Room temperature ionic liquids (RTIL's) have the advantages of low vapor pressure and non-flammability, but high cost and potential toxicity limits their use. Deep eutectic solvents (DES's), eutectic mixtures of ammonium salts and hydrogen bond donors, such as 1:2 choline chloride/urea or choline chloride/glycerol are an alternative to conventional RTIL's¹. In addition to having low vapor pressure and low flammability, DES's are composed of non-toxic and inexpensive materials including vitamins, amides, sugars, and alcohols. Many of these components are natural products or can be made from renewable materials, meaning that DES's are a sustainable alternative to common RTILs and to many petroleum-based organic solvents. Some DES's contain urea and/or high amounts of ammonium halide, so they do not appear to be likely solvents for biocatalysis. Surprisingly, we found that lipases are active in DES's for transesterification and aminolysis of ethyl valerate and butanol or butylamine². The transesterification and aminolysis activities of immobilized Candida antarctica lipase B (CALB) in 1:2 mixtures of choline chloride/urea (ChCl:U) and 1:2 choline chloride/glycerol (ChCl:Gly) were comparable to those in toluene and as much as five-fold higher than the RTIL's 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM[BF4]) and 1-butyl-methylimidazolum bis(trifluoromethane)sulfonamide (BMIM[Tf2N]). We found that the reactivity of the glycerol in ChCl:Gly was reduced by >200-fold compared to the transesterification of free glycerol by the lipase. Further, free CALB was at least 20 times more stable at 60 °C in a 1:2 mixture of choline chloride (5 M) and urea (10 M) than in aqueous 5 M choline chloride or 10 M urea. We hypothesize that the hydrogen-bonded structure of DES's prevents urea and halide from inactivating the enzyme and prevents the glycerol component of DES's from reacting. We also found that the addition of 10% to 25% ChCl:Gly to an aqueous medium could enhance esterase activity by as much as 3-fold for the pig liver esterase- and Rhizopus oryzae esterase-catalyzed hydrolysis of p-nitrophenyl acetate. We observed an even more dramatic 20-fold increase in conversion of styrene oxide to styrene glycol by an epoxide hydrolase from Agrobacterium radiobacter after addition of 25% ChCl:Gly to the reaction medium.

1. A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed, and V. Tambyrajah, 2003, Chem. Commun., 70.

2. J. T. Gorke, F. Srienc, and R. J. Kazlauskas, 2008, Chem. Commun., 1235.