Insights into the enhanced thermal stability of lysozyme with altered structure and activity induced by choline chloride-based deep eutectic solvents containing polyols and sugars

The stabilization of protein is crucial for its practical usage in industrial applications. This study sought to systematically determine how deep eutectic solvents (DESs) can stabilize proteins and affect their structure and activity by employing lysozyme as a model. Seven choline chloride (ChCl)-based DESs with polyols and sugars as hydrogen bond donors (HBDs) were evaluated as co-solvents at 10–75 wt% concentrations. The thermal stability (T_m) of lysozyme was improved by all DESs. The T_m increase had a positive correlation with the DES concentrations and the number of [sbnd]OH groups in HBDs. Increased α-helical contents in all conditions partially support the protein stabilization by DESs. Lysozyme could maintain its native tertiary structure and activity at 10 wt% DESs. At higher concentrations, the polyol-class DESs yielded higher T_m and more structure compaction with lower activity suppression than the sugar-class solvents, which displayed discernable changes in their tertiary structure with possible binding at the active site of lysozyme. The hydrogen-bonding ability of HBDs was suggested to play a key role in both classes of DESs by increasing the hydrogen bond network around the protein, which could recruit a protective hydration layer and lead to structure compaction.