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Mar 2009
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Open Access
Abstract: A molecular understanding of the unique aspects of dietary fructose metabolism may be the key to understanding and controlling the current epidemic of fructose-related obesity, diabetes and related adverse metabolic states in Western populations. Fructose catabolism is initiated by its phosphorylation to fructose 1-phosphate, which is performed by ketohexokinase (KHK). Here, the crystal structures of the two alternatively spliced isoforms of human ketohexokinase, hepatic KHK-C and the peripheral isoform KHK-A, and of the ternary complex of KHK-A with the substrate fructose and AMP-PNP are reported. The structure of the KHK-A ternary complex revealed an active site with both the substrate fructose and the ATP analogue in positions ready for phosphorylation following a reaction mechanism similar to that of the pfkB family of carbohydrate kinases. Hepatic KHK deficiency causes the benign disorder essential fructosuria. The effects of the disease-causing mutations (Gly40Arg and Ala43Thr) have been modelled in the context of the KHK structure.
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Mar 2009
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Abstract: The misfolding of proteins and peptides can lead to fibrillar aggregates which are often termed amyloid fibrils. These are characterized by a cross-beta structure, with -strands and polypeptide backbones running perpendicular to the fibril axis, which is coincident with the hydrogen bonding direction. Several types of amyloid fibrils have been shown by high resolution electron microscopy to comprise assemblies of individual twisted -sheets,2-5 of which there are two typesstapes or ribbons. The two are distinguished by the presence or absence of a hollow core in the fibril. The twist arises from packing constraints of the side groups of the residues within the beta-Strands.
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Oct 2008
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Abstract: A new high-pressure phase of pure nitric acid (HNO(3)) has been characterised at 1.6 GPa at room temperature by high-pressure neutron powder and X-ray single-crystal diffraction techniques. This is the first crystalline phase obtained upon compression of liquid nitric acid at room temperature and appears to be the stable phase up to pressures of at least 4 GPa. The crystal structure of this new phase shows some similarities to that of the low-temperature phase of nitric acid at ambient pressure, which has been redetermined as part of this study. Both structures share a herringbone packing of hydrogen-bonded molecular catemers, although the presence of disorder within the hydrogen bonds within one of the catemers of the low-temperature phase makes its structure comparatively more complex.
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Jan 2010
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Abstract: Energy-dispersive X-ray absorption spectroscopy is an increasingly powerful tool for the investigation of kinetic processes in chemical systems as an element-specific local structure and electronic-state probe. Advances in synchrotron radiation sources and detector technology are pushing the time resolution of the method to ever shorter periods, currently milliseconds to microseconds, while also providing a concomitant improvement in data quality that now makes feasible the identification of structural and electronic motifs characteristic of intermediate states in chemical processes. To maximize the value of the newly available high-quality time-resolved data, techniques for consistent data normalization and structural component analysis have been developed and here are illustrated in a model study of the electron-transfer reaction between [IrCl6]2- with [Co(CN)5]3-.
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Aug 2005
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Abstract: Time resolved energy dispersive X-ray absorption spectroscopy has been used to follow the structural evolution of the inner-sphere electron transfer reaction between [IrCl6]2? and [Co(CN)5]3?, and to characterise the local structure of the iridium metal centre in the bridged activated complex formed during the reaction.
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Sep 2005
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Jun 2008
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Abstract: Subtleties in the electronic structure of complex materials can be directly observed, in great detail, by means of the Bragg diffraction of x-rays whose energy matches an atomic resonance. Strange atomic multipoles can be encountered in the interpretation of measured Bragg intensities, e.g., chirality and magnetic charge. Additionally, the x-ray technique allows the direct observation of the enantiomorphic screw-axis in chiral crystals, such as tellurium, low quartz and berlinite.
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Nov 2009
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Jan 2010
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Abstract: Ordering of Fe3+ and Fe2+ cations between octahedral and tetrahedral sites in synthetic members of the magnetite-ulvospinel (Fe3O4-Fe2TiO4) solid-solution series was determined using Fe L-2,L-3-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-2,L-3-edge and Fe K-edge X-ray absorption spectroscopy (XAS), and unit-cell parameters. Microprobe analyses, cell edges, and chemical FeO determinations showed that bulk compositions were stoichiometric magnetite-ulvospinel solid solutions. XMCD showed that the surface was sensitive to redox conditions, and samples required re-equilibration with solid-solid buffers. Detailed site-occupancy analysis gave Fe2+/Fe3+ XMCD-intensity ratios close to stoichiometric values. L-2,L-3-edge XAS confirmed that Ti4+ was restricted to octahedral sites. XMCD showed that significant Fe2+ only entered the tetrahedral sites when Ti content was >0.40 atoms per formula unit (apfu), whereas Fe2+ in octahedral sites increased from 1 apfu in magnetite to a maximum of similar to 1.4 apfu when Ti content was 0.45 apfu. As Ti content increased, a steady increase in Fe2+ in tetrahedral sites was observable in the XMCD spectra, concurrent with a slow decrease in Fe2+ in octahedral sites. Calculated magnetic moments decreased rapidly from magnetite (4.06 mu(B)) to USP45 (1.5 mu(B)), then more slowly toward ulvospinel (0 mu(B)). Two synthesized samples were maghemitized by re-equilibrating with an oxidizing buffer. XMCD showed that Fe2+ oxidation, with concomitant vacancy formation, was restricted to octahedral sites. Through the direct measurement of Fe oxidation states, XMCD results can be used to rationalize the magnetic properties of titanomagnetites, along with oxidized titanomaghemitized analogs, in Earth's crustal rocks.
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Jan 2010
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