Jyotsana Lal

Interaction of polymer with discotic clay particles

Abstract Normally synthetic well defined monodisperse discotic laponite clays are known to form a gel phase at mass concentrations as low as a few percent in distilled water. Hydrosoluble polymer polyethylene oxide was added to this intriguing clay system, it was observed that it either prevents gelation or slows it down extremely depending on the polymer weight, concentration or the laponite concentration. Small Angle Neutron scattering (SANS) was used to study these systems because only by isotopic labeling can the structure of the adsorbed polymer layers be determined. The contrast variation technique is specifically used to determine separately the different partial structure factors of the clay and polymer. In this way the signal of the adsorbed chains is separated from the signal of the free chains in the dilute regime. Attempts have also been made to characterize the structure in the concentrated regime of laponite with polymer.

WAXS Studies of the Structural Diversity of Hemoglobin in Solution

Specific ligation states of hemoglobin are, when crystallized, capable of taking on multiple quaternary structures. The relationship between these structures, captured in crystal lattices, and hemoglobin structure in solution remains uncertain. Wide-angle X-ray solution scattering (WAXS) is a sensitive probe of protein structure in solution that can distinguish among similar structures and has the potential to contribute to these issues. We used WAXS to assess the relationships among the structures of human and bovine hemoglobins in different liganded forms in solution. WAXS data readily distinguished among the various forms of hemoglobins. WAXS patterns confirm some of the relationships among hemoglobin structures that have been defined through crystallography and NMR and extend others. For instance, methemoglobin A in solution is, as expected, nearly indistinguishable from HbCO A. Interestingly, for bovine hemoglobin, the differences between deoxy-Hb, methemoglobin and HbCO are smaller than the corresponding differences in human hemoglobin. WAXS data were also used to assess the spatial extent of structural fluctuations of various hemoglobins in solution. Dynamics has been implicated in allosteric control of hemoglobin, and increased dynamics has been associated with lowered oxygen affinity. Consistent with that notion, WAXS patterns indicate that deoxy-Hb A exhibits substantially larger structural fluctuations than HbCO A. Comparisons between the observed WAXS patterns and those predicted on the basis of atomic coordinate sets suggest that the structures of Hb in different liganded forms exhibit clear differences from known crystal structures.

Small angle neutron scattering (SANS) studies of a conjugated polyelectrolyte in aqueous solution

Abstract Small angle neutron scattering (SANS) by the conjugated polyelectrolyte, poly[5-methoxy-2-(4-sulfobutoxy)-1,4-phenylene-vinylene], was studied in semi-dilute solutions. A broad peak appears at high momentum transfer (q) for all concentrations. The peak position ( q m ) and intensity ( I ( q m )) scale with polymer concentration as q m ∼ c 1/2 and I ( q m )/ c ∼ c −1/2 , consistent with polyelectrolyte theory. The upturn of I ( q ) at q A −1 indicates chain aggregation. Addition of excess salt removes the broad peak from I ( q ). In high-salt solutions, I ( q )∼ q −1 implying a rod-like conformation with a persistence length larger than 800 A.

Neutron spin-echo studies of hemoglobin and myoglobin: multiscale internal dynamics.

Neutron spin-echo spectroscopy was used to study structural fluctuations that occur in hemoglobin (Hb) and myoglobin (Mb) in solution. Using neutron spin-echo data up to a very high momentum transfer q ( approximately 0.62 A(-)(1)), we characterized the internal dynamics of these proteins at the levels of dynamic pair correlation function and self-correlation function in the time range of several picoseconds to a few nanoseconds. In the same protein solution, data transition from pair correlation motion to self-correlation motion as the momentum transfer q increases. At low q, coherent scattering dominates; at high q, observations are largely due to incoherent scattering. The low q data were interpreted in terms of an effective diffusion coefficient; on the other hand, the high q data were interpreted in terms of mean square displacements. Comparison of data from the two homologous proteins collected at different temperatures and protein concentrations was used to assess the contributions made by translational and rotational diffusion and internal modes of motion to the data. The temperature dependence of decay times can be attributed to changes in the viscosity and temperature of the solvent, as predicted by the Stokes-Einstein relationship. This is true for contributions from both diffusive and internal modes of motion, indicating an intimate relationship between the internal dynamics of the proteins and the viscosity of the solvent. Viscosity change associated with protein concentration can account for changes in diffusion observed at different concentrations, but is apparently not the only factor involved in the changes in internal dynamics observed with change in protein concentration. Data collected at high q indicate that internal modes in Mb are generally faster than those in Hb, perhaps due to the greater surface-to-volume ratio of Mb and the fact that surface groups tend to exhibit faster motion than buried groups. Comparison of data from Hb and data from Mb at low q indicates an unexpectedly rapid motion of Hb alphabeta dimers relative to one another. Dynamic motion of subunits is increasingly perceived as important to the allosteric behavior of Hb. Our data demonstrate that this motion is highly sensitive to protein concentration, temperature, and solvent viscosity, indicating that great care needs to be exercised in interpreting its effect on protein function.

X-ray photon correlation spectroscopy studies of colloidal diffusion and the capillary modes of polymer films

Abstract The new method of X-ray photon correlation spectroscopy (XPCS) is introduced with reference to experiments studying diffusion in concentrated colloidal suspensions. It is shown that both the static structure and the diffusional dynamics of concentrated suspensions of polystyrene spheres in glycerol are well-described by theories developed for hard spheres. Next, XPCS experiments probing the dynamics of surface height fluctuations as a function of lateral length scale are described. Measurements were performed on polystyrene (PS) thin films of thicknesses varying from 840 to 3330 A at temperatures above the PS glass transition temperature. Within a range of wave vectors spanning 10−4– 10 −3 A −1 , good agreement is found between the measured surface dynamics and the theory of overdamped thermal capillary waves on thin films. Quantitatively, the data can be accounted for using the viscosity of bulk PS.

Synchrotron radiation studies of the dynamics of polymer films

X-ray photon correlation spectroscopy, an emerging technique for studying the slow dynamics of condensed matter, has been employed to probe surface fluctuations on thin supported polymer films as a function of in-plane wavevector, film thickness, temperature, and molecular weight of the polymer. The measurements were performed on thin polystyrene (PS) films of thicknesses ranging from 84 to 333 nm above the glass transition temperature. The lateral length scales probed are at least ten times smaller than those accessible in conventional dynamic light scattering. We find excellent agreement between the measured surface dynamics and the theory of overdamped thermal capillary waves on thin viscoelastic films. The values of the viscosity obtained from these data show good agreement with those of bulk PS.

Three-dimensional Imaging of Crystalline Inclusions Embedded in Intact Maize Stalks

Mineral inclusions in biomass are attracting increased scrutiny due to their potential impact on processing methods designed to provide renewable feedstocks for the production of chemicals and fuels. These inclusions are often sculpted by the plant into shapes required to support functional roles that include the storage of specific elements, strengthening of the plant structure, and providing a defense against pathogens and herbivores. In situ characterization of these inclusions faces substantial challenges since they are embedded in an opaque, complex polymeric matrix. Here we describe the use of Bragg coherent diffraction imaging (BCDI) to study mineral inclusions within intact maize stalks. Three-dimensional BCDI data sets were collected and used to reconstruct images of mineral inclusions at 50–100 nm resolution. Asymmetries in the intensity distributions around the Bragg peaks provided detailed information about the deformation fields within these crystal particles revealing lattice defects that result in distinct internal crystal domains.

New instruments based on flipper coils or modulation techniques for steady-state or pulsed neutron sources

We present concepts for two new instruments for steady state or pulsed neutron sources: • An ultra-small small angle scattering option for small angle scattering instruments - the principle might be applied for high resolution reflectometry as well; • A high resolution spin echo option for a long baseline SANS instrument. Further we present first data from a new resonance spin echo option, dedicated to enhance the resolution of thermal triple axis spectrometers by 2 orders of magnitude.

Modulation of Hemoglobin Dynamics by an Allosteric Effector

Hemoglobin (Hb) is an extensively studied paradigm of proteins that alter their function in response to allosteric effectors. Models of its action have been used as prototypes for structure‐function relationships in many proteins, and models for the molecular basis of its function have been deeply studied and extensively argued. Recent reports suggest that dynamics may play an important role in its function. Relatively little is known about the slow, correlated motions of hemoglobin subunits in various structural states because experimental and computational strategies for their characterization are challenging. Allosteric effectors such as inositol hexaphosphate (IHP) bind to both deoxy‐Hb and HbCO, albeit at different sites, leading to a lowered oxygen affinity. The manner in which these effectors impact oxygen binding is unclear and may involve changes in structure, dynamics or both. Here we use neutron spin echo measurements accompanied by wide‐angle X‐ray scattering to show that binding of IHP to HbCO results in an increase in the rate of coordinated motions of Hb subunits relative to one another with little if any change in large scale structure. This increase of large‐scale dynamics seems to be coupled with a decrease in the average magnitude of higher frequency modes of individual residues. These observations indicate that enhanced dynamic motions contribute to the functional changes induced by IHP and suggest that they may be responsible for the lowered oxygen affinity triggered by these effectors.

Scientific Review: Interactions of PEO with Monovalent Electrolytes in Solvents of Different Hydrogen Bonding Capacities

Small-Angle Neutron Scattering (SANS) has been used in order to investigate the ion binding capacities of PEO in different solvents. Aprotic (acetonitrile) as well as protic (methanol and water) solvents, have been studied where methanol and water form an average two and four tetrahedrally oriented hydrogen bonds per molecule, respectively [1]. Unusual behavior of PEO has been observed in aprotic solvents and solvents with moderate hydrogen bonding where monovalent ions associate to the polymer backbone leading to a polyelectrolyte-like (where a certain fraction of monomers are charged) behavior. This is in marked contrast to behavior in aqueous solutions where water molecules associate via hydrogen bonding to the polymer and the ions are more preferentially coordinated by the solvent than the polymer.