Proteins are important for a range of applications in the pharmaceutical, biological, chemical and food industries1-2. However, many proteins have low stability and solubility in aqueous buffer solutions. To remedy this, alternative solvents have been investigated in recent decades. Organic solvents can stabilize some proteins3,4, but are often detrimental to proteins and emit volatile organic compounds. Ionic liquids (ILs) on the other hand, due to their wide scope and tailorable properties have been reported to increase protein activity5-6, solubility7, long term and thermal stability8. However, the relationship between the structure of an IL and how it interacts with proteins in solution is unknown. In this study, a plethora of techniques will be used to characterise protein stability, solubility and IL physicochemical properties.
A library of 30 ammonium based ILs and 22 low melting point salts has been developed for use with proteins, and we have used hen egg white lysozyme and human lysozyme as model proteins. Analysis of protein stability and structural changes of lysozyme in IL-water mixtures was conducted using the Small Angle X-ray Scattering beamline at the Australian Synchrotron. These results showed that the structure of the IL, particularly the anion and its functional groups significantly contributed to the stability of the protein. Nitrate based ILs were seen to encourage unfolding of the protein at low concentrations (1-10 mol%) while ILs with hydroxyl groups on either cation or anion enhanced protein stability. These results were correlated with solvent characterisation methods, including Kamlet-Aboud-Taft parameters, physicochemical, and thermal properties of the neat ILs.