Protease-Catalyzed Synthetic Reactions in Ionic Liquids

Enzymes are very effective and biodegradable catalysts, and act under mild conditions such as room temperature, atmospheric pressure, and around pH 7. Consequently, the applications of enzymes to the organic synthesis have extensively been studied from the standpoint of the development of sustainable synthetic processes (Truesdell, 2005). Enzymes exhibit the high activity and specificity not only in conventional aqueous solutions but also in non-aqueous reaction media (Klibanov, 2001; Noritomi et al., 2007a). Enzymes in non-aqueous media have especially been applied to numerous synthetic processes because of the following benefits, although the enzyme has the low activity in organic solvents compared with that in water: (i) the solubilities of non-polar reactants and products are improved; (ii) the thermostability of enzymes is highly improved; (iii) the stereoselectivity of enzymes is markedly altered; (iv) the thermodynamic equilibria of many processes such the formation of peptide bond by protease are favorable; (v) the immobilization of enzymes is not necessary, and enzymes are easily recycled by recovering them with the filtration or the centrifugation, since enzymes are insoluble in organic solvents; (vi) the product is easily recovered with the evaporation when using the volatile organic solvent as the reaction medium; (vii) the contamination due to the growth of microorganisms is inhibited by organic solvents. Furthermore, it has been found that the activity and selectivity of enzymes can be manipulated by the choice of solvents or enzyme preparation, although they were changed only by protein engineering or enzyme screening prior to the advent of nonaqueous enzymology (Koskinen & Klibanov, 1996; Noritomi et al., 1996; Wescott et al., 1996). Proteases are stable hydrolytic enzymes with high selectivity, do not need expensive cofactors, and are used as useful synthetic tools for side-directed peptide cleavages, regiospecific ester hydrolyses, or the kinetic resolution of racemates (Bordusa, 2002). In order to improve undesired hydrolysis and substrate solubility in aqueous media, proteases have been used in organic solvents for organic synthesis. Proteases in organic solvents can catalyze water sensitive reactions, such as esterification, transesterification, and peptide synthesis, which are usually impossible in aqueous media. Ionic solvent that is liquid at room temperature has attracted increasing attention as a green solvent for the chemical processes because of the lack of vapor pressure, the thermal stability, and the high polarity (Welton, 1999; Greaves & Drummond, 2008). Chemical and physical properties of ionic liquids can be changed by the appropriate modification of