Enzymatic Synthesis of Esculin Ester in Ionic Liquids Buffered with Organic Solvents

The enzymatic esterification of esculin catalyzed by Candida antarctica lipase B (Novozym 435) was carried out in ionic liquid (IL)−organic solvent mixed systems in comparison with individual systems. The reaction behaviors in IL−organic solvents were systemically evaluated using acetone as a model solvent.

With organic solvents as media, the esterification rates of esculin depended mainly on its solubility in solvents; for the reactions in ILs, the reaction rates were generally low, and the anion part of the IL played a critical role in enzyme activity. Therefore, the esterification of esculin in IL−acetone mixtures made it possible to improve the solubility of esculin while the effects of ILs on lipase activity were minimized.

Following the benignity of ILs to lipase activity, the anions of ILs were ranked in the order as [Tf2N]− > [PF6]− > [BF4]− > [CF3SO3]− > [C4F9SO3]− > [TAF]− > [MDEGSO4]− > [OctSO4]− > [ES]− = [DMP]− = [OTs]− = Cl−. The reaction behaviors differed in different systems and largely depended on the properties of the ILs and organic solvents.

In general, improvements were observed in terms of both solubility and reaction efficiency. The knowledge acquired in this work gives a better understanding of multiple interactions in IL−organic solvent systems, which provide guidance for system design and optimization.