Monday, April 26, 2010

J. Am. Chem. Soc., 2010, 132 (16), pp 5556–5557
Fibrillar vs Crystalline Full-Length β-2-Microglobulin Studied by High-Resolution Solid-State NMR Spectroscopy
Emeline Barbet-Massin, Stefano Ricagno, Józef R. Lewandowski, Sofia Giorgetti, Vittorio Bellotti, Martino Bolognes, Lyndon Emsley and Guido Pintacuda

Elucidating the fine structure of amyloid fibrils as well as understanding their processes of nucleation and growth remains a difficult yet essential challenge, directly linked to our current poor insight into protein misfolding and aggregation diseases. Here we consider β-2-microglobulin (β2m), the MHC-1 light chain component responsible for dialysis-related amyloidosis, which can give rise to amyloid fibrils in vitro under various experimental conditions, including low and neutral pH. We have used solid-state NMR to probe the structural features of fibrils formed by full-length β2m (99 residues) at pH 2.5 and pH 7.4. A close comparison of 2D 13C−13C and 15N−13C correlation experiments performed on β2m, in both the crystalline and fibrillar states, suggests that, in spite of structural changes affecting the protein loops linking the protein β-strands, the protein chain retains a substantial share of its native secondary structure in the fibril assembly. Moreover, variations in the chemical shifts of the key Pro32 residue suggest the involvement of a cis−trans isomerization in the process of β2m fibril formation. Lastly, the analogy of the spectra recorded on β2m fibrils grown at different pH values hints at a conserved architecture of the amyloid species thus obtained.

J. Am. Chem. Soc., 2010, 132 (16), pp 5672–5676
NMR-Based Structural Modeling of Graphite Oxide Using Multidimensional 13C Solid-State NMR and ab Initio Chemical Shift Calculations
Leah B. Casabianca†, Medhat A. Shaibat†, Weiwei W. Cai‡, Sungjin Park‡, Richard Piner‡, Rodney S. Ruoff‡ and Yoshitaka Ishii†

Chemically modified graphenes and other graphite-based materials have attracted growing interest for their unique potential as lightweight electronic and structural nanomaterials. It is an important challenge to construct structural models of noncrystalline graphite-based materials on the basis of NMR or other spectroscopic data. To address this challenge, a solid-state NMR (SSNMR)-based structural modeling approach is presented on graphite oxide (GO), which is a prominent precursor and interesting benchmark system of modified graphene. An experimental 2D 13C double-quantum/single-quantum correlation SSNMR spectrum of 13C-labeled GO was compared with spectra simulated for different structural models using ab initio geometry optimization and chemical shift calculations. The results show that the spectral features of the GO sample are best reproduced by a geometry-optimized structural model that is based on the Lerf−Klinowski model (Lerf, A. et al. Phys. Chem. B 1998, 102, 4477); this model is composed of interconnected sp2, 1,2-epoxide, and COH carbons. This study also convincingly excludes the possibility of other previously proposed models, including the highly oxidized structures involving 1,3-epoxide carbons (Szabo, I. et al. Chem. Mater. 2006, 18, 2740). 13C chemical shift anisotropy (CSA) patterns measured by a 2D 13C CSA/isotropic shift correlation SSNMR were well reproduced by the chemical shift tensor obtained by the ab initio calculation for the former model. The approach presented here is likely to be applicable to other chemically modified graphenes and graphite-based systems.

J. Am. Chem. Soc., 2010, 132 (16), pp 5558–5559
Ultrafast MAS Solid-State NMR Permits Extensive 13C and 1H Detection in Paramagnetic Metalloproteins
Ivano Bertini*†‡, Lyndon Emsley§, Moreno Lelli†, Claudio Luchinat†‡, Jiafei Mao† and Guido Pintacuda§

We show here that by combining tailored approaches based on ultrafast (60 kHz) MAS on the CoII-replaced catalytic domain of matrix metalloproteinase 12 (CoMMP-12) we can observe and assign, in a highly paramagnetic protein in the solid state, 13C and even 1H resonances from the residues coordinating the metal center. In addition, by exploiting the enhanced relaxation caused by the paramagnetic center, and the low power irradiation enabled by the fast MAS, this can be achieved in remarkably short times and at very high field (21.2 T), with only less than 1 mg of sample. Furthermore, using the known crystal structure of the compound, we are able to distinguish and measure pseudocontact (PCS) contributions to the shifts up to the coordinating ligands and to unveil structural information.

J. Am. Chem. Soc., 2010, 132 (16), pp 5607–5609
Toward Flexibility−Activity Relationships by NMR Spectroscopy: Dynamics of Pin1 Ligands
Andrew T. Namanja†, Xiaodong J. Wang‡, Bailing Xu‡, Ana Y. Mercedes-Camacho‡, Brian D. Wilson†, Kimberly A. Wilson†, Felicia A. Etzkorn‡ and Jeffrey W. Peng*†

Drug design involves iterative ligand modifications. For flexible ligands, these modifications often entail restricting conformational flexibility. However, defining optimal restriction strategies can be challenging if the relationship between ligand flexibility and biological activity is unclear. Here, we describe an approach for ligand flexibility−activity studies using Nuclear Magnetic Resonance (NMR) spin relaxation. Specifically, we use 13C relaxation dispersion measurements to compare site-specific changes in ligand flexibility for a series of related ligands that bind a common macromolecular receptor. The flexibility changes reflect conformational reorganization resulting from formation of the receptor−ligand complex. We demonstrate this approach on three structurally similar but flexibly differentiated ligands of human Pin1, a peptidyl-prolyl isomerase. The approach is able to map the ligand dynamics relevant for activity and expose changes in those dynamics caused by conformational locking. Thus, NMR flexibility−activity studies can provide information to guide strategic ligand rigidification. As such, they help establish an experimental basis for developing flexibility−activity relationships (FAR) to complement traditional structure−activity relationships (SAR) in molecular design.

J. Phys. Chem. A, 2010, 114 (16), pp 5365–5371
NMR and Quantum Chemistry Study of Mesoscopic Effects in Ionic Liquids
Vytautas Balevicius*†, Zofia Gdaniec‡, Kestutis Aidas§ and Jelena Tamuliene

1H, 13C, and 81Br NMR spectra of the neat room-temperature ionic liquid (RTIL), namely, 1-decyl-3-methyl-imidazolium bromide ([C10mim][Br]) as well as its solutions in acetonitrile, dichloromethane, methanol, and water have been investigated. The most important observation of the present work is the significant broadening of 81Br NMR signal in the solutions of [C10mim][Br] in organic solvents, which molecules tend to associate into hydrogen bond networks and the appearance of the complex contour of 81Br NMR signal in the neat RTIL as well as in the liquid crystalline (LC) ionogel formed in RTIL/water solution. The complex structure of 81Br signal changes upon heating and dilution in water. It disappears at ca. 353 K and in the aqueous solution below ca. 0.1 mol fraction of RTIL. Several new 1H NMR signals appear at the [C10mim][Br]/water compositions just before the solidification of the sample (0.3 mol fraction of [C10mim][Br]). These additional peaks can be attributed to the H2O protons placed in inhomogeneous regions of the sample or due to the appearance of nonequivalent water sites in LC ionogel, the exchange between which is highly restricted or even frozen. The complex shape of 81Br NMR signal can originate from the presence of supra-molecular structures (mesoscopic domains) that live over the period of the NMR time-scale due to a very high viscosity of [C10mim][Br]. These domains exhibit some features of partially disordered solids (liquid- or plastic crystals). To evaluate the static and dynamic contributions into the relaxation rate of 81Br nuclei, the quantum chemistry calculations of the electronic structure, magnetic shielding, and electric field gradient (EFG) tensors of [C10mim][Br] and related model systems (Br−·6H2O cluster, with addition of the dipoles (hydrogen fluoride) and charged particles − cations: H+ or C1mim+) were performed.

J. Phys. Chem. A, 2010, 114 (16), pp 5279–5286
DFT Calculations of Indirect 29Si−1H Spin−Spin Coupling Constants in Organoalkoxysilanes
Jyothirmai Ambati and Stephen E. Rankin

The performance of four basis sets (6-311+G(2d,p), IGLO-III, cc-PVTZ, and 6-31G) is evaluated in order to find a quantum mechanical technique that can be used to accurately estimate 29Si−1H spin−spin coupling constants in organoalkoxysilanes. The 6-31G basis set with the B3LYP functional is found to be an accurate, efficient, and cost-effective density functional theory method for predicting spin−spin coupling constants of organoalkoxysilanes. Knowledge of these scalar coupling constants and their dependence on structural variations is important to be able to fine-tune NMR experiments that rely on polarization transfer among nuclei, such as 29Si distortionless enhancement by polarization transfer (DEPT). The effects of size and the number of unhydrolyzable alkyl groups attached to silicon and the effects of substitution of alkoxy groups with hydroxyl groups on 29Si−1H spin−spin coupling constants are investigated using this DFT method. The results show that the predicted scalar coupling between silicon and organic groups depends weakly on the degree of hydrolysis of the alkoxysilanes. The effectiveness of this method is also illustrated for the determination of spin−spin coupling constants in a species containing a siloxane bond.

J. Am. Chem. Soc., 2010, 132 (16), pp 5538–5539

Breaking the T1 Constraint for Quantitative Measurement in Magic Angle Spinning Solid-State NMR Spectroscopy
Guangjin Hou, Shangwu Ding, Limin Zhang and Feng Deng

Quantitative solid-state NMR experimental schemes that break the conventional T1 constraint are described. The combination of broad-band homonuclear recoupling techniques and the conventional single pulse or cross-polarization (CP) schemes (referred as QUSP or QUCP) render the long T1 of low-γ spins no longer a constraint for obtaining quantitative NMR spectra. During the mixing time when dipolar recoupling occurs, the nonuniformly CP enhanced or recovered spin magnetization is redistributed under the reintroduced homonuclear dipole−dipole interactions so that uniformly enhanced or recovered magnetization is achieved when the system reaches the quasi-equilibrium state. It is shown that quantitative NMR spectra can be obtained for the recycle delays substantially shorter than the conventionally required 5T1. In addition, the high efficiency gain can be achieved in QUSP and QUCP experiments with a relatively short recycle delay.

J. Am. Chem. Soc., 2010, 132 (15), pp 5479–5493

Solid-State 115In and 31P NMR Studies of Triarylphosphine Indium Trihalide Adducts
Fu Chen, Guibin Ma, Guy M. Bernard, Ronald G. Cavell, Robert McDonald, Michael J. Ferguson and Roderick E. Wasylishen

Solid-state 115In and 31P NMR spectroscopy, relativistic density functional theory (DFT) calculations, and single-crystal X-ray diffraction were used to investigate a series of triarylphosphine indium(III) trihalide adducts, X3In(PR3) and X3In(PR3)2 (X = Cl, Br or I; PR3 = triarylphosphine ligand). The electric field gradient tensors at indium as well as the indium and phosphorus magnetic shielding tensors and the direct and indirect 115In−31P spin−spin coupling were characterized; for complexes possessing a C3 symmetry axis, the anisotropy in the indirect spin−spin coupling, ΔJ(115In,31P), was also determined. The 115In quadrupolar coupling constants, CQ(115In), range from ±1.25 ± 0.10 to −166.0 ± 2.0 MHz. For any given phosphine ligand, the indium nuclei are most shielded for X = I and least shielded for X = Cl, a trend also observed for other group-13 nuclei in M(III) complexes. This experimental trend, attributed to spin−orbit effects of the halogen ligands, is reproduced by the DFT calculations. The spans of the indium magnetic shielding tensors for these complexes, δ11 − δ33, range from 40 ± 7 to 710 ± 60 ppm; those determined for phosphorus range from 28 ± 1.5 to 50 ± 3 ppm. Values of 1J(115In,31P) range from 550 ± 20 to 2500 ± 20 Hz. For any given halide, the 1J(115In,31P) values generally increase with increasing basicity of the PR3 ligand. Calculated values of 1J(115In,31P) and ΔJ(115In,31P) duplicate experimental trends and indicate that both the Fermi-contact and spin−dipolar Fermi-contact mechanisms make important contributions to the 1J(115In,31P) tensors.

J. Am. Chem. Soc., 2010, 132 (15), pp 5443–5455

Si═X Multiple Bonding with Four-Coordinate Silicon? Insights into the Nature of the Si═O and Si═S Double Bonds in Stable Silanoic Esters and Related Thioesters: A Combined NMR Spectroscopic and Computational Study
Jan. D. Epping, Shenglai Yao, Miriam Karni, Yitzhak Apeloig and Matthias Driess

The electronic structures and nature of silicon−chalcogen double bonds Si═X (X = O, S) with four-coordinate silicon in the unique silanoic silylester 2 and silanoic thioester 3 have been investigated for the first time, by 29Si solid state NMR measurements and detailed DFT and ab initio calculations. 29Si solid state NMR spectroscopy of the precursor silylene 1 was also carried out. The experimental and computational study of 2 and 3, which was also supported by a detailed computational study of smaller model systems with Si═O and Si═S bonds, provides a deeper understanding of the isotropic and tensor components of their NMR chemical shifts. The general agreement between the experimental NMR spectra and the calculations strongly support our previous NMR assignment deduced from experiment. The calculations revealed that in 2 δ(29Si(═O))iso is shifted upfield relative to H2Si═O by as much as 175 ppm; the substituents are responsible for ca. 100 ppm of this shift, while the remaining upfield shift is caused by change in the coordination number from three to four at the Si═O moiety. The change in coordination number leads to a nearly cylindrical symmetry in the plane which is perpendicular to the Si═O molecular axis (δ11 ≈ δ22), in contrast to the significant anisotropy found in this plane in typical doubly bonded compounds. The change in r(Si═O) or in the degree of pyramidality at the Si═O center which accompanies the change in coordination number has practically no effect on the chemical shift. δ(29Si(═S))iso in 3 is shifted downfield significantly relative to that in 2, and a similar trend is found in smaller models with Si═S vs those with Si═O subunits. This downfield shift can be explained by the smaller σ−π* energy difference in the Si═S bond, relative to that of the Si═O bond. The NMR measurements of 2 and 3 having a four-coordinate silicon−chalcogen moiety, and the calculations of their tensor components, their bond polarities, and their Wiberg bond indices revealed that the Si═X moieties in both 2 and 3 have a significant π(Si═X) character; yet, in both molecules there is a substantial contribution from a zwitterionic Si+—X− resonance structure, which is more pronounced in 2.

J. Am. Chem. Soc., 2010, 132 (15), pp 5387–5393

Molecular Wheels as Nanoporous Materials: Differing Modes of Gas Diffusion through Ga10 and Ga18 Wheels Probed by Hyperpolarized 129Xe NMR Spectroscopy
Chi-Yuan Cheng, Theocharis C. Stamatatos, George Christou and Clifford R. Bowers

The study of crystals of molecular wheels as nanoporous materials is reported. Hyperpolarized 129Xe NMR spectroscopy has been used to characterize the mode of molecular diffusion and Xe interactions within the supramolecular nanochannels formed upon crystallization of the molecular wheels [Ga10(OMe)20(O2CMe)10] and [Ga18(pd)12(pdH)12(O2CMe)6(NO3)6](NO3)6. In agreement with expectations based on the collision diameter of the Xe atom relative to the differing internal diameters of the two types of gallium wheels, single-file diffusion occurs in the Ga10 channels, whereas in the Ga18 system the data are consistent with normal, Fickian diffusion. Information about the electronic environment inside the channels was probed by the Xe chemical shift. The interaction of the gas with the channel walls is found to be substantially stronger than the interaction in organic nanotubes and zeolites. The results establish the ability of crystals of molecular wheel compounds to function as a new class of porous nanotubular materials, and ones of a known and variable diameter, for studying the channel diameter dependence of molecular exchange and unidirectional diffusion on the micrometer length scale.

J. Phys. Chem. A, 2010, 114 (15), pp 5119–5131

A Solid-State 11B NMR and Computational Study of Boron Electric Field Gradient and Chemical Shift Tensors in Boronic Acids and Boronic Esters
Joseph W. E. Weiss and David L. Bryce

The results of a solid-state 11B NMR study of a series of 10 boronic acids and boronic esters with aromatic substituents are reported. Boron-11 electric field gradient (EFG) and chemical shift (CS) tensors obtained from analyses of spectra acquired in magnetic fields of 9.4 and 21.1 T are demonstrated to be useful for gaining insight into the molecular and electronic structure about the boron nucleus. Data collected at 21.1 T clearly show the effects of chemical shift anisotropy (CSA), with tensor spans (Ω) on the order of 10−40 ppm. Signal enhancements of up to 2.95 were achieved with a DFS-modified QCPMG pulse sequence. To understand the relationship between the measured tensors and the local structure better, calculations of the 11B EFG and magnetic shielding tensors for these compounds were conducted. The best agreement was found between experimental results and those obtained from GGA revPBE DFT calculations. A positive correlation was found between Ω and the dihedral angle (CCBO), which describes the orientation of the boronic acid/ester functional group relative to an aromatic system bound to boron. The small boron CSA is discussed in terms of paramagnetic shielding contributions as well as diamagnetic shielding contributions. Although there is a region of overlap, both Ω and the 11B quadrupolar coupling constants tend to be larger for boronic acids than for the esters. We conclude that the span is generally the most characteristic boron NMR parameter of the molecular and electronic environment for boronic acids and esters, and show that the values result from a delicate interplay of several competing factors, including hydrogen bonding, the value of CCBO, and the electron-donating or withdrawing substituents bound to the aromatic ring.

J. Am. Chem. Soc., 2010, 132 (16), pp 5779–5788

Proton-Evolved Local-Field Solid-State NMR Studies of Cytochrome b5 Embedded in Bicelles, Revealing both Structural and Dynamical Information
Ronald Soong, Pieter E. S. Smith, Jiadi Xu, Kazutoshi Yamamoto, Sang-Choul Im, Lucy Waskell and Ayyalusamy Ramamoorthy

Structural biology of membrane proteins has rapidly evolved into a new frontier of science. Although solving the structure of a membrane protein with atomic-level resolution is still a major challenge, separated local field (SLF) NMR spectroscopy has become an invaluable tool in obtaining structural images of membrane proteins under physiological conditions. Recent studies have demonstrated the use of rotating-frame SLF techniques to accurately measure strong heteronuclear dipolar couplings between directly bonded nuclei. However, in these experiments, all weak dipolar couplings are suppressed. On the other hand, weak heteronuclear dipolar couplings can be measured using laboratory-frame SLF experiments, but only at the expense of spectral resolution for strongly dipolar coupled spins. In the present study, we implemented two-dimensional proton-evolved local-field (2D PELF) pulse sequences using either composite zero cross-polarization (COMPOZER-CP) or windowless isotropic mixing (WIM) for magnetization transfer. These PELF sequences can be used for the measurement of a broad range of heteronuclear dipolar couplings, allowing for a complete mapping of protein dynamics in a lipid bilayer environment. Experimental results from magnetically aligned bicelles containing uniformly 15N-labeled cytochrome b5 are presented and theoretical analyses of the new PELF sequences are reported. Our results suggest that the PELF-based experimental approaches will have a profound impact on solid-state NMR spectroscopy of membrane proteins and other membrane-associated molecules in magnetically aligned bicelles.

J. Am. Chem. Soc., 2010, 132 (14), pp 5179–5185

Elucidation of Structure and Dynamics in Solid Octafluoronaphthalene from Combined NMR, Diffraction, And Molecular Dynamics Studies
Andrew J. Ilott, Sebastian Palucha, Andrei S. Batsanov, Mark R. Wilson and Paul Hodgkinson

X-ray diffraction (XRD), molecular dynamics simulations (MD), and 19F NMR have been used to investigate structure and dynamics in solid octafluoronaphthalene, C10F8. Two distinct processes are observed via measurements of 19F relaxation times as a function of temperature; a faster process from T1 relaxation with a correlation time of the order of ns at ambient temperature (fitting to Arrhenius-type parameters Ea = 20.6 ± 0.4 kJ mol−1 and τ0 = 8 ± 1 × 10−14 s) and a much slower process from T1ρ relaxation with a correlation time of the order of μs (fitting to Ea = 55.1 ± 1.3 kJ mol−1 and τ0 = 4 ± 2 × 10−16 s). Atomistic molecular dynamics reveals the faster process to involve a small angle jump of 40° of the molecules, which is in perfect agreement with the X-ray diffraction study of the material at ambient temperature. The MD study reveals the existence of more extreme rotations of the molecules, which are proposed to enable the full rotation of the octafluoronaphthalene molecules. This explains both the T1ρ results and previous wide-line 19F NMR studies. The experimental measurements (NMR and XRD) and the MD computations are found to be strongly complementary and mutally essential. The reasons why a process on the time scale of microseconds, and associated with such a large activation barrier, can be accessed via classical molecular dynamics simulations are also discussed.

J. Am. Chem. Soc., 2010, 132 (15), pp 5357–5363

Sensitivity Enhanced Heteronuclear Correlation Spectroscopy in Multidimensional Solid-State NMR of Oriented Systems via Chemical Shift Coherences
T. Gopinath, Nathaniel J. Traaseth, Kaustubh Mote and Gianluigi Veglia

We present new sensitivity enhanced schemes for heteronuclear correlation spectroscopy (HETCOR) in solid-state NMR of oriented systems. These schemes will enhance the sensitivity of the HETCOR by 40% for the two-dimensional experiments (SE-HETCOR) and up to 180% for the 3D HETCOR-separated local field version (SE-PISEMAI-HETCOR). The signal enhancement is demonstrated for a single crystal of (15N)N-acetylleucine and the integral membrane protein sarcolipin oriented in lipid bicelles. These methods will significantly reduce the time needed to acquire multidimensional experiments for membrane proteins oriented in magnetically or mechanically aligned lipid bilayers as well as liquid crystalline materials.

J. Am. Chem. Soc., 2010, 132 (14), pp 5143–5155

Solid-State 17O NMR and Computational Studies of C-Nitrosoarene Compounds
Gang Wu, Jianfeng Zhu, Xin Mo, Ruiyao Wang and Victor Terskikh

We report the first solid-state 17O NMR determination of the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for four 17O-labeled C-nitrosoarene compounds: p-[17O]nitroso-N,N-dimethylaniline ([17O]NODMA), SnCl2(CH3)2([17O]NODMA)2, ZnCl2([17O]NODMA)2, and [17O]NODMA·HCl. The 17O quadrupole coupling constants (CQ) observed in these C-nitrosoarene compounds are on the order of 10−15 MHz, among the largest values found to date for organic compounds. The 17O CS tensor in these compounds exhibits remarkable sensitivity toward the nitroso bonding scheme with the chemical shift anisotropy (δ11 − δ33) ranging from just 350 ppm in [17O]NODMA·HCl to over 2800 ppm in [17O]NODMA. This latter value is among the largest 17O chemical shift anisotropies reported in the literature. These extremely anisotropic 17O NMR interactions make C-nitrosoarene compounds excellent test cases that allow us to assess the detection limit of solid-state 17O NMR. Our results suggest that, at 21.14 T, solid-state 17O NMR should be applicable to all oxygen-containing organic functional groups. We also show that density functional theory (DFT) calculations can reproduce reasonably well the experimental 17O QC and CS tensors for these challenging molecules. By combining quantum chemical calculations with experimental solid-state 17O NMR results, we are able to determine the 17O QC and CS tensor orientations in the molecular frame of reference for C-nitrosoarenes. We present a detailed analysis illustrating how magnetic field-induced mixing between individual molecular orbitals (MOs) contributes to the 17O shielding tensor in C-nitrosoarene compounds. We also perform a Townes−Dailey analysis for the observed 17O QC tensors and show that 17O CS and QC tensors are intrinsically related through the π bond order of the N═O bond. Furthermore, we are able for the first time to examine the parallelism between individual 17O and 15N CS tensor components in C-nitrosoarenes.

J. Am. Chem. Soc., 2010, 132 (13), pp 4653–4668

DFT-NMR Investigation and 51V 3QMAS Experiments for Probing Surface OH Ligands and the Hydrogen-Bond Network in a Polyoxovanadate Cluster: The Case of Cs4[H2V10O28]·4H2O

Lionel A. Truflandier, Florent Boucher, Christophe Payen, Redouane Hajjar, Yannick Millot, Christian Bonhomme and Nathalie Steunou

This work shows that the combination of first-principles calculations and 51V NMR experiments is a powerful tool to elucidate the location of surface hydroxyl groups and to precisely describe the hydrogen bond network in the complex decavanadate cluster Cs4[H2V10O28].4H2O, enhancing the strength of NMR crystallography. The detailed characterization of H-bond networks for these kinds of inorganic compounds is of primary importance and should benefit from the DFT-NMR predictions by considering explicitly the periodic boundary conditions. The determination of the Cs4[H2V10O28]·4H2O structure by single-crystal X-ray diffraction was not sufficiently accurate to provide the location of protons. From available diffraction data, five different protonated model structures have been built and optimized using DFT-based methods. The possible interconversion of two decavanadate isomers through a proton exchange is evaluated by calculating the energy barrier and recording variable-temperature 1H MAS NMR spectra. First-principles calculations of 51V NMR parameters clearly indicate that these parameters are very sensitive to the local intermolecular hydrogen-bonding interactions. Considering the DFT error limits, the fairly good agreement between calculated and experimental NMR parameters arising from the statistical modeling of the data allows the unambiguous assignment of the five 51V NMR signals and, thus, the location of OH surface ligands in the decavanadate cluster. In particular, first-principles calculations accurately reproduce the 51V quadrupolar parameters. These results are fully consistent with 51V 3QMAS NMR spectra recorded with and without 1H decoupling. Finally, correlations are established between local octahedral VO6 deformations and 51V NMR parameters (Cq and Δδ), which will be useful for the characterization of a wide range of chemical species containing vanadium(V).

Friday, April 16, 2010

Journal of Physical Chemistry B and C, Vol. 114, Issues 14 and 15

7Li NMR Knight Shifts in Li−Sn Compounds: MAS NMR Measurements and Correlation with DFT Calculations

Emilie Bekaert†, Florent Robert‡, Pierre Emmanuel Lippens‡ and Michel Mntrier*†

J. Phys. Chem. C, 2010, 114 (14), pp 6749–6754
DOI: 10.1021/jp100365u

Abstract: Several Li−Sn crystalline phases, LiSn, Li7Sn3, Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5, were prepared by ball-milling and studied by 7Li MAS NMR spectroscopy with silica as a diluting agent to avoid field penetration limitations. All phases except for LiSn exhibit exchanged NMR signals at room temperature for the various types of Li present in the unit cells, in the 10 to 100 ppm range. Electronic structure calculations based on first-principles method led to a rather good correlation between the participation of the Li 2s orbital to the density of states (DOS) at the Fermi level and the corresponding NMR Knight shift for the two Li crystallographic types in the case of LiSn, and for the weighted average of the different crystallographic types in the case of the NMR-exchanged signals for the other compounds.

A Novel Phase Transformation Phenomenon in Mesostructured Aluminophosphate

Wanling Shen†, Shenhui Li†, Jun Xu†, Hailu Zhang†, Wei Hu†, Dan Zhou‡, Jianan Zhang‡, Jihong Yu‡, Wujun Xu§, Yao Xu§ and Feng Deng*†

J. Phys. Chem. C, 2010, 114 (15), pp 7076–7084
DOI: 10.1021/jp911959u

Abstract: A novel phase transformation phenomenon that involves two successive phase transformation events was found for the first time in the synthesis of mesostructured aluminophosphate and studied by XRD, TEM, and multinuclear solid-state NMR techniques. The results showed that a hexagonal phase Hex and two lamellar phases, L1 and L2, were formed after hydrothermal treatment for 1, 3, and 50 h, respectively. The status of the surfactant was found to be arrayed interdigitated in a bilayer with a tilt angle in L1 phase but upright in L2 phase. A mechanism that the exciting of the alkane tail of the surfactant together with the condensation of aluminophosphate cooperatively promoted the phase transformation was proposed for the observed phenomenon. Additionally, a ZON microporous structure was found for the first time existing in the framework of the mesostructured aluminophosphate.

Tuesday, April 13, 2010

Solid State Nuclear Magnetic Resonance

The partial 1H NMR spectra of Al-OH and H2Omol in hydrous aluminosilicate glasses: Component-Resolved analysis of 27Al-1H cross polarization and 1H spin-echo spectra

Publication year: 2010
Source: Solid State Nuclear Magnetic Resonance, In Press, Accepted Manuscript, Available online 9 April 2010
Wim J., Malfait , Xianyu, Xue
The Component-Resolved methodology was applied to 1H spin-echo and 27Al-1H cross polarization data of aluminosilicate glasses. The method was able to resolve two components with different T2 relaxation rates, hydroxyl groups (OH) and molecular water (H2Omol), from the spin-echo data and to determine partial spectra and the relative abundances of OH and H2Omol. The algorithm resolved two to three components with different 27Al-1H CP dynamics from the 27Al-1H cross polarization data; the obtained partial NMR spectra for Al-OH are in excellent agreement with those obtained previously from the difference spectra and confirm previous quantitative results and models for the Al-OH, Si-OH and H2Omol speciation (Malfait and Xue, 2010).

Nuclear hyperpolarization in solids and the prospects for nuclear spintronics

Publication year: 2010
Source: Solid State Nuclear Magnetic Resonance, In Press, Accepted Manuscript, Available online 7 April 2010
Jeffrey A., Reimer
Nuclear hyperpolarization can be achieved in a number of ways. This article focuses on the use of coupling of nuclei to (nearly) pure quantum states, with particular emphasis on those states obtained by optical excitation in bulk semiconductors. I seek an answer to this question: ”What is to prevent the design and analysis of nuclear spintronics devices that use the extremely long-lived hyperpolarized nuclear spin states, and their weak couplings to each other, to affect computation, memory, or informational technology schemes?” The answer, I argue, is in part because there remains a lack of fundamental understanding of how to generate and control nuclear polarization with schemes other than with rf coils.

Tuesday, April 06, 2010

PCCP, vol. 12, Issues 10 - 15

Phys. Chem. Chem. Phys., 2010, 12, 2895 - 2914, DOI: 10.1039/b915587b

The conformation and orientational order of a 1,2-disubstituted ethane nematogenic molecule (I22) in liquid crystalline and isotropic phases studied by NMR spectroscopy

James W. Emsley, Philippe Lesot, Anne Lesage, Giuseppina De Luca, Denis Merlet and Giuseppe Pileio

The structure, conformation and orientational order of the mesogen I22 have been studied by proton, carbon-13 and deuterium 1D and 2D-NMR spectroscopies at natural abundance and at various magnetic fields when in the nematic phase, the isotropic phase close to the nematic-isotropic phase transition, and as a solute in the chiral nematic solution comprised of the polypeptide PBLG dissolved in chloroform. It is concluded that 95% of conformers have a trans arrangement about the central C–C bond of the ethane fragment in all phases

Phys. Chem. Chem. Phys., 2010, 12, 2989 - 2998, DOI: 10.1039/b924666e

Dynamics on the microsecond timescale in hydrous silicates studied by solid-state 2H NMR spectroscopy

John M. Griffin, Andrew J. Miller, Andrew J. Berry, Stephen Wimperis and Sharon E. Ashbrook

Solid-state 2H NMR spectroscopy has been used to probe the dynamic disorder of hydroxyl deuterons in a synthetic sample of deuterated hydroxyl-clinohumite (4Mg2SiO4·Mg(OD)2), a proposed model for the incorporation of water within the Earths mantle. Both static and magic angle spinning (MAS) NMR methods were used. Static 2H NMR appears to reveal little evidence of the dynamic process, yielding results similar to those obtained from deuterated brucite (Mg(OD)2), where no dynamics on the relevant timescale are expected to be present. However, in 2H MAS NMR spectra, considerable line broadening is observed for hydroxyl-clinohumite and a 2H double-quantum (DQ) MAS NMR spectrum confirms that this is due to motion on the microsecond timescale. Using a model for dynamic exchange of the hydroxyl deuterons between two sites identified in previous diffraction studies, first-principles density functional theory (DFT) calculations of 2H (spin I = 1) quadrupolar NMR parameters, and a simple analytical model for dynamic line broadening in MAS NMR experiments, we were able to reproduce the observed motional line broadening and use this to estimate a rate constant for the dynamic process. From analysis of the observed 2H linewidths in variable-temperature MAS experiments, an activation energy for the exchange process was also determined. A simulated static 2H NMR lineshape based on our dynamic model is consistent with the observed experimental static NMR spectrum, confirming that the motion present in this system is not easily detectable using a static NMR approach. Finally, a 2H DQMAS NMR spectrum of fluorine-substituted 2H-enriched hydroxyl-clinohumite shows how the dynamic exchange process is inhibited by O–DF- hydrogen-bonding interactions.

Phys. Chem. Chem. Phys., 2010, 12, 3254 - 3259, DOI: 10.1039/b925326b

14N NQR and proton NMR study of ferroelectric phase transition and proton exchange in organic ferroelectric (H2-TPPZ)(Hca)2

Janez Seliger, Veselko agar, Tetsuo Asaji and Yumi Hasegawa

The complete 14N nuclear quadrupole resonance spectrum has been measured in ferroelectric (H2-TPPZ)(Hca)2 using nuclear quadrupole double resonance. The quadrupole coupling tensors are assigned to various nitrogen positions in the crystal structure. Two types of asymmetric N–H+N hydrogen bonds are observed in the ferroelectric phase. A slow dynamics influencing the 14N NQR spectrum and relaxation has been observed in the paraelectric phase. The analysis of the 14N NQR spectra in the paraelectric phase shows that above Tc each hydrogen bond exchanges between the two types observed in the ferroelectric phase. The change of the type of hydrogen bond is associated with the transfer of protons within the bond.

Phys. Chem. Chem. Phys., 2010, 12, 3895 - 3903, DOI: 10.1039/b915401a

Extra-framework aluminium species in hydrated faujasite zeolite as investigated by two-dimensional solid-state NMR spectroscopy and theoretical calculations

Shenhui Li, Anmin Zheng, Yongchao Su, Hanjun Fang, Wanling Shen, Zhiwu Yu, Lei Chen and Feng Deng

Extra-framework aluminium (EFAL) species in hydrated dealuminated HY zeolite were thoroughly investigated by various two-dimensional solid-state NMR techniques as well as density functional theoretical calculations. 27Al MQ MAS NMR experiments demonstrated that five-coordinated and four-coordinated extra-framework aluminium subsequently disappeared with the increase of water loading, and the quadrupole interaction of each aluminium species decreased gradually during the hydration process. 1H double quantum MAS NMR revealed that the EFAL species in the hydrated zeolite consisted of three components: a hydroxyl AlOH group, and two types of water molecule (rigid and mobile water). 1H–27Al LG-CP HETCOR experiments indicated that both the extra-framework and the framework Al atoms were in close proximity to the rigid water in the fully rehydrated zeolite. The experimental results were further confirmed by DFT theoretical calculations. Moreover, theoretical calculation results further demonstrated that the EFAL species in the hydrated zeolite consisted of the three components and the calculated 1H NMR chemical shift for each component agreed well with our NMR observations. It is the rigid water that connects the extra-framework aluminium with the four-coordinated framework aluminium through strong hydrogen bonds

Monday, April 05, 2010

J. Mag. Reson.

Optimal transfer of spin-order between a singlet nuclear pair and a heteronucleus

from ScienceDirect Publication: Journal of Magnetic Resonance
Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 6 March 2010
Stephen, Kadlecek , Kiarash, Emami , Masaru, Ishii , Rahim, Rizi
Intramolecular spin-order transfer is a useful technique for signal enhancement of insensitive and low-concentration molecular species. We present a closed-form, optimized pulse sequence which maximizes the efficiency of transfer between a singlet (para) nuclear pair and a vicinal heteronucleus. Neglecting the decay of coherences while the nuclei are in the transverse plane, the scheme is unity efficient for all combinations of internuclear scalar couplings. Efficiency loss due to T2-like decay is also minimized by keeping the sequence as short as possible. We expect this result to be useful for hyperpolarization experiments in which the spin-order originates in parahydrogen, as well as studies of singlet state decay aimed at longer-term storage of spin-order in hyperpolarized Magnetic Resonance Imaging.

Carbon-13 NOESY and equivalent protons: methyl iodide dynamics

from ScienceDirect Publication: Journal of Magnetic Resonance
Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript,Dmytro Kotsyubynskyy, Jozef Kowalewski, Pekka Tallavaara, Ville-Veikko Telkki, Jukka Jokisaari,and Evgeny Polyakov
We have shown that proton-coupled carbon-13 2D NOESY experiments, performed on degenerate spin systems, can provide unique quantitative information about anisotropic reorientational motions and molecular geometry. Relevant theory for AX2 and AX3 spin systems is presented, assuming the dipole–dipole and random field relaxation mechanisms of 13C nucleus, and demonstrated on methyl iodide solution in chloroform. Agreement with experimental intensities of all the six independent peaks is very good in the whole range of mixing times (up to 45 s).

Quantitative determination of NOE rates in perdeuterated and protonated proteins: practical and theoretical aspects

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 15 March 2010
Beat, Vögeli , Michael, Friedmann , Dominik, Leitz , Alexander, Sobol , Roland, Riek
Precision and accuracy are the limiting factors in extracting structural and dynamic information from experimental NOEs. In this study, error sources at all stages of such an analysis are identified and errors are estimated. The data set of HN-HN cross-relaxation rates obtained from triple labeled ubiquitin presented in [Vögeli, B.; Segawa, T.F.; Leitz, D., Sobol, A.; Choutko, A.; Trzesniak, D.; van Gunsteren, W.; Riek, R., J. Am. Chem. Soc. 131 (47), 17215–17225, 2009] is extended to rates obtained from a double labeled sample. Analog data sets are presented for GB3. It is shown that quantitative NOE rates can be determined with high accuracy from both triple-labeled as well as double-labeled samples. The quality of experimental cross-relaxation rates obtained from 3D HXQC-NOESY and NOESY-HXQC experiments is discussed. It is shown that NOESY-HXQC experiments provide rates of the same quality as HXQC-NOESY if both diagonal and cross peaks for a spin pair can be resolved. Expressions for cross-relaxation rates for anisotropically tumbling molecules exhibiting fast and slow motion are derived. The impact of anisotropy on the prediction of cross-relaxation rates and on the conversion of experimental rates into effective distances is discussed. For molecules with anisotropy DII/Dperpendicular up to 5 the distance error is smaller than 2%. Finally, “averaged order parameters” are calculated for specific secondary structural elements showing similar trends for ubiquitin and GB3.

NMR q-Space Imaging of Macroscopic Pores using Singlet Spin States

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 19 March 2010
Nirbhay N., Yadav , Allan M., Torres , William S., Price
NMR q-space imaging is a powerful non-invasive technique used to determine structural characteristics of pores in applications ranging from medical to material science. To date, the application of q-space imaging has primarily been limited to microscopic pores in part because of limitations of the effective observation time due to relaxation. Here we report on the use of singlet spin states for NMR q-space imaging, which allow significantly greater observation times. This opens the way for studying larger pores in materials such as biological tissue, emulsions, and rocks.

Quantitative Rapid Scan EPR Spectroscopy at 258 MHz

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 21 March 2010
Richard W., Quine , George A., Rinard , Sandra S., Eaton , Gareth R., Eaton
Experimental data obtained with an electron paramagnetic resonance (EPR) rapid scan spectrometer were translated through the reverse transfer functions of the spectrometer hardware to the sample position. Separately, theoretical calculations were performed to predict signal and noise amplitudes at the sample position for specified experimental conditions. A comparison was then made between the translated experimental values and the calculated values. Excellent agreement was obtained.

Solid State Nuclear Magnetic Resonance

Controversy about the reference compound for correct chemical shift determination of 195Pt nuclei

Publication year: 2010
Source: Solid State Nuclear Magnetic Resonance, In Press, Accepted Manuscript, Available online 27 March 2010
Bogdan, Nowak
Several groups exploring the 195Pt NMR in solids, including metallic and magnetic materials, use different standards for chemical shift (Knight shift) determination. Commonly applied H2PtCl6 and Na2PtCl6 (IUPAC δ scale) lead to considerable underestimation of the shifts since H2PtCl6 has considerable own 195Pt NMR shift due to its Van Vleck paramagnetism.In this Letter new results on 195Pt NMR in heavy fermion system CeInPt4 are presentedand rationalized scale for the Knight shift determination is discussed.

Off-resonance effects in two-dimensional exchange NMR at zero- field

Publication year: 2010
Source: Solid State Nuclear Magnetic Resonance, In Press, Accepted Manuscript, Available online 15 March 2010
Mariusz, Maćkowiak , Nicolay, Sinyavsky

The effects of off-resonance irradiation in 2D exchange NMR at zero-field are analysed. A theoretical treatment of the 2D exchange NMR pulse sequence is presented and applied to the quantitative study of exchange processes in molecular crystals. It takes into account the off-resonance irradiation, which critically influences the spin dynamics. The response of a system of spins I=3/2 in zero applied field to the three-pulse sequence is analysed. The mixing dynamics by exchange and the expected cross-peak intensities as a function of the frequency offset has been derived. It is shown that the off-resonance effects are of crucial importance for the quantitative description of the exchange spectra. The theory is successfully tested for the exchange spectra of hindered trichloromethyl groups of p-chloroanilinium trichloroacetate, where the conventional approach without taking into account the off-resonance phenomena has failed.

Thursday, April 01, 2010

J. Phys. Chem. B and C, v114, Issues 13

Distinguishing Polymorphs of the Semiconducting Pigment Copper Phthalocyanine by Solid-State NMR and Raman Spectroscopy

Medhat A. Shaibat†, Leah B. Casabianca†, Diana Y. Siberio-Prez§‡, Adam J. Matzger*‡ and Yoshitaka Ishii*†

Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, and Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109
J. Phys. Chem. B, 2010, 114 (13), pp 4400–4406

Abstract: Cu(II)(phthalocyanine) (CuPc) is broadly utilized as an archetypal molecular semiconductor and is the most widely used blue printing pigment. CuPc crystallizes in six different forms; the chemical and physical properties are substantially modulated by its molecular packing among these polymorphs. Despite the growing importance of this system, spectroscopic identification of different polymorphs for CuPc has posed difficulties. This study presents the first example of spectroscopic distinction of α- and β-forms of CuPc, the most widely used polymorphs, by solid-state NMR (SSNMR) and Raman spectroscopy. 13C high-resolution SSNMR spectra of α- and β-CuPc using very-fast magic angle spinning (VFMAS) at 20 kHz show that hyperfine shifts sensitively reflect polymorphs of CuPc. The experimental results were confirmed by ab initio chemical shift calculations. 13C and 1H SSNMR relaxation times of α- and β-CuPc under VFMAS also showed marked differences, presumably because of the difference in electronic spin correlation times in the two forms. Raman spectroscopy also provided another reliable method of differentiation between the two polymorphs.