Currently buffered aqueous salt solutions are used as solvents for proteins, but these do not sufficiently control protein solubility and stability, which adversely affects protein activity, folding-unfolding transitions, aggregation and crystallisation. Therefore, there is a need for new solvents which can control protein and biomolecule solubility and stability. Protic ionic liquids (PILs) are cost efficient “designer” solvents which can be tailored to have properties suitable for a broad range of applications.[1] These are liquid salts which are typically liquid at room temperature and miscible with water. Certain aqueous PIL solutions have beneficial properties, including stabilising biomolecules, suppressing aggregation and enhancing protein crystal growth [2]. However, there is a lack of understanding about the interactions present, which prevents solvent design for specific protein applications [3].
Recently, we have combined results from solution SAXS and protein crystallography, using Australian Synchrotron SAXS/WAXS and MX2 beamlines, to obtain a deeper understanding of ionic liquid-protein interactions. In these studies, we have used hen egg white lysozyme as a model protein in a selection of aqueous ionic solutions. Specifically, we have identified conformational changes of the protein in solution due to changes in the ionic liquid chemical structure and/or concentrations. We have also identified the ion-binding sites of the ionic liquid solvated cations and anions. From these results we have clearly shown that the anion has significantly more interactions with the protein, and preferentially binds to positively charged and aromatic side chains, whereas few of the cations were identified in the solvation layer. We have been able to relate the ion-binding properties of ionic liquid solutions to their ability to stabilise and maintain protein structure. This is significant progress towards being able to design ionic liquid solutions for specific proteins.