Ionic Liquids Induced Structural Changes of Bovine Serum Albumin in Aqueous Media: A Detailed Physicochemical and Spectroscopic Study

Abstract

Structural changes of a globular protein, bovine serum albumin (BSA), as a consequence of interaction with the surface active ionic liquids (ILs)-3-methyl-1-octylimidazolium chloride, [C(8)mim][Cl], and 1-butyl-3-methylimidazolium octylsulfate, [C(4)mim][C(8)OSO(3)]-have been investigated using various physicochemical and spectroscopic techniques such as tensiometry, conductometry, steady-state fluorescence, far-UV circular dichroism spectroscopy (CD), and dynamic light scattering (DLS). The interactional behavior of ILs (monomers and self-assembled structures) toward BSA in different IL concentration regimes at the air/solution interface as well as in the bulk is investigated and discussed depending upon the nature of ions of ILs. CD combined with the steady state fluorescence spectroscopy provided valuable insights into the unfolding of BSA as a consequence of IL binding. The complementary results obtained from the multitechnique approach proved very useful in drawing out the mechanism of interaction between ILs and BSA in different IL concentration regimes.

Ionic Liquid Surfactant Mediated Structural Transitions and Self-Assembly of Bovine Serum Albumin in Aqueous Media: Effect of Functionalization of Ionic Liquid Surfactants

“…The smaller increase in D h from ≈ 8.0 nm to ≈ 11.0 nm in case of BSA-ILS-1 system is assigned to the monomeric interaction of ILS with BSA via electrostatic and hydrophobic interactions leading to partial unfolding of its secondary structure forming ILS-1-MCs. Such marginal increase in D h due to unfolding in monomeric concentration regime has also been reported for other imidazolium based ILSs bearing simple alkyl chain as well as for conventional surfactants 18,33,34. However, in case of BSA-ILS-2 and BSA-ILS-3 systems, a substantial increase in D h from ≈8 nm to 126 nm, and to ≈207 nm, respectively via intermediate structures having D h ≈40 nm up to C 1 is observed.…”

“…From Figure S2B (supporting information), it has been found that N for different ILSs is 57 (ILS-1), 76 (ILS- based ILSs. 33,34 The increase in hydrophobic interactions between alkyl chains of ILSs forming hemi-micelles lead to decrease in interactions between BSA and ILSs stabilizing the ACs between C 2 and C 3 . This destabilizes the ACs leading to structural changes.…”

“…In this regard, our previous report established the destabilizing effect of imidazolium based ILSs, 3-methyl-1-octylimidazolum chloride, [C 8 OSO 3 ], on the native structure of BSA. 33 On the similar lines, Bharmoria et al, reported biamphiphilic ILSs which induce significant folding alterations in structure of BSA, wherein the native BSA has been stabilized by vesicular structures formed by ILSs. 34 Geng et al, have reported the interactional behavior of ILS, 1-tetradecyl-3-methyimidazolium bromide, [C 14 mim] [Br] with BSA, where stabilization of secondary structure along with destabilization of tertiary structure of BSA have been reported at low concentration of ILS.…”

Amino acid-based cationic gemini surfactant–protein interactions

“…The Trp 134 present at the surface of the protein in domain I is surrounded by a number of negatively charged aspartic (Asp) and glutamic (Glu) acid residues to which the cationic gemini (C 12 Cys) 2 can bind by electrostatic interactions thus perturbing the native conformation of BSA [60]. The observed changes suggest a shift of the fluorophores to a more hydrophobic environment that can be attributed to internalization of the fluorophore towards the protein hydrophobic core due to protein refolding (which is possible when protein gets stabilized [59,61]) or to hydrophobic interactions between the dodecyl chains of (C 12 Cys) 2 and hydrophobic amino acid residues around Trp 134, such as leucine (Leu), Tyr and Phe [60], leading to a decrease in intrinsic fluorescence intensity [26,28,57,62,63]. For surfactant concentrations above the CAC (0.30 g L −1 ) the blue shift vanishes and the intensity of the emission reaches a constant value at a surfactant concentration of 0.70 g L −1 .…”

“…The fluorescence quenching process can be described by the Stern-Volmer equation [26,27,50,58,61,66]:…”