Ionic Liquids: Alternative Reactive Media for Oxidative Enzymes

Abstract

During the last decades, biotechnology has attracted a great interest from academic and industrial environments due to the progress made in the discovery of new and efficient biocatalysts for many different applications. Due to the increasing variety of these applications, the aqueous medium became limiting. Biocatalysis in nonaqueous media offers unique capabilities and thus plays a major role in biotransformation technologies. It is well known that when enzymes are introduced in a non-aqueous medium, a number of factors can alter their native structure (critically at the active centre of the protein) and so alter their biological functions, thus causing reversible or irreversible inactivation of the enzymes. In order to overcome these problems, a high number of papers related to biocatalysis in organic solvents are published every year, in several areas of knowledge, investigating the effects of different organic media in enzyme activity, stability, structure and kinetics. It is important to highlight that good results have been reported for several enzymes, involving many different reaction types and a wide variety of organic solvents (Carrea & Riva, 2000). As in aqueous enzymatic reactions, the enzymology in non-aqueous media also presents specific and unique advantages like: reaction enantioselectivity, resistance to contamination by microorganisms, enhanced thermo-stability of the enzyme. Additionally, the solubility of hydrophobic substrates and/or products can be increased with the right selection of the solvent. The latter decreases diffusional barriers for the reactions, thus improving their yields. The study of enzymatic reactions in non-aqueous media started more than 100 years ago (Halling & Kvittingen, 1999), but did not receive attention until the 1970s with the pioneering works of Berezin and co-workers (Klyosov et al., 1975; Klibanov et al., 1977; Martinek et al., 1981) and today either water and organic solvents are conventional media for enzymes. Klyosov et al. (1975) studied the hydrolysis of p-nitrophenyl esters by means of α-chymotrypsin and related proteins in four different organic solvents (dimethylsulfoxide, dimethylformamide, formamide, and N-methylacetamide). The work of Klibanov et al. (1977) was based on enzymatic reactions in a “water–water-immiscible organic solvent” biphasic system with bovine chymotrypsin using chloroform, benzene, ether, acetone, ethanol, dimethylsulfoxide and dioxane as organic solvents. On the other hand, Martinek et al. (1981) investigated the behaviour of α-chymotrypsin, trypsin, pyrophosphatase,