Reactive natural deep eutectic solvents increase selectivity and efficiency of lipase catalyzed esterification of carbohydrate polyols

A novel approach for the enzymatic esterification of carbohydrate polyols with fatty acids was investigated, with reactive natural deep eutectic solvents (R-NADES) as reaction medium and source of reagents. Three binary hydrophilic R-NADES consisting of choline chloride (ChCl) as hydrogen bond acceptor (HBA) and sugar alcohols (D-sorbitol, xylitol, D-arabitol) as hydrogen bond donors (HBD), were prepared and characterized. The carbohydrate polyol-based R-NADES were durable viscous fluids between 40 and 80⁰C, the common temperature range for an enzymatic reaction. The commercially accessible lipase B from Candida antarctica immobilized on acrylic resins (LAR) showed significant esterification activity and exceptional thermal stability in all three tested R-NADES and successfully catalyzed the synthesis of polyol esters. Box Behnken factorial design with three levels-three variables was used for optimizing the reaction conditions. The preparative esterification of D-arabitol with lauric acid (LA) at predicted optimal values for enzyme load, (i.e., 800 U/gram D-arabitol), temperature (70°C) and LA/D-arabitol molar ratio of 1), attained 80 mol% LA conversion after 24 h reaction time. Structural analysis based on mass, infrared and nuclear magnetic resonance spectroscopy demonstrated that the reaction product is solely the diester 1,5-dilauryl-D-arabitol (1,5-DLDA). The 1,5-DLDA product, with an NMR-purity above 99.9%, was isolated in 95% yield. Under similar conditions, 1,5-dilauryl-xylitol (1,5-DLX) and 1,6-dilauryl-D-sorbitol (1,6-DLDS) were obtained, at a LA conversion of 56 mol% and 62 mol%, respectively. A combination of docking and molecular dynamics simulations allowed to rationalize the structural stability of the lipase B from Candida antarctica (CalB) active site in R-NADES, as well as the selectivity of the catalyzed esterification.