| Hydrolases are important enzymes for stereoselective and environmentally benign synthesis. In nature, hydrolases cleave bonds with water. When used in organic solvents, these enzymes can make synthetically useful bonds through condensation and the release of a small molecule, usually water or an alcohol. Many organic solvents that preserve enzyme activity, such as toluene, can be environmentally damaging or toxic. Room temperature ionic liquids, poorly coordinating salts that are liquid at temperatures below 100 °C, are a potential alternative to organic solvents for hydrolase-catalyzed reactions because of their low volatility, moderate polarity, and recyclability. However, many commonly used ionic liquids are orders of magnitude more expensive than conventional organic solvents and may also cause adverse environmental effects if released into aquatic environments. We demonstrate that ionic liquids are effective solvents for the lipase-catalyzed polymerization of epsilon-caprolactone and other poly(hydroxyalkanoates) and are effective in enhancing the electrical conductivity of carotenoid-containing polymers produced enzymatically. However, we found that they were not as effective as toluene for enzyme catalysis, and strove to find better alternative solvents for biotransformations.;We discovered that deep eutectic solvents, mixtures of ammonium or metal salts such as choline chloride and hydrogen bond donors such as urea or glycerol, were exceptional low-cost, biodegradable alternatives to organic solvents for hydrolase-catalyzed reactions. These physical mixtures may be thought of as ionic liquids, because they share similar physical properties to those solvents. Though they are composed of potential denaturants such as urea or halide anions, deep eutectic solvents stabilize enzymes. This stabilization is likely due to a preference for intra-solvent hydrogen bonding compared to enzyme-solvent hydrogen bonding. Deep eutectic solvents enhanced enzyme activity for a number of lipases either as pure solvents for reactions such as transesterification or polyesterification; or as additives in aqueous reactions such as epoxide ring opening or ester hydrolysis. We have preliminary evidence that deep eutectic solvents may induce a conformational change in enzymes that can alter reaction rates. These changes appear to be distinct from those caused by denaturing solvents. |
