Monday, July 27, 2009

Al's Literature Update

Symmetry-based dipolar recoupling by optimal control: Band-selective experiments for assignment of solid-state NMR spectra of proteins
J. Chem. Phys. 131, 025101 (2009); DOI:10.1063/1.3157737
Anders Bodholt Nielsen, Morten Bjerring, Jakob Toudahl Nielsen, and Niels Chr. Nielsen

We present design of novel low-power homonuclear dipolar recoupling experiments for magic-angle-spinning solid-state NMR studies of proteins. The pulse sequences are developed by combining principles of symmetry-based dipolar recoupling and optimal control-based pulse sequence design. The scaffold of the pulse sequences is formed by known CN-type recoupling sequences, while the intrinsic sequence elements are designed using optimal control. This procedure allows for the development of high-performance pulse sequences demanding significantly weaker rf fields than previous symmetry-based pulse sequences while compensating for rf inhomogeneity and providing excitation over relevant ranges of chemical shifts for biological applications. The new recoupling experiments, referred to as optimal control CN (OCCN), are demonstrated numerically and experimentally by two-dimensional (2D) 13C–13C and three-dimensional (3D) 15N–13C–13C chemical shift correlation experiments on uniformly 13C, 15N-labeled ubiquitin. Exploiting the double-quantum, band-selective dipolar recoupling properties of the OCCN experiments, we demonstrate significant sensitivity enhancement for 2D and 3D correlation spectra showing exclusively one- or two-bond correlations.

Effect of glass-forming biopreservatives on head group rotational dynamics in freeze-dried phospholipid bilayers: A 31P NMR study
J. Chem. Phys. 131, 025102 (2009); DOI:10.1063/1.3170927
P. Jain, S. Sen, and S. H. Risbud

31P NMR spectroscopy has been used to elucidate the role of glass-forming sugars in the preservation of dipalmitoylphosphatidylcholine (DPPC) lipid bilayers. 31P wideline NMR spectra of freeze-dried pure DPPC, DPPC/trehalose, DPPC/glucose, and DPPC/hydroxyethyl starch (HES) mixtures collected in the temperature range of 25–80 °C have been simulated to obtain quantitative information about rotational dynamics and orientation of the lipid head groups in these media. In the case of pure DPPC, DPPC/glucose, and DPPC/HES, the gel-to-liquid crystalline phase transition of DPPC bilayer is characterized by a sudden increase in the rate of rotational diffusion of the PO4 head groups near 40 °C. The corresponding rotational jump frequency increases from a few kilohertz in the gel phase to at least several megahertz in the liquid crystalline phase. On the other hand, in the case of DPPC/trehalose mixture the temperature of this onset of rapid head group dynamics is increased by ~10 °C. Trehalose reduces the lipid head group motions most effectively in the temperature range of T50 °C relevant for biopreservation. Additionally, and possibly more importantly, trehalose is found to strongly restrict any change in the orientation of the diffusion axis of the PO4 head groups during the phase transformation. This unique ability of trehalose to maintain the dynamical and orientational rigidity of lipid head groups is likely to be responsible for its superior ability in biopreservation.

Structural and Dynamic Properties of BaInGeH: A Rare Solid-State Indium Hydride
Inorg. Chem., 2009, 48 (13), pp 5602–5604

Michael J. Evans, Verina F. Kranak, Francisco J. Garcia-Garcia, Gregory P. Holland, Luke L. Daemen, Thomas Proffen, Myeong H. Lee, Otto F. Sankey and Ulrich Hussermann

BaInGeH was synthesized by hydrogenating the intermetallic compound BaInGe. The crystal structure determination from the powder neutron diffraction data of BaInGeD [P3m1, Z = 1, a = 4.5354(3) Å, c = 5.2795(6) Å] reveals the presence of hydrogen in tetrahedral voids defined by three Ba atoms and one In atom.

Sodium Ion Mobility in NaxCoO2 (0.6 < x < 0.75) Cobaltites Studied by 23Na MAS NMR

Inorg. Chem., 2009, 48 (15), pp 7018–7025
Dany Carlier, Maxime Blangero, Michel Mntrier, Michal Pollet, Jean-Pierre Doumerc and Claude Delmas

Various P2 and P′3-NaxCoO2 phases, with x ranging approximately from 0.6 to 0.75, have been studied by variable-temperature 23Na magic angle spinning (MAS) NMR. Signal modification versus temperature plots clearly show that Na+ ions are not totally mobile at room temperature on the NMR time scale. As the temperature increases, the line shape change of the 23Na MAS NMR signal differs for the P2 and P′3 stackings and is interpreted by the differences of Na+ ion sites and of sodium diffusion pathways in the two structures.

Reactivity of NH4H2PO4 toward LaCl3 in LiCl-KCl Melt Flux. Step by Step Formation of Monazite-Like LaPO4.
Inorg. Chem., 2009, 48 (15), pp 7141–7150
Damien Hudry, Aydar Rakhmatullin, Catherine Bessada, Isabelle Bardez, Florence Bart, Stphane Jobic and Philippe Deniard
The synthesis of lanthanum phosphates in molten LiCl-KCL eutectic was chosen to address the preliminary treatment of chlorinated wastes containing fission products that are already present in a Li/Cl eutectic. The obtained monazite compound shows interesting properties to be considered as a good candidate to trap lanthanum for a long-time. The synthesis route based on LaCl3 reaction with NH4H2PO4 in a stoichiometric amount is a key point to obtain monazite as a pure phase. Hence, the salt composition is not modified during the synthesis reaction. The chemical reactivity of ammonium dihydrogenphosphate (NH4H2PO4, hereafter abbreviated ADP) toward lanthanum chloride (LaCl3) in molten LiCl-KCl eutectic is probed by NMR spectroscopy to follow the formation of LaPO4. Formally, a direct transformation of the two aforementioned precursors into LaPO4, NH4Cl and HCl can be discarded on the basis of the low thermal stability of ADP. To shed some light on the formation of LaPO4, in situ and ex situ NMR experiments were carried out on LiCl-KCl/LaCl3/ADP, as well as LiCl-KCl/ADP, KCl/ADP, and LiCl/ADP mixtures. First, the reactivity of the precursors in contact with the eutectic was studied from room temperature to 600 °C by means of 31P, 35Cl, and 139La high temperature NMR. Second, ex situ room temperature magic angle spinning (MAS) and RadioFrequency driven recoupling (RFDR) 31P solid-state NMR experiments were carried out on solid samples prepared in different conditions (i.e., temperature and atmosphere) and quenched at room temperature to identify frozen intermediate species in their metastable state. On the basis of this approach, we propose a model for the LaPO4 formation based on a multistep mechanism which highlights the strong reactivity of ADP toward the alkaline salts but without final change in the composition of the solvent.

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