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Acta Cryst. (2014). A70, C856
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Cubic crystals have been studied in detail ever since the advent of X-ray crystallography 101 years ago when Lawrence Bragg first solved the cubic structure of rocksalt. Despite this long history they still hold a fascination with many topical materials such as C60, PbTe-based thermoelectrics, ZrW2O8-related negative thermal expansion materials and several ice phases, all crystallising into cubic structures. This talk will briefly outline the early heritage of (cubic) crystallography and will then highlight the key role of complexity on the physical properties of many cubic crystals. I define complexity as a local breaking of the overall crystal symmetry, rather than the creation of ever larger unit cells, and therefore accurate understanding cannot be gained through analysis of Bragg diffraction alone. Instead I will describe methods for quantifying this complexity via models refined by reverse Monte Carlo analysis of total scattering (pair distribution function) data and using a range of different examples, some taken from the list given above.

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Acta Cryst. (2014). A70, C1204
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Enzymes continue to expand their role in industry as a "green" option for the synthesis of value-added products. They are targeted for the design of drugs in pharmaceutical applications and also for protein engineering in industry to improve their efficiency, stability, and specificity. Knowledge of the exact mechanisms of enzymatic reactions may provide essential information for more effective drug design and enzyme engineering. For the first time, we are employing a joint X-ray/neutron (XN) protein crystallographic technique in combination with high-performance computing, including QM and QM/MM calculations, MD and Rosetta simulations, to investigate the mechanisms of several enzymes that are important to renewable energy and chemical synthesis. D-xylose isomerase (XI) is an enzyme which can be used to increase the production of biofuels from lignocellulosic biomass and also to synthesize rare sugars for pharmaceutical industry. XI catalyzes the reversible multi-stage sugar inter-conversion reaction facilitated by the presence of two divalent metal cations in its active site. It primarily catalyzes the isomerization of the aldo-sugar D-xylose to the keto-isomer D-xylulose, but can also epimerize L-arabinose into L-ribose, albeit much less efficiently. The reaction involves moving hydrogen atoms between the protein residues, sugar and water molecules, and can only be understood if hydrogen atoms are visualized at each reaction stage. We have obtained a number of joint XN structures of XI complexes representing snapshots along the reaction path with D-glucose, D-xylose and L-arabinose. The suggested reaction mechanism has been verified by QM calculations using the novel O(N) methodology. We are using this structural and mechanistic information to re-design XI to be more efficient on D-xylose and L-arabinose for biofuels and biomedical applications by employing QM/MM, MD, and Rosetta methodologies.

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Acta Cryst. (2014). A70, C1544
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Frustrated spin systems can exhibit a macroscopic degeneracy of magnetic ground states which suppresses periodic magnetic order [1]. The importance of understanding these correlations lies primarily in determining the origin of exotic phenomena that emerge from frustrated spin systems, e.g. the evolution of high-temperature superconductivity from spin liquids and the ability of spin ices to support emergent magnetic charges [2]. The magnetic structures of ordered magnets can usually be determined by crystallographic analysis of the magnetic Bragg scattering observed in neutron diffraction experiments. However, the suppression of periodic magnetic order in frustrated magnets means that only magnetic diffuse scattering is observed. This has led to a consensus that single-crystal data are needed for a detailed understanding of spin correlations in frustrated systems. In my presentation, I will explore how the powder magnetic diffuse scattering pattern I(Q) can be converted robustly into a magnetic-structure model. Our approach is to consider simulated I(Q) data for a number of test cases. These data are then fitted using the reverse Monte Carlo (RMC) method. Finally, the quality of the models obtained is assessed by calculating the 3D scattering function I(Q). I will show that the extent of information loss during spherical averaging of I(Q) is surprisingly minimal, and that the full I(Q) is recoverable from powder diffraction data for each frustrated system that we explore [3]. I will go on to discuss real examples where we have used the RMC approach to obtain insight into frustrated materials. First, I will examine the interplay between geometrical frustration and low dimensionality in the paramagnetic phase of the spin-chain compound Ca3Co2O6. Finally, I will show how the RMC method can be extended for the analysis of large single-crystal diffuse scattering datasets, using as an example the canonical antiferromagnet MnO.
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