Contrasting Effects of Ionic Liquids of Varying Degree of Hydrophilicity on the Conformational and Interfacial Properties of a Globular Protein
Krishna Prasad Ghanta, Souvik Mondal, Sandip Mondal, Sanjoy Bandyopadhyay
Abstract
Ionic liquids (ILs), depending on their cation–anion combinations, are known to influence the conformational properties and activities of proteins in a nonuniform manner. To obtain microscopic understanding of such influence, it is important to characterize protein–IL interactions and explore the modified solvation environment around the protein. In this work, molecular dynamics (MD) simulations of the globular protein α-lactalbumin have been carried out in aqueous IL solutions containing 1-butyl-3-methylimidazolium cations (BMIM+) in combination with a series of anions with varying degree of hydrophilicity, namely, hexafluorophosphate (PF6–), ethyl sulfate (ETS–), acetate (OAc–), chloride (Cl–), dicyanamide (DCA–), and nitrate (NO3–) . The calculations revealed that ILs with hydrophobic and hydrophilic anions have contrasting influence on conformational flexibility of the protein. It is further observed that the BMIM+ cations exhibit site-specific orientations at the interface depending on the hydrophilicity of the anion component. Most importantly, the results demonstrated enhanced propensity of hydrophilic ILs to replace relatively weaker protein–water hydrogen bonds by stronger protein–IL hydrogen bonds at the protein surface as compared to the hydrophobic ILs. Such breaking of protein–water hydrogen bonds at a greater extent leads to greater loss of water hydrating the protein in the presence of hydrophilic ILs, thereby reducing the protein’s stability.