Biamphiphilic Ionic Liquid Induced Folding Alterations in the Structure of Bovine Serum Albumin in Aqueous Medium

Abstract

3-Methyl-1-octylimidazolium dodecylsulfate, [C8mim][C12OSO3], a vesicle forming biamphiphilic ionic liquid (BAIL) (J. Phys. Chem. B 2012, 116, 14363-14374), has been found to induce significant folding alterations in the structure of bovine serum albumin (BSA) in the aqueous medium at pH 7.0. Such alterations have been investigated in detail using various physicochemical and spectroscopic techniques. Different concentration regimes (monomeric, shared aggregation, and post-vesicular) of [C8mim][C12OSO3]-BSA interactions have been defined through adsorption and binding isotherms using tensiometry and isothermal titration calorimetry (ITC). Fluorimetry, circular dichroism (CD), and dynamic light scattering (DLS) measurements have shown that [C8mim][C12OSO3] induces a small unfolding of BSA in the monomeric regime at low concentration (designated as C(f)), which is followed by a refolding up to critical aggregation concentration (CAC) (designated as C1). Above C1, i.e., in the shared aggregation concentration regime, again a small unfolding of BSA was observed up to critical vesicular concentration (CVC) (designated as C2). In the vesicular and post-vesicular regimes, the BSA remained stable against folding alterations. The kinetic stability of BSA in the vesicular concentration regimes was studied for a month using turbidimetry. It has been found that [C8mim][C12OSO3] stabilizes BSA against the aggregation which is the major cause of protein destabilization. The present study gives insights for the design of surface active ILs for protein stabilization as a potential replacement for the mixed micelles of conventional surfactants used in detergent industries for enzyme stabilization and as an artificial chaperone.

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.…”

“…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. 35,36 The unfolding of secondary structure of BSA by short chain imidazolium and pyrolidinium based ionic liquids (ILs) have also been reported.…”

Amino acid-based cationic gemini surfactant–protein interactions

“…BSA has 19 Tyr residues in different domains and two Trp residues, namely Trp 134, present at the protein surface in domain I, and Trp 213, present in the hydrophobic binding pocket of the protein in domain II, that act as intrinsic fluorophores [58,59]. At the excitation wavelength of 280 nm, the quenching in BSA mainly occurs from binding of the surfactant ion in the vicinity of the forementioned Trp residues [60].…”

“…BSA shows a strong fluorescence emission peak at 340 nm whose intensity gradually decreases upon addition of increasing surfactant concentrations, being accompanied by a blue shift of the maximum emission wavelength from 340 nm to 333 nm. 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].…”