Octacalcium phosphate (OCP, Ca8(PO4)4(HPO4)2.5H2O) is a notable calcium phosphate due to its biocompatibility, making it a widely studied material for bone substitution. It is known to be a precursor of bone mineral, but its role in biomineralisation remains unclear. While the structure of OCP has been the subject of thorough investigations (including using Rietveld refinements of X-ray diffraction data, and NMR crystallography studies), important questions regarding the symmetry and H-bonding network in the material remain. In this study, it is shown that OCP undergoes a lowering of symmetry below 200 K, evidenced by 1H, 17O, 31P and 43Ca solid state NMR experiments. Using ab-initio molecular dynamics (MD) simulations and Gauge Including Projected Augmented Wave (GIPAW) DFT calculations of NMR parameters, the presence of rapid motions of the water molecules in the crystal cell at room temperature is proved. This information leads to an improved description of the OCP structure at both low and ambient temperatures, and helps explain long-standing issues of symmetry. Remaining challenges related to the understanding of the structure of OCP are then discussed.
Membrane proteins in their native cellular membranes are accessible by dynamic nuclear polarization magic angle spinning solid-state NMR spectroscopy without the need of purification and reconstitution (see picture). Dynamic nuclear polarization is essential to achieve the required gain in sensitivity to observe the membrane protein of interest. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
Aktive Blockierung: Mit deuterierten Blockierungsreagentien werden oberflächenverstärkte DNP-NMR-Signale, die durch Methylgruppen enthaltende Substrate beeinträchtigt werden, wiederhergestellt; die Schutzgruppenfunktion auf der Oberfläche bleibt dabei erhalten (siehe Bild; DNP=dynamische Kernpolarisation). Unpolare Gruppen wie [D9]-Trimethylsiloxy (TMS) halten Radikale (gelb) von der Oberfläche fern und vermindern dadurch schädliche paramagnetische Effekte. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
Membranproteine in ihrer natürlichen Zellmembran können ohne vorherige Reinigung durch Kombination dynamischer Kernpolarisation mit Festkörper-NMR-Spektroskopie unter Probenrotation um den magischen Winkel untersucht werden (siehe Bild). Die Empfindlichkeit, die zum Beobachten der Membranproteine notwendige ist, wird durch dynamische Kernpolarisation erreicht. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
The structure and molecular order in the thermotropic ionic liquid crystal (ILC), [choline]+[geranate(H)octanoate]-, an analogue of Choline And GEranate (CAGE) which has potential for use as a broad-spectrum antimicrobial and transdermal and oral delivery agent, were investigated using magic-angle spinning (MAS) nuclear magnetic resonance (NMR), polarizing optical microscopy, small angle X-ray scattering (SAXS) and mass spectrometry. Mass spectrometry and the 1H NMR chemical shift reveal that CAGE-oct is a dynamic system, with metathesis (the exchange of interacting ions) and hydrogen exchange occurring between hydrogen-bonded/ionic complexes such as [(choline)(geranate)(H)(octanoate)], [(choline)(octanoate)2(H)] and [(choline)(geranate)2(H)]. These clusters, which are shown by mass spectrometry to be significantly more stable than expected for typical electrostatic ion clusters, involve hydrogen bonding between the carboxylic acid, carboxylate and hydroxyl groups, with rapid hydrogen bond breaking and reforming observed to average the 1H chemical shifts. The formation of a partial bilayer liquid crystal (LC) phase was identified by SAXS and polarizing optical microscopy at temperatures below ~293 K. The occurrence of this transition close to room temperature could be utilized as a temperature-induced ‘switch’ of the anisotropic properties for particular applications. The presence of an isotropic component of approximately 25% was observed to coexist with the LC phase, as detected by polarizing optical microscopy and quantified by both 1H-13C dipolar-chemical shift correlation (DIPSHIFT) and 1H double-quantum (DQ) experiments. At temperatures above the LC-to-isotropic transition, intermediate-range order (clustering of polar and non-polar domains), a feature of many ILs, persists. Site-specific order parameters for the LC phase of CAGE-oct were obtained from the measurement of the partially averaged 13C-1H dipolar couplings (DCH) by cross-polarization (CP) build-up curves and DIPSHIFT experiments, and 1H-1H dipolar couplings (DHH) by DQ build-up curves. The corresponding order parameters, SCH and SHH, are in the range 0 to 0.2, and are lower compared to those for smectic (i.e., layered) phases of conventional non-ionic liquid crystals, resembling those of lamellar phases formed by lyotropic surfactant-solvent systems.
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