Friday, February 20, 2009

J. Am. Chem. Soc., 2009, 131 (6), pp 2058–2059

Adsorption−Desorption Induced Structural Changes of Cu-MOF Evidenced by Solid State NMR and EPR Spectroscopy

Yijiao Jiang, Jun Huang, Besnik Kasumaj, Gunnar Jeschke, Michael Hunger, Tamas Mallat and Alfons Baiker

Adsorption−desorption induced structural changes of Cu(bpy)(H2O)2(BF4),(bpy) (bpy = 4,4′-bipyridine) [Cu-MOF] have been evidenced by combined NMR and EPR spectroscopy. Upon adsorption of probe molecules even at a few mbar, EPR spectra show that they are activated to form complexes at Cu(II) sites, which results in a change of the Cu-MOF’s structure as indicated by a high-field shift of the 11B MAS NMR. After desorption, both EPR and 11B MAS NMR spectra evidenced that the structure of the Cu-MOF reversibly shifted to the original state. This observation indicates that MOFs can undergo structural changes during processes where adsorption−desorption steps are involved such as gas storage, separation, and catalysis.

Thursday, February 19, 2009

Nature: MRI at a Distance

Nature 457, 994-998 (19 February 2009) doi:10.1038/nature07752; Received 25 September 2008; Accepted 24 December 2008

Travelling-wave nuclear magnetic resonance
David O. Brunner
1, Nicola De Zanche1, Jürg Fröhlich2, Jan Paska2 & Klaas P. Pruessmann1
Institute for Biomedical Engineering, University of Zürich and ETH Zürich, Gloriastrasse 35, 8092 Zürich, Switzerland
Laboratory for Electromagnetic Fields and Microwave Electronics, ETH Zürich, Gloriastrasse 35, 8092 Zürich, Switzerland

Nuclear magnetic resonance
1, 2 (NMR) is one of the most versatile experimental methods in chemistry, physics and biology3, providing insight into the structure and dynamics of matter at the molecular scale. Its imaging variant—magnetic resonance imaging4, 5 (MRI)—is widely used to examine the anatomy, physiology and metabolism of the human body. NMR signal detection is traditionally based on Faraday induction6 in one or multiple radio-frequency resonators7, 8, 9, 10 that are brought into close proximity with the sample. Alternative principles involving structured-material flux guides11, superconducting quantum interference devices12, atomic magnetometers13, Hall probes14 or magnetoresistive elements15 have been explored. However, a common feature of all NMR implementations until now is that they rely on close coupling between the detector and the object under investigation. Here we show that NMR can also be excited and detected by long-range interaction, relying on travelling radio-frequency waves sent and received by an antenna. One benefit of this approach is more uniform coverage of samples that are larger than the wavelength of the NMR signal—an important current issue in MRI of humans at very high magnetic fields. By allowing a significant distance between the probe and the sample, travelling-wave interaction also introduces new possibilities in the design of NMR experiments and systems.


Editor's Summary
19 February 2009
Travelling-wave NMR
Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are widely used in the sciences and medicine. Although the implementation details differ from application to application, the underlying detection principle is the same: the need for intimate coupling (and hence usually close proximity) between nuclear magnetization in the sample and the detector. Brunner et al. show that it is possible to abandon this traditional detection principle, and that the nuclear magnetization signal can be excited and detected by long-range interaction using travelling radiofrequency waves sent and received by an antenna. This approach offers more uniform coverage of larger samples. And by freeing up space in the centre of the costly high-field magnets needed for MRI, it could potentially make the imaging experience more comfortable for human subjects.

J Phys Chem C, Vol 113, Issue 7

Ordered and Hydrothermally Stable Cubic Periodic Mesoporous Organosilicas with SBA-1 Mesostructures: Synthesis, Characterization, Solid-State NMR Spectroscopy, and DFT Calculations
Yu-Chi Pan
, Hao-Yiang Wu, Guang-Liang Jheng, Hui-Hsu Gavin Tsai* and Hsien-Ming Kao*

J. Phys. Chem. C, 2009, 113 (7), pp 2690–2698
Abstract: Periodic mesoporous organosilicas (PMOs) based on the cubic SBA-1 mesostructure (Pm3n mesophase) were synthesized by co-condensation of tetraethoxysilane (TEOS) and 1,2-bis(triethoxysilyl)ethane (BTEE) under acidic conditions using cetyltriethylammonium bromide (CTEABr) as a structure-directing agent. The ethane-bridged PMO materials thus obtained were characterized by powder X-ray diffraction (XRD), solid-state 13C and 29Si NMR spectroscopy, thermogravimetric analysis (TGA), and nitrogen sorption measurements. The maximum BTEE contents that can be incorporated into the pore wall without degrading the Pm3n mesostructure were up to 60% (based on silica). The resulting materials were hydrothermally stable up to 120 h in boiling water with only a slight decrease in their structural properties, whereas the structure of the pure silica counterpart SBA-1 material was completely collapsed after such treatment. The presence of the ethane groups in the mesoporous wall led to a more hydrophobic environment and thus enhancement of hydrothermal stability, as revealed by water adsorption. The combined results of 2D 29Si{1H} heteronuclear correlation (HETCOR) NMR and density functional theory calculations suggested that the T3−T2−Q4−Q3 motif could be the favorable framework building unit in PMOs.

Studies of the Active Sites for Methane Dehydroaromatization Using Ultrahigh-Field Solid-State 95Mo NMR Spectroscopy
Jian Zhi Hu
*, Ja Hun Kwak, Yong Wang, Charles H. F. Peden*, Heng Zheng§, Ding Ma and Xinhe Bao

J. Phys. Chem. C, 2009, 113 (7), pp 2936–2942
Abstract: In this contribution, we show that the spin−lattice relaxation time, T1, corresponding to zeolite exchanged molybdenum species in Mo/HZSM-5 catalysts is about 2 orders of magnitude shorter than the corresponding T1 for small MoO3 crystallites. Such a difference is utilized to differentiate the exchanged Mo species from MoO3 agglomerates in Mo/HZSM-5 catalysts and to readily estimate their relative fractions present in catalysts with varying Mo loading. A good linear correlation between the amount of zeolite exchanged species and the aromatics formation rate during catalytic methane dehydroaromatization is obtained. This result significantly strengthens our prior conclusion that the exchanged Mo species are the active centers for this reaction on Mo/HZSM-5 catalysts (J. Am. Chem. Soc. 2008, 130, 3722−3723). Of more general interest for Mo-exchanged zeolites, the results may provide useful data for analyzing the binding of exchanged Mo species in zeolite cages. In particular, the NMR data suggest a possible saturation loading for the exchanged Mo species at a Mo/Al ratio of approximately 0.5 for the ZSM-5 zeolite used in this study (Si/Al = 25). Furthermore, for polycrystalline MoO3 powder samples, the parameters related to the electric field gradient (EFG) tensor, the chemical shift anisotropy (CSA), and the three Euler angles required to align the CSA principal axis system with the quadrupolar principal axis system are determined by analyzing both the magic angle spinning (MAS) and static 95Mo spectra. The new results obtained from this study on MoO3 powders should help to clarify some of the contradictions in prior literature reports of studies of Mo-containing solids by 95Mo NMR.

J Phys Chem B, Vol. 113, Issue 7

NMR Measure of Translational Diffusion and Fractal Dimension. Application to Molecular Mass Measurement
Sophie Aug
, Pierre-Olivier Schmit, Christopher A. Crutchfield, Mohammad T. Islam, Douglas J. Harris, Emmanuelle Durand§, Martin Clemancey, Anne-Agathe Quoineaud§, Jean-Marc Lancelin, Yann Prigent, Francis Taulelle and Marc-Andr Delsuc*#

J. Phys. Chem. B, 2009, 113 (7), pp 1914–1918
Abstract: Experimental NMR diffusion measure on polymers and on globular proteins are presented. These results, complemented with results found in the literature, enable a general description of effective fractal dimension for objects such as small organic molecules, sugars, polymers, DNA, and proteins. Results are compared to computational simulations as well as to theoretical values. A global picture of the diffusion phenomenon emerges from this description. A power law relating molecular mass with diffusion coefficients is described and found to be valid over 4 orders of magnitude. From this law, the fractal dimension of the molecular family can be measured, with experimental values ranging from 1.41 to 2.56 in full agreement with theoretical approaches. Finally, a method for evaluating the molecular mass of unknown solutes is described and implemented as a Web page.

A Comparative 13C NMR Study of Local Ordering in a Homologous Series of Bent-core Liquid Crystals
Ronald Y. Dong

J. Phys. Chem. B, 2009, 113 (7), pp 1933–1939
Abstract:A 13C NMR study is carried out in a number of banana-shaped molecules belonging to a homologous series. The derivatives contain chlorine or bromine substituent(s) in the center ring of 1,3-phenylene bis[4′-alkenyloxy biphenyl]-4-carboxylate (nPBBC), and different terminal chain lengths are systematically compared in terms of their local order parameters and their ability to form an aligned nematic phase in the NMR magnet. The chemical shift anisotropy tensors measured from nPBBC by fitting the 13C powder patterns are now extensively used on these molecules to interpret their observed 13C chemical shifts in the nematic phase. The bend angles are estimated as a function of temperature in two members of the nPBBC series. Conformation twists in the bent-core region inferred from the local molecular biaxial paramters are discussed.

Revisiting Magnesium Chelation by Teichoic Acid with Phosphorus Solid-State NMR and Theoretical Calculations
Jason R. Wickham, Jeffrey L. Halye, Stepan Kashtanov, Jana Khandogin and Charles V. Rice

J. Phys. Chem. B, 2009, 113 (7), pp 2177–2183
Abstract:Teichoic acids are essential components of the Gram-positive bacterial cell wall. One of their many functions is metal binding, a vital process for bacterial growth. With the combination of phosphorus-31 solid-state NMR spectroscopy and theoretical calculations using density functional theory (DFT), we have determined that the binding mode between teichoic acids and magnesium involves bidentate coordination by the phosphate groups of teichoic acid. Measurement of chemical shift anisotropy tensors gave a reduced anisotropy (δ) of 49.25 ppm and an asymmetry (η) of 0.7. DFT calculations with diglycerol phosphate and triglycerol diphosphate model compounds were completed with Mg2+ in anhydrous as well as partially hydrated bidentate and fully hydrated monodentate, bidentate, and bridging binding modes. 31P CSA tensors were calculated from the energy-minimized model compounds using the combined DFT and GIAO methods, resulting in dramatically different tensor values for each binding mode. The anhydrous bidentate chelation mode was found to be a good approximation of the experimental data, an observation that alters the current monodentate paradigm for metal chelation by teichoic acids.

Thermal Denaturation of Hydrated Wool Keratin by 1H Solid-State NMR
Maria Baias
*, Dan E. Demco*§, Crisan Popescu*, Radu Fechete§, Claudiu Melian, Bernhard Blmich and Martin Mller

J. Phys. Chem. B, 2009, 113 (7), pp 2184–2192
Abstract:Thermal denaturation of hydrated keratin in wool was investigated by NMR using 1H wide-line spectra to obtain the phase composition and 1H spin-diffusion experiments using a double-quantum filter to obtain the domain sizes for the wool fibers. The denaturation process detected by DSC takes place for wool fibers in deuterated water in the temperature range 140−144 °C. The phase composition measured by 1H wide line NMR spectra reveals a rigid, semirigid and an amorphous phase for temperatures in the range 25−160 °C. A dramatic change in the phase composition was detected around 142 °C, corresponding to the denaturation temperature. The morphological domain sizes measured by 1H spin-diffusion NMR experiments were obtain from the solutions of the spin-diffusion equations for two-dimensional rectangular and cylindrical morphologies. The keratin mobility gradient in the interfacial region at different denaturation temperatures was measured from the 1H spin-diffusion data. A qualitative model describing the denaturation process of hydrated keratin protein was developed that explains the changes in domain thickness, spin diffusivities, phase composition, and thermodynamic parameters.

Friday, February 13, 2009

Cryst. Growth Des., 2009, 9 (2), pp 921–937

Solid-State NMR Analysis of Organic Cocrystals and Complexes

Frederick G. Vogt, Jacalyn S. Clawson, Mark Strohmeier, Andrew J. Edwards, Tran N. Pham‡ and Simon A. Watson

Solid-state NMR (SSNMR) is capable of providing detailed structural information about organic and pharmaceutical cocrystals and complexes. SSNMR nondestructively analyzes small amounts of powdered material and generally yields data with higher information content than vibrational spectroscopy and powder X-ray diffraction methods. These advantages can be utilized in the analysis of pharmaceutical cocrystals, which are often initially produced using solvent drop grinding techniques that do not lend themselves to single crystal growth for X-ray diffraction studies. In this work, several molecular complexes and cocrystals are examined to understand the capabilities of the SSNMR techniques, particularly their ability to prove or disprove molecular association and observe structural features such as hydrogen bonding. Dipolar correlation experiments between spin pairs such as 1H−1H, 1H−13C, and 19F−13C are applied to study hydrogen bonding, intermolecular contacts, and spin diffusion to link individual molecules together in a crystal structure and quickly prove molecular association. Analysis of the principal components of chemical shift tensors is also utilized where relevant, as these are more sensitive to structural effects than the isotropic chemical shift alone. In addition, 1H T1 relaxation measurements are also demonstrated as a means to prove phase separation of components. On the basis of these results, a general experimental approach to cocrystal analysis by SSNMR is suggested.

Monday, February 09, 2009

Chemical Physics, Volume 356, Issues 1-3, 17 February 2009, Pages 236-242

Restricted magnetically balanced basis applied for relativistic calculations of indirect nuclear spin–spin coupling tensors in the matrix Dirac–Kohn–Sham framework

Michal Repiskýa, Stanislav Komorovskýa, Olga L. Malkinaa and Vladimir G. Malkin

The relativistic four-component density functional approach based on the use of restricted magnetically balanced basis (mDKS-RMB), applied recently for calculations of NMR shielding, was extended for calculations of NMR indirect nuclear spin–spin coupling constants. The unperturbed equations are solved with the use of a restricted kinetically balanced basis set for the small component while to solve the second-order coupled perturbed DKS equations a restricted magnetically balanced basis set for the small component was applied. Benchmark relativistic calculations have been carried out for the X–H and H–H spin–spin coupling constants in the XH4 series (X = C, Si, Ge, Sn and Pb). The method provides an attractive alternative to existing approximate two-component methods with transformed Hamiltonians for relativistic calculations of spin–spin coupling constants of heavy-atom systems. In particular, no picture-change effects arise in our method for property calculations.

Chemical Physics, Volume 356, Issues 1-3, 17 February 2009, Pages 7-13

Orbital instabilities and spin-symmetry breaking in coupled-cluster calculations of indirect nuclear spin–spin coupling constants

Alexander A. Auera,and Jürgen Gaussb

The effect of orbital instabilities is investigated for spin-symmetry breaking perturbations, namely the Fermi-contact (FC) and spin–dipole (SD) contributions to the indirect nuclear spin–spin coupling constants. For the CO and N2 molecules the FC and SD contributions have been calculated and orbital-stability analyses for various interatomic distances have been carried out. This includes calculations at the Hartree–Fock self-consistent field (HF-SCF), coupled-cluster (CC) singles and doubles (CCSD), CC3, CCSD(T), CCSDT-4, CC singles, doubles, and triples (CCSDT) levels, and for the first time also at the CC singles, doubles, triples, and quadruples (CCSDTQ) level of theory. For calculations with relaxation of the reference orbitals in the presence of the perturbation, unphysical results are obtained over a wide range of the potential curve. This is due to a triplet instability of the Hartree–Fock reference determinant which leads to a pronounced pole in the FC and SD contributions. The effect of orbital instabilities in the relaxed methods is most dramatic for perturbative approaches like CCSD(T), while it is less pronounced for methods of the classical CC hierarchy. CC calculations without relaxation of the orbitals, i.e., so-called “unrelaxed” calculations, do not show any of these effects.

Friday, February 06, 2009

J. Am. Chem. Soc., 2009, 131 (5), pp 1915–1926

Accurate Measurement of Alpha Proton Chemical Shifts of Excited Protein States by Relaxation Dispersion NMR Spectroscopy

Patrik Lundström, D. Flemming Hansen, Pramodh Vallurupalli and Lewis E. Kay

Carr−Purcell−Meiboom−Gill relaxation dispersion NMR spectroscopy can provide detailed information about low populated, invisible states of protein molecules, including backbone chemical shifts of the invisible conformer and bond vector orientations that can be used as structural constraints. Notably, the measurement of 1Hα chemical shifts in excited protein states has not been possible to date because, in the absence of suitable labeling, the homonuclear proton scalar coupling network in side chains of proteins leads to a significant degradation in the performance of proton-based relaxation dispersion experiments. Here we have overcome this problem through a labeling scheme in which proteins are prepared with U−2H glucose and 50% D2O/50% H2O that results in deuteration levels of between 50−88% at the Cβ carbon. Effects from residual 1Hα−1Hβ scalar couplings can be suppressed through a new NMR experiment that is presented here. The utility of the methodology is demonstrated on a ligand binding exchanging system and it is shown that 1Hα chemical shifts extracted from dispersion profiles are, on average, accurate to 0.03 ppm, an order of magnitude better than they can be predicted from structure using a database approach. The ability to measure 1Hα chemical shifts of invisible conformers is particularly important because such shifts are sensitive to both secondary and tertiary structure. Thus, the methodology presented is a valuable addition to a growing list of experiments for characterizing excited protein states that are difficult to study using the traditional techniques of structural biology.

J. Am. Chem. Soc., 2009, 131 (5), pp 1820–1834

Probing Heteronuclear 15N−17O and 13C−17O Connectivities and Proximities by Solid-State NMR Spectroscopy

Ivan Hung†, Anne-Christine Uldry, Johanna Becker-Baldus, Amy L. Webber†, Alan Wong, Mark E. Smith, Siân A. Joyce, Jonathan R. Yates, Chris J. Pickard, Ray Dupree and Steven P. Brown

Heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing 15N−17O dipolar and J couplings are presented for [2H(NH3),1-13C,15N,17O2]glycine·2HCl and [15N2,17O2]uracil. Two-dimensional 15N−17O correlation spectra are obtained using the R3-HMQC experiment; for glycine·2HCl, the intensity of the resolved peaks for the CO and C−O2H 17O resonances corresponds to the relative magnitude of the respective 15N−17O dipolar couplings. 17O−15N REDOR curves are presented for glycine·2HCl; fits of the initial buildup (ΔS/S < 0.2) yield effective dipolar couplings in agreement with (±20%) the root-sum-squared dipolar couplings determined from the crystal structure. Experimental 15N−17O REAPDOR curves for the 15N resonances in glycine·2HCl and uracil fit well to the universal curve presented by Goldbourt et al. (J. Am. Chem. Soc. 2003, 125, 11194). Heteronuclear 13C−17O and 15N−17O J couplings were experimentally determined from fits of the quotient of the integrated intensity obtained in a heteronuclear and a homonuclear spin−echo experiment, SQ(τ) = SHET(τ)/SHOM(τ). For glycine·2HCl, 1JCO was determined as 24.7 ± 0.2 and 25.3 ± 0.3 Hz for the CO and C−O2H resonances, respectively, while for uracil, the average of the two NH···O hydrogen-bond-mediated J couplings was determined as 5.1 ± 0.6 Hz. In addition, two-bond intramolecular J couplings, 2JOO = 8.8 ± 0.9 Hz and 2JN1,N3 = 2.7 ± 0.1 Hz, were determined for glycine·2HCl and uracil, respectively. Excellent agreement was found with J couplings calculated using the CASTEP code using geometrically optimized crystal structures for glycine·HCl [1JCO(CO) = 24.9 Hz, 1JCO(COH) = 27.5 Hz, 2JOO = 7.9 Hz] and uracil [2hJN1,O4 = 6.1 Hz, 2hJN3,O4 = 4.6 Hz, 2JN1,N3 = 2.7 Hz].

J. Am. Chem. Soc., 2009, 131 (2), pp 563–569

Cryptophane Xenon-129 Nuclear Magnetic Resonance Biosensors Targeting Human Carbonic Anhydrase

Jennifer M. Chambers, P. Aru Hill, Julie A. Aaron, Zhaohui Han, David W. Christianson, Nicholas N. Kuzma and Ivan J. Dmochowski

129Xe NMR biosensors are promising agents for early disease detection, especially when their interactions with target biomolecules can perturb 129Xe chemical shifts well beyond the typical field inhomogeneity of clinical MRI. We introduce human carbonic anhydrase (CA) as a single-binding-site enzyme for studying xenon biosensor−protein interactions. A xenon-binding cryptophane was substituted with linkers of varying lengths to p-benzenesulfonamide to yield nondiastereomeric biosensors with a single 129Xe NMR resonance. X-ray crystallography confirmed binding of the eight-bond-linked biosensor containing a single xenon atom in the CAII active site. Biosensor dissociation constants (Kd = 20−110 nM) were determined by isothermal titration calorimetry (ITC) for isozymes CA I and II. The biosensor−CA complexes yielded “bound” hyperpolarized 129Xe NMR resonances of narrow line width that were shifted by 3.0−7.5 ppm downfield, signifying much larger shifts than seen previously. Moreover, isozyme-specific chemical shifts clearly differentiated CA I and II, despite their similar structures. Thus, xenon biosensors may provide a powerful strategy for diagnosing human diseases characterized by the upregulation of specific CA isozymes and other protein biomarkers.

J. Phys. Chem. B, 2009, 113 (2), pp 416–425

Network Dynamics and Species Exchange Processes in Aluminophosphate Glasses: An in situ High Temperature Magic Angle Spinning NMR View

Sebastian Wegner, Leo van Wüllen and Gregory Tricot

In this contribution, we present an in situ high temperature 27Al and 31P magic angle spinning (MAS) NMR study of binary and ternary phosphate glasses at temperatures above the glass transition temperature TG. For binary phosphate glasses, xK2O−(1 − x)P2O5 and ternary aluminophosphate glasses 30K2O−xAl2O3−(70 − x)P2O5 with 7 < x < 15 dynamic exchange processes between the various phosphate species (and aluminate species) present in the glasses could be identified in the temperature range between TG and the maximum achievable temperature Tmax of our high temperature MAS NMR setup, TG < T < Tmax. This observation indicates rapid P−O−P and P−O−Al bond formation and bond breaking in the (alumino)phosphate glasses. From a modeling of the temperature dependence of these exchange processes, the activation energy EA for the corresponding process could be determined. These local bond breaking and making processes are ultimately linked to the macroscopic viscous flow and may indeed form the basic microscopic local step of viscous flow.

J. Am. Chem. Soc., 2009, 131 (4), pp 1364–1365

4D 1H−13C NMR Spectroscopy for Assignments of Alanine Methyls in Large and Complex Protein Structures
Devon Sheppard, Chenyun Guo and Vitali Tugarinov

Alanine 13CH3 methyl groups can serve as a useful addition to the Ile, Leu, Val (ILV) selective isotope labeling methodology adopted for NMR studies of high-molecular-weight protein systems. A four-dimensional (4D) methyl-detected “out-and-back” NMR experiment has been developed that allowed us to obtain practically complete 1H−13C assignments of more than 70 alanine methyl sites in a 723-residue enzyme Malate Synthase G. It can be anticipated that the developed NMR methodology will promote the use of alanine methyls as important probes of molecular structure and dynamics in large proteins.

J. Am. Chem. Soc., 2009, 131 (4), pp 1426–1435

Nonaromatic Core−Shell Structure of Nanodiamond from Solid-State NMR Spectroscopy

XiaoWen Fang, JingDong Mao, E. M. Levin and Klaus Schmidt-Rohr

The structure of synthetic nanodiamond has been characterized by 13C nuclear magnetic resonance (NMR) spectral editing combined with measurements of long-range 1H−13C dipolar couplings and 13C relaxation times. The surface layer of these ∼4.8-nm diameter carbon particles consists mostly of sp3-hybridized C that is protonated or bonded to OH groups, while sp2-hybridized carbon makes up less than 1% of the material. The surface protons surprisingly resonate at 3.8 ppm, but their direct bonding to carbon is proved by fast dipolar dephasing under homonuclear decoupling. Long-range 1H−13C distance measurements, based on 13C{1H} dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains ∼60% of all carbons, predominantly resonate more than +8 ppm from the 37-ppm peak of bulk diamond (i.e., within the 45−80 ppm range). Nitrogen detected in 15N NMR spectra is mostly not protonated and can account for some of the high-frequency shift of carbon. The location of unpaired electrons (∼40 unpaired electrons per particle) was studied in detail, based on their strongly distance-dependent effects on T1,C relaxation. The slower relaxation of the surface carbons, selected by spectral editing, showed that the unpaired electrons are not dangling bonds at the surface. This was confirmed by detailed simulations, which indicated that the unpaired electrons are mostly located in the disordered shell, at distances between 0.4 and 1 nm from the surface. On the basis of these results, a nonaromatic core−shell structural model of nanodiamond particles has been proposed.

J. Am. Chem. Soc., 2009, 131 (3), pp 985–992

Protein Structure Refinement Using 13Cα Chemical Shift Tensors

Benjamin J. Wylie, Charles D. Schwieters, Eric Oldfield and Chad M. Rienstra

We have obtained the 13Cα chemical shift tensors for each amino acid in the protein GB1. We then developed a CST force field and incorporated this into the Xplor-NIH structure determination program. GB1 structures obtained by using CST restraints had improved precision over those obtained in the absence of CST restraints and were also more accurate. When combined with isotropic chemical shifts, distance, and vector angle restraints, the root-mean squared error with respect to existing X-ray structures was better than ∼1.0 Å. These results are of broad general interest since they show that chemical shift tensors can be used in protein structure refinement, improving both structural accuracy and precision, opening up the way to accurate de novo structure determination.

Thursday, February 05, 2009

Macromolecules, Volume 42, Issues 2 and 3

Dynamics of a Polyphosphazene Melt Studied by Solid-State 2H NMR

Barbara Koch
Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstr. 30, 48149 Münster, Germany
Michael Vogel
Institut für Festkörperphysik, Technische Universität Darmstadt, Hochschulstr. 6, 64829 Darmstadt, Germany
Macromolecules, 2009, 42 (2), pp 531–536
DOI: 10.1021/ma802421e

Abstract: Poly[bis(methoxy)phosphazene] (PBMP) is used as a model to investigate the backbone dynamics of polyphosphazenes approaching their glass transitions. Specifically, we study PBMP featuring deuterated methyl groups so that, as a consequence of fast rotation of the methyl groups, 2H NMR probes the reorientation of their 3-fold symmetry axes and, thus, of the inorganic backbone. Combining 2H NMR spin−lattice relaxation, line-shape, and stimulated-echo analyses, we follow the slowdown of the segmental motion upon cooling over a broad temperature/time range. Comparison of present and previous results provides no evidence that polymers featuring inorganic and organic backbones, respectively, show fundamentally different dynamical behaviors during vitrification. In particular, typical of glass-forming polymer melts, we find for the α-process of PBMP that its temperature dependence deviates from an Arrhenius law and its time dependence differs from a single-exponential function. 2H NMR three-time correlation functions indicate that both homogeneous and heterogeneous dynamics contribute to the nonexponential relaxation. In addition, 2H NMR spin−lattice relaxation and line-shape analyses reveal the existence of some large-angle anisotropic precursor motion in the moderately viscous melt, which may be a peculiarity of polyphosphazene.

Polyolefin Blend Miscibility: Polarization Transfer versus Direct Excitation Exchange NMR
Marcin Wachowicz, Lance Gill and Jeffery L. White
Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
Macromolecules, 2009, 42 (2), pp 553–555
DOI: 10.1021/ma802531f
Publication Date (Web): December 29, 2008
Copyright © 2008 American Chemical Society
* To whom all correspondence should be addressed. E-mail:

Relatively simple chemical constituents in polyolefin macromolecules belie the fact that their phase behavior in mixtures can be complex.(1) Polyolefins are obviously important economically as commodity polymers, but many specialty applications require unique formulations of multiple polyolefins with slightly different chemical structures. Predicting the details of the ultimate phase mixing is difficult and nonintuitive, and experimental verification of chain level behavior is challenging due to similar chemical and physical properties among varying polyolefin chain structures. Many investigators have approached this problem through multiple theoretical and experimental avenues in recent years.(2-7) We have recently described aspects of binary polyolefin blend phase behavior, relying extensively on advanced solid-state NMR methods to show that configurational entropy is an important thermodynamic parameter in controlling miscibility between different polyolefin structures.(8-10) Important general conclusions based on these advanced NMR experiments are often complicated by complex pulse sequences that employ an initial polarization transfer step. CODEX NMR experiments have proven particularly powerful for direct chain level interrogation of mixing and dynamics in amorphous polyolefin blends, but to date, all work has involved polarization transfer from protons to carbons (cross-polarization or CP) to generate the initial signal in the CODEX experiment.(11, 12) Concerns about nonrepresentative sampling of a subset of polymer chains by polarization transfer steps can arise in cross-polarization solid-state NMR methods; differential polymer chain dynamics may lead to nonuniform polarization transfer efficiency in that step, often preferentially emphasizing the more spatially constrained or rigid regions of the sample which preserve larger heteronuclear dipolar couplings.(13, 14) To address this in the context of amorphous polyolefin blend miscibility, we have devised a modified version of the experiment employing only direct carbon polarization as the initial step in the experiment. On the basis of quantitative comparisons of the modified direct polarization versus CP-based CODEX results over a wide temperature range (including Tg) for atactic polypropylene (aPP), we demonstrate that results representative of all polymer chains in the sample are obtained here as well as in previously published polarization-transfer-based results. Our work shows that CODEX-based exchange methods can provide chain-level information representative of the bulk mixing and miscibility in amorphous polyolefin blends.

J Phys Chem B, vol 113, issue 5 and 6

Solid-State Organization of Semifluorinated Alkanes Probed by 19F MAS NMR Spectroscopy
Young Joo Lee, Christopher G. Clark, Jr., Robert Graf, Manfred Wagner, Klaus Müllen and Hans Wolfgang Spiess*
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
J. Phys. Chem. B, 2009, 113 (5), pp 1360–1366

Abstract:Bulk-phase self-assembly of a series of semifluorinated alkanes (SFAs) with hydrocarbon chains of varying length has been investigated by 19F NMR spectroscopy. At room temperature, a single 19F resonance for the terminal CF3 group was observed at −81.7 ppm for perfluorododecylhexane (F12H6), whereas a CF3 resonance was seen at −82.5 ppm for perfluorododecyldodecane (F12H12) and perfluorododecyleicosane (F12H20). This difference in chemical shift position is ascribed to the different molecular packing geometries, i.e., a monolayer lamellar structure for F12H6 vs a bilayer lamellar organization for F12H12 and F12H20. Moreover, in F12H12, a solid−solid phase transition from bilayer to monolayer lamellae can be followed by 19F NMR spectroscopy. 1H/19F → 13C CPMAS experiments indicated that the phase transition is accompanied by disordering of hydrocarbon chains, but does not involve a significant conformational change in the fluorocarbon chains. Yet, a change in the 19F T1 relaxation times was found to occur at the phase transition temperature, suggesting a change in the packing environments of the fluorocarbon chains. Two-dimensional exchange NMR experiments yielded cross-peaks between terminal CF3 and inner CF2CH2 moieties for the high-temperature monolayer phase, providing clear evidence for the spatial proximity between these groups. On the basis of these findings, we propose a model for the phase transition involving bilayer lamellae and monolayer lamellae with hydrocarbon and fluorocarbon interdigitation.

J Phys Chem C, vol 113, Issue 5 and 6

Structure and Dynamics of Hydrous Surface Species on Alumina−Boria Catalysts and Their Precursors from 1H, 2H, 11B, and 27Al MAS NMR Spectroscopy

Michael Ryan Hansen, Hans J. Jakobsen and Jørgen Skibsted*
Instrument Centre for Solid-State NMR Spectroscopy and Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark
J. Phys. Chem. C, 2009, 113 (6), pp 2475–2486
DOI: 10.1021/jp807595m
Abstract: The hydrous surface of a pseudoboehmite precursor impregnated with 1.3 wt % boron, its dehydration upon gentle heat treatment, and the corresponding alumina boria catalyst obtained by calcination at 750 °C have been studied by 1H and 2H MAS NMR as well as double-resonance 1H{11B}, 1H{27Al} TRAPDOR, and 11B{1H}, 27Al{1H} CP/MAS NMR experiments. Resonances from bulk and surface AlOH groups, surface BOH sites, and water molecules adsorbed on the surface-aluminate and -borate species have been identified and characterized. The dynamics of the water molecules and their removal from the surface by heat treatment have been studied by the double-resonance experiments. Optimizations of the 1H −11B/27Al cross-polarization conditions for the individual 11B and 27Al sites demonstrate that the BO4 as well as the BO3 sites on the alumina surface exhibit hydroxyl groups in their near vicinity. The same holds for the AlO4, AlO5, and AlO6 sites on the γ-Al2O3 surface of the alumina−boria catalyst.

2J Si−O−Si Scalar Spin−Spin Coupling in the Solid State: Crystalline and Glassy Wollastonite CaSiO3
Pierre Florian
*, Franck Fayon and Dominique Massiot
CNRS, UPR3079 CEMHTI, 1D Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France, and Université d’Orléans, Avenue du Parc Floral, BP 6749, 45067 Orléans Cedex 2, France
J. Phys. Chem. C, 2009, 113 (6), pp 2562–2572
DOI: 10.1021/jp8078309

Abstract: We have investigated the 29Si−O−29Si 2J scalar spin−spin coupling constant in two crystalline polymorphs and in a glass of 29Si isotopically enriched wollastonite CaSiO3 composition. In the crystalline samples, line widths of less than 1 Hz are observed in the indirect dimension of a simple J-resolved NMR experiment leading to 2JSiOSi values of 1.5, 3.6, and 8.0 Hz determined with a high accuracy of ±0.1 Hz. The very high resolution obtained with those two compositions enabled us to evidence strong coupling effects in the J-resolved two-dimensional spectra. In particular, additional lines in J-spectra are seen here for the first time in solid-state NMR experiments of inorganic samples. Cluster ab initio calculations led us to propose a stereochemical analysis of 2J, leading to a close-to-linear relationship between 2JSiOSi and the Si−O−Si bonding angle Ω ≃ 3.41J + 127 which takes into account the predominant influence of the Ca atoms. In the glass, this relationship allowed the analysis of the distribution of angle within each Q(n) species. The classical relationship between the 29Si isotropic chemical shift and the average Si−O−Si bond angle is experimentally confirmed and quantified. The most probable set of Ω’s are 128.0° for Q(1), (131.8°,135.4°) for Q(2), and (132.5°,137.1°,144.3°) for Q(3), and the presence of three-membered rings is evidenced. An unexpectedly strong correlation between the bonding at each corner of a given Q(n) unit and the bonding at adjacent corners is found.

Synthesis, Spectroscopic Properties, and Stabilities of Ternary Europium Complex in SBA-15 and Periodic Mesoporous Organosilica: A Comparative Study
Xianmin Guo
, Huadong Guo, Lianshe Fu§, Ruiping Deng, Wan Chen, Jing Feng, Song Dang and Hongjie Zhang*
State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China, Graduate School of the Chinese Academy of Sciences, P. R. China, and Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
J. Phys. Chem. C, 2009, 113 (6), pp 2603–2610

Abstract: Ternary europium complex Eu(tta)3phen was covalently bonded with the general mesoporous material SBA-15 and SBA-15-type of periodic mesoporous organosilica (PMO) material via impregnation of Eu(tta)3·2H2O into phen-S15 and phen-PMO, respectively, through a ligand exchange reaction. The parent materials of phen-S15 and phen-PMO were synthesized by co-condensation of tetraethylorthosilicate (TEOS) or 1,2-bis(triethoxysilyl)ethane (BTESE) and the functionalized chelate ligand 5-(N,N-bis(3-triethoxysilyl)propyl)ureyl-1,10-phenanthroline (phen-Si) in the presence of Pluronic P123 surfactant as template, which were confirmed by SEM, XRD, FTIR, 29Si CP-MAS NMR, and N2 adsorption measurements. The photophysical properties of the hybrids, such as the photoluminescence (PL) spectra, PL intensities, symmetry properties, luminescence decay times, and Judd−Ofelt parameters, were investigated in detail. Compared to the sample of Eu(tta)3phen-PMO, the mesoporous hybrid material Eu(tta)3phen-S15 exhibited longer luminescent decay time and higher luminescence intensity, emission quantum efficiency (q), and absolute quantum yield (Φ), which were quantitatively stressed by the emission spectra and the calculated values of q of Eu3+ ion. Meanwhile, the result of thermal treatment demonstrated that the europium complex in Eu(tta)3phen-S15 material possessed a better thermal stability than that in Eu(tta)3phen-PMO. The above photoluminescence and thermal stability features indicated that SBA-15 is a better host material for lanthanide complex than SBA-15-type PMO material.

Tuesday, February 03, 2009

JACS: Nonaromatic Core−Shell Structure of Nanodiamond from Solid-State NMR Spectroscopy

Nonaromatic Core−Shell Structure of Nanodiamond from Solid-State NMR Spectroscopy
XiaoWen Fang, JingDong Mao, E. M. Levin and Klaus Schmidt-Rohr
J. Am. Chem. Soc., 2009, 131 (4), pp 1426–1435
DOI: 10.1021/ja8054063
Publication Date (Web): January 9, 2009

The structure of synthetic nanodiamond has been characterized by 13C nuclear magnetic resonance (NMR) spectral editing combined with measurements of long-range 1H−13C dipolar couplings and 13C relaxation times. The surface layer of these ∼4.8-nm diameter carbon particles consists mostly of sp3-hybridized C that is protonated or bonded to OH groups, while sp2-hybridized carbon makes up less than 1% of the material. The surface protons surprisingly resonate at 3.8 ppm, but their direct bonding to carbon is proved by fast dipolar dephasing under homonuclear decoupling. Long-range 1H−13C distance measurements, based on 13C{1H} dipolar dephasing by surface protons, show that seven carbon layers, in a shell of 0.63 nm thickness that contains ∼60% of all carbons, predominantly resonate more than +8 ppm from the 37-ppm peak of bulk diamond (i.e., within the 45−80 ppm range). Nitrogen detected in 15N NMR spectra is mostly not protonated and can account for some of the high-frequency shift of carbon. The location of unpaired electrons (∼40 unpaired electrons per particle) was studied in detail, based on their strongly distance-dependent effects on T1,C relaxation. The slower relaxation of the surface carbons, selected by spectral editing, showed that the unpaired electrons are not dangling bonds at the surface. This was confirmed by detailed simulations, which indicated that the unpaired electrons are mostly located in the disordered shell, at distances between 0.4 and 1 nm from the surface. On the basis of these results, a nonaromatic core−shell structural model of nanodiamond particles has been proposed.

MRI at the nanoscale

An interesting description of nanoscale magnetic resonance imaging at Chemistry World, RSC.

Monday, February 02, 2009

MRC: Magn. Reson. Chem. (Wiley) up to Feb. 2, 2009

MRC: Magn. Reson. Chem. (Wiley) up to Feb. 2, 2009

13C CP MAS NMR of halogenated (Cl, Br, I) pharmaceuticals at ultrahigh magnetic fields
from Magnetic Resonance in Chemistry by Victor V. Terskikh, Stephen J. Lang, Peter G. Gordon, Gary D. Enright, John A. Ripmeester
1 Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Canada K1A 0R6
2 Department of Chemistry, Carleton University, Ottawa, Canada K1S 5B6

This work reports significantly improved spectral resolution of 13C CP MAS NMR spectra of chlorinated, brominated and iodinated solid organic compounds when such spectra are recorded at ultrahigh magnetic field strengths. The cause of this is the residual dipolar coupling between carbon atoms and quadrupolar halogen nuclides (chlorine-35/37, bromine-79/81 or iodine-127), an effect inversely proportional to the magnetic field strength which declines in importance markedly at 21.1 T as compared to lower fields. In favorable cases, the fine structure observed can be used for spectral assignment, e.g. for Cl-substituted aromatics where the substituted carbon as well as the ortho-carbons show distinct doublets. The experimental results presented are supported by theoretical modeling and calculations. The improved spectral resolution in the studied systems and similar halogenated materials will be of particular interest and importance for polymorph identification, drug discovery and quality control in the pharmaceutical industry.

Digital Object Identifier (DOI) 10.1002/mrc.2399

Studies on drug-DNA complexes, adriamycin-d-(TGATCA)2 and 4[prime]-epiadriamycin-d-(CGATCG)2, by phosphorus-31 nuclear magnetic resonance spectroscopy
from Magnetic Resonance in Chemistry by Prashansa Agrawal, Girjesh Govil, Ritu Barthwal
1 Department of Biotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, India
2 Chemical Physics Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Navy Nagar, Colaba, Mumbai 400 005, India

The complexes of adriamycin-d-(TGATCA)2 and 4[prime]-epiadriamycin-d-(CGATCG)2 are studied by one- and two-dimensional 31P nuclear magnetic resonance spectroscopy (NMR) at 500 MHz in the temperature range 275-328 K and as a function of drug to DNA ratio (0.0-2.0). The binding of drug to DNA is clearly evident in 31P[bond]31P exchange NOESY spectra that shows two sets of resonances in slow chemical exchange. The phosphate resonances at the intercalating steps, T1pG2/C1pG2 and C5pA6/C5pG6, shift downfield up to 1.7 ppm and that at the adjacent step shift downfield up to 0.7 ppm, whereas the central phosphate A3pT4 is relatively unaffected. The variations of chemical shift with drug to DNA ratio and temperature as well as linewidths are different in each of the two complexes. These observations reflect change in population of BI/BII conformation, stretching of backbone torsional angle [zeta], and distortions in O[bond]P[bond]O bond angles that occur on binding of drug to DNA. To the best of our knowledge, there are no solution studies on 4[prime]-epiadriamycin, a better tolerated drug, and binding of daunomycin or its analogue to d-(TGATCA)2 hexamer sequence. The studies report the use of 31P NMR as a tool to differentiate various complexes. The specific differences may well be the reasons that are responsible for different antitumor action of these drugs due to different binding ability and distortions in DNA. Copyright © 2009 John Wiley & Sons, Ltd.

Digital Object Identifier (DOI) 10.1002/mrc.2398

Progress in NMR: Review: solid-state selenium-77 (77Se) NMR

Progress in NMR: Review: solid-state selenium-77 (77Se) NMR


Solid-state selenium-77 NMR

Bryan A. Demko and Roderick E. Wasylishen

Department of Chemistry, Gunning/Lemieux Chemistry Centre, University of Alberta, Edmonton, AB, Canada T6G 2G2

Keywords: Solid-state NMR; 77Se NMR; Chemical shift tensors; Indirect spin–spin coupling constants