Deep eutectic solvents (DESs) are utilized as green and inexpensive alternatives to the classical ionic liquids. It was known that some of DESs can be used as solvent in the enzymatic reactions to obtain very green chemical processes. DESs are almost poorly understood at the molecular level. Moreover, we do not know much about the enzyme microstructure in such systems. For example, how some hydrolase can remain active and stable in a deep eutectic solvent including 9M of urea? In this study, molecular dynamic of DESs as a liquid has been simulated at the molecular level. Urea: Choline chloride as a well-known eutectic mixture was chosen as model DESs. Behavior of the lipase as a biocatalyst was studied in this system. For comparison, the enzyme structure was also simulated in urea 8M. Thermal stability of the enzyme was also evaluated in DESs, water, and urea 8M. The enzyme showed very good conformational stability in urea:choline chloride mixture with about 66% urea (9M) even at high temperatures. The results are in good agreement with recent experimental observations. In contrast, complete enzyme denaturation occurred in urea 8M with only 12% urea in water. It was found that urea molecules denature the enzyme by interrupting the intra-chain hydrogen bonds in a “direct denaturation mechanism”. However, in urea:choline chloride deep eutectic solvent, as a result of hydrogen bonding with choline and chloride ions, urea molecules have a low diffusion coefficient and cannot reach to the protein domains. Interestingly, urea, choline, and chloride ions form hydrogen bonds with the surface residues of the enzyme which, instead of lipase denaturation, leads to more enzyme stability. To the best of our knowledge, this is the first study in which the microstructural properties of a macromolecule are examined in a deep eutectic solvent.

Keywords