Pankaj Bharmoria

Ionic liquids induced structural changes of bovine serum albumin in aqueous media: a detailed physicochemical and spectroscopic study.

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.

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

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.

Micellization behavior of morpholinium-based amide-functionalized ionic liquids in aqueous media.

Morpholinium-based amide-functionalized ionic liquids (ILs) [C(n)AMorph][Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior in aqueous medium using a variety of state of the art techniques. The adsorption and micellization behavior of [CnAMorph][Br] ILs at the air-solution interface and in the bulk, respectively, has been found to be much better compared to that observed for nonfunctionalized homologous ILs and conventional cationic surfactants, as shown by the comparatively higher adsorption efficiency, lower surface tension at the critical micelle concentraiton (γ(cmc)), and much lower critical micelle concentration (cmc) for [C(n)AMorph][Br] ILs. Conductivity measurements have been performed to obtain the cmc, degree of counterion binding (β), and standard free energy of micellization (ΔG(m)°). Isothermal titration calorimetry has provided information specifically about the thermodynamics of micellization, whereas steady-state fluorescence has been used to obtain the cmc, micropolarity of the cybotactic region, and aggregation number (N(agg)) of the micelles. Both dynamic light scattering and atomic force microscopy have provided insights into the size and shape of the micelles. 2D (1)H-(1)H nuclear Overhauser effect spectroscopy experiments have provided insights into the structure of the micelle, where [C16AMorph][Br] has shown distinct micellization behavior as compared to [C8AMorph][Br] and [C12AMorph][Br] in corroboration with observations made from other techniques.

Structural and functional stability of cellulase in aqueous-biamphiphilic ionic liquid surfactant solution.

In order to explore the potential of a biamphiphilic ionic liquid surfactant as an enzyme stabilizer in detergents, we have investigated the structural and functional stability of cellulase upon interaction with 3-methyl-1-octylimidazolium dodecylsulfate, [C8mim][C12OSO3], in aqueous medium at pH 4.8. Adsorption and binding isotherms determined from tensiometry and isothermal titration calorimetry indicated that [C8mim][C12OSO3] interacts with cellulase distinctly at the three critical concentrations, viz., aggregation, C1, saturation, C2, and vesicular, C3. Fluorescence (at λex = 280 nm), far UV-circular dichroism spectra, and dynamic light scattering results have shown that [C8mim][C12OSO3] alters the tertiary and secondary structure of cellulase with a slight initial unfolding in the monomeric regime (up to C1), refolding in the aggregation regime (up to C2), and unfolding in the shared aggregation regimes (below C3) and stabilizes the altered conformation in the post-vesicular regime with an overall variation of hydrodynamic diameter from 4.12 to 7.19 nm. A dinitrosalicylic acid sugar assay test showed excellent functional stability of cellulase with an activity of ≥1 unit/mg in all the concentration regimes. A very good surface activity (J. Phys. Chem. B 2012, 116, 14363) complied by the present results vindicates the candidature of [C8mim][C12OSO3] as a potential alternative of mixed micelles or nonionic surfactants for cellulase stabilization in detergent industries.

Spontaneous Formation of Multiarchitecture Vesicles of [C8mim]Br + [Na]DBS in Aqueous Medium: Synergic Interplay of Electrostatic, Hydrophobic, and π–π Stacking Interactions

A mixture of a cationic surface active ionic liquid, [C8mim]Br and anionic surfactant, [Na]DBS has been shown to form unilamellar vesicles in water over an exceptionally wide mole fraction range of [C8mim]Br (x1 = 0.2 to 0.8). Formation of vesicles has been evidenced from transmission electron microscopy (TEM), cryo-TEM and atomic force microscopy (AFM) imaging. Cryo-TEM imaging of an equimolar mixture showed multiarchitectural unilamellar vesicles (spherical, tubular, and ribbon). Such complex architectures were earlier reported for Janus dendrimers of different structures (Science, 2010, 328, 1014). The synergism between oppositely charged single chain surfactants to form bilayer structures has been explained based on the evidence of π-π stacking interaction from 2D NOESY measurements, Coulombic interactions from zeta potential measurements and magnitude of interaction parameter from the critical aggregation concentration. The aggregation concentrations were measured from tensiometry and fluorescence using pyrene as a polarity probe. The phase behavior at different mixture compositions has been revealed from turbidity measurements and visual inspection. Hydrodynamic radii of self-assembled structures in the bulk solution phase were measured from dynamic light scattering. Vesicles formed have been explored as delivery vehicles for proteins using bovine serum albumin (BSA) as model.

Temperature-Dependent Solubility Transition of Na2SO4 in Water and the Effect of NaCl Therein: Solution Structures and Salt Water Dynamics

Dual, aqueous solubility behavior of Na2SO4 as a function of temperatures is still a natural enigma lying unresolved in the literature. The solubility of Na2SO4 increases up to 32.38 °C and decreases slightly thereafter at higher temperatures. We have thrown light on this phenomenon by analyzing the Na2SO4-water clusters (growth and stability) detected from temperature-dependent dynamic light scattering experiments, solution compressibility changes derived from the density and speed of sound measurements, and water structural changes/Na2SO4 (ion pair)-water interactions observed from the FT-IR and 2D DOSY (1)H NMR spectroscopic investigations. It has been observed that Na2SO4-water clusters grow with an increase in Na2SO4 concentration (until the solubility transition temperature) and then start decreasing afterward. An unusual decrease in cluster size and solution compressibility has been observed with the rise in temperature for the Na2SO4 saturated solutions below the solubility transition temperature, whereas an inverse pattern is followed thereafter. DOSY experiments have indicated different types of water cluster species in saturated solutions at different temperatures with varying self-diffusion coefficients. The effect of NaCl (5-15 wt %) on the solubility behavior of Na2SO4 at different temperatures has also been examined. The studies are important from both fundamental and industrial application points of view, for example, toward the clean separation of NaCl and Na2SO4 from the effluent streams of textile and tannery industries.

Surface-Active Ionic Liquid Cholinium Dodecylbenzenesulfonate: Self-Assembling Behavior and Interaction with Cellulase

The conventional sodium dodecylbenzenesulfonate (NaDBS) has been converted into an efficient and nontoxic anionic surface-active ionic liquid, cholinium dodecylbenzenesulfonate (Cho[DBS]). We have investigated its self-assembling behavior, interaction with the enzyme cellulase, and ecotoxicity. The surface-active properties at the air–liquid interface and the aggregation behavior of Cho[DBS] in water have been determined using tensiometry, isothermal titration calorimetry (ITC), conductometry, and dynamic light scattering (DLS). The enzyme activity was observed using dinitro salicylic acid analysis. The enhanced enzyme activity was explained through active-site exfoliation and structural constancy of cellulase in the micellar medium using the results from fluorescence, circular dichroism, DLS, and ITC. The nontoxic nature was confirmed by toxicity analysis on the freshwater microalgae Scenedesmus sp.

Simple and Versatile Platform for Air-Tolerant Photon Upconverting Hydrogels by Biopolymer–Surfactant–Chromophore Co-assembly

Exploration of triplet-triplet annihilation based photon upconversion (TTA-UC) in aqueous environments faces difficulty such as chromophores insolubility and deactivation of excited triplets by dissolved oxygen molecules. We propose a new strategy of biopolymer-surfactant-chromophore coassembly to overcome these issues. Air-stable TTA-UC with a high upconversion efficiency of 13.5% was achieved in hydrogel coassembled from gelatin, Triton X-100 and upconverting chromophores (triplet sensitizer and emitter). This is comparable to the highest UC efficiency observed to date for air-saturated aqueous UC systems. Moreover, this is the first example of air-stable TTA-UC in the form of hydrogels, widening the applicability of TTA-UC in biological applications. The keys are two-fold. First, gelatin and the surfactant self-assemble in water to give a developed hierarchical structure with hydrophobic domains which accommodate chromophores up to high concentrations. Second, thick hydrogen-bonding networks of gelatin backbone prevent O2 inflow to the hydrophobic interior, as evidenced by long acceptor triplet lifetime of 4.9 ms. Air-stable TTA-UC was also achieved for gelatin with other nonionic surfactants (Tween 80 and Pluronic f127) and Triton X-100 with other gelling biopolymers (sodium alginate and agarose), demonstrating the versatility of current strategy.