Enzymatic synthesis of sugar esters using reactive natural deep eutectic solvents

Enzymatic synthesis of sugar esters in non-aqueous media has advantages over chemical synthesis, particularly regioselectivity. However, conventional non-aqueous media do not solubilise sugars well and can destabilise the enzyme. Natural deep eutectic solvents (NADES) are environmentally friendly alternatives, and reactive NADES (R-NADES), in which sugars act simultaneously as solvent components and substrates, are especially promising. We investigated the acetylation of α-d-glucose with vinyl acetate catalysed by Candida antarctica lipase B (CALB), comparing an R-NADES composed of choline chloride and α-d-glucose with a solvent-free system, under identical substrate molar ratios (1:1) and reaction time (72 h). The solvent-free system reached 82% conversion, but its volumetric productivity of 6-O-acetyl-α-d-glucopyranose was low (0.021 g L−1 h−1) due to limited glucose solubility. The anhydrous R-NADES achieved 44% conversion but exhibited higher volumetric productivity (1.05 g L−1 h−1), reflecting its higher substrate concentration, despite its high viscosity. Water was added at 10–30% (v/v) to reduce viscosity and the highest conversion (91%) and volumetric productivity (2.16 g L−1 h−1) were obtained with 20% water. The R-NADES with 20% water also had a 3.8-fold higher enzyme-normalised volumetric productivity than the solvent-free system. This enhancement results from reduced viscosity and favourable enzyme-solvent interactions. The results demonstrate that water-modified R-NADES significantly intensify enzymatic sugar-ester synthesis, reinforcing their potential as green reaction media for biocatalysis.