Improvement of Conversion Efficiency from d-Glucose to d-Allulose by Whole-Cell Catalysts with Deep Eutectic Solvents

The synthesis efficiency of d-allulose, an ideal sugar substitute and dietary supplement, from d-glucose with whole-cell catalysts was improved by three out of the four tested deep eutectic solvents (DESs) based on choline chloride (ChCl). The promotion effects on the product yields with increments of up to 2.6-fold were observed in reactions with the presence of DESs in the order ChCl/glycerol < ChCl/urea < ChCl/ethylene glycol. DES compositions displayed distinct influences, but all positive effects appeared in the reactions with low DES contents (1–10%). Higher DES contents produced negative effects, and 40% even totally suppressed the reaction. The enhancement of efflux of intracellular molecules was proved by an analysis of the nucleic acid and protein contents in the medium. DES assisted in the increase in cell membrane permeability rather than in the damage of cell wall integrity, accelerating quick contacts between the enzyme and substrate and improving the conversion rate.