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Abstract: For zeolite-type frameworks the focus has for a long time been put on producing new structures that can give optimized properties for a variety of different purposes. Many new structures have been produced on a trial and error basis. Open-framework germanates have played the role of forming many new structures as it is easier to form certain building units within the germanate system. It is time to start comparing synthesis mechanisms and building units to determine how we can control the synthesis. Here we will give an overview of some of the structures found within the open-framework germanate system and demonstrate that in order for more optimized systems to be synthesized there is a clear need for the more detailed comparison of the structural systematics of existing materials.
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Apr 2010
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Abstract: Expressions for dichroic signals in terms of electron multipoles have been used to analyse optical data gathered on a crystal of copper metaborate in the presence of a magnetic field. Calculated signals comply with the established crystal and magnetic structures of CuB2O4, and respect the global symmetries of parity-even and parity-odd dichroic signals in full. We have success in describing five different experiments in total. The claim by Saito et al (2008 Phys. Rev. Lett. 101 117402) that they observe magnetic control of crystal chirality in one of their five experiments is challenged.
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Apr 2009
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Aug 2009
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Abstract: Bacteria are abundant in many natural and engineered environments where they are thought to exert important controls on the cycling, mobility, bioavailability, and toxicity of metal contaminants. In order to probe their role in moderating the behavior of lanthanides, pH-dependent adsorption edges of 13 individual lanthanides and yttrium to the Gram-negative bacterium Pantoea agglomerans were used to generate discrete site surface complexation constants. The calculated surface complexation constants were compared with stability constants estimated using linear free energy relationships based on a number of hydroxyl-containing ligands. The experimental data suggests that lanthanide adsorption edges below pH 6.5 are consistent with adsorption to phosphate groups for the light and some of the middle lanthanides (La to Gd), whereas some of the middle and heavy lanthanides appear to favor carboxyl co-ordination (Tb to Yb), although exceptions occur in each grouping. The experimentally derived surface complexation constants for carboxyl coordination were of similar magnitude to stability constants estimated from linear free energy correlations using fulvic acid stability constants. The implication is that the adsorption of lanthanides to bacterial surfaces could be modeled reasonably well using lanthanide stability constants for natural organic matter, except perhaps at low pH where phosphate binding dominates.
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Jan 2010
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Moritz
Hoesch
,
Matteo
D’astuto
,
Matteo
Calandra
,
Geneviève
Loupias
,
Francesco
Mauri
,
Shuyun
Zhou
,
Jeff
Graf
,
Alessandra
Lanzara
,
Nicolas
Emery
,
Claire
Hérold
,
P.
Lagrange
,
Daniel
Petitgrand
Abstract: We report measurements of phonon dispersion in CaC6 using inelastic x-ray and neutron scattering. We find good overall agreement, particularly in the 50 meV energy region, between experimental data and first-principles density-functional-theory calculations. However, on the longitudinal dispersion along the (111) axis of the rhombohedral representation, we find an unexpected anticrossing with an additional longitudinal mode, at about 11 meV. At a comparable energy, we observe also unexpected intensity on the in-plane direction. These results resolve the previous incorrect assignment of a longitudinal phonon mode to a transverse mode in the same energy range. By calculating the electron susceptibility from first principles we show that this longitudinal excitation is unlikely to be due to a plasmon and consequently can probably be due to defects or vacancies present in the sample.
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Mar 2010
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Abstract: This study was designed to combine surface complexation modelling of macroscopic adsorption data with X-ray Absorption Spectroscopic (XAS) measurements to identify lanthanide sorption sites on the bacterial surface. The adsorption of selected representatives for light (La and Nd), middle (Sm and Gd) and heavy (Er and Yb) lanthanides was measured as a function of pH, and biomass samples exposed to 4 mg/L lanthanide at pH 3.5 and 6 were analysed using XAS. Surface complexation modelling was consistent with the light lanthanides adsorbing to phosphate sites, whereas the adsorption of middle and heavy lanthanides could be modelled equally well by carboxyl and phosphate sites. The existence of such mixed mode coordination was confirmed by Extended X-ray Absorption Fine Structure (EXAFS) analysis, which was also consistent with adsorption to phosphate sites at low pH, with secondary involvement of carboxyl sites at high adsorption density (high pH). Thus, the two approaches yield broadly consistent information with regard to surface site identity and lanthanide coordination environment. Furthermore, spectroscopic analysis suggests that coordination to phosphate sites is monodentate at the metal/biomass ratios used. Based oil the best-fitting pK(a) site, we infer that the phosphate sites are located on N-acetylglucosamine phosphate, the most likely polymer on gram-negative cells with potential phosphate sites that deprotonate around neutral pH.
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Jan 2009
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Abstract: The first layered iron borate, Fe5O5(B6O10(OH)(3)] center dot nH(2)O, has been prepared by the boric acid flux method, Its structure, determined by single crystal X-ray diffraction, contains a double FeO6-octahedral layer and an unusual [B6O13] chain. The rigid and cambered [B6O13] Chains bend the octahedral layers, resulting in a wave-like and sandwiched structure. Crystallographic study indicates the structural modulation is mainly from the [B6O13] chains because of the insertion of water molecules in between. Nevertheless, FeO6 layers in the average structure, which are well separated by borate chains, is still a reasonable model to understand the two-dimensional magnetism. The strong antiferromagnetic interactions and the complex Fe3+-net suggest a possible geometrically magnetic frustration, which may be the reason for the second-order temperature-induced magnetic transition at similar to 125 K. The condensed Fe3+ layers and the relatively low redox potential at about 1.25 V versus Li+/Li show its potentials as an anodic material.
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Oct 2009
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Open Access
Abstract: The transient complex between cytochrome f and plastocyanin from the cyanobacterium Nostoc sp. PCC 7119 has been analysed by X-ray Absorption Spectroscopy in solution, using both proteins in their oxidized and reduced states. Fe K-edge data mainly shows that the atypical metal coordination geometry of cytochrome f, in which the N-terminal amino acid acts as an axial ligand of the heme group, remains unaltered upon binding to its redox partner, plastocyanin. This fact suggests that cytochrome f provides a stable binding site for plastocyanin and minimizes the reorganization energy required in the transient complex formation, which could facilitate the electron transfer between the two redox partners.
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Jan 2007
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Abstract: For the initiation of DNA replication, dsDNA is unwound by helicases. Primases then recognize specific sequences on the template DNA strands and synthesize complementary oligonucleotide primers that are elongated by DNA polymerases in leading- and lagging-strand mode. The bacterial plasmid RSF1010 provides a model for the initiation of DNA replication, because it encodes the smallest known primase RepB? (35.9 kDa), features only 1 single-stranded primase initiation site on each strand (ssiA and ssiB, each 40 nt long with 5?- and 3?-terminal 6 and 13 single-stranded nucleotides, respectively, and nucleotides 7–27 forming a hairpin), and is replicated exclusively in leading strand mode. We present the crystal structure of full-length dumbbell-shaped RepB? consisting of an N-terminal catalytic domain separated by a long ?-helix and tether from the C-terminal helix-bundle domain and the structure of the catalytic domain in a specific complex with the 6 5?-terminal single-stranded nucleotides and the C7–G27 base pair of ssiA, its single-stranded 3?-terminus being deleted. The catalytic domains of RepB? and the archaeal/eukaryotic family of Pri-type primases share a common fold with conserved catalytic amino acids, but RepB? lacks the zinc-binding motif typical of the Pri-type primases. According to complementation studies the catalytic domain shows primase activity only in the presence of the helix-bundle domain. Primases that are highly homologous to RepB? are encoded by broad-host-range IncQ and IncQ-like plasmids that share primase initiation sites ssiA and ssiB and high sequence identity with RSF1010.
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Apr 2009
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Jan 2009
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