New insight into molecular interactions of imidazolium ionic liquids with bovine serum albumin.

The interactions of imidazolium ionic liquids (ILs), i.e., dibutylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium nitrate, with bovine serum albumin (BSA) were studied by monitoring the spectral behaviors of IL-BSA aqueous systems. The intrinsic fluorescence of BSA at 340 nm excited at 230 nm is obviously quenched by these ILs due to complex dynamic collision and their quenching constants are at the order of 10(2) L mol(-1). However, no fluorescence quenching is observed within the same region when excited at 280 nm, which is widely used for probing protein conformations. Thermodynamic investigations reveal that the combination between ILs and BSA is entropy driven by predominantly hydrophobic and electrostatic interactions, leading to the unfolding of polypeptides within BSA. The influence of the ILs on the conformation of BSA follows a sequence of BmimNO(3) > BmimCl ≈ BbimCl. Molecular docking shows that cationic imidazolium moieties of ILs enter the subdomains of protein and interact with the hydrophobic residues of domain III. An agreement between fluorescence spectroscopic investigations and molecular docking is reached. It is found that the fluorescence of BSA at λ(ex) 230 nm arising from aromatic amino acids Trp and Tyr is almost as sensitive as that achieved at λ(ex) 280 nm for elucidating the protein conformational changes, which provides a valid and new probe for the investigation of binding kinetics between molecules/ions and proteins.