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Abstract: This reference work provides a state-of-the-art, comprehensive overview of our current understanding of the fundamental properties of magnetically ordered materials, and their use in a wide range of sophisticated applications.
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Jul 2007
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Abstract: Context. Silicate dust grains exist in a wide range of astronomical environments and understanding the effect of these on grain structure is of great interest, particularly the effect of thermal annealing on amorphous silicates as a possible route to the formation of crystalline grains. Although laboratory simulations have largely focussed on IR-spectroscopic measurements, since these relate directly to observational data, laboratory synchrotron techniques such as X-ray absorption spectroscopy (XAS) and X-ray scattering are becoming increasingly routine in the analysis of recovered materials. With the increasing prospect of performing astronomical XAS observations, there is much to be gained from applying these techniques to laboratory analogues.
Aims. Diagnostic markers for medium-range order and the effect of thermal annealing on these in amorphous silicates of differing Mg content are characterised using synchrotron X-ray methods.
Methods. Three synthetic amorphous silicates with high, medium and low Mg:Si ratios were annealed at varying temperatures up to 1300 K. X-ray Absorption Near Edge Structure (XANES) spectra at the Si and Mg K-shell absorption edges and X-ray scattering patterns for low values of the X-ray scattering wavevector were recorded for these along with comparative data for commercially produced amorphous SiO2 and mineral samples of forsterite, enstatite and quartz.
Results. XANES features due to short- and medium-range structure are identified at both Mg and Si edges and a new temperature dependent behaviour observed in the medium-range structure surrounding Mg and Si. Based on changes to the morphological details of the XANES spectra, the medium-range structure changes between an enstatite-like and forsterite-like coordination with increasing temperature and appears to correlate directly with Mg content. Low wavevector X-ray scattering features were also found to be diagnostic of the type of medium-range structural ordering. However, these features depend on whether the relative arrangement of clusters of medium-range structure exhibit semi-periodic ordering over the longer-range, which can vary with annealing.
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Jun 2008
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Jul 2008
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Abstract: The high-pressure evolution of the cubic perovskite NH4MnCl3 has been studied in the 0-6 GPa range by angle-dispersive x-ray diffraction. A structural phase transition has been found at low pressure (P-C= 0.4 GPa), in agreement with previous reports. The high-pressure phase has been refined in the tetragonal P4/mbm and orthorhombic Pbnm space groups. In either case, the structure can be described in terms of a tilted perovskite model (one tilt a(0)a(0)c(+) and two tilts a(+)b(-)b(-), respectively). The average tilting under pressure has been estimated from the lattice parameters. The P-V relation has been fitted by a Murnaghan Equation-of-State with K-0=29(2) GPa and K-0'=9(1).
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Mar 2010
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Abstract: Experimental and computational searches for the crystal structures of the five commercially available isomers of dichloronitrobenzene and 3,4-dinitrochlorobenzene were performed to assess the relationship between functional group interactions and steric requirements in determining the solid forms. Experimentally, this resulted in the first crystal structure determination of 2,4-dichloronitrobenzene, two solvates of 3,4-dichloronitrobenzene and one of 3,4-dinitrochlorobenzene. Additionally, low temperature redeterminations of the crystal structures were obtained for 2,5-dichloronitrobenzene, 3,4-dichloronitrobenzene, and both the ?- and ?-forms of 3,4-dinitrochlorobenzene. The searches for energetically feasible structures of each of these compounds showed a wide variety of distributions leading to varying degrees of clarity of prediction of the solid state behavior. These range from 2,3-dichloronitrobenzene, which only adopts the crystal structure that was clearly the most thermodynamically stable of all five isomers, through complex systems, which show a range of low energy minima indicating possible polymorphism and solvate formation, to 2,4-dichloronitrobenzene, which can conformationally distort and adopts a complicated Z? = 2 crystal structure.
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Jan 2008
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Abstract: Two types of poly (epsilon-caprolactone (CLo))-co-poly (epsilon-caprolactam (CLa)) copolymers were prepared by catalyzed hydrolytic ring-opening polymerization. Both cyclic comonomers were added simultaneously in the reaction medium for the first type of materials where copolymers have a random distribution of counits, as evidenced by 1H and 13C NMR. For the second type of copolymers, the cyclic comonomers were added sequentially, yielding diblock poly(ester-amides). The materials were characterized by differential scanning calorimetry (DSC), wide- and small-angle X-ray scattering (WAXS and SAXS), and transmission and scanning electron microscopies (TEM and SEM). Their biodegradation in compost was also studied. All copolymers were found to be miscible by the absence of structure in the melt. TEM revealed that all samples exhibited a crystalline lamellar morphology. DSC and WAXS showed that in a wide composition range (CLo contents from 6 to 55%) only the CLa units were capable of crystallization in the random copolymers. The block copolymer samples only experience a small reduction of crystallization and melting temperature with composition, and this was attributed to a dilution effect caused by the miscible noncrystalline CLo units. The comparison between block and random copolymers provided a unique opportunity to distinguish the dilution effect of the CLo units on the crystallization and melting of the polyamide phase from the chemical composition effect in the random copolymers case, where the CLa sequences are interrupted statistically by the CLo units, making the crystallization of the polyamide strongly composition dependent. Finally, the enzymatic degradation of the copolymers in composted soil indicate a synergistic behavior where much faster degradation was obtained for random copolymers with a CLo content larger than 30% than for neat PCL.
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Aug 2009
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Abstract: The transient complexes of plastocyanin with cytochrome f and photosystem I are herein used as excellent model systems to investigate how the metal sites adapt to the changes in the protein matrix in transient complexes that are involved in redox reactions. Thus, both complexes from the cyanobacterium Nostoc sp. PCC 7119 (former Anabaena sp. PCC 7119) have been analysed by X-ray absorption spectroscopy. Our data are consistent with a significant distortion of the trigonal pyramidal geometry of the Cu coordination sphere when plastocyanin binds to cytochrome f, no matter their redox states are. The resulting tetrahedral geometry shows a shortening of the distance between Cu and the S(delta) atom of its ligand Met-97, with respect to the crystallographic structure of free plastocyanin. On the other hand, when plastocyanin binds to photosystem I instead of cytochrome f, the geometric changes are not significant but a displacement in charge distribution around the metal centre can be observed. Noteworthy, the electronic density around the Cu atom increases or decreases when oxidised plastocyanin binds to cytochrome f or photosystem I, respectively, thus indicating that the protein matrix affects the electron transfer between the two partners during their transient interaction.
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Nov 2006
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Abstract: The Hippo signaling pathway controls cell growth, proliferation, and apoptosis by regulating the expression of target genes that execute these processes. Acting downstream from this pathway is the YAP transcriptional coactivator, whose biological function is mediated by the conserved TEAD family transcription factors. The interaction of YAP with TEADs is critical to regulate Hippo pathway-responsive genes. Here, we describe the crystal structure of the YAP-interacting C-terminal domain of TEAD4 in complex with the TEAD-interacting N-terminal domain of YAP. The structure reveals that the N-terminal region of YAP is folded into two short helices with an extended loop containing the PXX?P motif in between, while the C-terminal domain of TEAD4 has an immunoglobulin-like fold. YAP interacts with TEAD4 mainly through the two short helices. Point mutations of TEAD4 indicate that the residues important for YAP interaction are required for its transforming activity. Mutagenesis reveals that the PXX?P motif of YAP, although making few contacts with TEAD4, is important for TEAD4 interaction as well as for the transforming activity.
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Feb 2010
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Abstract: The structure of the sodium-benzylhydantoin transport protein Mhp1 from Microbacterium liquefaciens comprises a five-helix inverted repeat, which is widespread among secondary transporters. Here, we report the crystal structure of an inward-facing conformation of Mhp1 at 3.8 angstroms resolution, complementing its previously described structures in outward-facing and occluded states. From analyses of the three structures and molecular dynamics simulations, we propose a mechanism for the transport cycle in Mhp1. Switching from the outward-to the inward-facing state, to effect the inward release of sodium and benzylhydantoin, is primarily achieved by a rigid body movement of transmembrane helices 3, 4, 8, and 9 relative to the rest of the protein. This forms the basis of an alternating access mechanism applicable to many transporters of this emerging superfamily.
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Jan 2010
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Abstract: A combined computational and experimental polymorph search was undertaken to establish the crystal forms of 7-fluoroisatin, a simple molecule with no reported crystal structures, to evaluate the value of crystal structure prediction studies as an aid to solid form discovery. Three polymorphs were found in a manual crystallisation screen, as well as two solvates. Form I (P21/c, Z? = 1), found from the majority of solvent evaporation experiments, corresponded to the most stable form in the computational search of Z? = 1 structures. Form III (P21/a, Z? = 2) is probably a metastable form, which was only found concomitantly with form I, and has the same dimeric R22(8) hydrogen bonding motif as form I and the majority of the computed low energy structures. However, the most thermodynamically stable polymorph, form II (P
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Jan 2008
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