title: High ionic liquid concentration-induced structural change of protein in aqueous solution: a case study of lysozyme.
abstract: The structural change of chicken egg white lysozyme in aqueous 1-butyl-3-methylimidazolium nitrate ([bmim][NO(3)]) solutions (0-24 M) has been investigated by optical spectroscopy and small-angle X-ray scattering (SAXS) methods. Fourier-transform infrared (FTIR) and circular dichroism (CD) spectra and SAXS profiles indicated that the addition of up to 6 M of [bmim][NO(3)] induces unfolding of lysozyme resulting from disruption of the α-helix by the NO(3)(-) ion. On the other hand, even with the addition of more than 10 M of [bmim][NO(3)], lysozyme aggregation is inhibited and the protein adopts a partially globular state (the secondary structure is partially refolded while the tertiary structure is disrupted). Observation of the structural features of the aqueous [bmim][NO(3)] solution by Raman OD stretching spectra indicated that bulk-like water still remains at concentrations above 10 M and form an "aggregated water" (water pool) in the nanoheterogeneous structure consisting of a polar domain (the high charge-density region) and nonpolar areas (the alkyl-chain region) in the IL. At these concentrations (above 10 M), lysozyme is not sufficiently hydrated because of the reduced number of water molecules. Consequently lysozyme above 10 M assumes the partially globular state. We propose that the changes of the unique IL solution structure (nanoheterogeneous) between the lower and higher [bmim][NO(3)] concentrations strongly correlated to the differences in the protein stability of the present results.
authors.affiliation: Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Japan 239-8686. take214@nda.ac.jp
authors.forename: Takahiro
authors.initials: T
authors.lastname: Takekiyo
authors.forename: Kumiko
authors.initials: K
authors.lastname: Yamazaki
authors.forename: Erika
authors.initials: E
authors.lastname: Yamaguchi
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Abe
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Yoshimura
mesh_terms: D000818:Animals; D002645:Chickens; D002942:Circular Dichroism; D004795:Enzyme Stability; D007093:Imidazoles / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D008958:Models, Molecular; D009113:Muramidase / Q000737:chemistry*; D011487:Protein Conformation; D058767:Protein Unfolding; D053838:Scattering, Small Angle; D017550:Spectroscopy, Fourier Transform Infrared; D014961:X-Ray Diffraction
chemical_list: C522540:1-butyl-3-methylimidazolium nitrate; D007093:Imidazoles; D052578:Ionic Liquids; C421864:hen egg lysozyme; D009113:Muramidase