X-ray and neutron scattering

Abstract:

A study of vapour transport equilibrated crystals of LiTaO3 shows periodic fringes that change with temperature. The origin of these fringes and their behaviour is explained in terms of domains with slanted walls. Micro-Raman measurements have been used to determine the Li2O concentration through the crystals, and by comparison with the observed fringe pattern the variation of birefringence with Li2O content has been determined.

Abstract:

We report neutron inelastic scattering measurements on the normal and superconducting states of single-crystalline Cs 0.8Fe 1.9Se 2. Consistent with previous measurements on Rb xFe 2-ySe 2, we observe two distinct spin excitation signals: (i) spin-wave excitations characteristic of the block antiferromagnetic order found in insulating A xFe 2-ySe 2 compounds, and (ii) a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane wave-vector of (0.25,0.5). The resonance peak increases below T c=27 K, and has a similar absolute intensity to the resonance peaks observed in other Fe-based superconductors. The existence of a magnetic resonance in the spectrum of Rb xFe 2-ySe 2 and now of Cs xFe 2-ySe 2 suggests that this is a common feature of superconductivity in this family. The low-energy spin-wave excitations in Cs 0.8Fe 1.9Se 2 show no measurable response to superconductivity, consistent with the notion of spatially separate magnetic and superconducting phases. © 2012 American Physical Society.

Abstract:

D-Xylose isomerase (XI) converts the aldo-sugars xylose and glucose to their keto analogs xylulose and fructose, but is strongly inhibited by the polyols xylitol and sorbitol, especially at acidic pH. In order to understand the atomic details of polyol binding to the XI active site, a 2.0 Å resolution room-temperature joint X-ray/neutron structure of XI in complex with Ni(2+) cofactors and sorbitol inhibitor at pH 5.9 and a room-temperature X-ray structure of XI containing Mg(2+) ions and xylitol at the physiological pH of 7.7 were obtained. The protonation of oxygen O5 of the inhibitor, which was found to be deprotonated and negatively charged in previous structures of XI complexed with linear glucose and xylulose, was directly observed. The Ni(2+) ions occupying the catalytic metal site (M2) were found at two locations, while Mg(2+) in M2 is very mobile and has a high B factor. Under acidic conditions sorbitol gains a water-mediated interaction that connects its O1 hydroxyl to Asp257. This contact is not found in structures at basic pH. The new interaction that is formed may improve the binding of the inhibitor, providing an explanation for the increased affinity of the polyols for XI at low pH.