Impact of imidazolium-based ionic liquids on the structure and stability of lysozyme

Various types of water-miscible aprotic ionic liquids (ILs) with different cations (1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-octyl-3-methylimidazolium) and anions (ethylsulfate and chloride) were used as co-solvents to investigate the stability of lysozyme. Different techniques such as fluorescence, thermal absorption, and circular dichroism (CD) spectroscopy have been used for the study. Fluorescence results reveal that the addition of ILs (1-ethyl-3-methylimidazolium ethyl sulfate and 1-ethyl-3-methylimidazolium) increases the hydrophobicity around the tryptophan environment in lysozyme. CD analysis and temperature-dependent studies were done to investigate the stability of the protein. From the CD analysis, it was observed that the ILs keep the native structure of protein intact. Thermal denaturation studies depicted that the melting temperature of the protein increased in the presence of ILs (1-ethyl-3-methylimidazolium ethyl sulfate and 1-ethyl-3-methylimidazolium), which indicates the stabilization of the protein.