyme-catalyzed biotransformation in friendly environmentally reaction media (1).

What are ILs? They are simply liquids composed entirely of ions at or close to room temperature. Using sodium chloride as an example, molten sodium chloride (up 800°C) is an IL, whereas an aqueous solution of this salt is an ionic solution (2). Typical room temperature ionic liquids (RTILs) are based on organic cations (e.g., 1,3-dialkylimidazolium, N-alkylpyridinium, tetraalkylammonium, and tetraalkylphosphonium) paired with a variety of anions that have a strongly delo-

258 Cations

258 Cations

Dialkylimidazolium Ionic Liquid

(Triflate or Tf)

Fig. 1. Structures of typical ions of ILs used for enzymatic reactions.

(Triflate or Tf)

Fig. 1. Structures of typical ions of ILs used for enzymatic reactions.

calized negative charge (e.g., BF4_, PF6-, SbF6-, triflate, bistriflimide; see Fig. 1), resulting in colorless, low-viscosity, and easily manipulable materials with very interesting properties as solvents. These properties includes negligible vapor pressure (they do not therefore evaporate), excellent thermal stability (up 300°C in many cases), a high ability to dissolve a wide range of organic and inorganic compounds, including gases (e.g., H2, CO2, CO,), nonflammable nature, high conductivity, and a large electrochemical window. Moreover, their polarities, hydrophilicities/hy-drophobicities and solvent miscibility can be tuned by selecting the appropriate cation and anion. ILs can be designed to be miscible or immiscible with water or some organic solvents (e.g., hexane, benzene, ether, i-PrOH), making them more useful for recovering products from the reaction mixture (Table 1) (3). Spectro-scopic measurements of solvatochromic and fluorescence probe molecules in ILs suggest that these solvents have a polarity comparable with that of the lower alcohols (e.g., methanol, ethanol). All of these properties, including the fact that they are re-usable and recyclable, allow ILs to be considered ideal solvents for green chemistry.

Enzymatic transformations in ILs have only recently been considered (first paper published in 2000 (4) and a wide number of applications have been tested. Some ILs have been shown to be by far the best nonaqueous media for enzyme-catalyzed reactions, because the enzymes display a high level of activity and stereoselectivity in synthesizing many different compounds (e.g., aspartame) (4), aliphatic and aromatic esters (5-8), aminoacid esters (9-11), quiral esters by kinetic resolution of racemic alcohols (12-20,25), carbohydrate esters (21,22), and polymers (23,24).

Table 1

Physical Properties of Some Ionic Liquids

Physical Properties of Some Ionic Liquids

Table 1

Cation

(°C)

Density at 20°C (g/mL)

Viscosity at 20°C (cSt)

Was this article helpful?

0 0
Detoxify the Body

Detoxify the Body

Need to Detoxify? Discover The Secrets to Detox Your Body The Quick & Easy Way at Home! Too much partying got you feeling bad about yourself? Or perhaps you want to lose weight and have tried everything under the sun?

Get My Free Ebook


Post a comment