Ions are important for modulating protein properties, including solubility and stability, through specific ion effects. Ionic liquids (ILs) are designer salts with versatile ion combinations which have great potential to control protein stability, conformation changes, crystallization and aggregation. The protein phase separation and metal ion (e.g., Ca2+)-binding sites in eukaryotes have been extensively studied [1], however, with some simple protein models, current understanding of specific ion effects on protein properties, interactions and ion binding is limited. There is a need for understanding the ion effect on proteins and developing ion-dominated systems to control protein behaviours.
This presentation explores using small angle X-ray scattering, protein crystallography and spectroscopies to understand the IL effect on model proteins, including hen egg white lysozyme, green fluorescence (GFP) [2, 3], and bovine insulin. In particular, this work unveils the molecular mechanisms underlying the interaction and ion binding between the ILs and protein, providing unprecedented insights into the interfacial phenomena associated with protein ion binding. We emphasize the specific ion binding can induce more flexible loop regions in the protein, while the ion binding in bulk phase can be more dynamic in solution and protein behaviour in ILs depend on the net effect of nonspecific interactions and specific ion binding. Overall, anions showed to have specific binding to the protein surface via direct electrostatic interactions with the charged side chains, and hydrogen bonding with polar and aromatic residues. These findings provide new insights into interfacial behaviour of proteins, protein-ions binding interaction and using ions to modulate protein solution behaviours.