Monday, October 27, 2008

Friday, October 24, 2008

J. Am. Chem. Soc., 130 (43), 14060–14061, 2008.

Dynamics of Ligand Binding from 13C NMR Relaxation Dispersion at Natural Abundance
John S. Zintsmaster, Brian D. Wilson, and Jeffrey W. Peng*

We show that Carr−Purcell−Meiboom−Gill (CPMG) 13Cα NMR relaxation dispersion measurements are a viable means for profiling μs−ms ligand dynamics involved in receptor binding. Critically, the dispersion is at natural 13C abundance; this matches typical pharmaceutical research settings in which ligand isotope-labeling is often impractical. The dispersion reveals ligand 13Cα nuclei that experience μs−ms modulation of their chemical shifts due to binding. 13Cα shifts are dominated by local torsion angles φ, ψ, χ1; hence, these experiments identify flexible torsion angles that may assist complex formation. Since the experiments detect the ligand, they are viable even in the absence of a receptor structure. The μs−ms dynamic information gained helps establish flexibility−activity relationships. We apply these experiments to study the binding of a phospho-peptide substrate ligand to the peptidyl-prolyl isomerase Pin1.

Tuesday, October 21, 2008

Physical Review Letters - Up to Vol 101 No 16

93Nb NMR Spin Echo Spectroscopy in Single Crystal NbSe3
S. Suh, W. G. Clark, P. Monceau, R. E. Thorne, and S. E. Brown
Phys. Rev. Lett. 101, 136407 (2008)
We report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe3 using 93Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E=(0.96)ET. The polarizations have a broad distribution extending up to phase angles of order 60° for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable.

Boroxol Rings in Liquid and Vitreous B2O3 from First Principles
Guillaume Ferlat, Thibault Charpentier, Ari Paavo Seitsonen, Akira Takada, Michele Lazzeri, Laurent Cormier, Georges Calas, and Francesco Mauri
Phys. Rev. Lett. 101, 065504 (2008)
We investigate the structural and vibrational properties of glassy B2O3 using first-principles molecular dynamics simulations. In particular, we determine the boroxol rings fraction f for which there is still no consensus in the literature. Two numerical models containing either a low or a high level of boroxol rings are tested against a gamut of experimental probes (static structure factor, Raman, 11B and 17O NMR data). We show that only the boroxol-rich model (f=75%) can reproduce the full set of observables. Total-energy calculations show that at the glass density, boroxol-rich structures are favored by about 6 kcal/(mol boroxol). Finally, the liquid state is explored in the 2000–4000 K range and a reduction of f to 10%–20% is obtained.

Physical Review B - Up to Vol 78 No 12

Nuclear magnetic resonance linewidth and spin diffusion in 29Si isotopically controlled silicon
Hiroshi Hayashi, Kohei M. Itoh and Leonid S. Vlasenko
Phys. Rev. B 78, 153201 (2008)
A nuclear magnetic resonance (NMR) study was performed with n-type silicon single crystals containing 29Si isotope abundance f ranges from 1.2% to 99.2%. The nuclear spin diffusion coefficient D has been determined from the linewidth of significantly enhanced 29Si NMR signals utilizing a developed dynamic nuclear polarization (DNP) method. The 29Si NMR linewidth depends linearly on f, at least when f<10%, and approaches f1/2 dependence when f>50%. The estimated 29Si nuclear spin diffusion time Tsd between phosphorus atoms used for DNP is more than ten times shorter than the nuclear polarization time T of 29Si nuclei around phosphorus. Therefore, the regime of “rapid spin diffusion” is realized in the DNP experiments.

Magnetization and Cu nuclear magnetic resonance study of Sr0.9La0.1Cu1−xNixO2
G. V. M. Williams, S. Richter, J. Haase, C. U. Jung, Hye-Gyong Lee, and Sung-Ik Lee
Phys. Rev. B 78, 104522 (2008)
We report the results from a Cu nuclear magnetic resonance (NMR) and magnetization study of the electron-doped high-temperature superconducting cuprate (HTSC) Sr0.9La0.1Cu1−xNixO2. This compound shows a large suppression of superconductivity by Ni [Tc/x=18 K/%(Ni)] that is comparable to that observed in underdoped and hole-doped HTSCs. We find small effective moments of 0.06µB/Cu for the pure sample and 0.76µB/Ni for the 1% Ni sample in the paramagnetic regime. The partial substitution of Ni for Cu causes the Cu NMR linewidth to increase and there is a further reduction in the Cu NMR intensity with decreasing temperature over and above that observed in the pure compound. This can be interpreted in terms of Ni inducing additional spin disorder and wipeout of the Cu NMR intensity for Cu sites near the Ni impurities.

19F NMR investigation of the iron pnictide superconductor LaFeAsO0.89F0.11
K. Ahilan, F. L. Ning, T. Imai, A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, and D. Mandrus
Phys. Rev. B 78, 100501(R) (2008)
We report on 19F nuclear magnetic resonance (NMR) investigation of the high-temperature superconductor LaFeAsO0.89F0.11 (Tc~28 K). We demonstrate that low-frequency spin fluctuations exhibit pseudogap behavior above Tc. We also deduce the London penetration depth from NMR line broadening below Tc.

Atomic-scale measurement of ultraslow Li motions in glassy LiAlSi2O6 by two-time 6Li spin-alignment echo NMR correlation spectroscopy
M. Wilkening, A. Kuhn, and P. Heitjans
Phys. Rev. B 78, 054303 (2008)
6Li spin-alignment echo (SAE) nuclear-magnetic-resonance (NMR) spectroscopy is used to monitor single-particle two-time correlation functions in LiAlSi2O6 glass. The method, here applied in the temperature range from 300 to 400 K, is sensitive to ultraslow Li hopping processes with rates (1/SAE) down to 10 jumps/s. The use of a sample with natural 6Li abundance allowed the measurement of pure NMR spin-alignment echoes which are damped with increasing mixing time exclusively by slow Li jumps, i.e., free of influences arising from, e.g., interfering spin-diffusion effects. The considerably stretched correlation functions reveal the presence of a broad distribution of jump rates. The results are comprehensively compared with those recently obtained from both 7Li SAE and 7Li spin-lattice relaxation NMR as well as from dc conductivity measurements. Interestingly, the activation energy of the latter, which are sensitive to long-range Li transport parameters, is in good agreement with that microscopically probed by 6Li SAE NMR, here.

Journal of Magnetic Resonance - Vol 195 Issue 1

Solid state water motions revealed by deuterium relaxation in 2H2O-synthesized kanemite and 2H2O-hydrated Na+-Zeolite A
Bernie O’Hare, Michael W. Grutzeck, Seong H. Kim, David B. Asay and Alan J. Benesi
Deuterium NMR relaxation experiments, low temperature deuterium NMR lineshape analysis, and FTIR spectra are consistent with a new model for solid state jump dynamics of water in 2H2O-synthesized kanemite and 2H2O-hydrated Na+-Zeolite A. Exchange occurs between two populations of water: one in which water molecules are directly coordinated to sodium ions and experience C2 symmetry jumps of their OH bonds, and a population of interstitial water molecules outside the sodium ion coordination sphere that experience tetrahedral jumps of their OH bonds. For both samples the C2 jump rate is much faster than the tetrahedral jump rate. 2H NMR relaxation experiments match well with the fast exchange regime of the model over a wide range of temperatures, including room temperature and above.

Coherence selection in double CP MAS NMR spectroscopy
Jen-Hsien Yang, Fang-Chieh Chou and Der-Lii M. Tzou
Applications of double cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy, via 1H/15N and then 15N/13C coherence transfers, for 13C coherence selection are demonstrated on a 15N/13C-labeled N-acetyl-glucosamine (GlcNAc) compound. The 15N/13C coherence transfer is very sensitive to the settings of the experimental parameters. To resolve explicitly these parameter dependences, we have systematically monitored the 13C{15N/1H} signal as a function of the rf field strength and the MAS frequency. The data reveal that the zero-quantum coherence transfer, with which the 13C effective rf field is larger than that of the 15N by the spinning frequency, would give better signal sensitivity. We demonstrate in one- and two-dimensional double CP experiments that spectral editing can be achieved by tailoring the experimental parameters, such as the rf field strengths and/or the MAS frequency.

Journal of Magnetic Resonance - Vol 194 Issue 2

The utility of phase alternated pulses for the measurement of dipolar couplings in 2D-SLF experiments
Bibhuti B. Das, N. Sinha and K.V. Ramanathan
The measurement of hetero-nuclear dipolar coupling using two-dimensional separated local field (SLF-2D) NMR experiments is a powerful technique for the determination of the structure and dynamics of molecules in the solid state and in liquid crystals. However, the experiment is sensitive to a number of factors such as the Hartmann–Hahn match condition, proton frequency off-set and rf heating. It is shown here that by the use of phase alternated pulses during spin-exchange the effect of rf mismatch on the dipolar coupling measurement can be compensated over a wide range of off-sets. Phase alternation together with time and amplitude modulation has also been considered and incorporated into a pulse scheme that combines spin exchange with homonuclear spin decoupling based on magic sandwich sequence and named as SAMPI4. Such time and amplitude averaged nutation experiments use relatively low rf power and generate less sample heating. One of these schemes has been applied on liquid crystal samples and is observed to perform well and yield spectra with high resolution.

Practical aspects of Lee–Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: Implementation and applications
Cristina Coelho, João Rocha, P.K. Madhu and Luís Mafra
Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because 1H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution 1H MAS techniques. 1H Lee–Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic–organic hybrid materials, rich in 1H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the 1H NMR spectra of the different parameters at play, for example LG (Lee–Goldburg) pulses, radio-frequency (rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H4pmida] [I], and inorganic–organic hybrid materials (C4H12N2)[Ge2(pmida)2OH2]·4H2O [II] and (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O [III].

Double-acquisition: Utilization of discarded coherences in a 2D separation experiment using the States method
Masashi Fukuchia, Munehiro Inukai, Kazuyuki Takeda and K. Takegoshi
We propose a new data-acquisition scheme for 2D separation experiments to save the spectrometer time by 1/2. This scheme, referred to as a double-acquisition scheme, is applicable to most of separation experiments with the hypercomplex time-domain data-acquisition scheme (the States method) for data collection.

Friday, October 10, 2008

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja801789t

Raftlike Mixtures of Sphingomyelin and Cholesterol Investigated by Solid-State 2H NMR Spectroscopy

Tim Bartels, Ravi S. Lankalapalli, Robert Bittman, Klaus Beyer, and Michael F. Brown*

Sphingomyelin is a lipid that is abundant in the nervous systems of mammals, where it is associated with putative microdomains in cellular membranes and undergoes alterations due to aging or neurodegeneration. We investigated the effect of varying the concentration of cholesterol in binary and ternary mixtures with N-palmitoylsphingomyelin (PSM) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using deuterium nuclear magnetic resonance (2H NMR) spectroscopy in both macroscopically aligned and unoriented multilamellar dispersions. In our experiments, we used PSM and POPC perdeuterated on the N-acyl and sn-1 acyl chains, respectively. By measuring solid-state 2H NMR spectra of the two lipids separately in mixtures with the same compositions as a function of cholesterol mole fraction and temperature, we obtained clear evidence for the coexistence of two liquid-crystalline domains in distinct regions of the phase diagram. According to our analysis of the first moments M1 and the observed 2H NMR spectra, one of the domains appears to be a liquid-ordered phase. We applied a mean-torque potential model as an additional tool to calculate the average hydrocarbon thickness, the area per lipid, and structural parameters such as chain extension and thermal expansion coefficient in order to further define the two coexisting phases. Our data imply that phase separation takes place in raftlike ternary PSM/POPC/cholesterol mixtures over a broad temperature range but vanishes at cholesterol concentrations equal to or greater than a mole fraction of 0.33. Cholesterol interacts preferentially with sphingomyelin only at smaller mole fractions, above which a homogeneous liquid-ordered phase is present. The reasons for these phase separation phenomena seem to be differences in the effects of cholesterol on the configurational order of the palmitoyl chains in PSM-d31 and POPC-d31 and a difference in the affinity of cholesterol for sphingomyelin observed at low temperatures. Hydrophobic matching explains the occurrence of raftlike domains in cellular membranes at intermediate cholesterol concentrations but not saturating amounts of cholesterol.

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja805839y

Dynamics of Ligand Binding from 13C NMR Relaxation Dispersion at Natural Abundance

John S. Zintsmaster, Brian D. Wilson, and Jeffrey W. Peng*

We show that Carr−Purcell−Meiboom−Gill (CPMG) 13Cα NMR relaxation dispersion measurements are a viable means for profiling μs−ms ligand dynamics involved in receptor binding. Critically, the dispersion is at natural 13C abundance; this matches typical pharmaceutical research settings in which ligand isotope-labeling is often impractical. The dispersion reveals ligand 13Cα nuclei that experience μs−ms modulation of their chemical shifts due to binding. 13Cα shifts are dominated by local torsion angles φ, ψ, χ1; hence, these experiments identify flexible torsion angles that may assist complex formation. Since the experiments detect the ligand, they are viable even in the absence of a receptor structure. The μs−ms dynamic information gained helps establish flexibility−activity relationships. We apply these experiments to study the binding of a phospho-peptide substrate ligand to the peptidyl-prolyl isomerase Pin1.

Wednesday, October 08, 2008

J. Am. Chem. Soc., 130 (41), 13664–13672, 2008.

Nanochannels of Two Distinct Cross-Sections in a Porous Al-Based Coordination Polymer
Angiolina Comotti, Silvia Bracco, Piero Sozzani, Satoshi Horike, Ryotaro Matsuda, Jinxi Chen, Masaki Takata, Yoshiki Kubota, and Susumu Kitagawa

A new aluminum naphthalenedicarboxylate Al(OH)(1,4-NDC)·2H2O compound has been synthesized. The crystal structure exhibits a three-dimensional framework composed of infinite chains of corner-sharing octahedral Al(OH)2O4 with 1,4-naphthanedicarboxylate ligands forming two types of channels with squared-shape cross-section. The large channels present a cross-section of 7.7 × 7.7 Å2, while the small channels are about 3.0 × 3.0 Å2. When water molecules are removed, no structural transformation occurs, generating a robust structure with permanent porosity and remarkable thermal stability. 2D 1H−13C heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, were applied, for the first time, to porous coordination polymers revealing the spatial relationships between the 1H and 13C spin-active nuclei of the framework. To demonstrate the open pore structure and the easy accessibility of the nanochannels to the gas phase, highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution, has been applied. Xenon can diffuse selectively into the large nanochannels, while the small ones show no substantial uptake of xenon due to severe restrictions along the channels that prevent the diffusion. Two-dimensional exchange experiments showed the exchange time to be as short as 15 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the large nanochannel space and surface, a physisorption energy of 10 kJ mol−1, and the chemical shift value of xenon probing the internal surfaces. The large pore channels are straight, parallel, and independent, allowing one-dimensional anisotropic diffusion of gases and vapors. Their walls are composed of the naphthalene moieties that create an unique environment for selective sorption. These results prompted us to measure the storage capacity toward methanol, acetone, benzene, and carbon dioxide. The selective adsorption of methanol and acetone vs that of water, together with the permanent porosity and high thermal stability, makes this compound a suitable matrix for separation and purification.

Friday, October 03, 2008

ASAP Cryst. Growth Des., ASAP Article, 10.

1H, 13C, and 15N Solid-State NMR Studies of Imidazole- and Morpholine-Based Model Compounds Possessing Halogen and Hydrogen Bonding Capabilities

Karim Bouchmella,† Sylvain G. Dutremez,*† Bruno Alonso,‡ Francesco Mauri,§ and Christel Gervais*#

The halogen and hydrogen bonding interactions present in solid 1-(2,3,3-triiodoallyl)imidazole (1), morpholinium iodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)imidazole·morpholinium iodide (3), morpholine (4), imidazole (5), and 1-(3-iodopropargyl)imidazole (6) have been investigated by solid-state 1H, 13C, and 15N NMR spectroscopies. Comparison of the 15N CP MAS NMR spectrum of 3 with that of 2 indicates that protonated morpholine is present in solid 3, but this conclusion must be taken with caution as GIPAW calculations predict a 15N chemical shift for morpholine similar to that of the morpholinium cation. Conclusive evidence for the presence of a morpholinium cation in crystalline 3 was obtained by recording the static 15N NMR spectrum of this host−guest complex and comparing the morpholinium/morpholine part of the spectrum with the static spectra of 3 and 4 as obtained from ab initio calculations of NMR parameters based on the X-ray structures of these compounds. Concerning the imidazolyl group, 15N NMR spectroscopy has proven quite valuable to identify changes in the bonding situation of the C−NC nitrogen on passing from 1 to 3. In addition, slight differences are observed between the 15N chemical shifts of 1 and 6 that are ascribed to differences in halogen bond strengths between the two compounds. Attempts have also been made to study halogen bonding by 13C NMR spectroscopy, but this method did not provide exploitable results as signals corresponding to the sp and sp2 carbon atoms bonded to iodine could not be observed experimentally. 1H NMR spectroscopy is a powerful tool to study hydrogen bonding interactions of moderate energies such as +NH2···X (X = N, O, I). Indeed, we have found that the chemical shifts of the NH hydrogens were quite sensitive to the nature of X and to the N−H···X distance. This is demonstrated by the fact that the chemical shifts of the +NH2 protons of the morpholinium cation in 2 and 3 are noticeably different.

Wednesday, October 01, 2008

J. Am. Chem. Soc., 130 (40), 13425–13432, 2008.

In Situ Observation of the Internal Structure and Composition of Biomineralized Emiliania huxleyi Calcite by Solid-State NMR Spectroscopy
Ronen Gertman, Ira Ben Shir, Shifi Kababya, and Asher Schmidt*

Biomineralization, particularly the formation of calcium carbonate structures by organisms under ambient conditions, is of vast fundamental and applied interest. Organisms finely control all aspects of the formation of the biomaterials: composition, polymorph, morphology, and macroscopic properties. While in situ molecular-level characterization of the resulting biominerals is a formidable task, solid-state magic angle spinning NMR is one of the most powerful analytical techniques for this purpose. It is employed in this study to elucidate the structure and composition of biogenic calcite formed by Emiliania huxleyi, a unicellular alga distinguished by its exquisitely sculptured calcite cell coverings known as coccoliths. Strain 371 (CCMP) was grown and harvested from 15N- and 13C-enriched growth medium, with biosynthetic labeling to enhance the sensitivity of the NMR measurements. Crystalline and interfacial calcite environments were selectively probed using direct and indirect (cross-polarized) 13C excitation, respectively. Different crystalline environments, in particular structural defect sites at concentrations of up to 1.4% with P and N moieties incorporated, were identified using 13C rotational-echo double-resonance (REDOR) NMR. REDOR-derived geometrical constraints show that the P and N atoms at the defect sites are 3.2 and 2.3 (0.2) Å apart from a crystalline carbon carbonate. The phosphorus and nitrogen moieties within the biogenic calcite are identified as small, non-protonated moieties, attributed to inorganic ions such as PO43− and NO3−. The carbonates adjacent to these defects are chemically indistinguishable from bulk crystalline carbonates, yet their immediate environments experience reduced rigidity, as reflected by substantial T1(13CO32−) shortening. Interfacial carbonates, on the other hand, reside in structurally/chemically perturbed environments, as reflected by heterogeneous line broadening. This study is the first to directly unravel evidence on the incorporation of P/N moieties as structural defects within E. huxleyi biogenic calcite, and on the state of the adjacent crystalline carbonates.