Unveiling the Influence of Hydrated Deep Eutectic Solvents on the Dynamics of Water-Soluble Proteins

Deep eutectic solvents (DESs) are mixtures of two or more pure compounds (e.g., Lewis or Brønsted acids and bases, anionic and/or cationic species) in a well-defined stoichiometric proportion, with a melting point lower to that of an ideal liquid mixture. These neoteric solvents are highly tunable through varying the structure or relative ratio of parent components and have been evaluated as solvents able to improve biomolecules' performance, specifically their stability and biocatalytic properties. Inspired by a recent crystallographic study, we have explored through molecular dynamics (MD) simulations the dynamic properties of two different proteins (hen egg-white lysozyme and the human VH antibody fragment HEL4) in a (20% w/w) hydrated solution of choline chloride-glycerol (1:2). We have developed proper force fields to account for DES, protein, and DES-protein interactions, which have been calibrated using pair distribution function measurements of pure DES solutions. MD results show that the presence of DES quenches the protein motion, increasing the rigidity of the overall protein structure. Specific interactions among DES components and protein residues, such as those between choline ions and two Tryptophan residues of lysozyme, may amplify the protein-DES interactions and lead to protein crystallization in the presence of hydrated DES. These findings open new horizons to improve or achieve control on protein properties by a proper choice of hydrated DESs used as solvents.