Aruna Dhathathreyan

Reversible and irreversible conformational transitions in myoglobin: role of hydrated amino acid ionic liquid.

Hydrated phenylalanine ionic liquid (Phe-IL) has been used to solubilize myoglobin (Mb). Structural stability of Mb in Phe-IL analyzed using fluorescence and circular dichroism spectroscopy shows that for low levels of hydration of Phe-IL there is a large red shift in the fluorescence emission wavelength and the protein transforms to complete β sheet from its native helical conformation. Rehydration or dilution reverses the β sheet to an α helix which on aging organizes to micrometer-sized fibrils. At concentrations higher than 200 μM, the protein changes from β to a more random coiled structure. Organization of the protein in Phe-IL in a Langmuir film at the air/water interface has been investigated using the surface pressure-molecular area isotherm and shows nearly the same surface tension for both pure Mb and Mb in Phe-IL. Scanning electron microscopy of the films of Mb in Phe-IL transferred using the Langmuir-Blodgett film technique show layered morphology. This study shows that the conformation of Mb is completely reversible going from β → helix → β sheet up to 200 μM of Phe-IL. Similar surface tension values for Mb in water and in Phe-IL suggests that direct ion binding interactions with the protein coupled with the change in local viscosity from the IL seems to not only alter the secondary structure of individual proteins but also drives the self-assembly of the protein molecules leading finally to fibril formation.

Tuning size and catalytic activity of nano-clusters of cobalt oxide

Cobalt oxides were prepared by three different methods: (1) by reacting cobalt nitrate with oxalic acid, (2) co-precipitating cobalt nitrate with sodium carbonate, and (3) using sodium dodecyl sulphate as organic surfactant. All three samples were characterized before and after calcination by solvent extraction and the resulting products examined by IR spectroscopy. In the case of method 3, the removal of surfactant was followed by TGA studies. Products from all three methods were identified by XRD. Peaks in low angle XRD indicate the porous nature of the oxides. The morphology of the pores was studied by transmission electron microscopy. Some irregular pore structures were obtained for samples from methods 1 and 2, with an average size of 4–6 nm. Only the product from method 3 using SDS as template showed ordered structure and optimum size, and Brunauer-Emmet-Teller surface areas of the as-prepared, as well as the treated samples, exhibited H3 type hysteresis. The samples from the three methods were used as catalysts in the oxidation reaction of cyclohexane under mild conditions and the catalytic efficiency of the cobalt oxide was comparable with mesoporous cobalt oxides.

Surface energy of aqueous solutions of Hofmeister electrolytes at air/liquid and solid/liquid interface

In this study, the effect of Hofmeister electrolytes in aqueous solution and their effect on the interfacial structure is analyzed by measuring their surface tension, zeta and surface potential. Gradients of surface tension with concentration versus the molar entropy of hydration of the various ions and the dispersion component of the solid/liquid surface energy of glass (high surface energy, HSFE) and Teflon (low surface energy, LSFE) show the Hofmeister effect clearly. This provides a rationale for the inclusion of dispersion forces in the evaluation of surface tension numerically and in turn on the overall studies of surface energy.

Microviscosity-induced conformational transition in β-lactoglobulin in the presence of an ionic liquid.

This study reports on the helix-beta conformation transition of bovine β-lactoglobulin (βLG) prepared at two different pH conditions (pH 4 and 7.5) and in the presence of the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate (IL-emes). The investigation was carried out by combining a range of techniques such as circular dichroic (CD) spectroscopy, steady-state fluorescence spectroscopy, isothermal titration calorimetry (ITC), and transmission electron microscopy. The influence of microviscosity induced by IL-emes on the secondary structure of βLG was studied using a quartz crystal microbalance and correlated with the steady-state fluorescence emission. The effect of heat on the helix-beta transition in βLG was directly measured by ITC by titrating βLG with IL-emes. The net effect of heat after subtraction of the heat of dilution was negative in both cases, suggesting that the protein moves to a stable conformation. The changes in the overall aggregated structures were confirmed by transmission electron microscopy, where a shift in the size and morphology of aggregates was found, from large clusters (size of 70 nm) at pH 4 to smaller aggregates (size of 20 nm) at pH 7.5, which reduced to 7 nm in the presence of the IL. The transformation of helical to beta structure at pH 4 show that the folding pathway in the presence of the ionic liquid is hierarchical, whereas at neutral pH, it appeared to be nonhierarchical and the final native structure was acquired by nonlocal interactions through typical forces involved in the stabilization of the tertiary structure.

Contact angle hysteresis of liquid drops as means to measure adhesive energy of zein on solid substrates

Adhesion of zein to solid substrates has been studied using surface energy profiles as indices and by adhesion mapping using atomic force microscopy (AFM). Different plasticizers like glycerol and sorbitol have been used to form mixed films with zein and properties of these films are studied using surface energy profiles. Comparison of the results from the different mixed samples with those from the pure zein films showed that force mapping could identify areas rich in protein. The adhesion maps produced were deconvoluted from sample topography and contrasted with the data obtained from contact angle measurements. A comparison of the two methods shows that the extent of contact angle hysteresis is indicative of both hydrophobicity of the surface as well as the force of adhesion. Mechanical properties and microstructure of zein films prepared by casting from solutions and using Langmuir-Blodgett film technique have been investigated. Pure zein seemed brittle and exhibited an essentially linear relationship between stress and strain. Films with plasticizer were tougher than these films. In general, mixed films showed better mechanical properties than pure films and had higher ultimate tensile strength and increased per cent elongation. Further, the mixed films of zein showed a higher force of adhesion compared to the pure films.

Investigations on geometrical features in induced ordering of collagen by small molecules

Binding energies of the interaction of collagen like triple helical peptides with a series of polyphenols, viz. gallic acid, catechin, epigallocatechingallate and pentagalloylglucose have been computed using molecular modelling approaches. A correlation of calculated binding energies with the interfacial molecular volumes involved in the interaction is observed. Calculated interface surface areas for the binding of polyphenols with collagen-like triple helical peptides vary in the range of 60–210 Å2 and hydrogen bond lengths vary in the range of 2.7–3.4 Å. Interfacial molecular volumes can be calculated from the solvent inaccessible surface areas and hydrogen bond lengths involved in the binding of polyphenols to collagen. Molecular aggregation of collagen in the presence of some polyphenols and chromium (III) salts has been probed experimentally in monolayer systems. The monolayer arrangement of collagen seems to be influenced by the presence of small molecules like formaldehyde, gluteraldehyde, tannic acid and chromium (III) salts. A fractal structure is observed on account of two-dimensional aggregation of collagen induced by tanning species. Atomic force microscopy has been employed to probe the topographic images of two-dimensional aggregation of collagen induced by chromium (III) salts. A case is made that long-range ordering of collagen by molecular species involved in its stabilisation is influenced by molecular geometries involved in its interaction with small molecules.

Interaction of chromium(III) complexes with model lipid bilayers: implications on cellular uptake.

To understand molecular cytotoxicity of chromium(III) and how it affects the stability of biological membranes, studies on the interaction of chromium(III) complexes aquapentaminechromium complex (complex I) and trans- [Cr(5-methoxysalcyclohex) (H(2)O) (2)] ClO(4) (complex II) with model biomembranes have been carried out. Langmuir films of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidic acid (DPPA), dioctadecyldimethylammoniumbromide (DOMA) at air/water interface interacting with the chromium(III) complexes have been characterized using the surface pressure-molecular area (π-A) isotherms. Initial surface pressures changes for the two complexes show that the chromium(III) complexes inserted in the Langmuir films and complex I interacted strongly compared to complex II. Supported bilayers (SB) of the lipids on solid substrates formed by hydrating their Langmuir-Blodgett films (LB films) have been characterized using linear dichroic spectra, low angle X-ray diffraction and steady state fluorescence anisotropy. Depending on the geometry of the ligands and concentration, the complexes either insert in the alkyl or in the head group region of the SB and sometimes in both regions. The Supported lipid bilayers are well-layered and at low concentration, the metal complexes are incorporated near the head group region. Order and increase in lamellar spacing show stronger interaction of complex I with the lipids compared with complex II. This study provides some insights into the mechanism of chromium(III) toxicity and uptake of chromium(III) by the cells.

Assembling fibrinogen at air/water and solid/liquid interfaces using Langmuir and Langmuir-Blodgett films.

Langmuir films of pure fibrinogen (Fg) and Fg spread at the air/buffer interface and subphase containing electrolytes, NaCl, KCl, CaCl(2), and ZnCl(2), have been analyzed to understand the role of the surface/interface in mediating the organization of the protein eventually to fibrils. These films have been characterized by the surface pressure and surface potential-molecular area ((pi-A) and (DeltaV-A)) isotherms and Brewster angle microscopy (BAM). The Langmuir-Blodgett (LB) films of the protein transferred to the solid substrates have been characterized by scanning electron microscopy (SEM) and circular dichroism (CD). Our results suggest that fibrils are formed during organization at air/solution interface and also in LB films. The rate of formation of the fibril is the maximum for Fg with ZnCl(2). Adsorption of Fg to surfaces coated with a neutral lipid, dimyristoylphosphatidylcholine (DMPC), and a cationic lipid, dioctadecyldimethylammonium bromide (DOMA), from a range of solution concentrations has been studied using a quartz crystal microbalance (QCM). The work of adhesion of the protein on the solid surface shows fibril formation and positive adhesion for Fg in the presence of electrolytes. SEM results show that the adherent protein exhibits the widely reported nodulelike structure in the presence of CaCl(2) and ZnCl(2). These results provide definite evidence that specifically designed surfaces can promote adhesion of Fg and also activate fibril formation even in the absence of thrombin.

Langmuir–Blodgett film properties of a donor–acceptor Schiff base

Abstract A low-molecular weight Schiff base with donor–acceptor substituent on the Schiff base has been synthesized in order to study the Langmuir–Blodgett (LB) film properties. This is the first report wherein we demonstrate a short chain, Schiff base forms stable monolayers which have been transferred onto solid substrates to prepare Langmuir–Blodgett multilayers. Spectroscopic measurements of this Schiff base in chloroform as well as its LB film show changes in the spectral features indicating the changes in the environment around the chromophore.

Insight into structural transition, aggregation and folding of glycoprotein in the presence of chromium(III) complexes by spectroscopic, microscopic and kinetic studies

An octahedral complex of chromium, tris(ethylenediamine)chromium(III) chloride which is known to exhibit ionic interaction with the protein has been found to bring about structural transition in the protein conformation. The binding of tris(ethylenediamine)chromium(III) chloride to mucin was found to bring about transition of native random coil structure to α helix with the resultant formation of aggregates as evidenced by circular dichroic and infrared spectroscopy and atomic force microscopy. This conformational change was accompanied by protein folding as depicted by change in the fluorescence intensity of the protein fluorophore in the presence of tris(ethylenediamine)chromium(III) chloride.