BACKGROUND: Natural deep eutectic solvents (NADESs), a class of green solvents which completely accords to 12 principles of green chemistry, have proven to have great potential applications in enzymatic reactions. Despite strong interest, the role of NADESs in these processes, and the molecular interaction between enzymes and NADESs, still remain ambiguous. In the present study, the stability and activity of Candida Antarctica lipase B (CALB) were studied, and the mechanism by which CALB was activated in NADESs was explored systematically from both molecular- and macroscopic-scale perspectives. RESULTS: The results suggested that the activity of CALB in all NADESs was significantly higher than that of the control (ethanol). Moreover, the stability of CALB increased to 115.48 +/- 1.36% and 108.54 +/- 1.26% in betaine-glycerin (B-Gly) and choline chloride-glycerol (C-Gly), but decreased to 91.69 +/- 3.26% and 92.31 +/- 3.36% in betaine-xylitol (B-X) and choline chloride-xylitol (C-X), respectively. The results of circular dichroism (CD), fluorescence spectroscopy and molecular dynamics studies (MD) indicated that there was no significant change in the secondary structure of CALB. Furthermore, the results of MD provide some information supporting that CALB was stabilized by the hydrogen(H)-bonding interaction between surface amino residues of CALB and NADESs and was activated via the H-bonding interaction between substrate and NADESs in the acyl-binding pocket. CONCLUSION: The mechanism by which CALB was activated and stabilized via the H-bonding interactions in NADESs was revealed in the present study. This provides a scientific basis from which to further explore the potential application of NADESs in enzymatic reactions in food engineering and health-related fields. (C) 2019 Society of Chemical Industry