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Small-angle X-ray scattering measurements on native pig thyroglobulin in phosphate buffer, pH6.9, yield a radius of gyration of 6.4nm (64A), a particle volume of approx. 1.5x10(3)nm(3) (1.5x10(6)A(3)), an axial ratio of 2.2:1 (assuming an ellipsoidal shape), and a solvation of 0.63g of solvent/g of protein.
Small angle x-ray scattering measurements on dimeric yeast hexokinase B at pH 5.5 in acetate buffer yield a radius of gyration of 31.28 +/- 0.23 angstrom. This measured value is comparable to the radius of gyration of 31.5 angstrom calculated from the refined coordinates of the dimer in the BII crystal form. The hexokinase dimer found in the BI crystal form has a radius of gyration of 42 angstrom calculated from the atomic coordinates. Thus, the measured radius of gyration is consistent with the BII dimer being the predominant species in solution and rules out the existence of the BI dimer as a major species under these conditions.
The initial stages of SBA-15 synthesis have been studied by using in situ time-resolved small-angle X-ray scattering with a synchrotron radiation source. The quantitative analysis of X-ray scattering and diffraction intensities allows the structures of intermediates to be identified at the different stages of SBA-15 synthesis. Following tetraethylorthosilicate (TEOS) addition, an intense small-angle scattering and an associated secondary maximum are observed, which are attributed to non-interacting surfactant template micelles encrusted with silicate species. After 25-30 min of the reaction, the broad scattering disappears and narrow Bragg diffraction peaks typical of hexagonally ordered structure are observed. The cylindrical micelles identified from X-ray scattering data appear to be the direct precursors of 2D hexagonal SBA-15 structure. Just after the formation of the SBA-15 hexagonal phase, the cylindrical micelles are only weakly linked in the hexagonal structure. As the synthesis proceeds, the solvent in the void volume between the cylindrical micelles is gradually replaced by more dense silicate species. The unit cell parameter of SBA-15 is progressively decreasing during the SBA-15 synthesis, which can be related to the condensation and densification of silicate fragments in the spaces between the cylinders. The volume fraction of the 2D hexagonally ordered phase is sharply growing during the first 2 h of the reaction. The inner core radius of SBA-15 material remains almost constant during the whole synthesis and is principally affected by the size of the poly(propylene oxide) inner core in the original cylindrical micelles.
A filtering technique to remove parasitic scattering from X-ray absorption spectra that are acquired in energy-dispersive mode has been developed and tested at the European Synchrotron Radiation Facility. The improved set-up removes small-angle scattering of the sample or the windows of sample cells which may spoil the energy resolution or reduce the intensity of prominent features in the absorption spectrum, such as the white line at the Pt L(III) edge. The sample is placed behind the curved monochromator and between two plane perfect crystals in the Bonse-Hart configuration. The dispersion of the Bonse-Hart double-crystal camera is matched to the dispersion of the curved monochromator by inclining the scattering planes of the two optical elements against each other.
The spatial arrangement of Tet repressor dimer, both free and in complex with an 80 bp DNA fragment spanning the wild-type Tn10-encoded tet transcriptional control sequence containing a tandem repeat of two operators, has been determined by neutron small-angle scattering. The active, free Tet repressor dimer is an elongated and flat molecule with a maximum dimension of 11 +/- 1.5 mm which can be approximated by an ellipsoid with the half-axes 6 nm, 2.5 nm and 1 nm. The overall conformation undergoes no detectable change when the repressor dimer is bound to a DNA fragment containing a single tet operator. The normal distance between the centre of gravity of the protein and the DNA axis is 3.0 +/- 0.1 nm, indicating that the repressor dimer is mainly located on one side of the DNA. When bound to the wild type tet control DNA, the two repressor dimers have a centre-to-centre distance of 11.0 +/- 0.5 nm. Their minimal distance is 5 +/- 2 nm. Protein-protein contacts via loop formation of the DNA by repressor binding is excluded. The repressors are well separated and have no direct contact. A model is proposed where the two repressor dimers are located on opposite sides of the DNA and the DNA is not strongly bent in the complex.
The effects of enzymatic action on human low density lipoproteins (LDL) occurring during in vitro incubation of plasma have been studied by chemical analysis, analytical ultracentrifugation, negative stain electron microscopy and X-ray small angle scattering. Chemically, the action of cholesteryl ester exchange and transfer proteins(s) (CEPT) leads to a relative increase in trigylcerides at the expense of cholesteryl esters. Morphologically, the particles maintain their characteristic features detectable by X-ray small angel scattering. Additional action of lecithin/cholesterol acyl transferase (LCAT) causes mainly a decrease in polar lipid contents and a reduction in particle size. The associated changes in the thermotropic transition were found to be strongly correlated to the triglyceride/cholesteryl ester ratio.
Controlling interparticle interactions, aggregation and cluster formation is of central importance in a number of areas, ranging from cluster formation in various disease processes to protein crystallography and the production of photonic crystals. Recent developments in the description of the interaction of colloidal particles with short-range attractive potentials have led to interesting findings including metastable liquid-liquid phase separation and the formation of dynamically arrested states (such as the existence of attractive and repulsive glasses, and transient gels). The emerging glass paradigm has been successfully applied to complex soft-matter systems, such as colloid-polymer systems and concentrated protein solutions. However, intriguing problems like the frequent occurrence of cluster phases remain. Here we report small-angle scattering and confocal microscopy investigations of two model systems: protein solutions and colloid-polymer mixtures. We demonstrate that in both systems, a combination of short-range attraction and long-range repulsion results in the formation of small equilibrium clusters. We discuss the relevance of this finding for nucleation processes during protein crystallization, protein or DNA self-assembly and the previously observed formation of cluster and gel phases in colloidal suspensions.
The effect of alpha-tocopherol on the thermotropic phase behaviour of aqueous dispersions of dipalmitoylphosphatidylethanolamine in mixtures containing 0, 2.5, 5, 10 and 20 mol% alpha-tocopherol was examined using synchrotron X-ray diffraction methods. Dispersions were equilibrated for at least 12 h at 4 degrees C before measurement. The phospholipid alone undergoes a lamellar gel to liquid-crystalline phase transition at 66 degrees C during the initial heating scan. When codispersed with alpha-tocopherol there was evidence of phase separation of alpha-tocopherol-rich domains from bilayers of pure phospholipid. The alpha-tocopherol-rich domains were identified from the relationship between scattering intensity of the phase and the proportion of alpha-tocopherol in the mixture. In initial heating scans alpha-tocopherol-rich domains were characterized by broad lamellar repeat spacings (5.0 nm at 55 degrees C) in the small-angle scattering region which first appears at approximately 40 degrees C and increases in intensity with increasing temperature. In the presence of more than 5 mol% alpha-tocopherol the wide-angle scattering region showed two scattering profiles consisting of multiple peaks typical of lamellar crystal phases. The low-temperature crystal phase, designated Lc1, was transformed into Lc2 phase at approximately 50 degrees C. On further heating the Lc2 phase was transformed directly into inverted hexagonal phase at approximately 56 degrees C and coexisted with the lamellar gel phase of the pure phospholipid. The inverted hexagonal phase coexists with the lamellar phase of the pure phospholipid, after transformation into lamellar liquid-crystal phase at 66 degrees C, at least up to 70 degrees C. In cooling scans the d-spacing of the inverted hexagonal phase decreases progressively and the scattering intensity weakens on cooling below the liquid-crystal to gel phase transition temperature of the pure phospholipid. There is no evidence of formation of lamellar crystal phases during cooling so that the alpha-tocopherol either mixes with the lamellar gel phase of the phospholipid without change in the structural parameters of the pure phospholipid or is completely phase separated from the phospholipid. The stoichiometry of phospholipid/alpha-tocopherol in the alpha-tocopherol-rich lamellar crystal and inverted hexagonal phases is estimated to be approximately 4 : 1. The structural changes observed in the study are related to the calorimetric transitions reported in independent work on these dispersions.
We have studied how the dynamic components of laser light scattered from human spermatozoa depend on the scattering angle. This was done by investigating the halfwidth of the intensity autocorrelation function. A model of the spermatozoa as freely rotating and translating linear objects was adequate to describe the scattered light. Rotational motions determined the halfwidth of the intensity autocorrelation function at very small scattering angles and contribution from translational motions was dominant at scattering angles larger than 20 degrees. The contribution from translational motions increased with increasing scattering angle. We found a nearly linear relationship between the translation speed and the rotation frequency. However, the ratio between the two properties varied more than expected from the methodological error. Therefore we introduced a propelling efficacy as a concept to describe the swimming efficiency. This property might contain important information about the swim characteristics.
The key carboxylating enzyme of the reductive pentose phosphate cycle, D-ribulose-1,5-bisphosphate carboxylase/oxygenase [RuBisCO] isolated from the chemolithoautotrophic, H2-oxidizing bacterium Alcaligenes eutrophus H16 has been analyzed by several different techniques that allow conclusions about structure and function-dependent structural changes. The techniques include a novel approach in which the enzyme was induced to form 2D-crystals suitable for electron microscopy in each of its three stable functional states: as active enzyme [Ea] (in the presence of Mg2+ and HCO3-); as inactivated enzyme [Eia] (in the absence of Mg2+ and HCO3-) and as enzyme locked in an in vitro transition state [CABP-E] (Ea fully saturated with the transition state analogue 2-carboxy-D-arabinitol-1,5-bisphosphate [CABP-E]). In conjunction with X-ray crystallography, X-ray small angle scattering and other biophysical and biochemical data, the results obtained by electron microscopy support the idea that drastic configurational changes occur. Upon transition from Ea to the CABP-E the upper and lower L4S4 halves of the molecule consisting of eight large and eight small subunits (L8S8; MW = 536,000 Da) are assumed to be laterally shifted by as much as 3.6 nm relative to one another while the location of the small subunits on top of the large subunits, and relative to them, remains the same. For the Eia a similar sliding-layer configurational change in the range of 2-2.5 nm is proposed and in addition it is suggested that other configurational/conformational changes take place. The proposed structural changes are discussed with respect to the current model for the tobacco enzyme and correlated with data obtained for various other plant and (cyano) bacterial L8S8 RuBisCOs leading to speculations about structure-function relationships.
Uniform sized PMMA-beads were deposited as a monolayer on silicon substrates using dip-coating techniques. High-resolution grazing incidence X-ray small angle scattering experiments were performed using a micrometer sized beam (mu-GISAXS) to determine the structure of a highly ordered monolayer with two-dimensional hexagonal arrays. A clear and strong interference pattern coming from the reflection and refraction effects of particles on flat surfaces with small uncorrelated roughnesses is shown. The quantitative analysis and simulations of the X-ray scattering pattern have been performed, and a detailed explanation of the analysis is reported. The results were directly compared and verified with atomic force microscopy (AFM) measurements and their resulting FFT spectra.
Scattering of a quasi-monochromatic electron beam by a crystal with a defect is described with the use of the mutual coherency function and the formalism of quasi-Bloch waves. An expression correlating the mutual intensity on the exit and entrance surfaces of the crystal in terms of the scattering matrix has been found. The matrix elements are determined by a system of integro-differential equations, which have been obtained without using the column approximation. It has been shown that calculations of the matrix elements can be significantly simplified when the approximation of the small-angle scattering of quasi-Bloch waves by the defect displacement field is satisfied. Such an approximation can be applied in many cases, e.g. to a crystal with a dislocation. The mutual intensity on the crystal entrance surface has been found for the general case of defocused illumination. As an example of applying the new approach, expressions for the intensity in convergent-beam electron diffraction (CBED) and large-angle CBED (LACBED) patterns have been obtained. The LACBED patterns of a crystal with a dislocation have been simulated. It has been shown that the developed approach allows a more exact simulation of the LACBED than do the conventional approaches using the column approximation and the approximation of independent plane waves filling the illumination cone.
The reversal of magnetic moments of nanoparticles in concentrated Co ferrofluids was monitored in an oscillating magnetic field by new time-resolved stroboscopic small-angle neutron-scattering techniques. Time resolution in the micros range was achieved by using a pulsed beam technique, TISANE, while in continuous mode resolution was limited by the wavelength spread to about 1 ms. The frequency dependence of anisotropic scattering patterns has been modeled using Langevin dynamics. The dynamics follows a two step mechanism: field-induced ordering is governed by fast Brownian rotation of nanoparticles with a characteristic time of about 160 micros. Magnetic relaxation of locally ordered domains of about 100 nm in size takes place within a few seconds by Brownian rotation or by Néel type rotation of magnetic moments.
A discrete ordinates code is developed with which to compute the beam spread function (BSF) without invoking the small-angle scattering approximation or performing Monte Carlo calculations. The computed BSF is used to predict the response of a detector versus its distance to the origin of a highly collimated beam, its angle with respect to the beam, and the two local angles that specify the detector orientation. Numerical results have been obtained for water models that simulate a clear ocean, a coastal ocean, and a turbid harbor. Six orders of magnitude or more change in the detector response caused by scattered photons can be predicted for different detector locations while simultaneously obtaining small changes for different detector orientations. This capability is useful for assessment of the sensitivity of the detector response to the interpretation of time-independent underwater imaging systems or visibility models.
A detailed interpretation of the grossly distorted ultraviolet absorption and circular dichroism spectra of the intact red blood cell is given, including an evaluation of the effects of protein conformation, detector geometry, cell hemoglobin content, and refractive index on the calculated cell spectra. The origins of the major differences between cell and hemoglobin solution spectra were quantitatively accounted for in terms of differential scatter and absorption flattening, with the latter effect dominating the picture. The relatively low sensitivity of the red blood cell suspension circular dichroism spectrum to hemoglobin conformation is due to the order of magnitude flattening of circular dichroism intensity. The importance of accounting for instrumental light detection geometry and the intense small angle scattering (less than 8 degrees) for a range of particle sizes (0.1-5 mum) is made clear.
The interface geometry of Vycor porous glass was examined by adsorption of water in conjunction with small-angle scattering of X rays and neutrons. When the sample is dry, a fractal surface with a fractal dimension D approximately 2.5 was detected. When wet, a soft hydrogel is formed, resulting in a diffuse boundary. To observe a fractal surface, thorough drying of the sample is necessary, however. It was shown that a small amount of water, 3% w/w, is sufficient to render the surface smooth. Copyright 1997Academic Press
Particles of micrometer to nanometer size often aggregate to form branched structures. Such materials include metals and metal oxides as well as biological and polymeric materials (considering the persistence length as a primary unit). Characterization of such structures is difficult since they typically display disordered, irregular features in three dimensions. Branched aggregates display two limiting size scales: that of the primary particle, R1 and that of the aggregate, R2. The mass-fractal model is often used to describe such structures where the aggregate mass, z=M2/M1, is related to the aggregate size, r=R2/R1, through a scaling relationship z=alpha r (d(f)), where the lacunarity alpha is close to 1 and may depend on the growth mechanism. Scattering of x rays, light and neutrons yields a direct measure of the mass-fractal dimension since I(q) approximately q(-d(f)) for 1/R2<q<1/R1 using scaling arguments. For linear, monodisperse aggregates with convoluted chain paths, analytic functions describing both the scaling and larger-size aggregate scattering regimes have been reported. For example, the Debye function for linear, Gaussian coils describes scattering when d(f)=2. Real, mass-fractal aggregates, however, can display variability from the linear chain, monodisperse model. Often the branch content is of vital importance to understanding both the growth of aggregates and their physical properties, especially dynamic properties. An approach is presented for the analysis of aggregate branching from static small-angle scattering. Comparison is made with analytic, simulation, and experimental results from the literature.
As an electron beam (of order 100 keV) travels through a crystalline solid it can be channeled down a zone axis of the crystal to form a channeling peak centered on the atomic columns. The channeling peak can be similar in size to the outer atomic orbitals. Electron energy loss spectroscopy (EELS) measures the losses that the electron experiences as it passes through the solid yielding information about the unoccupied density of states in the solid. The interaction matrix element for this process typically produces dipole selection rules for small angle scattering. In this paper, a theoretical calculation of the EELS cross section in the presence of strong channeling is performed for the silicon L23 edge. The presence of channeling is found to alter both the intensity and selection rules for this EELS signal as a function of depth in the solid. At some depths in the specimen small but significant non-dipole transition components can be produced, which may influence measurements of the density of states in solids.