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Science Highlights

Neutrons Reveal Structural Tunability of Magnetic Exchange Couplings

Dynamic magnetic structure factor for BaFe2As2 (top) and NaFeAs (bottom)

Inelastic neutron scattering experiments reveal that the effective magnetic exchange couplings in NaFeAs are smaller and more isotropic than those in the heavily studied AFe2As2 family. These results provide evidence that the energy scale of the magnetic excitations, which are thought to influence the superconducting transition temperature, is controlled by the position of As atoms above the Fe layers.

Spin waves, measured using the ARCS chopper spectrometer at SNS, were studied in the antiferromagnetic ordered phase of NaFeAs, the parent compound of NaFe1−x CoxAs family of iron pnictide superconductors. NaFeAs was chosen because it has weak AF ordered moment, low superconducting transition temperature, and large As height, significantly different from AFe2As2.

C. Zhang, L.W. Harriger, Z. Yin, W. Lv, M. Wang, G. Tan, Y. Song, D. L. Abernathy, W. Tian, T. Egami, K. Haule, G. Kotliar, and P. Dai, “Effect of Pnictogen Height on Spin Waves in Iron Pnictides”. Phys. Rev. Lett. 112 (2014): 217202. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.217202

Unlocking Enzyme Synthesis of Rare Sugars to Create Drugs with Fewer Side Effects

An artist’s rendering of the enzyme D-xylose isomerase as it isomerizes L-arabinose into rare sugars not found in nature. The enzyme acts as a filter by capturing and performing catalysis only on the high-energy 5S1 conformation of L-arabinose, while remaining inactive on other more abundant sugar conformations. Neutron macromolecular crystallography has unequivocally demonstrated how this high-energy conformer of L-arabinose binds in the enzyme active site and is converted to the linear intermediate form. Simulations provide evidence for the experimental results. Image credit: Genevieve Martin/ORNL

In a paper published in Structure, a team led by the U.S. Department of Energy’s Oak Ridge National Laboratory reported the pioneering use of neutron and X-ray crystallography and high performance computing to study how the enzyme D-xylose isomerase, or XI, can cause a biochemical reaction in natural sugar to produce rare sugars. Unlike drugs made from natural sugar compounds, drugs made from rare sugars do not interfere with cellular processes. As a result, rare sugars have important commercial and biomedical applications as precursors for the synthesis of different antiviral and anti-cancer drugs with fewer side effects. Using X-ray and neutron crystallography combined with theoretical calculations, the team figured out how the enzyme isomerizes L-arabinose into the rare sugar L-ribulose and then epimerizes the latter into another rare sugar L-ribose. Importantly, L-ribose is the enantiomer, a mirror image, of the ubiquitous D-ribose that is a building block of DNA and RNA.

Langan , A.K. Sangha, T. Wymore, J.M. Parks, Z.K. Yang, B.L. Hanson, Z. Fisher, S.A. Mason, M.P. Blakeley, V.T. Forsyth, J.P. Glusker, H.L. Carrell, J.C. Smith, D.A. Keen, D.E. Graham, A. Kovalevsky. “L-Arabinose Binding, Isomerization, and Epimerization by D-Xylose Isomerase: X-Ray/Neutron Crystallographic and Molecular Simulation Study,” Structure. 22:9 (2014): 1287–1300. http://dx.doi.org/10.1016/j.str.2014.07.002

Improving Plant-based Battery with Neutrons, Simulation

Researchers at ORNL and the University of Tennessee are studying the structure of plant-based battery materials by combining neutron experiments and supercomputer simulations. The molecular model pictured above shows the battery anode’s composition: amorphous carbon (blue), crystalline carbon (green), and hydrogen (white.)

A team from Oak Ridge National Laboratory and the University of Tennessee studied carbon fibers made from lignin, a woody plant polymer known as, and found a mixture of perfectly spherical nanoscale crystallites distributed within a fibrous matrix. They found the lignin fiber’s unique structure could make it useful as a battery anode, potentially improving upon graphitic materials found in most lithium-ion batteries. Lignin, a low-cost byproduct of the pulp, paper and biofuels industries, could be transformed into a cheaper version of highly engineered graphite through a simple and industrially scalable manufacturing process. The team ran neutron scattering experiments at ORNL’s Spallation Neutron Source (SNS) to analyze how lignin-based fiber samples reacted with lithium. Using neutron data, they developed computational models and ran simulations on supercomputers including the Oak Ridge Leadership Computing Facility’s (OLCF) Titan, supported by DOE’s Office of Science, and UT’s Kraken, supported by the National Science Foundation. The team detailed its approach in the Journal of Applied Crystallography. The initial combination of neutron experiments and simulation gave the UT-ORNL team a first glimpse into how the material’s structure affects its overall performance. Now that they have confirmed their model’s accuracy, the researchers plan on applying the technique to how the material’s structure changes with and without added lithium, mimicking the charging and discharging cycles of a real battery.

N.W. McNutt, O. Rios, M. Feygenson, T.  E. Proffen and D. J. Keffer. “Structural analysis of lignin-derived carbon composite anodes,” Journal of Applied Crystallography. 47:5 (2014): 1577-1584. http://journals.iucr.org/j/issues/2014/05/00/pd5045/index.html

Study compares structures of Huntington’s disease protein

Researchers used neutron scattering experiments to clarify structural differences between the normal (left) and pathological (right) forms of huntingtin protein aggregates. The pathological form of this protein is implicated in Huntington’s disease

Neutron scattering research at the Department of Energy’s Oak Ridge National Laboratory has revealed clear structural differences in the normal and pathological forms of a protein involved in Huntington’s disease. Huntington’s disease, an incurable neurodegenerative disorder, starts as a genetic mutation that leads to an overabundance of “huntingtin” protein fragments, which form clumps in the brain. Valerie Berthelier of the University of Tennessee Graduate School of Medicine, who co-led the study published in Biophysical Journal with ORNL’s Chris Stanley, said the goal was to establish a baseline understanding of huntingtin’s structure in order to eventually determine the true structural basis of Huntington’s disease. The study’s results showed key differences in the ways mutant and normal huntingtin proteins take shape. The disease protein, for instance, initially forms aggregates of one to two peptides, whereas the normal version makes bigger aggregates, gathering seven or eight peptides together. These data on the very early stages of protein aggregate formation support a growing focus of the research in the amyloid field. Amyloid disorders, such as Parkinson’s, Alzheimer’s and Huntington’s, all involve protein aggregation phenomena leading to a disease.

Perevozchikova, Tatiana et al. “Investigating the Structural Impact of the Glutamine Repeat in Huntingtin Assembly,” Biophysical Journal. 107:2 (2014): 411 – 421. http://www.cell.com/biophysj/abstract/S0006-3495(14)00602-X

Neutrons Explore the Quantum Nature of Magnetically Frustrated Systems

Temp. dependence of the INS intensity (0.5 > Q > 2.0 Å-1) for (a) Lu2Mo2O7 and (b) Lu2Mo2O5N2. The response of the oxide at low-T indicates spin glass behavior. In the oyxnitride, the featureless response at all temperatures is consistent with a quantum spin liquid.

Neutron scattering and heat capacity techniques were used to investigate two frustrated magnets with similar structure, but different sized spins (S).The research team, led by McMaster University, found the molybdate pyrochlore oxide material, Lu2Mo2O7 (S=1) undergoes a transition to a spin glass ground state, while the oxynitride Lu2Mo2O5N2 (S=1/2) possesses a quantum spin liquid ground state. The smaller spin of the oxynitride is subjected to larger quantum fluctuations that favor a quantum spin liquid state. The preparation of oxynitride samples makes it possible to study magnetic frustration for different spin magnitudes without changing the magnetic lattice. Inelastic neutron scattering conducted at the Cold Neutron Chopper Spectrometer (CNCS) instrument at the Spallation Neutron Source of the Oak Ridge National Laboratory was used to probe the length and energy scales of spin fluctuations.

L. Clark, G. J. Nilsen, E. Kermarrec, G. Ehlers, K. S. Knight, A. Harrison, J. P. Attfield and B. D. Gaulin. “From Spin Glass to Quantum Spin Liquid Ground States in Molybdate Pyrochlores.” Phys. Rev. Lett. 113 (2014): 117201. http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.117201

Spatiotemporal stress and structure evolution in dynamically sheared polymer-like micellar solutions

The complex, nonlinear flow behavior of soft materials transcends industrial applications, smart material design and non-equilibrium thermodynamics. A long-standing, fundamental challenge in soft-matter science is establishing a quantitative connection between the deformation field, local microstructure and macroscopic dynamic flow properties i.e., the rheology. Here, a new experimental method is developed using simultaneous small angle neutron scattering (SANS) and nonlinear oscillatory shear rheometry to investigate the spatiotemporal microstructure evolution of a polymer-like micellar (PLM) solution. We demonstrate the novelty of nonlinear oscillatory shear experimental methods to create and interrogate metastable material states. These include a precursory state to the shear banded condition as well as a disentangled, low viscosity state with an inhomogeneous supra-molecular microstructure flowing at high shear rates. This new experimental evidence provides insight into the complexities of the shear banding phenomenon often observed in sheared complex fluids and provides valuable data for quantitatively testing non-equilibrium theory.

A. Kate Gurnon, Carlos R. Lopez-Barron, Aaron P. R. Eberle, Lionel Porcar and Norman J. Wagner, Soft Matter, 2014,10, 2889-2898
DOI:10.1039/c3sm53113a

Neutrons Used to Study Model Vascular Systems

Actin cytoskeleton of endothelium remodels under simulated vascular flow conditions.

In what may be the first use of neutron scattering to study complex bio-medical systems under dynamic conditions, Los Alamos researchers and collaborators mimicked blood flow by engineering a layer of human endothelial cells (the cells that cover the inner surface of blood vessels) and subjecting them to shear stress. Simultaneously, the team used neutrons at the Lujan Neutron Scattering Center’s Surface Profile Analysis Reflectometer (SPEAR) to understand changes in the cell’s properties. The technique, which relies on neutron reflectometry to reveal the behavior and composition of the cells, provides a new means to explore conditions that affect human vascular health. The American Journal of Physiology-Lung Cellular and Molecular Physiology* has published the research. This research was chosen as one of the scientific news by MedicalXpress (Jan., 23rd, 2014): http://medicalxpress.com/news/2014-01-neutrons-vascular.html.

*: “Tuning Endothelial Adhesion with Temperature and Fluid Shear Stress: A Neutron Reflectivity Study”, Luka Pocivavsek, Ann Junghans, Nouredine Zebda, Konstantin Birukov, Jaroslaw Majewski, Am. J. of Physiology, vol. 306   Issue: 1   Pages: L1-L9, Jan. 2014.

Glass-like thermal transport in AgSbTe2: nano-scale insights to improve thermoelectric efficiency

A combination of neutron scattering and microscopy is used to reveal that a spontaneously forming nanostructure is gives rise to the extremely low glass-like thermal conductivity of AgSbTe2. AgSbTe2 has long been known to have an extremely low thermal conductivity, even in crystalline form, which enables high-performance thermoelectric modules that can be used for recovery of waste heat.  Inelastic neutron scattering measurements were performed at both SNS and HFIR and electron microscopy work was performed at MIT.

J. Ma, O. Delaire, A. F. May, C. E. Carlton, M. A. McGuire, L. H. VanBebber, D. L. Abernathy, G. Ehlers, T. Hong, A. Huq, W. Tian, V. M. Keppens, Y. Shao-Horn, and B. C. Sales, Nature Nanotechnology, 8, 445 (2013).

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