Monday, January 21, 2008

Joel's Journal Updates

Synthesis, Characterization, and Catalytic Activity of a Well-Defined Rhodium Siloxide Complex Immobilized on Silica
Bogdan Marciniec, Karol Szubert, Marek J. Potrzebowski, Ireneusz Kownacki, Kinga Leszczak
Angwandte (2007)47, 541.

Rhodium siloxide surface complex 2, which was obtained directly by reaction of molecular precursor 1 with aerosil silica and characterized inter alia by solid-state NMR spectroscopy, is a highly active and stable catalyst for hydrosilylation reactions.

NMR Spectroscopy: Pushing the Limits of Sensitivity
Hans Wolfgang Spiess
Angwandte (2007)47, 639.


Impressively sensitive: To meet the challenges of miniaturization and limited sample quantity, new approaches for substantially increasing the sensitivity of NMR spectroscopy for gases, liquids, and solids have been developed (for two examples, see picture). These and future developments will assure that NMR spectroscopy remains a major tool for chemistry and other fields for years to come.

Li6PS5X: A Class of Crystalline Li-Rich Solids With an Unusually High Li+ Mobility
Hans-Jörg Deiseroth, Shiao-Tong Kong, Hellmut Eckert, Julia Vannahme, Christof Reiner, Torsten Zaiß, Marc Schlosser
Angwandte (2007)47, 755.


Mobile metal ions: Halide-substituted lithium argyrodites form a new class of Li-rich solids with an unusually high Li mobility. Single-crystal X-ray studies (see picture; Li black, S yellow, I red) at room temperature and MAS-NMR measurements in a wide temperature range provide insights into the Li+ ion dynamics.

The dynamics and orientation of a lipophilic drug within model membranes determined by 13C solid-state NMR
Martin P. Boland and David A. Middleton


Methods for determining how a drug interacts with cellular membranes at the molecular level can give valuable insight into the mode of action of the drug and its absorption, distribution and metabolism profile. A procedure is described here to determine the orientation and location of the lipophilic drug trifluoperazine (TFP) intercalated into dimyristoylphosphatidylcholine (DMPC) bilayers, by using a novel combination of high-resolution solid-state nuclear magnetic resonance (SSNMR) methods to observe signals from 13C within the drug at natural abundance. SSNMR measurements of 1H–13C dipolar couplings for TFP and selective broadening of 13C NMR peaks by paramagnetic Mn2+ together suggest a model for the location, orientation and dynamics of the drug within lipid bilayers that offers an explanation for the lysoprotective effect of the drug at low concentrations. The experiments described are straightforward to implement and can be used for the routine analysis of drug–membrane interactions to provide useful information for drug design and structure refinement.

Isotropic chemical shifts in magic-angle spinning NMR spectra of proteins
Benjamin J. Wylie, Lindsay J. Sperling and Chad M. Rienstra


Here we examine the effect of magic-angle spinning (MAS) rate upon lineshape and observed peak position for backbone carbonyl (C) peaks in NMR spectra of uniformly-13C,15N-labeled (U–13C,15N) solid proteins. 2D N–C spectra of U–13C,15N microcrystalline protein GB1 were acquired at six MAS rates, and the site-resolved C lineshapes were analyzed by numerical simulations and comparison to spectra from a sparsely labeled sample (derived from 1,3-13C–glycerol). Spectra of the U–13C,15N sample demonstrate large variations in the signal-to-noise ratio and peak positions, which are absent in spectra of the sparsely labeled sample, in which most 13C sites do not possess a directly bonded 13CA. These effects therefore are a consequence of rotational resonance, which is a well-known phenomenon. Yet the magnitude of this effect pertaining to chemical shift assignment has not previously been examined. To quantify these effects in high-resolution protein spectra, we performed exact numerical two- and four-spin simulations of the C lineshapes, which reproduced the experimentally observed features. Observed peak positions differ from the isotropic shift by up to 1.0 ppm, even for MAS rates relatively far (a few ppm) from rotational resonance. Although under these circumstances the correct isotropic chemical shift values may be determined through simulation, systematic errors are minimized when the MAS rate is equivalent to 85 ppm for 13C. This moderate MAS condition simplifies spectral assignment and enables data sets from different labeling patterns and spinning rates to be used most efficiently for structure determination.

Molybdenum magnetic shielding and quadrupolar tensors for a series of molybdate salts: a solid-state 95Mo NMR study
Michelle A. M. Forgeron and Roderick E. Wasylishen


A series of molybdate, MoO42–, salts have been studied using solid-state 95Mo NMR spectroscopy at applied magnetic field strengths of 11.75, 17.63 and 21.14 T. In contrast to previous investigations, the principal components of the Mo shielding and EFG tensors have been obtained, as well as their relative orientations. At the fields employed, the anisotropic Mo shielding and quadrupolar interactions make significant contributions to the observed 95Mo central transition NMR lineshapes. Based on available structural data, the extent of distortion of the MoO42– anion from Td symmetry is reflected in the observed 95Mo nuclear quadrupolar coupling constants for the molybdate salts with divalent cations (i.e., Ca2+, Sr2+, Cd2+, Ba2+, Pb2+), but no correlation is found for molybdate salts containing the monovalent alkali metal (Li+, K+, Rb+, Cs+) cations.

Probing the surface structure of hydroxyapatite using NMR spectroscopy and first principles calculations
Helen Chappell, Melinda Duer, Nicholas Groom, Chris Pickard and Paul Bristowe

The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix.

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