Designing enzyme-compatible ionic liquids that can dissolve carbohydrates

Abstract

Recently, there is a rising interest in dissolving a variety of carbohydrates (such as sugars, starch and cellulose) in ionic liquids (ILs). The solutions of carbohydrates are then conveniently subject to chemical or physical modifications. However, one serious disadvantage of these ILs is their strong tendency in denaturing enzymes. This drawback prohibits the dissolved carbohydrates from being transformed by enzymatic reactions. In the present study, we designed a series of ILs that are able to dissolve carbohydrates but do not considerably inactivate the immobilized lipase B from Candida antarctica. These ILs consist of glycol-substituted cations and acetate anions. They could dissolve more than 10% (wt) cellulose and up to 80% (wt) D-glucose. The transesterification activities of the lipase in these ILs are very comparable with those in hydrophobic ILs. The hydrogen-bond forming anions, oxygen-containing cations, and low cation bulkiness promote the carbohydrate dissolution, while the low anion concentration appears essential for the enzyme stabilization. Therefore, an optimization could be achieved through a fine design of IL structures. To demonstrate the potential applications of these ILs, we performed the enzymatic transesterifications of methyl methacrylate with D-glucose and cellulose, respectively, both fully dissolved in ionic media. In the case of D-glucose, conversions up to 80% were obtained; and in the case of cellulose, conversions up to 89% and isolated yields up to 66% were achieved.