Janaky Narayanan

Determination of agarose gel pore size: Absorbance measurements vis a vis other techniques

The absorbance measurements in the wavelength range 700 nm to 800 nm were used to probe the agarose gel topology evolution and extract the pore size of the trapped solvent. By following the changes in absorbance and pore size, the gelation process could be clearly divided into three stages - induction stage, gelation stage and pseudo-equilibrium stage. The gelation mechanism is explained as a nucleation and growth process. Following the kinetics of gelation using dynamic light scattering is complicated by multiple scattering (for high concentrations) and large fluctuations in intensity and relaxation time. Comparatively, scanning the absorption spectrum is fast and the method is suitable for a wide range of concentrations and setting temperatures. Pore size determination using absorbance is a fast and non-invasive method when compared to the DNA electrophoresis measurements, which extend over several hours and use probe diffusion.

Protein Interactions in Undersaturated and Supersaturated Solutions: A Study Using Light and X-Ray Scattering

Protein interactions in undersaturated and supersaturated solutions were investigated using static and dynamic light scattering and small angle x-ray scattering. A morphodrom of lysozyme crystals determined at 35 degrees C and pH = 4.6 was used as a guideline in selecting the protein and precipitant concentrations. The osmotic second virial coefficient, B(22), was determined by static and dynamic light scattering. At low ionic strengths for which no crystals were formed, B(22) was positive indicating repulsive interactions between the protein molecules. Negative B(22) at higher ionic strengths corresponds to attractive interactions where crystallization becomes possible. At two extreme salt concentrations, small angle x-ray scattering data were collected and fitted with a statistical mechanical model based on Derjaguin-Landau-Verwey-Overbeek potential using Random Phase Approximation. This model accounted well for the small angle x-ray scattering data at undersaturated condition with constant potential parameters. At very high salt concentration corresponding to supersaturated solution this model seems to fail, possibly due to the presence of non-Derjaguin-Landau-Verwey-Overbeek hydration repulsion between the molecules.

Structural evolution in catanionic mixtures of cetylpyridinium chloride and sodium deoxycholate

Sodium deoxycholate (NaDC) is a water soluble bile salt commonly used in applications ranging from cell lysis, liposome preparation and isolation of membrane proteins. We present the microstructural evolution in aqueous mixtures of biocompatible cationic surfactant cetylpyridinium chloride (CPC) and bile salt NaDC using dynamic light scattering (DLS), small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS). When the total concentration of the mixture (CT) is less than 370 mM, associative phase separation occurs, near the equimolar ratio, which vanishes at high concentrations (>370 mM). The associative phase separation observed at low CT has been explained on the basis of competition between electrostatic attraction and entropy of mixing of the components. Pure CPC micelles undergo shape transition from prolate to oblate, as the concentration increases from 50 mM to 400 mM. Small addition of NaDC to 400 mM of CPC leads to marginal size change in the oblate micelles. On the other hand, pure NaDC micelles are prolate ellipsoids for which the micellar size increases by incorporation of CPC. The observed structural transition is explained in terms of the electrostatic binding of bile salts to cationic surfactants and the incorporation of the steroidal skeleton in the bile salt at the micelle core-head group interface. Microstructure evolution in catanionic mixtures comprising biocompatible surfactants offers potential pharmaceutical applications.

A small-angle X-ray scattering study of the structure of lysozyme-sodium dodecyl sulfate complexes

The structure of lysozyme-sodium dodecyl sulfate (SDS) complexes in solution is studied using small-angle X-ray scattering (SAXS). The SAXS data cannot be explained by the necklace and bead model for unfolded polypeptide chain interspersed with surfactant micelles. For the protein and surfactant concentrations used in the study, there is only marginal growth of SDS micelles as they complex with the protein. Being a small and rather rigid protein, lysozyme can penetrate the micellar core which is occupied by flexible and disordered paraffin chains and also the shell occupied by the hydrated head groups. A partially embedded swollen micellar model seems appropriate and describes well the scattering data. The SAXS intensity profiles are analyzed by considering the change in the electron scattering length density of the micellar core and shell due to complexation with protein and treating the intermicellar interaction using rescaled mean spherical approximation (RMSA) for charged spheres.

Aggregates from cetyltrimethylammoniumhydroxynaphthalene carboxylate (CTAHNC): a light scattering study

Abstract A light scattering study of cetyltrimethylammoniumhydroxynaphthalene carboxylate (CTAHNC) is presented. In the concentration range studied (0.07–0.6 mM) the structure of the micellar aggregate is likely to be vesicles of size ∼ 100 nm. HNC − is expected to act as a co-surfactant which reduces the bending rigidity, k c , of the bilayer membrane leading to a rough and undulating surface for the vesicles. The fractal dimension d ≈2–2.6, which fits the light scattering data, supports this view. A scanning electron micrograph at a higher concentration indicates that the aggregates are vesicles.

An Analysis of FtsZ Assembly Using Small Angle X-ray Scattering and Electron Microscopy

Small angle X-ray scattering (SAXS) was used for the first time to study the self-assembly of the bacterial cell division protein, FtsZ, with three different additives: calcium chloride, monosodium glutamate and DEAE-dextran hydrochloride in solution. The SAXS data were analyzed assuming a model form factor and also by a model-independent analysis using the pair distance distribution function. Transmission electron microscopy (TEM) was used for direct observation of the FtsZ filaments. By sectioning and negative staining with glow discharged grids, very high bundling as well as low bundling polymers were observed under different assembly conditions. FtsZ polymers formed different structures in the presence of different additives and these additives were found to increase the bundling of FtsZ protofilaments by different mechanisms. The combined use of SAXS and TEM provided us a significant insight of the assembly of FtsZ and microstructures of the assembled FtsZ polymers.

Colloidal phase transition driven by alternating electric field

The transverse two-dimensional assembly of colloidal particles near an electrode surface subjected to ac polarization is studied by varying the frequency and field strength in the absence and presence of an added electrolyte. The variation of the translational and bond-orientational correlation functions with frequency suggests the existence of a hexatic phase in which the particles retain the remnants of the crystalline long-range orientational order, but has a liquidlike translational order. The electrohydrodynamic (EHD) flow is analyzed in the light of the existing theoretical models. The equilibrium distribution of particles is considered to be the resultant of the two opposing forces--Stokes force due to EHD flow and the screened Coulomb interaction between the colloidal particles. Several features of the experimental results are discussed, such as the role played by the EHD flow in the particle aggregation, the dependence of the equilibrium interparticle separation on ionic strength, zeta potential, and particle size.

Nanoconfinement of Water Layers in Lamellar Structures Prepared in the Presence and Absence of Organic Solvent

An attempt is made to draw a line of comparison between the extent of rigidity of the hydration layers bound to the interfacial region of lamellar structures of Aerosol OT (AOT, sodium bis(2-ethylhexyl) sulfosuccinate) in water, in the presence and absence of an organic solvent using POM, SAXS, cryo-TEM, and time-resolved fluorescence spectroscopy. These systems are ternary mixtures of AOT, water, and n-heptane containing lamellar structures in an aqueous layer at higher w(0) values (w(0) = 300 and 150) and a binary solution of 20 and 50% AOT in neat water (w/w). The solvation shells residing at the vicinity of these lamellar structures are monitored using two different coumarin probes (C153 and C500). It is intended to envisage a comparative solvation dynamics study of the restricted aqueous region confined in lamellar structures formed in ternary mixture and binary solution. Though steady state measurements show a similar microenvironment probed by the fluorophores in lamellar structures formed in the two different aqueous phases, temporal evolution of the solvent correlation function C(t) unveils the existence of lamellar structures with different degrees of confinement of water layers in these two systems. A slower relaxation of the restricted aqueous region in lamellar structures of binary solution signifies the presence of more rigid interfacially bound water layers at the lamellar interface than in the ternary mixture having a similar weight percentage of AOT in water. The present investigation concludes that the lamellar structures formed under two different conditions provide a similar hydrophobic environment with different extents of localized water populations at the lamellar interface as manifested by the solvent relaxation time in agreement with SAXS and cryo-TEM images.

Understanding of hydrogel network formation and its application in the architecture of significantly enhanced hydrogel

An understanding of the physical hydrogel network formation has been obtained by dynamic rheological experiments. The evidence shows that the network formation turns out to be a nucleation-controlled process. It was found that there exists a critical temperature Tc; fiber branching is greatly enhanced when the network formation is performed in the regime of T<Tc (T, the final setting temperature). This finding enables the authors to build significantly enhanced gel networks. So far G′ (elastic modulus) of the hydrogel network has been enhanced by 187% while the formation period can be greatly shortened to only 1∕20 of the previous process.An understanding of the physical hydrogel network formation has been obtained by dynamic rheological experiments. The evidence shows that the network formation turns out to be a nucleation-controlled process. It was found that there exists a critical temperature Tc; fiber branching is greatly enhanced when the network formation is performed in the regime of T<Tc (T, the final setting temperature). This finding enables the authors to build significantly enhanced gel networks. So far G′ (elastic modulus) of the hydrogel network has been enhanced by 187% while the formation period can be greatly shortened to only 1∕20 of the previous process.

Helix – Rod transition in a nanospring

A model of helical spring is described whose effective pitch and length are decided by the balance of the attractive van der Waals and repulsive hydration and electrical double layer forces. Also the electric contribution to the curvature free energy is taken into account. The stability of the nanospring is investigated and it is shown that the spring becomes unstable and jumps to its extended state on increase of electrostatic repulsion beyond a limit implying a jump in viscosity of suspension of springs in the solution. The magnitude of the jump and threshold potential are investigated.