Isothermal aggregation of Bi atoms embedded in a soda borate glass: Coarsening of liquid nanodroplets and atomic diffusion

The process of nucleation and growth of liquid Bi nanodroplets embedded in a soda borate glass submitted to isothermal annealing at different temperatures was studied by small-angle x-ray scattering (SAXS) and transmission-electron microscopy. The experimental results indicate that the formation and growth of Bi droplets occur in two successive stages after a short incubation period. The first is characterized by the nucleation and growth of spherical droplets promoted by atomic diffusion and aggregation of isolated Bi atoms and the second one by a subsequent droplet coarsening. The experimental functions describing the time variation of the droplet average radius and density number at advanced stages of the growth process agree with the classical Lifshitz-Slyozov-Wagner (LSW) theory. However, the radius distribution was demonstrated to be well described by a log-normal function thus differing from the prediction of the LSW model. The atomic diffusion coefficient of Bi was determined from SAXS results for several annealing temperatures and, from it, the activation energy for the diffusion process was inferred.67

Structure characterization and mechanism of growth of PbTe nanocrystals embedded in a silicate glass

A nanocomposite consisting of PbTe nanocrystals embedded in a silicate glass was studied by small-angle x-ray scattering during the early stage of isothermal annealing at 793 K. A theoretical function based on a model of spherical PbTe nanocrystals surrounded by a Pb and Te depleted shell fits well to all experimental curves. The time dependences of the nanocrystal radius and size of the depleted shell agree with the prediction of the theory of nucleation and growth by the classical mechanism of atomic diffusion.892

Structure of PbTe(SiO2)/SiO2 multilayers deposited on Si(111)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The structure of thin films composed of a multilayer of PbTe nanocrystals embedded in SiO2, named as PbTe(SiO2), between homogeneous layers of amorphous SiO2 deposited on a single-crystal Si( 111) substrate was studied by grazing-incidence small-angle X-ray scattering (GISAXS) as a function of PbTe content. PbTe(SiO2)/SiO2 multilayers were produced by alternately applying plasma-enhanced chemical vapour deposition and pulsed laser deposition techniques. From the analysis of the experimental GISAXS patterns, the average radius and radius dispersion of PbTe nanocrystals were determined. With increasing deposition dose the size of the PbTe nanocrystals progressively increases while their number density decreases. Analysis of the GISAXS intensity profiles along the normal to the sample surface allowed the determination of the period parameter of the layers and a structure parameter that characterizes the disorder in the distances between PbTe layers. (C) 2010 International Union of Crystallography Printed in Singapore - all rights reserved433385393Brazilian Synchrotron Light Laboratory (LNLS)PRONEXConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Structure and melting of Bi nanocrystals embedded in a B2O3-Na2O glass

A composite material consisting of spherical Bi nanoclusters (nanocrystals and/or liquid nanodroplets) embedded in a 28Na(2)O-72B(2)O(3) glass was studied by the wide-angle x-ray scattering (WAXS) and small-angle x-ray scattering (SAXS) techniques over the temperature range in which the Bi crystal-liquid transition occurs. Because of the wide radius distribution of Bi clusters and due to the dependence of the melting temperature on crystal radius, the overall transition occurs over a wide range, from 365 up to 464 K. In this transition range, large Bi nanocrystals coexist with small liquid droplets. A weak contraction in a and c lattice parameters of rhombohedral Bi nanocrystals with respect to the bulk crystal was detected. As expected, the average radius of crystalline Bi clusters, deduced from WAXS data, increases for increasing temperatures over the whole solid-to-liquid transition range. The SAXS spectrum recorded at different temperatures within the transition range is essentially invariant, indicating that the radius distribution of Bi nanoclusters (nanocrystals and nanodroplets) is temperature independent. The volume distribution of Bi nanoclusters is a single-mode function with the radius ranging from about 15 up to 41 Angstrom with a maximum at 28 Angstrom. The integral of Bragg peaks of Bi nanocrystals decreases for increasing temperatures as a consequence of the progressive melting of nanocrystals of increasing size. By combining the results of WAXS and SAXS experiments, we determined the melting temperature of the nanocrystals as a function their radius suppressing unwanted size dispersion effects. Our results clearly indicate a linear dependence of the melting temperature on nanocrystal reciprocal radius, thus confirming previous theoretical predictions.651

Low resolution structural study of two human HSP40 chaperones in solution - DjA1 from subfamily A and DjB4 from subfamily B have different quaternary structures

Proteins that belong to the heat shock protein (Hsp) 40 family assist Hsp70 in many cellular functions and are important for maintaining cell viability. A knowledge of the structural and functional characteristics of the Hsp40 family is therefore essential for understanding the role of the Hsp70 chaperone system in cells. In this work, we used small angle x-ray scattering and analytical ultracentrifugation to study two representatives of human Hsp40, namely, DjA1 (Hdj2/dj2/HSDJ/Rdj1) from subfamily A and DjB4 (Hlj1/DnaJW) from subfamily B, and to determine their quaternary structure. We also constructed low resolution models for the structure of DjA1-(1-332), a C-terminal-deleted mutant of DjA1 in which dimer formation is prevented. Our results, together with the current structural information of the Hsp40 C-terminal and J-domains, were used to generate models of the internal structural organization of DjA1 and DjB4. The characteristics of these models indicated that DjA1 and DjB4 were both dimers, but with substantial differences in their quaternary structures: whereas DjA1 consisted of a compact dimer in which the N and C termini of the two monomers faced each other, DjB4 formed a dimer in which only the C termini of the two monomers were in contact. The two proteins also differed in their ability to bind unfolded luciferase. Overall, our results indicate that these representatives of subfamilies A and B of human Hsp40 have different quaternary structures and chaperone functions.28014136711368

Clay-containing block copolymer nanocomposites with aligned morphology prepared by extrusion

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Clay-containing nanocomposites of polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) copolymers having cylindrical domains were obtained by melt extrusion using a tape die. One type of sample (SEBS-MA) had maleic anhydride attached to the middle block. Two types of organoclays were added, namely Cloisite 20A and Cloisite 30B. Small angle X-ray scattering and transmission electron microscopy (TEM) analyses showed that the addition of 20A clay to SEBS and SEBS-MA resulted in nanocomposites with intercalated and partially exfoliated structures, respectively. The addition of 30B clay to SEBS and SEBS-MA promoted the formation of composites containing relatively large micron-sized and partially exfoliated clay particles, respectively. Our TEM analysis revealed that clay particles embedded in SEBS are preferably in contact with the polystyrene cylindrical domains, while in SEBS-MA they are in contact with the maleated matrix. The extrusion processing promoted alignment of the axes of the polystyrene cylinders along the extrusion direction in all samples, and the basal planes of the clay particles were mostly parallel to the main external surfaces of the extruded tapes. (c) 2013 Society of Chemical Industry632184194Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Free human mitochondrial GrpE is a symmetric dimer in solution

The co-chaperone GrpE is essential for the activities of the Hsp70 system, which assists protein folding. GrpE is present in several organisms, and characterization of homologous GrpEs is important for developing structure-function relationships. Cloning, producing, and conformational studies of the recombinant human mitochondrial GrpE are reported here. Circular dichroism measurements demonstrate that the purified protein is folded. Thermal unfolding of human GrpE measured both by circular dichroism and differential scanning calorimetry differs from that of prokaryotic GrpE. Analytical ultracentrifugation data indicate that human GrpE is a dimer, and the sedimentation coefficient agrees with an elongated shape model. Small angle x-ray scattering analysis shows that the protein possesses an elongated shape in solution and demonstrates that its envelope, determined by an ab initio method, is similar to the high resolution envelope of Escherichia coli GrpE bound to DnaK obtained from single crystal x-ray diffraction. However, in these conditions, the E. coli GrpE dimer is asymmetric because the monomer that binds DnaK adopts an open conformation. It is of considerable importance for structural GrpE research to answer the question of whether the GrpE dimer is only asymmetric while bound to DnaK or also as a free dimer in solution. The low resolution structure of human GrpE presented here suggests that GrpE is a symmetric dimer when not bound to DnaK. This information is important for understanding the conformational changes GrpE undergoes on binding to DnaK.27837353373534

Complementary methods for cluster size distribution measurements: supported platinum nanoclusters in methane reforming catalysts

Pt cluster size strongly influences the reactivity of surfaces for C-H bond activation during CH4 steam reforming. C-H bond activation turnover rates for H2O reforming of CH4 increased with decreasing Pt cluster size, suggesting that coordinatively unsaturated surface Pt atoms are more reactive than Pt atoms in low-index surfaces prevalent in larger clusters. Near-edge fine structure in X-ray absorption spectra clearly shows electronic changes in Pt clusters relative to bulk Pt crystallites, though differences in the 5d electronic density among samples with different cluster size could not be related directly to Pt surface atom coordination. These results suggest that catalytic consequences of cluster size arise from surface structure changes leading to coordinative unsaturation of surface atoms. X-ray absorption and small angle X-ray scattering techniques were used to measure the size of Pt metal nanoclusters and the results are compared with H-2 chemisorption and transmission electron microscopy data. (C) 2004 Elsevier B.V. All rights reserved