Lipase-catalysed esterification in a reactive natural deep eutectic solvent leads to lauroylcholine chloride rather than glucose ester†

Abstract

Enzymatic esterification of glucose with lauric acid catalyzed by lipase B from Candida antarctica immobilized on acrylic resin (LAR) was investigated in hydrophilic reactive natural deep eutectic solvents (R-NADESs), composed of choline chloride (ChCl) as the hydrogen bond acceptor (HBA) and glucose (Glc) and water as hydrogen bond donors (HBDs) in different molar ratios. Surprisingly, no glucose esters were obtained, the only esterification product being lauroylcholine chloride, obtained in the ChCl : Glc : H₂O (2 : 1 : 1) ternary R-NADES. Molecular dynamic simulations clearly explained this unexpected selectivity, showing that the lipase-catalyzed synthesis of glucose lauryl esters is hindered by the manifold and strong interactions in the H-bond network and the formation of voluminous adducts of glucose with the chloride ion, which cannot access the alcohol catalytic subsite. The free choline chloride, not involved in the H-bond network of the ChCl : Glc : H₂O (2 : 1 : 1) R-NADES, did enter the CalB catalytic pocket and was converted to the corresponding lauroylcholine ester.