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Abstract: The polydentate ligand 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine (dpyatriz) in combination with the Cu(ClO4)2/CuX2 salt mixtures (X? = Cl?, Br?, or N3?) leads to the formation of molecular coordination aggregates with formulas [Cu3Cl3(dpyatriz)2](ClO4)3 (2), [Cu3Br3(dpyatriz)2](ClO4)3 (3), and [Cu4(N3)4(dpyatriz)2(DMF)4(ClO4)2](ClO4)2 (4). These complexes consist of two dpyatriz ligands bridged via coordination to CuII and disposed either face-to-face in an eclipsed manner (2 and 3) or parallel and mutually shifted in one direction. The copper ions complete their coordination positions with Cl? (2), Br? (3), or N3?, ClO4?, and N,N-dimethylformamide (DMF) (4) ligands. All complexes crystallize together with noncoordinate ClO4? groups that display anion···? interactions with the triazine rings. These interactions have been studied by means of high level ab initio calculations and the MIPp partition scheme. These calculations have proven the ClO4?···[C3N3] interactions to be favorable and have revealed a synergistic effect from the combined occurrence of ??? stacking of triazine rings and the interaction of these moieties with perchlorate ions, as observed in the experimental systems.
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May 2008
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Abstract: We performed a combined theoretical and experimental investigation of the orbital magnetism and magnetocrystalline anisotropy of isolated Co and Fe adatoms on Pd(111) and Rh(111). Theoretical calculations of the spin and orbital moments are based on ab initio spin-polarized density-functional theory (DFT) including a self-consistent treatment of spin-orbit coupling. The calculations use a slab model to represent the adsorbate/substrate complex and allow for a complete structural relaxation leading to a strong inward displacement of the adatom and modest vertical and lateral relaxations in the substrate atoms. Compared to an idealized geometry where the atoms are kept on bulk lattice positions up to the surface, relaxation leads to a much stronger adatom/ligand hybridization. This is also reflected in the results for orbital moments and magnetocrystalline anisotropy energy (MAE). The enhanced hybridization leads to strong quenching of the adatom orbital moments but also to the formation of large induced spin and orbital moments in the substrate. As a consequence, we find that the substrate contribution to the MAE is much more important than estimated before on the basis of studies using an idealized geometry. We also find the surprising result that the MAE strongly depends on the adsorption site. The magnitude and even the sign of the MAE change for adatoms on face-centered cubic with respect to the ones on hexagonal close-packed hollow sites on the (111) surface. The dependence of the MAE on the combination of adatom and substrate has been analyzed in terms of the electronic structure, leading to a sound physical picture of the origin of the MAE. A fundamental problem, however, is the correct prediction of the size of the orbital moments of the adatoms. We suggest that this problem can be solved only via post-DFT corrections introducing an orbital dependence of the exchange potential. The theoretical results are compared to site-averaged, element-specific x-ray magnetic circular dichroism (XMCD) measurements. Low-temperature XMCD spectra and magnetization curves reveal weak out-of-plane anisotropy for Fe adatoms on both substrates. Interestingly, Co adatoms on Rh(111) present in-plane anisotropy with MAE of about -0.6 meV, contrary to the known out-of-plane anisotropy of Co on Pd(111) and Pt(111). The orbital to spin magnetic-moment ratio measured by XMCD shows that the Co adatoms present much stronger orbital magnetization components compared to Fe. The connection between orbital moments and MAE is discussed at the theoretical level including the contribution of the induced substrate magnetization.
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Mar 2010
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Abstract: Cardiovascular stents are commonly made from 316L stainless steel and are deployed within stenosed arterial lesions using balloon expansion. Deployment involves inflating the balloon and plastically deforming the stent until the required diameter is obtained. This plastic deformation induces static stresses in the stent, which will remain for the lifetime of the device. In order to determine these stresses, finite element models of the unit cells of geometrically different, commercially available balloon expandable stents have been created, and deployment and elastic recoil have been simulated. In this work the residual stresses associated with deployment and recoil are compared for the various stent geometries, with a view to establishing appropriate initial stress states for fatigue loading for the stents. The maximum, minimum, and mean stresses induced in the stent due to systolic/diastolic pressure are evaluated, as are performance measures such as radial and longitudinal recoil.
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Apr 2007
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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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Abstract: We report on the capillary flow behaviour of thermotropic liquid crystal mixtures containing 4-n-octyl-4?-cyanobiphenyl (8CB) and 4-n-pentyl-4?-cyanobiphenyl (5CB). The liquid crystal mixtures are studied in the Nematic (N) and Smectic (SA) phases at room temperature. Polarised optical microscopy (POM), rheology and simultaneous X-ray diffraction (XRD)/capillary flow experiments are performed to characterise the system.
Polarised optical microscopy reveals a dramatic change in optical texture when the 5CB content is increased from 20 to 30% in the mixtures. X-ray diffraction results show that the system goes through a SA–N phase transition, such that the mixtures are smectic for 10–20% 5CB and nematic for 30–90% 5CB. Smectic mixtures flow with the layers aligned along the flow direction (mesogens perpendicular to flow) while nematic mixtures flow with the mesogens aligned in the flow direction. Simultaneous XRD/shear flow experiments show that the SA–N transition is independent of the flow rate in the range 1–6 ml min?1.
The correlation length of the liquid crystal order decreases with increasing 5CB content. Rheology is used to prove that the correlation length behaviour is related to a reduction in the viscosity of the mixture.
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Apr 2009
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Abstract: Background: Despite the existence of over 50 subtypes and circulating recombinant forms of HIV-1, Subtype C dominates the heterosexual pandemic causing approximately 56% of all infections.Objective: To evaluate whether viral genetic factors may contribute to the observed subtype-C predominance.Methods: Chimeric viruses were generated using V1-V3 envelope fragments from a subtype-A/C dually infected woman with preferential genital replication of subtype C. Viral adaptation, spread and cell fusion ability were evaluated in vitro using peripheral blood mononuclear cells and HeLa-CD4-CCR5 cell lines, sequencing and cloning. Structural modeling was performed using a crystal structure of gp120-CD4-X5. Phylogenetic analysis was done using subtype-A, subtype-B and subtype-C sequences from blood and cervix of 37 infected women and database sequences.Results: We identified two envelope motifs, compact V1-V2 loops and V3-316T, which are found at high frequency throughout subtype-C evolution and affect gp120 interactions with CD4 and CCR5, respectively. When a V1-Delta 5 deletion or V3-A316T was incorporated into subtype A, each increased viral fusion and spread several fold in peripheral blood mononuclear cell and cell lines with low CCR5 expression. Structural modeling suggested the formation of an additional hydrogen bond between V3 and CCR5. Moreover, we found preferential selection of HIV with 316T and/or extremely short V1-V2 loops in cervices of three women infected with subtypes A/C, B or C.Conclusion: As CD(4+)-CCR(5+)-T cells are key targets for genital HIV infection and cervical selection can favor compact V1-V2 loops and 316T, which increase viral infectivity, we propose that these conserved subtype-C motifs may contribute to transmission and spread of this subtype. (C) 2009 Wolters Kluwer Health vertical bar Lippincotl Williams & Wilkins
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Feb 2009
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Nathan
Zaccai
,
Lester G.
Carter
,
Nick S.
Berrow
,
Sarah
Sainsbury
,
Joanne E.
Nettleship
,
Thomas S.
Walter
,
Ray J.
Owens
,
Keith S.
Wilson
,
Robert S.
Esnouf
,
David
Stuart
,
Karl
Harlos
Abstract: The Oxford Protein Production Facility (OPPF) was established to develop methods for high-throughput protein production and crystallization. As part of these developments, and in the context of the European SPINE project, a pilot study was undertaken on 48 proteins from Bacillus anthracis from protein families which were well conserved across a wide range of bacteria.1, 2Bacillus anthracis, the causative agent of anthrax, is a large, gram-positive, spore-bearing bacterium. The genome of the Ames strain of the bacterium has been sequenced3 showing two plasmids, pXO1 and pXO2, that carry the major virulence factors, as well as normal chromosomal DNA of 5.23 megabases, predicted to code for about 5311 genes. The set of 48 proteins chosen for study1 are all encoded for by the chromosomal DNA.
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Jan 2008
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Abstract: We have designed and constructed a hybrid scanning near-field optical microscope (SNOM)-scanning tunnelling microscope (STM) instrument which operates under ultrahigh vacuum (UHV) conditions. Indium tin oxide (ITO)-coated fibre-optic tips capable of high quality STM imaging and tunnelling spectroscopy are fabricated using a simple and reliable method which foregoes the electroless plating strategy previously employed by other groups. The fabrication process is reproducible, producing robust tips which may be exchanged under UHV conditions. We show that controlled contact with metal surfaces considerably enhances the STM imaging capabilities of fibre-optic tips. Light collection (from the cleaved back face of the ITO-coated fibre-optic tip) and optical alignment are facilitated by a simple two-lens arrangement where the in-vacuum collimation/collection lens may be adjusted using a slip-stick motor. A second in-air lens focuses the light (which emerges from the UHV system as a parallel beam) onto a cooled CCD spectrograph or photomultiplier tube. The application of the instrument to combined optical and electronic spectroscopy of Au and GaAs surfaces is discussed.
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Nov 2008
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Abstract: We describe the synthesis, crystal structure and conducting properties of two new BEDT-TTF charge transfer salts of tris(oxalato)ferrate(III) containing slightly different numbers of water molecules, (BEDT-TTF)12[Fe(C2O4)3]2·xH2O [x = 15 (I) and 16 (II)]. I crystallizes in the monoclinic space groupC2/c, a = 36.017(10), b = 30.880(10), c = 36.183(15) Å, ? = 90, ? = 93.66(3), ? = 90°, V = 40161(23) Å3, T = 120(2) K, Z = 8, R1 = 0.0842 [F2 > 2?(F2)]; II crystallizes in the same space group, a = 17.9695(15), b = 15.4988(13), c = 36.115(3) Å, ? = 90, ? = 93.720(2), ? = 90°, V = 10037.1(14) Å3, T = 150(2) K, Z = 4, R1 = 0.0673 [F2 > 2?(F2)]. Electrical resistivity measurements show that I and II are semiconductors below 275 K.
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Oct 2007
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Abstract: Commensal lactobacilli frequently produce bile salt hydrolase (Bsh) enzymes whose roles in intestinal survival are unclear. Twenty-six Lactobacillus salivarius strains from different sources all harbored a bsh1 allele on their respective megaplasmids. This allele was related to the plasmid-borne bsh1 gene of the probiotic strain UCC118. A second locus (bsh2) was found in the chromosomes of two strains that had higher bile resistance levels. Four Bsh1-encoding allele groups were identified, defined by truncations or deletions involving a conserved residue. In vitro analyses showed that this allelic variation was correlated with widely varying bile deconjugation phenotypes. Despite very low activity of the UCC118 Bsh1 enzyme, a mutant lacking this protein had significantly lower bile resistance, both in vitro and during intestinal transit in mice. However, the overall bile resistance phenotype of this and other strains was independent of the bsh1 allele type. Analysis of the L. salivarius transcriptome upon exposure to bile and cholate identified a multiplicity of stress response proteins and putative efflux proteins that appear to broadly compensate for, or mask, the effects of allelic variation of bsh genes. Bsh enzymes with different bile-degrading kinetics, though apparently not the primary determinants of bile resistance in L. salivarius, may have additional biological importance because of varying effects upon bile as a signaling molecule in the host.
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Sep 2009
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