(5ai) Enzymatic Synthesis in Deep Eutectic Solvents

Enzymes are desirable catalysts to study because they give high selectivity with minimal byproducts, and industrial enzymatic processes are becoming increasingly common. Often, non-aqueous conditions are required for desired reactions, particularly with industrial hydrolases. These processes commonly use volatile organic solvents (VOS), which are often flammable, toxic, and environmentally harmful. Ionic liquids (ILs) are molten salts that are recyclable, non-volatile, and non-flammable alternatives to VOS that initially showed promise in enzymatic reactions. ILs offer advantages in enzymatic reactions such as high enzyme stability, ease of product separation, and recyclability; but their high cost is prohibitive, and the lack of sustainable materials is undesirable. However, current ILs are mainly prepared from petrochemically-derived, toxic materials such as imidazole, fluorinated amides, and halogenated alkanes. Further, ILs require high purity, are not available on large scales, and are typically orders of magnitude more expensive than VOS, limiting their widespread use. Deep eutectic solvents (DES's) are physical mixtures of ammonium or metal salts and hydrogen bond donors, such as 1:2 choline chloride (a naturally occurring vitamin used in chicken feed): urea (fertilizer or animal waste product) or 1:2 choline chloride:glycerol (biodiesel byproduct) are promising replacements for both VOS and ILs. In addition to offering the recyclability, low volatility, and low flammability of ILs, DES's are composed of non-toxic and inexpensive materials including vitamins, amides, sugars, and alcohols that have costs and production scales comparable to VOS. Many of these components are natural products or can be made from renewable materials, meaning that DES's are a sustainable alternative to common ILs and to many petroleum-based organic solvents. Further, DES's are trivial to synthesize compared to most ILs, Figure 1. We have found that DES's are effective replacements for a number of hydrolase-catalyzed reactions, including transesterification, epoxide hydrolysis, and ring-opening polymerization. This efficacy was not obvious, however. The use of DES's in enzymatic reactions was initially questionable due to the presence of strong denaturants such as chloride anions strong hydrogen bond donors that inhibit enzyme activity in ILs. However, we hypothesized that intrasolvent hydrogen bonding between DES components would be preferred over hydrogen bonding between solvent and enzyme, a key step in denaturation. We have found conversions, selectivity, and activities in DES's comparable to those found under similar conditions in VOS. For example, we found nearly identical polymerization activity of Candida antarctica lipase B in the production of polycaprolactone from ε-caprolactone in 1:2 choline chloride:urea compared to in toluene. We also found a 20-fold conversion enhancement of epoxide hydrolase-catalyzed hydrolysis of styrene oxide to styrene glycol (an important reaction in pharmaceutical production) in 25% 1:2 choline chloride:glycerol compared to in water or water/VOS mixtures.