New Generations of Ionic Liquids Applied to Enzymatic Biocatalysis

Abstract

Ionic liquids are salts in a liquid state, combinations of cations and anions that are liquid at temperatures below 100 oC. Thus, they have been called Room-Temperature Ionic Liquids (RTILs, or just ILs) in order to differentiate them from traditional salts, which melt at much higher temperatures and receive the name of “molten salts”. In contrast to conventional or‐ ganic solvents, ILs usually have extremely low volatility. Indeed, vapor pressures for ILs are scarce in the literature exactly because they are extremely low (< 1 Pa) and have to be ob‐ tained at high temperatures (400-500 K) [1]. For this ”negligible” vapor pressure, ILs are of‐ ten said to be “green“ solvents when compared to traditional, environmentally harmful volatile organic compounds (VOCs). A big goal in the use of ILs in enzymatic reactions is the replacement of VOCs by ILs. In addition, ILs have other potential advantageous proper‐ ties such as reasonable thermal stability; ability to dissolve a wide range of organic, inorgan‐ ic and organometallic compounds; controlled miscibility with organic solvents (which is relevant for applications in biphasic systems) among others. All these properties make them very attractive non-aqueous solvents for biocatalysis. As they have been extensively descri‐ bed, ILs offer new possibilities for the application of solvent engineering to enzymatic reac‐ tions. Biocatalysis with ILs as reaction medium was first showed in the beginning of 2000 [2-4]. During the last decade, ILs have fast increased their attention as reaction media for en‐ zymes with some remarkable results [2-4]. The advantage of using ILs in enzymatic biocatal‐ ysis, as compared to VOCs, is the enhancement in the solubility of substrates or products without inactivation of the enzymes, high conversion rates and high activity and stability [5]. ILs are also being used as co-solvents in aqueous biocatalytic reactions, since ILs help to