Small-angle X-ray Scattering Studies Research Articles

A SAXS Study of Hydrogenated Nanocrystalline Silicon Thin Films

AbstractWe have used small-angle x-ray scattering (SAXS) in conjunction with X-ray diffraction (XRD) to study the nanostructure of hydrogenated nanocrystalline silicon (nc-Si:H). The crystallite size in the growth direction, as deduced from XRD data, is 24 nm with a preferred [220] orientation in the growth direction of the film. Fitting the SAXS intensity shows that the scattering derives from electron density fluctuations of both voids in the amorphous phase and H-rich clusters in the film, probably at the crystallite interfaces. The SAXS results indicate ellipsoidal shaped crystallites about 6 nm in size perpendicular to the growth direction. We annealed the samples, stepwise, and then measured the SAXS and ESR. At temperatures below 350◦C, we observe an overall increase in the size of the scattering centers on annealing but only a small change in the spin density, which suggests that bond reconstruction on the crystallite surfaces takes place with high efficacy.

UV and X‐ray structural studies of a 101‐residue long Tat protein from a HIV‐1 primary isolate and of its mutated, detoxified, vaccine candidate

The 101-residue long Tat protein of primary isolate 133 of the human immunodeficiency virus type 1 (HIV-1), wt-Tat(133) displays a high transactivation activity in vitro, whereas the mutant thereof, STLA-Tat(133), a vaccine candidate for HIV-1, has none. These two proteins were chemically synthesized and their biological activity was validated. Their structural properties were characterized using circular dichroism (CD), fluorescence emission, gel filtration, dynamic light scattering, and small angle X-ray scattering (SAXS) techniques. SAXS studies revealed that both proteins were extended and belong to the family of intrinsically unstructured proteins. CD measurements showed that wt-Tat(133) or STLA-Tat(133) underwent limited structural rearrangements when complexed with specific fragments of antibodies. Crystallization trials have been performed on the two forms, assuming that the Tat(133) proteins might have a better propensity to fold in supersaturated conditions, and small crystals have been obtained. These results suggest that biologically active Tat protein is natively unfolded and requires only a limited gain of structure for its function.

Small-Angle X-ray Scattering Study of Water Free Fuel Cell Membranes Containing Ionic Liquids

The effect of polymeric membranes containing ionic liquids in replacement of water on the morphology and the formation of ionic aggregates has been studied using mainly small-angle X-ray scattering...

Comb-like poly(4-vilylpyridinium) salts with dodecylsulfate, sodium bis(2-ethylhexyl) sulfosuccinate and bromide counter ions. Small-angle X-ray scattering and dynamic light scattering study

Poly(4-vinylpyridine) derivatives alkylated with dimethylsulfate, octylbromide and dodecylbromide were synthesized. Bromide anions in the polymer salts were substituted with octylsulfate, dodecylsulfate or bis(2-ethylhexyl)sulfosuccinate (AOT) anions using ion-exchange reactions. The small-angle X-ray scattering (SAXS) studies of the structure of the comb-like polysalts demonstrated their highly-ordered structure. Loading the comb-like polycation with the surfactant anions leads to additional ordering of the polysalts structure. In the case of large AOT anions the transition from lamella to hexagonal structure is observed. Polyelectrolyte – surfactant complexes (PEC) of poly(4-vinyl-1-methylpyridinium) cation with octylsulfate and dodecylsulfate counter ions demonstrate a higher degree of order than the comb-like polymers having a similar chemical structure. The SAXS studies were complemented by dynamic light scattering study of the solutions of the polymers in chloroform. In the solution the comb-like polymers and their complexes with linear surfactants have coil conformations with the size of the coils close to that of the original poly-4-vinylpyridine. The conformations of PECs with octylsulfate and dodecylsulfate counter ions are more unfolded than those of the corresponding comb-like polymers.

μ-PIXE and SAXS studies at the bone–cartilage interface

Micro Proton Induced X-ray Emission (micro-PIXE) analysis has been employed herein in investigating and quantifying the distribution of a number of essential elements in thin human diseased articular cartilage sections affected by osteoarthritis (OA). Various cations Ca, P and Zn have been reported to play an important role both in the normal growth and remodelling of articular cartilage and subchondral bone as well as in the degenerative and inflammatory processes associated with the disease; they act as co-factors of a class of enzymes known as metalloproteinases which are believed to be active during the initiation, progress and remodelling processes associated with osteoarthritis. Other important enzymes such as alkaline phosphatase are associated with cartilage mineralization. Synchrotron radiation X-ray fluorescence (SR-XRF) for mapping of elemental distributions in bone and cartilage has also been employed by the present group and others. In the current investigations using the cSAXS beamline at the Swiss light source, Small-Angle X-ray Scattering (SAXS) was carried out on decalcified human articular cartilage to explore the structural and organizational changes of collagen networks in diseased articular cartilage.

The FTIR and SAXS studies of influence of a morpholine derivatives on the DMPC-based biological membrane systems

Fourier transformed infrared (FTIR) spectroscopy and small-angle scattering of synchrotron radiation have been used to analyze the influence of two zwitterionic surfactants of the sulfobetaine structure (3-(N-decylmorpholine)-1-propansulfonate and 3-(N-tetradecylmorpholine)-1-propansulfonate) on the conformational dynamics of CH 2 and O–P–O groups of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and the structure of phospholipid bilayers. The presence of the surfactant causes changes in the DMPC phase transition temperature. Our small-angle X-ray scattering (SAXS) data analysis implies a gradual disappearance of the lamellar phase typical of DMPC and a probable formation of the bicellar phase or cubic phase in DMPC/surfactants system.

EXAFS and SAXS studies of ZrCo alloy doped with Hf, Sc and Ti atoms

The local structures of ZrCo alloy doped with Hf, Sc and Ti atoms before and after hydrogenation were studied by extended X-ray absorption fine structure (EXAFS) technique. For all the samples, the length of the Co–Zr bond increased and the coordination numbers of Co reduced after hydrogenation. As the Hf, Sc and Ti atoms came into the ZrCo alloy, the lengths of the Co–Zr bond varied. The ingoing Ti atoms had shortened the Co–Zr bond lengths from 2.74 to 2.62 Å in the pre-hydrogenation, whereas all of the doped atoms had no obvious effect in the post-hydrogenation. Also the size distribution of the nanoparticles in the as-cast and hydrogenated Zr–Co alloy was investigated by small-angle X-ray scattering (SAXS) technique.

Particle size distribution study by small-angle X-ray scattering technique and photoluminescence property of ZnO nanoparticles

ZnO nanoparticles have been synthesised by thermal decomposition of zinc acetate at ∼800°C. The structural characteristics and size distribution of ZnO nanoparticles have been investigated by X-ray powder diffraction (XRD) and small-angle X-ray scattering (SAXS), respectively. SAXS study reveals nanoparticles are of different sizes: namely 23 wt% of 8 nm, 19 wt% of 21 nm and 58 wt% of 51 nm. These ZnO nanoparticles possess yellow visible emission at 552 nm. The polydispersity and single emission peak at 552 nm in ZnO nanoparticles suggest that the yellow emission might be a bulk property instead of having a surface origin in nanostructured ZnO. The surface impurities are characterised by Fourier-transform infrared spectroscopy. The quenching of band edge emission in ZnO nanoparticles seems due to the presence of surface impurities.

Structure of the Noncatalytic Domains and Global Fold of the Protein Disulfide Isomerase ERp72

Protein disulfide isomerases are a family of proteins that catalyze the oxidation and isomerization of disulfide bonds in newly synthesized proteins in the endoplasmic reticulum. The family includes general enzymes such as PDI that recognize unfolded proteins, and others that are selective for specific classes of proteins. Here, we report the X-ray crystal structure of central non-catalytic domains of a specific isomerase, ERp72 (also called CaBP2 and protein disulfide-isomerase A4) from Rattus norvegicus. The structure reveals strong similarity to ERp57, a PDI-family member that interacts with the lectin-like chaperones calnexin and calreticulin but, unexpectedly, ERp72 does not interact with calnexin as shown by isothermal titration calorimetry and nuclear magnetic resonance (NMR) spectroscopy. Small-angle X-ray scattering (SAXS) of ERp72 was used to develop models of the full-length protein using both rigid body refinement and ab initio simulated annealing of dummy atoms. The two methods show excellent agreement and define the relative positions of the five thioredoxin-like domains of ERp72 and potential substrate or chaperone binding sites.

π-conjugation and conformation in a semiconducting polymer: small angle x-ray scatteringstudy

Small angle x-ray scattering (SAXS) in a poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) solution has shown the important role ofπ-electron conjugation in controlling the chain conformation and assembly. Byincreasing the extent of conjugation from 30 to 100%, the persistence length (lp) increases from 20 to 66 Å. Moreover, a pronounced second peak in the pair distributionfunction has been observed in a fully conjugated chain, at larger length scales. This featureindicates that the chain segments tend to self-assemble as the conjugation along the chainincreases. Xylene enhances the rigidity of the PPV backbone to yield extended structures,while tetrahydrofuran solvates the side groups to form compact coils in which thelp is much shorter.

Structure of Nonionic Surfactant (Glycerol α-Monomyristate) Micelles in Organic Solvents: A SAXS Study

In this paper we present the study of reverse micellar aggregates formed by glycerol alpha-monomyristate (C(14)G(1)) in different organic solvents. We have investigated the structure (mainly shape and size) of the monomyristin reverse micelles depending on alkyl chain length of the oils, temperature, and the surfactant concentration. Moreover, we present how addition of trace polar additives such as water or glycerol and the hydrophilic size of the surfactant affect the structure of the micelles. Monomyristin could not form any self-organized structure in nonpolar media at room temperature because of its smaller headgroup size and tends to separate from the nonpolar media. In dilute regions this surfactant forms a solid dispersion, which upon heating transforms into an isotropic reverse micellar solution. For the structural characterization of the reverse micelles formed at elevated temperatures, we have performed small-angle X-ray scattering (SAXS) measurements, and the data were analyzed utilizing the generalized indirect Fourier transformation (GIFT) method. The SAXS data have shown that the size of the reverse micelles increases with the alkyl chain length of oils and surfactant concentration but decreases with temperature. A dramatic growth to the micelles could be achieved when the hydrophilic size of the surfactant is increased. Furthermore, addition of trace water or glycerol induces a significant change in the structure of micelles in terms of a cross-section and maximum length. The added water tends to form a water pool in the micellar core and results in swollen micelles. Model-free cross-section analysis of the SAXS data provided complementary information on the structural parameters. Thus, the present study has highlighted the possible ways of controlling the structure, mainly the shape and the size of the inverse micelles.

Unidirectional Binding of Clostridial Collagenase to Triple Helical Substrates

Histotoxic clostridia produce collagenases responsible for extensive tissue destruction in gas gangrene. The C-terminal collagen-binding domain (CBD) of these enzymes is the minimal segment required to bind to collagen fibril. Collagen binding efficiency of CBD is more pronounced in the presence of Ca(2+). We have shown that CBD can be functional to anchor growth factors in local tissue. A (1)H-(15)N HSQC NMR titration study with three different tropocollagen analogues ((POG)(10))(3), ((GPOG)(7)PRG)(3), and (GPRG(POG)(7)C-carbamidomethyl)(3), mapped a saddle-like binding cleft on CBD. NMR titrations with three nitroxide spin-labeled analogues of collagenous peptide, (PROXYL-G(POG)(7)PRG)(3), (PROXYL-G(POG)(7))(3), and (GPRG(POG)(7)C-PROXYL)(3) (where PROXYL represents 2,2,5,5-tetramethyl-l-pyrrolidinyloxy), unambiguously demonstrated unidirectional binding of CBD to the tropocollagen analogues. Small angle x-ray scattering data revealed that CBD binds closer to a terminus for each of the five different tropocollagen analogues, which in conjunction with NMR titration studies, implies a binding mode where CBD binds to the C terminus of the triple helix.

Small Angle X-ray Scattering Study of Doped ZnO Nanocrystals Using Generalized Indirect Fourier Transformation

Small Angle X-ray Scattering Study of Doped ZnO Nanocrystals Using Generalized Indirect Fourier Transformation

A SAXS Study of Glucagon Fibrillation

Protein amyloid formation proceeds through a number of different stages. Oligomeric species observed at early stages have aroused particular interest because of evidence for their involvement in cytotoxic processes such as membrane permeabilization. It is unclear whether these oligomers are obligate precursors to fibrils or represent “dead-end” species that impede fibrillation. Because of the many interconverting species present during amyloid formation, it is important to study the process as non-invasively as possible. Small angle X-ray scattering (SAXS) measurements allow us to monitor structural changes in solution for a population of different species over time. Here, SAXS was used to provide a detailed structural description of the fibrillation of the 29 residue peptide hormone glucagon at pH 2.5 from the monomer and early oligomers to mature fibers. Investigation of the pseudo-equilibrium behavior in the lag phase before fibrillation at several concentrations showed that glucagon is present in a monomeric form below about 5.1 mg/mL, while larger oligomers with average aggregation numbers of about three and seven, are formed at 6.4 and 10.7 mg/mL, respectively. Applying several modeling tools to the experimental data, it is shown that the early oligomerization states can be described as associations between glucagon molecules. After the lag phase, a short rod-like protofibril (radius of ∼ 16 Å and length > 300 Å) is formed and subsequently grows to > 1000 Å in length and assembles into long triple-bundled mature fibers. The protofibril shares many features with the elongated oligomer proposed to be the structural nucleus for insulin fibrils. We propose that on-pathway fibrillar intermediates share this elongated shape that easily allows them to be incorporated into mature fibrils. This contrasts with the annular shape, which is suggested to be involved in cytotoxic membrane permeabilization and may represent a dead-end species off the fibrillar pathway.

Intrinsic parameters for structural variation of reverse micelles in nonionic surfactant (glycerol α-monolaurate)/oil systems: a SAXS study

Using Small-Angle X-ray scattering (SAXS), we have studied self-assembled structures of glycerol alpha-monolaurate (C(12)G(1)) in different nonpolar solvents, such as cyclohexane, n-octane, n-decane, n-tetradecane, and n-hexadecane, as a function of composition, temperature, and added polar additives. The dilute mixtures of C(12)G(1) with the aforementioned oils are essentially a solid dispersion at lower temperatures without forming any liquid crystalline phases, which are transformed into isotropic reverse micellar solutions upon heating. The generalized indirect Fourier transformation (GIFT) analysis of the SAXS data has drawn a clear picture of the structural variations of the reverse micellar aggregates in the isotropic solution phase. A notable effect on the micellar growth emerges when cyclohexane is replaced with the linear-chain hydrocarbon oils; a gradual increase of the carbon number of the hydrocarbon skeleton from octane to hexadecane, ellipsoidal prolate-like micelles are eventually transformed to long rod-like ones. Far more pronounced structural enhancement could be achieved by increasing the hydrophilicity of the surfactant, as short-rod micelles observed in the C(12)G(1)/decane system are replaced with long cylindrical ones when a more hydrophilic diglycerol monolaurate (C(12)G(2)) is used. This fact also supports a scenario that regulates the ability to form elongated to reverse structures. Lowering the temperature and increasing the surfactant concentration similarly lead to micellar growth while the cross-sectional structure remains essentially unchanged. In contrast, polar additive-induced growth is accompanied by the rapid swelling of the micellar core. Our data demonstrate that optimization of the oil properties and/or other outer conditions provide possible routes to flexibly controlling the size, shape, and internal structure of the reverse micelles despite lower universality for a shorter hydrophilic chain surfactant.

Transport and time dependent small angle X-ray morphology in metal cation and polymer modified perfluorocarbon ionomers

Transport and absorption in metal cation exchanged and polyvinylpyrrolidone (PVP) modified Nafion® films were studied by various techniques. To understand the microscopic basis for permeation, time resolved small angle X-ray scattering (SAXS) was used to characterize the ionic domain morphology while films were exposed to vapors of water, dimethyl methylphosphonate (DMMP), or triethyl phosphate. Macroscopic weight uptake studies of DMMP vapor into PVP-Nafion® were also used to help explain the SAXS absorption studies and DMMP permeation properties. The SAXS results were correlated with macroscopic permeation rates of DMMP, Soman, and water through several different membranes. To provide additional basis for the SAXS derived morphologies, tapping-mode AFM was also used to image the 3–5 nm diameter ionic domains. A goal for protective suit applications is to find films with a balance of high moisture permeation rate for comfort, and low DMMP permeation. The best balance of properties in this context was found with PVP- and zirconyl-modified films. In another extreme, the permeation rates of both water and DMMP through cesium-modified Nafion® were low. SAXS studies were used to explain this where the ionic domains of cesium-modified Nafion® did not “expand” when exposed to DMMP or water vapor. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 284–298, 2009

Effect of Osmolytes on Pressure‐Induced Unfolding of Proteins: A High‐Pressure SAXS Study

Herein, we explore the effect of different types of osmolytes on the high-pressure stability and tertiary structure of a well-characterized monomeric protein, staphylococcal nuclease (SNase). Changes in the denaturation pressure and the radius of gyration are obtained in the presence of different concentrations of trimethylamine N-oxide (TMAO), glycerol and urea. To reveal structural changes in the protein upon compression at various osmolyte conditions, small-angle X-ray scattering (SAXS) experiments were carried out. To this end, a new high-pressure cell suitable for high-precision SAXS studies at synchrotron sources was built, which allows one to carry out scattering experiments up to maximum pressures of about 7 kbar. Our data clearly indicate that the osmolytes that stabilize proteins against temperature-induced unfolding drastically increase their pressure stability and that the elliptically shaped curve of the pressure-temperature-stability diagram of proteins is shifted to higher temperatures and pressures with increasing osmolyte concentration. A drastic stabilization is observed for the osmolyte TMAO, which exhibits not only a significant stabilization against temperature-induced unfolding, but also a particularly strong stabilization of the protein against pressure. In fact, such findings are in accordance with in vivo studies (for example P. J. Yancey, J. Exp. Biol. 2005, 208, 2819-2830), where unusually high TMAO concentrations in some deep-sea animals were found. Conversely, chaotropic agents such as urea have a strong destabilizing effect on both the temperature and pressure stability of the protein. Our data also indicate that sufficiently high TMAO concentrations might be able to largely offset the destabilizing effect of urea. The different scenarios observed are discussed in the context of recent experimental and theoretical studies.

Synchrotron Small-Angle X-ray Scattering Studies of the Structure of Porcine Pepsin under Various pH Conditions

Structural characteristics of various conformational states of porcine pepsin in solution under different pH conditions were investigated in terms of size and shape by small-angle X-ray scattering (SAXS). Low-resolution structural models of porcine pepsin were reconstructed from SAXS data, which were made inside the search volume of maximum dimension (Dmax), calculated from the pair distance distribution function p(r). The reconstructed structural models were obtained without imposing any restrictions on the symmetry or anisometry of the pepsin molecule. Under conditions emulating those for physiological activity of the enzyme, the reconstructed structural models exhibited a more extended C-terminal domain compared to the crystal structure. The differences between the solution and crystal structures of pepsin can be explained by inherent conformations of the flexible subdomain in the C-terminal domain under the solution pH conditions. Under mild acidic conditions where the enzyme is inactive, the reconstructed structural models revealed a compact globular conformation similar in overall shape to the crystal structure. These results indicate that the changes in fluorescence and circular dichroism curves observed under acidic conditions could also arise from the inherent conformation of the flexible subdomain, which has a tendency to roll into a sphere in the overall structure, but without affecting the stability of internal structure. Furthermore, the conformational changes in the subdomain might explain the inactivity of pepsin under mildly acidic conditions. Finally, compared to neutral denaturing conditions, pepsin under alkaline denaturing conditions had a larger expanded vertical conformation in the reconstructed model, as a consequence of alkaline denaturation of the N-terminal domain and a fully extended conformation of the C-terminal domain. The structural evidence presented here may have important implications for understanding the relationship between the structure of porcine pepsin and enzymatic function.

Preparation, microstructure and hydrogen sorption properties of nanoporous carbonaerogels under ambient drying

Organic aerogels are prepared by the sol–gel method from polymerization of resorcinol withfurfural. These aerogels are further carbonized in nitrogen in order to obtaintheir corresponding carbon aerogels (CA); a sample which was carbonized at900 °C was also activated in a carbon dioxide atmosphere at900 °C. The chemical reaction mechanism and optimum synthesis conditions areinvestigated by means of Fourier transform infrared spectroscopy and thermoanalyses(thermogravimetric/differential thermal analyses) with a focus on the sol–gel process. Thecarbon aerogels were investigated with respect to their microstructures, using small anglex-ray scattering (SAXS), transmission electron microscopy (TEM) and nitrogen adsorptionmeasurements at 77 K. SAXS studies showed that micropores with a radius of gyration of<0.35 ± 0.07 to0.55 ± 0.05 nm were present, and TEM measurements and nitrogen adsorption showed that largermesopores were also present. Hydrogen storage properties of the CA were alsoinvestigated. An activated sample with a Brunauer–Emmett–Teller surface area of1539 ± 20 m2 g−1 displayed a reasonably high hydrogen uptake at 77 K with a maximum hydrogen sorption of3.6 wt% at 2.5 MPa. These results suggest that CA are promising candidate hydrogenstorage materials.

Small-Angle X-ray Scattering Study on Poly(ethyl acrylate-co-itaconate) Ionomers Neutralized with Various Cations

The morphology of poly(ethyl acrylate-co-itaconate) ionomers neutralized partly and fully with Na(I), Li(I) and Zn(II) was studied using a small-angle X-ray scattering (SAXS) technique. It was found that the SAXS peak position for the ionomers shifted to lower angle with increasing degree of neutralization for the ionomers containing 5.8 mol% of ionic repeat units. In the case of 11.9 mol% ionomers, with increasing degree of neutralization from 50% to 100% the SAXS peak shifted to slightly higher angles. With the density data and the space-filling concept for a simple cubic lattice, the average radii of multiplets of the ionomers were calculated. For 5.8 mol% ionomers, the radius of the Li-ionomer increased from 0.35 nm to 0.43 nm with increasing neutralization degree from 50% to 100%, and that of the Na-ionomer changed from 0.34 to 0.53 nm. In the case of the 11.9 mol% ionomers, the radius changed from 0.51 to 0.49 nm for the Li-ionomer and from 0.42 nm to 0.47 nm for the Na-ionomer. For the Zn-ionomers, only 100% neutralized ionomers showed a SAXS peak. From the SAXS profiles, the persistence lengths of the polymer chains attached to the scattering centers were also obtained.