Anomalous Small-angle X-ray Scattering Data Research Articles

ASAXS measurements on ferritin and apoferritin at the bioSAXS beamline P12 (PETRA III, DESY).

Small-angle X-ray scattering is widely utilized to study biological macromol-ecules in solution. For samples containing specific (e.g. metal) atoms, additional information can be obtained using anomalous scattering. Here, measuring samples at different energies close to the absorption edges of relevant elements provides specific structural details. However, anomalous small-angle X-ray scattering (ASAXS) applications to dilute macromolecular solutions are challenging owing to the overall low anomalous scattering effect. Here, pilot ASAXS experiments from dilute solutions of ferritin and cobalt-loaded apoferritin are reported. These samples were investigated near the resonance X-ray K edges of Fe and Co, respectively, at the EMBL P12 bioSAXS beamline at PETRA III, DESY. Thanks to the high brilliance of the P12 beamline, ASAXS experiments are feasible on dilute protein solutions, allowing one to extract the Fe- or Co-specific anomalous dispersion terms from the ASAXS data. The data were subsequently used to determine the spatial distribution of either iron or cobalt atoms incorporated into the ferritin/apoferritin protein cages.

Improving Electrostatic Descriptions of Ions Around Double-Stranded DNA

Ions play an essential role in governing the structure and function of nucleic acids, due to the large negative charge associated with the nucleic acid backbone. The addition of even small numbers of multivalent, positively charged ions induces intra-strand attraction in DNA and thus efficiently packages the extended polymer into compact toroids. Anomalous small angle X-ray scattering (ASAXS) has emerged as a powerful technique to report the spatial distribution of ions associated to nucleic acids with unprecedented levels of detail and resolution. To determine more detailed information about these highly mobile ions relative to the underlying nucleic acid surface requires tight coordination with theoretical or computational tools. Presently, very few robust theoretical or computational tools exist for understanding ion-nucleic interactions. Current atomically-detailed computational approaches that represent the solvent environment explicitly, as discrete water molecules and ions, are prohibitively expensive for systematic studies of biologically relevant structures on time scales needed to fully understand these interactions. The alternative approach of “implicit solvent” models, that represent solvent implicitly as a continuum, could potentially overcome this difficulty. However, most “implicit solvent” models reduce computational effort at the expense of simplifying assumptions that preclude their application to highly charged systems in the presence of concentrated salt solutions or multivalent ions. We have been examining the ability of traditional and “size-modified” Poisson-Boltzmann models to predict the distribution of multivalent ions at low concentration around B-form DNA. These predictions are tested through direct comparison with ASAXS data on similar systems. The goal of this work is to examine the role of size-exclusion in the well-known failure of traditional Poisson-Boltzmann approaches for describing multivalent ions in highly charged systems. This work lays the foundation for a systematic approach to improve implicit solvent models for nucleic acid systems.

Dietary iron-loaded rat liver haemosiderin and ferritin: in situ measurement of iron core nanoparticle size and cluster structure using anomalous small-angle x-ray scattering

The morphology, particle size distribution and cluster structure of the hydrated iron(III) oxyhydroxide particles associated with haemosiderin and ferritin in dietary iron-loaded rat liver tissue have been investigated using transmission electron microscopy (TEM) and anomalous small-angle x-ray scattering (ASAXS). Rat liver tissue was removed from a series of female Porton rats which had been fed an iron-rich diet until sacrifice at various ages from 2–24 months. Hepatic iron concentrations ranged from 1 to 65 mg Fe g−1 dry tissue. TEM studies showed both dispersed and clustered iron-containing nanoparticles. The dispersed particles were found to have mean sizes (±standard deviation) of 54 ± 8 Å for the iron-loaded animals and 55 ± 7 Å for the controls. Superposition of particles in TEM images prevented direct measurement of nanoparticulate size in the clusters. The ASAXS data were modelled to provide a quantitative estimate of both the size and spacing of iron oxyhydroxide particles in the bulk samples. The modelling yielded close-packed particles with sizes of 60 to 78 Å which when corrected for anomalous scattering suggests sizes from 54 to 70 Å. Particle size distributions are of particular importance since they determine the surface iron to core iron ratios, which in turn are expected to be related to the molar toxicity of iron deposits in cells.

Metalated Diblock and Triblock Poly(ethylene oxide)-block-poly(4-vinylpyridine) Copolymers: Understanding of Micelle and Bulk Structure

The paper provides new insights into the structure of Pt-containing diblock and triblock copolymers based on poly(ethylene oxide) (PEO) and poly(4-vinylpyridine) (P4VP), using a combination of atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and anomalous small-angle X-ray scattering (ASAXS). Parallel studies using methods contributing supplemental structural information allowed us to comprehensively characterize sophisticated polymer systems during metalation and to exclude possible ambiguity of the data interpretation of each of the methods. AFM and TEM make available the determination of sizes of the micelles and of the Pt-containing micelle cores, respectively, while a combination of XRD, TEM, and ASAXS reveals Pt-nanoparticle size distributions and locations along with the structural information about the polymer matrix. In addition, for the first time, ASAXS revealed the organization of Pt-nanoparticle-filled diblock and triblock copolymers in the bulk. The nanoparticle characteristics are mainly determined by the type of block copolymer system in which they are found: larger particles (2.0-3.0 nm) are formed in triblock copolymer micelles, while smaller ones (1.5-2.5 nm) are found in diblock copolymer micelles. This can be explained by facilitated intermicellar exchange in triblock copolymer systems. For both systems, Pt nanoparticles have narrow particle size distributions as a result of a strong interaction between the nanoparticle surface and the P4VP units inside the micelle cores. The pH of the medium mainly influences the particle location rather than the particle size. A structural model of Pt-nanoparticle clustering in the diblock PEO-b-P4VP and triblock P4VP-b-PEO-b-P4VP copolymers in the bulk was constructed ab initio from the ASAXS data. This model reveals that nearly spherical micellar cores of about 10 nm in diameter (filled with Pt nanoparticles) aggregate forming slightly oblate hollow bodies with an outer diameter of about 40 nm.

Anomalous small‐angle X‐ray scattering characterization of composites based on sulfonated poly(ether ether ketone), zirconium phosphates, and zirconium oxide

AbstractThe morphology and distribution of zirconium oxide and zirconium phosphates in a matrix of sulfonated poly(ether ether ketone) (SPEEK) were investigated with anomalous small‐angle X‐ray scattering (ASAXS) and electron microscopy. ASAXS revealed that ZrO2 was distributed in the SPEEK matrix in the form of nanoparticles smaller than 13 Å. A decrease in the conductivity suggested that the sulfonic groups were bound to the zirconium oxo species at the particle surface. Furthermore, two kinds of membranes containing zirconium phosphate were investigated. In one case, the phosphate was directly dispersed in the polymer solution for the casting of the membrane. In the other case, the phosphate was previously treated with n‐propyl ammonium and polybenzimidazole. From ASAXS data, the fractal dimension could be estimated. Mass‐fractal behavior was confirmed for the SPEEK membrane containing previously exfoliated zirconium phosphate, with aggregates of 6.3–165 Å. Surface‐fractal behavior was detected for membranes with untreated phosphates, with aggregates of 6.4–185 Å. The untreated phosphates caused an increase in the permeability, without changing the proton conductivity much. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 567–575, 2004

Precipitation in low temperature grown GaAs

The precipitation of arsenic in GaAs epitaxially grown at low temperature (LT GaAs) has been studied as a function of the post-growth annealing temperature by three independent methods: transmission electron microscopy (TEM), Raman scattering, and for the first time the anomalous small angle X-ray scattering (ASAXS). All the results agree that the average size increases with increasing annealing temperature but the ASAXS data indicate that the precipitates can be divided into two parts described by separate size distributions. The number density of precipitates around 5 nm size has been estimated to be at least two orders of magnitude higher than that of larger precipitates.

An analysis of the partial structure functions in unmixed Cu-Ni-Fe alloys studied by anomalous small-angle X-ray scattering

The solute partitioning and the unmixing kinetics of a Cu0.43Ni0.42Fe0.15 alloy have been studied by anomalous small-angle X-ray scattering (ASAXS). From ASAXS data, partial structure factors (PSF) have been extracted: the homoatomic PSF curves cannot account for a pseudobinary system and one has to consider a Ni (or Fe) segregation at the interfaces between the Cu-rich phase and the Ni-Fe phase. The main contribution to the PSF comes nevertheless from the two-phase term which behaves as in the late stage of deep quenches of concentrated binary systems: PSF present a hump which increases and shifts toward low q values as power laws of the aging time: qmvaries as t-0.25, Smvaries as t0.7.