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Abstract: Advanced spectroscopic techniques provide new and unique tools for unraveling the nature of the electronic structure of actinide materials. Inelastic neutron scattering experiments, which address temporal aspects of lattice and magnetic fluctuations, probe electromagnetic multipole interactions and the coupling between electronic and vibrational degrees of freedom. Nuclear magnetic resonance clearly demonstrates different magnetic ground states at low temperature. Photoemission spectroscopy provides information on the occupied part of the electronic density of states and has been used to investigate the momentum-resolved electronic structure and the topology of the Fermi surface in a variety of actinide compounds. Furthermore, x-ray absorption and electron energy-loss spectroscopy have been used to probe the relativistic nature, occupation number, and degree of localization of 5f electrons across the actinide series. More recently, element- and edge-specific resonant and non-resonant inelastic x-ray scattering experiments have provided the opportunity of measuring elementary electronic excitations with higher resolution than traditional absorption techniques. Here, we will discuss results from these spectroscopic techniques and what they tell us of the electronic and magnetic properties of selected actinide materials.
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Nov 2010
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Abstract: Meeting the challenges of Moore´s Law, predicting ambitious miniaturization rates of integrated circuits, requires to go beyond the traditional top down approaches, and to employ synthetic chemistry methods, to use bottom-up techniques. During the recent decades, it has been demonstrated that open shell coordination compounds may exhibit intra-molecular spontaneous magnetization, thus offering promising prospects for storage and processing of digital information. Against this background we regarded it rewarding to implement similar magnetic centers into a ceramic material, which would provide better long term mechanical and chemical durability. Here we present new robust inorganic compounds containing separate DyO+ ions in an apatite matrix, which behave like single-molecule magnets. The materials exhibit a blocking temperature of 11 K and an energy barrier for spin reversal of a thousand cm-1 which is among the highest values ever achieved.
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Sep 2017
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Abstract: The co-adsorption of hydrogen with a simple chiral modifier, alanine, on Ni{111} was studied using Density Functional Theory in combination with ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy at temperatures of 300~K and above, which are representative of chiral hydrogenation reactions. Depending on the hydrogen pressure, the surface enables protons to "pop on and off" the modifier molecules, thus significantly altering the adsorption geometry and chemical nature of alanine from anionic tridentate in ultra-high vacuum to predominantly zwitterionic bidentate at hydrogen pressures above 0.1 Torr. This hydrogen-stabilised modifier geometry allows alternative mechanisms for proton transfer and the creation of enatioselective reaction environments.
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Mar 2018
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I03-Macromolecular Crystallography
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Abstract: Kinesins comprise a superfamily of molecular motors that drive a wide variety of cellular physiologies, from cytoplasmic transport to formation of the bipolar spindle in mitosis. These differing roles are reflected in corresponding polymorphisms in key kinesin structural elements. One of these is a unique loop and stem motif found in all kinesins and referred to as loop 5 (L5). This loop is longest in the mitotic kinesin Eg5 and is the target for a number of small molecule inhibitors, including ispinesib, which is being used in clinical trials in patients with cancer. In this study, we have used x-ray crystallography to identify a new structure of an Eg5-ispinesib complex and have combined this with transient state kinetics to identify a plausible sequence of conformational changes that occur in response to ispinesib binding. Our results demonstrate that ispinesib-induced structural changes in L5 from Eg5 lead to subsequent changes in the conformation of the switch II loop and helix and in the neck linker. We conclude that L5 in Eg5 simultaneously regulates the structure of both the ATP binding site and the motor's mechanical elements that generate force.
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May 2013
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B16-Test Beamline
I22-Small angle scattering & Diffraction
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Abstract: The combination of X-ray imaging and diffraction techniques provides a unique tool for structural and mechanical analysis of engineering components. A variety of modes can be employed in terms of the spatial resolution (length-scale), time resolution (frequency), and the nature of the physical quantity being interrogated. This thesis describes my contributions towards the development of novel X-ray “rich” imaging experimental techniques and data interpretation. The experimental findings have been validated via comparison with other experimental methods and numerical modelling.
The combination of fast acquisition rate and high penetration properties of X-ray beams allows the collection of high-resolution 3-D tomographic data sets at submicron resolution during in situ deformation experiments. Digital Volume Correlation analysis tools developed in this study help understand crack propagation mechanisms in quasi-brittle materials and elasto-plastic deformation in co-sprayed composites.
For the cases of crystalline specimens where the knowledge of “live” or residual elastic strain distributions is required, diffraction techniques have been advanced. Diffraction Strain Tomography (DST) allows non-destructive reconstruction of the 2-D (in-plane) variation of the out-of-plane strain component. Another diffraction modality dubbed Laue Orientation Tomography (LOT), a grain mapping approach has been proposed and developed based on the translate-rotate tomographic acquisition strategy. It allows the reconstruction of grain shape and orientation within polycrystalline samples, and provides information about intragranular lattice strain and distortion. The implications of this method have been thoroughly investigated.
State-of-the-art engineering characterisation techniques evolve towards scrutinising submicron scale structural features and strain variation using the complementarity of X-ray imaging and diffraction. The first successful feasibility study is reported of in operando stress analysis in an internal combustion engine.
Finally, further advancement of ‘rich’ imaging techniques is illustrated via the first successful application of Time-of-Flight Neutron Diffraction Strain (TOF-NDST) tomography for non-destructive reconstruction of the complete strain tensor using an inverse eigenstrain formulation.
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Sep 2014
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B23-Circular Dichroism
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Diamond Proposal Number(s):
[7530]
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Oct 2012
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I03-Macromolecular Crystallography
I04-1-Macromolecular Crystallography (fixed wavelength)
I04-Macromolecular Crystallography
I24-Microfocus Macromolecular Crystallography
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Diamond Proposal Number(s):
[18069]
Open Access
Abstract: Protein ADP-ribosylation is a highly dynamic post-translational modification. The rapid turnover is achieved, among others, by ADP-(ribosyl)hydrolases (ARHs), an ancient family of enzymes that reverses this modification. Recently ARHs came into focus due to their role as regulators of cellular stresses and tumor suppressors. Here we present a comprehensive structural analysis of the enzymatically active family members ARH1 and ARH3. These two enzymes have very distinct substrate requirements. Our data show that binding of the adenosine ribose moiety is highly diverged between the two enzymes, whereas the active sites harboring the distal ribose closely resemble each other. Despite this apparent similarity, we elucidate the structural basis for the selective inhibition of ARH3 by the ADP-ribose analogues ADP-HPD and arginine-ADP-ribose. Together, our biochemical and structural work provides important insights into the mode of enzyme-ligand interaction, helps to understand differences in their catalytic behavior, and provides useful tools for targeted drug design.
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Nov 2018
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Open Access
Abstract: Hybrid halide perovskites have recently emerged as a highly efficient class of light absorbers; however, there are increasing concerns over their long-term stability. Recently, incorporation of SCN– has been suggested as a novel route to improving stability without negatively impacting performance. Intriguingly, despite crystallizing in a 2D layered structure, (CH3NH3)2Pb(SCN)2I2 (MAPSI) possesses an ideal band gap of 1.53 eV, close to that of the 3D connected champion hybrid perovskite absorber, CH3NH3PbI3 (MAPI). Here, we identify, using hybrid density functional theory, the origin of the smaller than expected band gap of MAPSI through a detailed comparison with the electronic structure of MAPI. Furthermore, assessment of the MAPSI structure reveals that it is thermodynamically stable with respect to phase separation, a likely source of the increased stability reported in experiment.
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Nov 2015
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B18-Core EXAFS
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Diamond Proposal Number(s):
[14239]
Abstract: The rutiles (M,Ru)O2 (M = Mg, Zn, Co, Ni, Cu) are formed directly under hydrothermal conditions at 240 °C from potassium perruthenate and either peroxides of zinc or magnesium, or poorly crystalline oxides of cobalt, nickel or copper. The polycrystalline powders consist of lath-shaped crystallites, tens of nanometres in maximum dimension. Powder neutron diffraction shows that the materials have expanded a axis and contracted c axis compared to the parent RuO2, but there is no evidence of lowering of symmetry to other AO2-type structures, supported by Raman spectroscopy. Rietveld refinement shows no evidence for oxide non-stoichiometry and provides a formula (MxRu1-x)O2 with 0.14 < x < 0.2, depending on the substituent metal. This is supported by energy-dispersive X-ray analysis on the transmission electron microscope, while Ru K-edge XANES spectroscopy shows that upon inclusion of the substituent the average Ru oxidation state is increased to balance charge. Variable temperature magnetic measurements provide evidence for atomic homogeneity of the mixed metal materials, with suppression of the high temperature antiferromagnetism of RuO2 and increased magnetic moment. The new rutiles all show enhanced electrocatalysis compared to reference RuO2 materials for oxygen evolution in 1 M H2SO4 electrolyte at 60 °C, with higher specific and mass activity (per Ru) than a low surface area crystalline RuO2, and with less Ru dissolution over 1000 cycles compared to an RuO2 with a similar surface area. Magnesium substitution provides the optimum balance between stability and activity, despite leaching of the Mg2+ into solution, and this was proved in membrane electrode assemblies.
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Jun 2020
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I24-Microfocus Macromolecular Crystallography
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Diamond Proposal Number(s):
[17844]
Abstract: The stringent response alarmones pppGpp and ppGpp are essential for rapid adaption of bacterial physiology to changes in the environment. In Escherichia coli, the nucleosidase PpnN (YgdH) regulates purine homeostasis by cleaving nucleoside monophosphates and specifically binds (p)ppGpp. Here, we show that (p)ppGpp stimulates the catalytic activity of PpnN both in vitro and in vivo causing accumulation of several types of nucleobases during stress. The structure of PpnN reveals a tetramer with allosteric (p)ppGpp binding sites located between subunits. pppGpp binding triggers a large conformational change that shifts the two terminal domains to expose the active site, providing a structural rationale for the stimulatory effect. We find that PpnN increases fitness and adjusts cellular tolerance to antibiotics and propose a model in which nucleotide levels can rapidly be adjusted during stress by simultaneous inhibition of biosynthesis and stimulation of degradation, thus achieving a balanced physiological response to constantly changing environments.
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Apr 2019
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