CHAPTER 2. Biocatalysis in Ionic Liquids

Abstract

The different scenarios for performing biocatalysis in ionic liquids (ILs) are outlined. Reactions can be performed using whole cells suspended in a two-phase water–IL mixture or a free enzyme dissolved in a water phase and the substrate in a hydrophobic IL phase. Alternatively, the reaction is conducted with a free enzyme dissolved in a single phase consisting of water mixed with a hydrophilic IL or in a single IL phase using either an immobilized enzyme or dissolved free enzyme. Strictly speaking, the term biocatalysis applies only to reactions in an IL in the absence of water. First generation ILs for biocatalysis generally consisted of dialkylimidazolium cations and tetrafluoroborate or hexafluorophosphate anions. More recently, second generation ILs have been developed that exhibit less ecotoxicity, are produced from renewable resources and are biodegradable and more compatible with enzymes. The various benefits to be gained from conducting biotransformations in ILs—increased activities, selectivities and stabilities—are discussed with examples. The use of supercritical carbon dioxide as a mobile phase provides the possibility for continuous product extraction. One area that is particularly interesting for performing biocatalysis in anhydrous ILs is in the conversion of highly polar substrates, such as carbohydrates, that are sparingly soluble in most common organic solvents. For example, transesterifications of carbohydrates cannot be performed in water but can readily be conducted in ILs. Hence, the current focus of attention on the production of chemicals from renewable resources provides a driving force for the further development of environmentally benign ILs as non-aqueous reaction media for biotransformations.