Lysozyme Solution-State Tertiary Structure Measured in Protic and Aprotic Ionic Liquid Aqueous Mixtures

Aqueous protic ionic liquid (PIL) mixtures are interesting solvent media for the stabilization of proteins over wide temperature ranges, which may help develop long term protein storage methods or high temperature catalysis applications. The stabilization and shift in the thermodynamics of hen egg white lysozyme (HWL) was previously demonstrated using differential scanning calorimetry (DSC). Understanding the relation between structure and function of HWL in PIL-based media will help design improved bioprotective solvents. The tertiary structure of HWL in mixtures of PIL ethylammonium nitrate (EAN), water, and an imidazolium-based aprotic ionic liquid (APIL) were measured using small-angle x-ray scattering (SAXS) over temperatures of 25-75 °C. APIL was introduced to confirm reports that APILs destabilize proteins, and to understand the role of aqueous solvation on all the dissolved species. A structural transition of HWL was observed between 50 and 55 °C. Results from models which delineate contributions due to protein unfolding from changes due to irreversible aggregation will be presented and compared with DSC results. The impact of water content on overall miscibility was studied using light scattering methods and the resulting protein structures will be discussed.