Dynamics of cytochrome c in surface active ionic liquid: A study of preferential interactions towards denaturation

Abstract Surface active ionic liquid (SAIL) 1-butyl-3-methylimidazolium octyl sulfate ([C 4 mim][C 8 OSO 3 ]) having octyl tail on anionic moiety have been very less investigated with proteins. Herein, we present a study of the structural change of horse heart cytochrome c (h-cyt c ) in aqueous solutions of SAIL ([C 4 mim][C 8 OSO 3 ]) by optical spectroscopy and molecular docking methods. Time-resolved fluorescence, UV–vis and circular dichroism (CD) spectra indicated that the addition of up to 25 mM of [C 4 mim][C 8 OSO 3 ] induces structural changes of h-cyt c resulting from disruption of tertiary structure and after 25 mM of [C 4 mim][C 8 OSO 3 ], h-cyt c denatures completely and loses its tertiary structure. Thermodynamic parameters for denaturation of h-cyt c by [C 4 mim][C 8 OSO 3 ] were also obtained having Δ G ° D and m -value of the unfolded state. The values were found to be lower when compared to the reported for urea denatured h-cyt c . The midpoint concentration of unfolding of h-cyt c by [C 4 mim][C 8 OSO 3 ] was found ~25 mM. The results showed that the presence of alkyl chain on the anionic moiety destabilizes the protein. The docking results confirm the preferential interactions of ions of SAIL to residues of h-cyt c and suggest the dominant role of anion [C 8 OSO 3 ] − by the electrostatic interaction that drives local destabilization of the h-cyt c .