BACKGROUND Deep eutectic solvents (DESs) derived from bio-available materials are promising eco-friendly solvents in the field of biocatalysis. However, the type of bio-based DES is still limited and its application in bioprocess has not been well-explored. We adopted a design strategy via the combination of choline and amino acids to synthesize novel bio-based DESs, aimed to enhance biocatalytic efficacy by exploiting the developed DESs as co-solvents. RESULTS Various bio-based DESs were synthesized, where the two components are both bio-available (choline and amino acids). Their performances in the bioreduction of 2-chloro-1-(3,4-difluorophenyl)ethanone (CFPO) were subsequently evaluated with recombinant Escherichia coli cells. The efficient asymmetric production of (S)-2-chloro-1-(3,4-difluorophenyl)ethanol ((S)-CFPL) was successfully achieved in a developed choline acetate/lysine (ChAc/Lys)-containing medium. ChAc/Lys can strengthen coenzyme regeneration and properly improve cell membrane permeability during bioreduction, which account for high conversion. After optimized reaction parameters were conducted in the ChAc/Lys-buffer system, 87.0% (S)-CFPL yield was observed with > 99.9% enantiomeric excess (ee) value at 1 mol L-1 CFPO content. The substrate loading in the established ChAc/Lys-buffer system was increased by 3.3-fold compared to aqueous medium. This bioprocess was also proved to be feasible at 500 mL preparative-scale with a satisfactory conversion. CONCLUSION ChAc/Lys bio-DES is a promising and biocompatible co-solvent. Introducing ChAc/Lys to the reaction system is an effective strategy for challenging high substrate loading for bioreduction. Our results give a valuable insight for developing green and benign sustainable bio-DESs for biotransformation. (c) 2020 Society of Chemical Industry