Friday, May 30, 2008

ASAP Chem. Mater., ASAP Article,

Characterization of Layered γ-Titanium Phosphate (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O Intercalate: A Combined NMR, Synchrotron XRD, and DFT Calculations Study
Luís Mafra* and João Rocha*

Organically templated titanium phosphate, (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O, has been prepared by hydrothermal synthesis from titanium(IV) chloride, phosphoric acid, and ethylamine. The structure of this material has been characterized by synchrotron X-ray powder diffraction, 1H, 13C, 15N, and 31P (including 1H{FS-LG}-1H HOMCOR, 1H{FS-LG}-31P HETCOR, 1H-1H DQ-SQ, 13C{31P} REDOR) MAS NMR, and FTIR spectroscopies and thermal analyses (TG and DSC). Its triclinic structure was solved in the space group P1 (No. 1) with the following final unit cell parameters: a = 5.1126(1), b = 6.3189(2), c = 12.0396(5) Å, α = 100.931(2), β = 97.211(2), γ = 90.683(3)°, and V = 378.62(2) Å3 (Z = 1). This pseudo-three-dimensional compound is built up of anionic titanium phosphate layers, similar to those present in the γ-type titanium phosphate, and ethylammonium cations residing in the interlayer one-dimensional channels. On the basis of empirical (particularly NMR and powder XRD) and theoretical (DFT calculations using plane waves basis set) data, the 1H NMR spectrum has been assigned, and evidence was found for a very strong interlayer P−O···H···O−P interaction.

Monday, May 26, 2008

Applied Magnetic Resonance - Vol 33 Number 4

First-Principles Calculation of Electric Field Gradients in Metals, Semiconductors, and Insulators
J. W. Zwanziger and M. Torrent
A scheme for computing electric field gradients within the projector augmented wave (PAW) formalism of density functional theory is presented. On the basis of earlier work (M. Profeta, F. Mauri, C.J. Pickard, J. Am. Chem. Soc. 125, 541, 2003) the present implementation handles metallic cases as well as insulators and semiconductors with equal efficiency. Details of the implementation, as well as applications and the discussion of the limitations of the PAW method for computing electric field gradients are presented.

A Quantum-Chemical Investigation of the Geometry and NMR Chemical Shifts of Bilirubin
T. Metzroth, M. Lenhart and J. Gauss
A computational investigation using density-functional-theory methods has been performed concerning the structure and nuclear magnetic resonance (NMR) chemical shifts of bilirubin with a special emphasis on the hydrogen bonds. Solid-state effects on the NMR spectra are investigated by considering a trimeric model derived from the available X-ray structure. Satisfactory agreement between theory and experiment is found with ring-current effects playing only a minor role for the interpretation of the solid-state NMR spectra.

Journal of Magnetic Resonance - Vol 192 Issue 2

Indirect determination of chemical shift by coupling evolution during adiabatic pulses
Peter W.A. Howe

The use of adiabatic 180° X-pulses within INEPT refocusing periods results in chemical shift-dependent evolution of J-couplings. This has been viewed as a disadvantage and several methods of overcoming it have been suggested. This article shows that there is the potential to use this chemical shift dependence to determine heteronuclear chemical shift without a heteronuclear evolution time. In this way, it possible to estimate heteronuclear chemical shift indirectly from a single one-dimensional proton-observe spectrum and determine it with high accuracy from a extensively-folded two-dimensional proton-observe spectrum.

Sensitivity enhancement of the central-transition signal of half-integer spin quadrupolar nuclei in solid-state NMR: Features of multiple fast amplitude-modulated pulse transfer
Mithun Goswami
and P.K. Madhu
Sensitivity enhancement of solid-state NMR spectrum of half-integer spin quadrupolar nuclei under both magic-angle spinning (MAS) and static cases has been demonstrated by transferring polarisation associated with satellite transitions to the central m = −1/2 → 1/2 transition with suitably modulated radio-frequency pulse schemes. It has been shown that after the application of such enhancement schemes, there still remains polarisation in the satellite transitions that can be transferred to the central transition. This polarisation is available without having to wait for the spin system to return to thermal equilibrium. We demonstrate here the additional sensitivity enhancement obtained by making use of this remaining polarisation with fast amplitude-modulated (FAM) pulse schemes under both MAS and static conditions on a spin-3/2 and a spin-5/2 system. Considerable signal enhancement is obtained with the application of the multiple FAM sequence, denoted as m-FAM. We also report here some of the salient features of these multiple FAM sequences with respect to the nutation frequency of the pulses and the spinning frequency.

MAS NMR spectra of quadrupolar nuclei in disordered solids: The Czjzek model
Jean-Baptiste d’Espinose de Lacaillerie
Christian Fretigny and Dominique Massiot
Structural disorder at the scale of two to three atomic positions around the probe nucleus results in variations of the EFG and thus in a distribution of the quadrupolar interaction. This distribution is at the origin of the lineshape tailing toward high fields which is often observed in the MAS NMR spectra of quadrupolar nuclei in disordered solids. The Czjzek model provides an analytical expression for the joint distribution of the NMR quadrupolar parameters υQ and η from which a lineshape can be predicted. This model is derived from the Central Limit Theorem and the statistical isotropy inherent to disorder. It is thus applicable to a wide range of materials as we have illustrated for 27Al spectra on selected examples of glasses (slag), spinels (alumina), and hydrates (cement aluminum hydrates). In particular, when relevant, the use of the Czjzek model allows a quantitative decomposition of the spectra and an accurate extraction of the second moment of the quadrupolar product. In this respect, it is important to realize that only rotational invariants such as the quadrupolar product can make sense to describe the quadrupolar interaction in disordered solids.

Friday, May 23, 2008

Al's Journal Update

Synthesis, NMR and Vibrational Spectroscopic Characterization, and Computational Study of the cis-IO2F32− Anion
Inorg. Chem., 47 (8), 3243–3247

Johnathan P. Mack, Jerry A. Boatz, and Michael Gerken
The N(CH3)4+ salt of the cis-IO2F32− anion was synthesized from [N(CH3)4][IO2F2] and excess [N(CH3)4][F] in CH3CN solvent. The [N(CH3)4]2[IO2F3] salt was characterized by Raman, infrared, and 19F solid-state MAS NMR spectroscopy. Geometry optimization and calculation of the vibrational frequencies at the DFT level of theory corroborated the experimental finding that the IO2F32− anion exists as a single isomer with a cis-dioxo and mer-trifluoro arrangement. The fluorine atom in IO2F32− that is trans to one of the oxygen atoms is weakly bound with a calculated bond length of 228.1 pm. The IO2F32− anion is only the second example of an AEO2F3 species after XeO2F3−.

DFT Calculation of 1J(119Sn,13C) and 2J(119Sn,1H) Coupling Constants in Di- and Trimethyltin(IV) Compounds
Inorg. Chem., 47 (11), 4796–4807, 200
Girolamo Casella, Francesco Ferrante, and Giacomo Saielli
We have tested several computational protocols, at the nonrelativistic DFT level of theory, for the calculation of 1J(119Sn,13C) and 2J(119Sn,1H) spin−spin coupling constants in di- and trimethyltin(IV) derivatives with various ligands. Quite a good agreement with experimental data has been found with several hybrid functionals and a double-ζ basis set for a set of molecules comprising tetra-, penta-, and hexa-coordinated tin(IV). Then, some of the protocols have been applied to the calculation of the 2J(119Sn,1H) of the aquodimethyltin(IV) ion and dimethyltin(IV) complex with D-ribonic acid and to the calculation of 1J(119Sn,13C) and 2J(119Sn,1H) of the dimethyltin(IV)−glycylglycine and glycylhistidine complexes in water solutions. Solvent effects have been considered in these cases by including explicit water molecules and/or the solvent reaction field, resulting in a good agreement with experimental data. The proposed protocols constitute a helpful tool for the structural determination of di- and triorganotin(IV) derivatives.

67Zn Solid-State NMR Spectroscopy of {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3]. The Importance of the Anion [HOB(C6F5)3]−
Inorg. Chem., 47 (12), 5184–5189, 2008
Andrew S. Lipton, Melissa M. Morlok, Gerard Parkin, and Paul D. Ellis
One of the paradigms of Zn2+ metallobiochemistry is that coordination of water to Zn2+ provides a mechanism of activation that involves lowering the pKa by approximately 7 pH units. This idea has become central to the development of mechanisms of action for zinc metalloproteins. However, the direct measurement of the pKa of water bound to Zn2+ in a metalloprotein has yet to be accomplished. Developing models for Zn2+−OH2 species has been a significant challenge, but we have utilized solid-state 67Zn NMR spectroscopy as a means to characterize one of the few examples of water bound to mononuclear tetrahedral Zn2+: {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3]. The measured quadrupole coupling (Cq) constant is 4.3 MHz with an asymmetry parameter of ηq of 0.6. Likewise, due to the small value of Cq, anisotropic shielding also contributed to the observed 67Zn NMR lineshape. As expected, the computed values of the magnetic resonance parameters depend critically on the nature of the anion. The predicted value of Cq for {[TpBut,Me]Zn(OH2)}[HOB(C6F5)3] is –4.88 MHz. We discuss the results of these calculations in terms of the nature of the anion, the local electrostatics, and its subsequent hydrogen bonding to [TpBut,Me]Zn(OH2)+.

NMR implementation of adiabatic SAT algorithm using strongly modulated pulses
J. Chem. Phys. 128, 124110 (2008)

Avik Mitra, T. S. Mahesh, and Anil Kumar
NMR implementation of adiabatic algorithms face severe problems in homonuclear spin systems since the qubit selective pulses are long and during this period, evolution under the Hamiltonian and decoherence cause errors. The decoherence destroys the answer as it causes the final state to evolve to mixed state and in homonuclear systems, evolution under the internal Hamiltonian causes phase errors preventing the initial state to converge to the solution state. The resolution of these issues is necessary before one can proceed to implement an adiabatic algorithm in a large system where homonuclear coupled spins will become a necessity. In the present work, we demonstrate that by using “strongly modulated pulses” (SMPs) for the creation of interpolating Hamiltonian, one can circumvent both the problems and successfully implement the adiabatic SAT algorithm in a homonuclear three qubit system. This work also demonstrates that the SMPs tremendously reduce the time taken for the implementation of the algorithm, can overcome problems associated with decoherence, and will be the modality in future implementation of quantum information processing by NMR.

Spin dynamics in the modulation frame: Application to homonuclear recoupling in magic angle spinning solid-state NMR
J. Chem. Phys. 128, 124503 (2008)

Gaël De Paëpe, Józef R. Lewandowski, and Robert G. Griffin
We introduce a family of solid-state NMR pulse sequences that generalizes the concept of second averaging in the modulation frame and therefore provides a new approach to perform magic angle spinning dipolar recoupling experiments. Here, we focus on two particular recoupling mechanisms—cosine modulated rotary resonance (CMpRR) and cosine modulated recoupling with isotropic chemical shift reintroduction (COMICS). The first technique, CMpRR, is based on a cosine modulation of the rf phase and yields broadband double-quantum (DQ) 13C recoupling using >70 kHz ω1,c/2π rf field for the spinning frequency ωr/2=10–30 kHz and 1H Larmor frequency ω0,H/2π up to 900 MHz. Importantly, for p ≥5, CMpRR recouples efficiently in the absence of 1H decoupling. Extension to lower p values (3.5 ≤ p <>

Analyzing NMR shielding tensors calculated with two-component relativistic methods using spin-free localized molecular orbitals
J. Chem. Phys. 128, 164112 (2008)

Jochen Autschbach
A recently developed analysis method [J. Chem. Phys. 127, 124106 (2007)] for NMR spin-spin coupling constants employing two-component (spin-orbit) relativistic density functional theory along with scalar relativistic natural localized molecular orbitals (NLMOs) and natural bond orbitals (NBOs) has been extended for analyzing NMR shielding tensors. Contributions from a field-dependent basis set (gauge-including atomic orbitals) have been included in the formalism. The spin-orbit NLMO/NBO nuclear magnetic shielding analysis has been applied to methane, plumbane, hydrogen iodide, tetracholoplatinate(II), and hexachloroplatinate(IV).

Measuring distances between half-integer quadrupolar nuclei and detecting relative orientations of quadrupolar and dipolar tensors by double-quantum homonuclear dipolar recoupling nuclear magnetic resonance experiments
J. Chem. Phys. 128, 204503 (2008)
Gregor Mali, Venčeslav Kaučič, and Francis Taulelle

We studied the possibility of using double-quantum homonuclear dipolar recoupling magic angle spinning nuclear magnetic resonance experiments for structural analysis of systems of half-integer quadrupolar nuclei. We investigated symmetry-based recoupling schemes R221 and R221R22-1 and showed that the obtained double-quantum filtered signals depend substantially on magnitudes and relative orientations of dipolar and quadrupolar tensors. Experimental results measured on aluminophosphate molecular sieve AlPO4-14, containing dipolar-coupled spin-5/2 aluminum nuclei, were compared to results of time-consuming numerical simulations. The comparison for short mixing times allowed us to roughly measure internuclear Al–Al distances, if constraints about relative tensor orientations were available. Inspection of relative orientations of dipolar and quadrupolar tensors, using known distances between nuclei, required experimental and simulated data for long mixing times and yielded less accurate results. Two experimental protocols were employed for measuring double-quantum filtered curves, the symmetric protocol, in which excitation and reconversion periods are incremented simultaneously, and the asymmetric protocol, in which only the length of the excitation period is incremented and the length of the reconversion period is kept constant. The former experimental protocol was more convenient for the detection of internuclear distances, and the latter one was more appropriate for the inspection of relative orientations of interaction tensors.

Structural ordering in CdxPb1−xF2 alloys: A combined molecular dynamics and solid state NMR study
J. Chem. Phys. 128, 224705 (2008)

Adalberto Picinin, Rashmi R. Deshpande, Andrea S. S. de Camargo, José Pedro Donoso, José Pedro Rino, Hellmut Eckert, and Maurício A. P. Silva
Molecular dynamics (MD) simulations of binary CdxPb1−xF2 alloys have been carried out, using a two-body Buckingham interaction potential, leading to a correct description of structural properties as a function of composition and pointing towards an understanding of the eutectic phenomenon. The simulation data can be analyzed in terms of five local fluorine environments Q(n) (4≥n≥0), where n is the number of Pb nearest-neighbor environments. The results suggest a highly nonstatistical population distribution, suggesting an intrinsic phase segregation tendency in the undercooled melt, during the cooling process. This prediction has been tested experimentally for six representative compositions (0.2 ≤ x ≤0.7) on the basis of high-resolution 19F solid state NMR data, revealing important similarities between theory and experiment. While the NMR data confirm that the population distribution is, indeed, nonstatistical for all compositions, the results are only found to be consistent with an intrinsic segregation tendency of PbF2-rich domains. This tendency manifests itself in substatistical populations of Q(3) units, resulting in preferred Q(2) and Q(4) formations.

A study of the phase transitions and proton dynamics of the superprotonic conductor Cs5H3(SO4)4·0.5H2O single crystal with 1H and 133Cs nuclear magnetic resonance
Journal of Solid State Chemistry Volume 181, Issue 4, April 2008, Pages 796-801
Ae Ran Lim
The phase transitions and proton dynamics of Cs5H3(SO4)4·0.5H2O single crystals were studied by measuring the NMR line shape, the spin-lattice relaxation time, T1, and the spin–spin relaxation time, T2, of the 1H and 133Cs nuclei. The “acid” protons and the “water” protons in Cs5H3(SO4)4·0.5H2O were distinguished. The loss of water protons was observed above TC1, whereas the content of water protons was found to recover above TC2. Therefore, the water protons play a special role in the stability of the superprotonic phase at high temperatures. The mechanism of fast proton conduction was found to consist of hydrogen-bond proton transfer involving the breakage of the weak part of the hydrogen bond and the formation of a new hydrogen bond. Thus, these structural phase transitions probably involve significant reorientation of the SO4 tetrahedra and dynamical disorder of the hydrogen bonds between them.

Local structure and disorder in crystalline Pb9Al8O21
Journal of Solid State ChemistryVolume 181, Issue 5, May 2008, Pages 1087-1102
Alex C. Hannon, Emma R. Barney, Diane Holland and Kevin S. Knight

Crystalline Pb9Al8O21 is a model compound for the structure of non-linear optical glasses containing lone-pair ions, and its structure has been investigated by neutron powder diffraction and total scattering, and 27Al magic angle spinning NMR. Rietveld analysis (space group
(No. 205), a=13.25221(4) Å) shows that some of the Pb and O sites have partial occupancies, due to lead volatilisation during sample preparation, and the non-stoichiometric sample composition is Pb9−δAl8O21−δ with δ=0.54. The NMR measurements show evidence for a correlation between the chemical shift and the variance of the bond angles at the aluminium sites. The neutron total correlation function shows that the true average Al–O bond length is 0.8% longer than the apparent bond length determined by Rietveld refinement. The thermal variation in bond length is much smaller than the thermal variation in longer interatomic distances determined by Rietveld refinement. The total correlation function is consistent with an interpretation in which AlO3 groups with an Al–O bond length of 1.651 Å occur as a result of the oxygen vacancies in the structure. The width of the tetrahedral Al–O peak in the correlation function for the crystal is very similar to that for lead aluminate glass, indicating that the extent of static disorder is very similar in the two phases.

Exploring Helical Folding of Oligoureas During Chain Elongation by High-Resolution Magic-Angle-Spinning (HRMAS) NMR Spectroscopy

Chem. Eur. J. Volume 14 Issue 13, Pages 3874 - 3882

Aude Violette, Dr., Nathalie Lancelot, Dr., Alexander Poschalko, Dr., Martial Piotto, Dr., Jean-Paul Briand, Dr., Jesus Raya, Dr., Karim Elbayed, Dr., Alberto Bianco, Dr., Gilles Guichard, Dr. Abstract:
The development of novel folding oligomers (foldamers) for biological and biomedical applications requires both precise structural information and appropriate methods to detect folding propensity. However, the synthesis and the systematic conformational investigation of large arrays of oligomers to determine the influence of factors, such as chain length, side chains, and surrounding environment, on secondary structure can be quite tedious. Herein, we show for 2.5-helical N,N-linked oligoureas (-peptide lineage) that the whole process of foldamer characterization can be accelerated by using high-resolution magic-angle-spinning (HRMAS) NMR spectroscopy. This was achieved by monitoring a simple descriptor of conformational homogeneity (e.g., chemical shift difference between diastereotopic main chain CH2 protons) at different stages of oligourea chain growth on a solid support. HRMAS NMR experiments were conducted on two sets of oligoureas, ranging from dimer to hexamer, immobilized on DEUSS, a perdeuterated poly(oxyethylene)-based solid support swollen in solvents of low to high polarity. One evident advantage of the method is that only minute amount of material is required. In addition, the resonance of the deuterated resin is almost negligeable. On-bead NOESY spectra of high quality and with resolution comparable to that of liquid samples were obtained for longer oligomers, thus allowing detailed structural characterization.

Thursday, May 22, 2008

J. Am. Chem. Soc., 130 (19), 6224–6230, 2008.

A QM/MM Approach to Interpreting 67Zn Solid-State NMR Data in Zinc ProteinsAndrew S. Lipton, Robert W. Heck, Greg R. Staeheli, Marat Valiev, Wibe A. De Jong, and Paul D. Ellis

We present here a 67Zn solid-state NMR investigation of Zn2+ substituted rubredoxin. The sample has been prepared as both a dry powder and a frozen solution to determine the effects of static disorder on the NMR line shape. Low-temperature experiments have been performed at multiple fields to determine the relative contributions to the NMR line shape from the electric field gradient and the anisotropic shielding tensors. Finally we present the theoretical interpretation of the experimental results utilizing a combined quantum mechanical molecular mechanics (QM/MM) approach. Theory predicts a sizable contribution from anisotropic shielding as compared with previously examined model systems. This is in good agreement with the experimental data.

Wednesday, May 21, 2008

Aaron's Journal Update

J. Phys. Chem. C, 112 (14), 5552 -5562, 2008. 10.1021/jp7107973 S1932-7447(71)00797-0
Multinuclear Solid-State NMR Studies of Ordered Mesoporous Bioactive Glasses
Ekaterina Leonova, Isabel Izquierdo-Barba, Daniel Arcos, Adolfo López-Noriega, Niklas Hedin, Maria Vallet-Regí, and Mattias Edén*
Abstract: The local structures of highly ordered mesoporous bioactive CaO-SiO2-P2O5 glasses were investigated for variable Ca contents. 1H NMR revealed a diversity of hydrogen-bonded and "isolated" surface silanols as well as adsorbed water molecules. The structural roles of Si and P were explored using a combination of 29Si and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques; the proximities of Si and P to protons were studied through cross-polarization-based experiments, including 1H-29Si and 1H-31P hetero-nuclear two-dimensional correlation spectroscopy. The results are consistent with SiO2 being the main pore-wall component, whereas P is present as a separate amorphous calcium orthophosphate phase, which is dispersed over the pore wall as nanometer-sized clusters. The excess Ca that is not consumed in the phosphate phase modifies the silica glass network where it associates at/near the mesoporous surface. This biphasic structural model of the pore wall leads to the high accessibility of both Ca and P to body fluids, and its relation to the experimentally demonstrated high in vitro bioactivities of these materials is discussed.

J. Phys. Chem. C, 112 (14), 5672 -5678, 2008. 10.1021/jp7108708 S1932-7447(71)00870-7
Conductivity and Fluoride Ion Dynamics in -PbSnF4; 19F Field-Cycling NMR and Diffraction Studies
Eoin Murray, Dermot F. Brougham,* Jovan Stankovic, and Isaac Abrahams
Abstract: Fast-field cycling 19F NMR relaxometry has been applied to investigate fluoride ion dynamics in the layered anionic conductor PbSnF4. Two dynamic processes, on different timescales, were shown to drive the 19F relaxation. By considering the temperature dependencies of the NMR, conductivity, and diffraction data, a complete mechanism for fluoride transport can for the first time be proposed. The slower process is due to anion exchange between equivalent sites (F(2)) in the conducting fluoride plane, which lie between Sn and Pb layers. This is a diffusive process related directly to the mechanism of electrical conduction. The activation barriers for this motion agree closely with those determined from the temperature dependence of the DC conductivity. The faster process is due to non-diffusive exchange between the occupied sites (F(2)) and nominally vacant sites (F(1)), which lie between Sn planes. While this process does not directly limit conductivity, it generates vacancies on the F(2) sites and partial occupancy of the F(1) sites. Above 340 K, the fast process shows an increase in activation energy, as increased occupancy of F(1) sites necessitates the formation of a Frenkel defect on a significant proportion of the F(1) sites prior to an F(2)-F(1) jump. In this temperature range, the slow process shows a decrease in the activation energy, also observed in the conductivity data, due to increased numbers of vacancies in the F(2) sites which provide additional diffusive pathways through the conducting fluoride plane. The results demonstrate that -PbSnF4 is essentially a two-dimensional (anisotropic) conductor, in which nondiffusive fluoride exchange into sites normal to the conducting plane provides a high population of vacancies within the conducting plane, resulting in unusually high conductivity.

J. Phys. Chem. C, 112 (15), 6151 -6159, 2008. 10.1021/jp711438v S1932-7447(71)01438-9
Intermediate-Range Order of Alkali Disilicate Glasses and Its Relation to the Devitrification Mechanism
James G. Longstaffe, Ulrike Werner-Zwanziger, Jose F. Schneider, Marcio L. F. Nascimento, Edgar D. Zanotto, and Josef W. Zwanziger*
Abstract: There are two general mechanisms of devitrification in glass: heterogeneous nucleation of crystals from surfaces and impurities and homogeneous nucleation from the volume. It is thought that structural similarities between glass and crystal at the intermediate-range level influence the mechanism followed; however, there are scarce experimental studies to test this hypothesis. In this paper solid-state nuclear magnetic resonance spectroscopy is used to probe intermediate-range order in sodium and lithium disilicate glasses through measurement of the second moment of the distribution of dipolar couplings. These two glasses undergo heterogeneous and homogeneous nucleation, respectively. The second moments measured for the lithium glass closely follow the trend established by the layered structures of the isochemical crystalline phases, while the same measurements for the sodium glass do not. This observation supports the hypothesis that glasses capable of homogeneous nucleation are structurally more similar to the resulting crystalline phases than those glasses that exhibit only heterogeneous nucleation.

J. Phys. Chem. C, 112 (15), 6165 -6172, 2008. 10.1021/jp7114498 S1932-7447(71)01449-3
19F MAS NMR Investigation of Strontium Substitution Sites in Ca2+/Sr2+ Fluorapatite Solid Solutions
Gyunggoo Cho, # Chung-Nin Chau, and James P. Yesinowski*
Abstract: The partial replacement of Ca2+ by Sr2+ in the fluorapatite lattice results in additional peaks in the 19F MAS NMR spectra at 9.4 T other than the main resonance of Ca10F2(PO4)6 at 64.0 ppm (from hexafluorobenzene). The assignment of these peaks to specific structural configurations is possible in the sample containing the least strontium, with a composition of Ca8.97Sr1.03F2(PO4)6. The solid-solution character of this sample is established by the observation of spectral spin diffusion between various peaks in the SPARTAN (selective population anti-z and rate of transfer to adjacent nuclei) experiment. Calculations based upon modeling the 19F chemical shift tensor show that this process is facilitated for most crystallites by the close approach of two peaks' resonances during the rotor cycle. A peak and set of spinning sidebands with an isotropic chemical shift of 79.6 ppm is assigned to fluoride ions in the center of a triangle of Ca2Sr ions (the so-called Ca(2) sites occurring as Ca3F in the fluorapatite lattice). Smaller chemical shift perturbations observed by deconvolution of two shoulders at 61.2 and 58.8 ppm on the main 64.0 ppm resonance are assigned to a Ca3F configuration that has, respectively, either one or multiple Sr2+ neighbors in the adjacent Ca(2) sites. Quantitative peak intensity measurements relative to the main 64 ppm resonance of both the 79.6 ppm peak as well as the deconvoluted peaks separately indicate that Sr2+ ions preferentially occupy the Ca(2) site at a level 23% greater than that expected for random substitution in the above solid solution. A sample having the composition Ca4.95Sr5.05F2(PO4)6 has a qualitatively similar site preference and has peaks assigned to Ca2SrF at 70 ppm, CaSr2F at 87 ppm, and Sr3F at 105 ppm.

J. Phys. Chem. C, 112 (16), 6430 -6438, 2008. 10.1021/jp7119087 S1932-7447(71)01908-3
H2O and Cation Structure and Dynamics in Expandable Clays: 2H and 39K NMR Investigations of Hectorite
Geoffrey M. Bowers,* David L. Bish, and R. James Kirkpatrick
Abstract: Variable temperature 39K and 2H nuclear magnetic resonance (VT NMR) spectroscopy of K+-saturated hectorite, a prototypical smectite clay, provides new insight into the relationships between the structural and dynamical behavior of K+ and H2O in confinement and at surfaces. In d = 10 Å K-exchanged hectorite, interlayer K+ is rigidly held by the silicate rings, probably in 12-coordinate inner-sphere sites as in muscovite mica. In a 1/1.5 by weight hectorite/water paste, K+ occurs on interlayer and external surface sites that are indistinguishable by 39K NMR. The K+ environments experience changes in dynamical behavior over the temperature range from -50 to 60 C that are directly related to H2O dynamics. 39K NMR of the paste sample shows dynamic line narrowing at low temperatures due to modulation of the electric field gradient (EFG) at frequencies of the order of the static line width ( 20 kHz) and two "melting"-type dynamic transitions near -10 C, one for surface and one for confined K+. At and above 0 C, K+ remains closely associated with the clay surfaces and experiences motion at frequencies greater than 200 kHz and less than 10 MHz, as revealed by 39K T1 relaxation behavior, nutation behavior, and the 39K quadrupolar product. These data are consistent with rapid exchange between inner- and outer-sphere K+ sites reported previously for K-montmorillonite based on molecular dynamics simulations. Deuterium NMR shows the presence of two unique H2O environments in the system: one structurally and dynamically consistent with bulk water between particles and one attributable to H2O confined in the interlayer. Confined H2O experiences anisotropic motion between -50 and 0 C via fast rotation (>2 MHz) about a single axis oriented 127.5 ± 0.5 from the principal axis of the 2H EFG, potentially due to C2 rotation. This motion does not affect the 39K EFG significantly. Melting of free and confined H2O occurs between -10 and 0 C and near 0 C, respectively, similar to the melting behavior of K+ and likely reflecting the onset of molecular diffusion. At and above 10 C, all H2O environments experience motion near or in excess of 300 kHz through at least three NMR-indistinguishable mechanisms, including Brownian motion of free water, exchange of free and confined H2O near particle edges, and diffusive motion of H2O that remains confined on the experimental time scale. The correlation between the rates of 2H and 39K motion and the observed melting transitions for both spin populations strongly suggest that 39K melting and dynamics above the melting transition are linked to an increase in the motional freedom of H2O.

J. Phys. Chem. C, 112 (19), 7503 -7508, 2008. 10.1021/jp711762n S1932-7447(71)01762-X
Temperature Dependence of Spontaneous Polarization in Order-Disorder Pyridinium Periodate Extracted from 2H NMR Data
J. W sicki, A. Pajzderska,* and Z. Fojud
Abstract: On the basis of occupancy factor data for nitrogen atoms in the pyridinium cation obtained from the structural study of PyHIO4 by neutron diffraction,4 the shape of the potential for cation reorientation about the axis perpendicular to its plane was found. The 2H NMR spectra of d5PyHIO4 taken over a wide range of temperatures have allowed the determination of potential minima populations. These data were used to obtain the temperature dependence of spontaneous polarization. The agreement between the spontaneous polarization calculated in this way and the measured values is very good and indicates that the ferroelectric crystal is of the order-disorder type.

J. Phys. Chem. B, 112 (13), 3927 -3930, 2008. 10.1021/jp711433d S1520-6106(71)01433-0
Water Diffusion in Nanoporous Glass: An NMR Study at Different Hydration Levels
Domenico Majolino,* Carmelo Corsaro, Vincenza Crupi, Valentina Venuti, and Ulderico Wanderlingh
Abstract: By pulsed field gradient nuclear magnetic resonance measurements, we investigated the translational diffusion of water confined in the 200 Å diameter pores of a sol-gel silica glass. The experiments, performed as a function of the hydration level, showed an enhancement of the self-diffusion coefficient when the water content corresponds to one or fewer monolayers. An explanation for this occurrence has been given in terms of a two-phase process involving a fast molecular exchange between the liquid and the vapor phase. Moreover, in partially filled pores, the surface water diffusion coefficient was measured, and was 4 times lower than the diffusion of liquid confined water in saturated spaces.

J. Phys. Chem. B, 112 (15), 4496 -4505, 2008. 10.1021/jp709739v S1520-6106(70)09739-0
Theoretical Predictions of 31P NMR Chemical Shift Threshold of Trimethylphosphine Oxide Absorbed on Solid Acid Catalysts
Anmin Zheng, Hailu Zhang, Xin Lu, Shang-Bin Liu,* and Feng Deng*
Abstract: The 31P NMR chemical shifts of adsorbed trimethylphosphine oxide (TMPO) and the configurations of the corresponding TMPOH+ complexes on Br nsted acid sites with varying acid strengths in modeled zeolites have been predicted theoretically by means of density functional theory (DFT) quantum chemical calculations. The configuration of each TMPOH+ complex was optimized at the PW91/DNP level based on an 8T cluster model, whereas the 31P chemical shifts were calculated with the gauge including atomic orbital (GIAO) approach at both the HF/TZVP and MP2/TZVP levels. A linear correlation between the 31P chemical shift of adsorbed TMPO and the proton affinity of the solid acids was observed, and a threshold for superacidity (86 ppm) was determined. This threshold for superacidity was also confirmed by comparative investigations on other superacid systems, such as carborane acid and heteropolyoxometalate H3PW12O40. In conjunction with the strong correlation between the MP2 and the HF 31P isotropic shifts, the 8T cluster model was extended to more sophisticated models (up to 72T) that are not readily tractable at the GIAO-MP2 level, and a 31P chemical shift of 86 ppm was determined for TMPO adsorbed on zeolite H-ZSM-5, which is in good agreement with the NMR experimental data.

J. Phys. Chem. B, 112 (16), 4943 -4947, 2008. 10.1021/jp7110082 S1520-6106(71)01008-3
Preparation and Characterization of New Glassy System As2P2S8-Ga2S3
Silvia H. Santagneli, José Schneider, Igor Skripachev, Sidney J. L. Ribeiro, and Younès Messaddeq*
Abstract: Glasses having the composition (100 - x)As2P2S8-xGa2S3 with x ranging from 0 to 50% were investigated to determine the compositional effect on properties and local structure. The glass transition temperature (Tg) and the stability parameter against crystallization (Tx - Tg) increased with the addition of Ga2S3. The structure of these glasses was probed by Raman scattering, Fourier transform infrared (FT-IR) and 31P nuclear magnetic resonance. On the basis of the observed vibrations and the strength of the 31P-31P homonuclear magnetic dipolar coupling, two scenarios can be proposed for the structural evolution induced by the addition of Ga2S3. For x 20% we may have the formation of GaS4E- groups (E = nonbonding electron), and for x 30% we have depolymerization of the As2P2S8 units and the formation of a network of GaPS4 units with each PS4/2 unit (Q4) species carrying a single positive formal charge

J. Phys. Chem. B, 112 (18), 5813 -5823, 2008. 10.1021/jp800580n S1520-6106(80)00580-9
51V NMR Chemical Shifts from Quantum-Mechanical/Molecular-Mechanical Models of Vanadium Bromoperoxidase
Mark P. Waller, K. R. Geethalakshmi, and Michael Bühl*
Abstract: According to quantum-mechanical/molecular-mechanical (QM/MM) optimizations, the active-site geometries of vanadium-dependent bromoperoxidase (VBPO) and vanadium-dependent chloroperoxidase (VCPO) are very similar. 51V NMR chemical shifts calculated from QM/MM-optimized models of VBPO are critically compared to VCPO and are found to be very similar for the two related proteins. The primary difference between these related structures, the presence of a His411 in VBPO whereas Phe397 is located at that position in VCPO, is studied via analysis of the respective theoretical 51V NMR spectra. The long-range electrostatic effects from more distal residues are also studied to establish their effect. Similar results are obtained for the two active sites of the VBPO homodimer. The experimentally observed shielding of the isotropic 51V NMR chemical shift on going from VCPO to VBPO is somewhat underestimated in the QM/MM models studied. NMR and NQC tensors of both enzymes are predicted to show noticeable differences, suggesting that precise solid-state 51V NMR data, when they become available, can be a sensitive probe for subtle differences in structural details between these enzymes.

J. Phys. Chem. B, 112 (19), 5881 -5882, 2008. 10.1021/jp801687s S1520-6106(80)01687-2
A Tribute to Attila Szabo
There are many biological NMR papers to be found in this special issue (Issue 19) for those who are interested.

J. Phys. Chem. B, 112 (19), 6114 -6121, 2008. 10.1021/jp076808o S1520-6106(07)06808-3
Theory of Stochastic Dipolar Recoupling in Solid-State Nuclear Magnetic Resonance
Robert Tycko*
Abstract: Dipolar recoupling techniques in solid-state nuclear magnetic resonance (NMR) consist of radio frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create nonzero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter fmax) increases; (2) in a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large fmax, with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) quantum mechanical interferences among noncommuting pairwise dipole-dipole couplings, which are a complicating factor in solid-state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large fmax, provided that coupled nuclei have distinct NMR chemical shifts.

J. Phys. Chem. B, 112 (20), 6285–6287, 2008. 10.1021/jp800646k
Fluorine-19 NMR Chemical Shift Probes Molecular Binding to Lipid Membranes
Eduard Y. Chekmenev,*†‡ Siu-Kei Chow,† Daniel Tofan,‡ Daniel P. Weitekamp,‡ Brian D. Ross,† and Pratip Bhattacharya†
Abstract: The binding of amphiphilic molecules to lipid bilayers is followed by 19F NMR using chemical shift and line shape differences between the solution and membrane-tethered states of −CF3 and −CHF2 groups. A chemical shift separation of 1.6 ppm combined with a high natural abundance and high sensitivity of 19F nuclei offers an advantage of using 19F NMR spectroscopy as an efficient tool for rapid time-resolved screening of pharmaceuticals for membrane binding. We illustrate the approach with molecules containing both fluorinated tails and an acrylate moiety, resolving the signals of molecules in solution from those bound to synthetic dimyristoylphosphatidylcholine bilayers both with and without magic angle sample spinning. The potential in vitro and in vivo biomedical applications are outlined. The presented method is applicable with the conventional NMR equipment, magnetic fields of several Tesla, stationary samples, and natural abundance isotopes.

Macromolecules, 41 (7), 2514–2519, 2008. 10.1021/ma702815k
Restricted Segmental Mobility Can Facilitate Medium-Range Chain Diffusion: A NMR Study of Morphological Influence on Chain Dynamics of Polyethylene
Y.-F. Yao,† R. Graf,† H. W. Spiess,*† and S. Rastogi†‡
Abstract: The influence of the morphology on the local motion in the noncrystalline regions and chain diffusion between crystalline and noncrystalline regions is studied in ultrahigh molecular weight linear polyethylene: The behaviors of samples of the same material crystallized from the melt and from solution are compared. The geometrical restrictions of the conformational transitions are probed via anisotropic NMR interactions, i.e., 13C−1H dipole–dipole couplings and 13C chemical shift anisotropy. As they are averaged out under MAS, recoupling techniques are applied to yield sideband patterns or quasi-static NMR spectra, from which residual anisotropies are determined. Chain diffusion is probed by 13C exchange NMR. As expected, the local conformational transitions are more restricted in the solution crystallized sample compared with the melt crystallized one. Moreover, the motional narrowing observed by both techniques indicates that the local mobility in the noncrystalline regions of the solution crystallized sample consists mainly of effectively axial motion of extended trans-conformers around their local chain axes. This facilitates the chain diffusion between the crystalline and the noncrystalline regions, being significantly faster in solution crystallized compared to melt crystallized samples, where the local mobility is much more isotropic. The implications of these results for the understanding of crystal thickening and cold drawing are discussed.

Tuesday, May 20, 2008

J. Am. Chem. Soc., 130 (18), 5922–5928, 2008.

Characterization of Alzheimer’s-like Paired Helical Filaments from the Core Domain of Tau Protein Using Solid-State NMR SpectroscopyOvidiu C.
Andronesi, Martin von Bergen, Jacek Biernat, Karsten Seidel, Christian Griesinger, Eckhard Mandelkow, and Marc Baldus

The polymerization of the microtubule-associated protein tau into paired helical filaments (PHFs) represents one of the hallmarks of Alzheimer’s disease. We employed solid-state nuclear magnetic resonance (NMR) to investigate the structure and dynamics of PHFs formed in vitro by the three-repeat-domain (K19) of protein tau, representing the core of Alzheimer PHFs. While N and C termini of tau monomers in PHFs are highly dynamic and solvent-exposed, the rigid segment consists of three major β-strands. Combination of through-bond and through-space ssNMR transfer methods with water-edited (15N,13C) and (13C,13C) correlation experiments suggests the existence of a fibril core that is largely built by repeat unit R3, flanked by surface-exposed units R1 and R4. Solid-state NMR, circular dichroism, and the fibrillization behavior of a K19 mutant furthermore indicate that electrostatic interactions play a central role in stabilizing the K19 PHFs.

J. Am. Chem. Soc., 130 (18), 5880–5882, 2008.

Mapping the Evolution of Adsorption of Water in Nanoporous Silica by in situ Solid-State 1H NMR Spectroscopy
Mingcan Xu, Kenneth D. M. Harris, and John Meurig Thomas


A recently developed technique for carrying out in situ solid-state NMR studies of adsorption processes in nanoporous materials is applied to map the evolution of the adsorption of water on the widely used siliceous nanoporous host material MCM-41. The technique allows the very earliest stages of the adsorption process to be probed, and using solid-state 1H NMR, the time resolution is of the order of a few hundred seconds. This work reveals that different water environments are populated at different stages of the adsorption process and reveals insights regarding both the sequence in which these different environments become populated and the exchange of water molecules between these environments. The results also provide access to information on the kinetics of the adsorption process, revealing an initial regime of rapid water adsorption up to ca. 1 wt % water, followed by a regime of slower water adsorption. In both regimes, the amount of water adsorbed increases linearly with time. As demonstrated by these results, the approach employed in this work creates new opportunities for investigating, in unprecedented detail, adsorption processes in nanoporous materials.

J. Am. Chem. Soc., 130 (18), 5870–5871, 2008.

2-Fluoro-ATP as a Versatile Tool for 19F NMR-Based Activity Screening
Brian J. Stockman

19F NMR-based methods have found utility in activity-based screening assays. However, because enzymes catalyze a diverse set of reactions, a large variety of fluorinated substrates would need to be identified to target each one separately. We have developed a more streamlined approach that is applicable to many enzymes that utilize ATP as a substrate. In this method, a fluorine-containing ATP analogue, 2-fluoro-ATP, is used to monitor the reaction. Applications are described for nicotinamide adenine dinucleotide synthetase and 3-phosphoinositide dependent kinase-1. Fragment screening results for the latter indicate that this technique can identify compounds that inhibit as well as activate reactions. The present results, together with previous biochemical studies from other laboratories, have shown that 2-fluoro-ATP can serve as a substrate for nine enzymes that are representative of three of the six enzyme subclasses, namely the transferases, hydrolases, and ligases. This suggests that 2-fluoro-ATP is suitable as a universal tool for screening ATP-requiring enzymes. Importantly, 2-fluoro-ATP has been determined to be a valid substrate for a variety of kinases, including both small molecule and protein kinases, suggesting that it may be useful for investigating the large number of pharmaceutically relevant kinases.

Wednesday, May 14, 2008

J. Am. Chem. Soc., 130 (20), 6306–6307, 2008.

NMR Crystallography of Zeolites: Refinement of an NMR-Solved Crystal Structure Using ab Initio Calculations of 29Si Chemical Shift Tensors
Darren H. Brouwer

An NMR structure refinement method for the NMR crystallography of zeolites is presented and demonstrated to give an NMR-determined crystal structure for the zeolite Sigma-2 that is in very good agreement with the single-crystal X-ray diffraction structure. The Si coordinates of the zeolite framework were solved from 29Si double-quantum NMR data obtained at a low magnetic field strength (7.0 T) and the Si and O coordinates were subsequently refined using the principal components of 29Si chemical shift tensors experimentally measured at an ultrahigh-field (21.1 T) and calculated using ab initio quantum chemical methods.

J. Am. Chem. Soc., 130 (17), 5618–5619, 2008.

Toward the Characterization of Peptidoglycan Structure and Protein−Peptidoglycan Interactions by Solid-State NMR Spectroscopy
Thomas Kern, Sabine Hediger,Patrick Müller,§ Cécile Giustini, Bernard Joris, Catherine Bougault,Waldemar Vollmer, and Jean-Pierre Simorre

Solid-state NMR spectroscopy is applied to intact peptidoglycan sacculi of the Gram-negative bacterium Escherichia coli. High-quality solid-state NMR spectra allow atom-resolved investigation of the peptidoglycan structure and dynamics as well as the study of protein−peptidoglycan interactions.

Monday, May 12, 2008

J. Am. Chem. Soc., 130 (16), 5459–5471, 2008.

Solid-State NMR Spectroscopy of Human Immunodeficiency Virus Fusion Peptides Associated with Host-Cell-Like Membranes: 2D Correlation Spectra and Distance Measurements Support a Fully Extended Conformation and Models for Specific Antiparallel Strand Registries
Wei Qiang, Michele L. Bodner, and David P. Weliky*

The human immunodeficiency virus (HIV) is “enveloped” by a membrane, and infection of a host cell begins with fusion between viral and target cell membranes. Fusion is catalyzed by the HIV gp41 protein which contains a functionally critical ~20-residue apolar “fusion peptide” (HFP) that associates with target cell membranes. In this study, chemically synthesized HFPs were associated with host-cell-like membranes and had “scatter-uniform” labeling (SUL), that is, only one residue of each amino acid type was U−13C, 15N labeled. For the first sixteen HFP residues, an unambiguous 13C chemical shift assignment was derived from 2D 13C/13C correlation spectra with short mixing times, and the shifts were consistent with continuous β-strand conformation. 13C−13C contacts between residues on adjacent strands were derived from correlation spectra with long mixing times and suggested close proximity of the following residues: Ala-6/Gly-10, Ala-6/Phe-11, and Ile-4/Gly-13. Specific antiparallel β-strand registries were further tested using a set of HFPs that were 13CO-labeled at Ala-14 and 15N-labeled at either Val-2, Gly-3, Ile-4, or Gly-5. The solid-state NMR data were fit with 50–60% population of antiparallel HFP with either Ala-14/Gly-3 or Ala-14/Ile-4 registries and 40–50% population of structures not specified by the NMR experiments. The first two registries correlated with intermolecular hydrogen bonding of 15–16 apolar N-terminal residues and this hydrogen-bonding pattern would be consistent with a predominant location of these residues in the hydrophobic membrane interior. To our knowledge, these results provide the first residue-specific structural models for membrane-associated HFP in its β-strand conformation.

J. Am. Chem. Soc., 130 (18), 5886–5900, 2008.

Dynamics of Silica-Supported Catalysts Determined by Combining Solid-State NMR Spectroscopy and DFT Calculations
Frédéric Blanc, Jean-Marie Basset, Christophe Copéret,† Amritanshu Sinha, Zachary J. Tonzetich, Richard R. Schrock, Xavier Solans-Monfort,Eric Clot,Odile Eisenstein, Anne Lesage, and Lyndon Emsley

The molecular dynamics of a series of organometallic complexes covalently bound to amorphous silica surfaces is determined experimentally using solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory calculations (DFT). The determination is carried out for a series of alkylidene-based catalysts having the general formula [(SiO)M(ER)(CHtBu)(R′)] (M = Re, Ta, Mo or W; ER = CtBu, NAr or CH2tBu; R′ = CH2tBu, NPh2, NC4H4). Proton−carbon dipolar coupling constants and carbon chemical shift anisotropies (CSA) are determined experimentally by solid-state NMR. Room-temperature molecular dynamics is quantified through order parameters determined from the experimental data. For the chemical shift anisotropy data, we validate and use a method that integrates static values for the CSA obtained computationally by DFT, obviating the need for low-temperature measurements. Comparison of the room-temperature data with the calculations shows that the widths of the calculated static limit dipolar couplings and CSAs are always greater than the experimentally determined values, providing a clear indication of motional averaging on the NMR time scale. Moreover, the dynamics are found to be significantly different within the series of molecular complexes, with order parameters ranging from = 0.5 for [(SiO)Ta(CHtBu)(CH2tBu)2] and [(SiO)Re(CtBu)(CHtBu)(CH2tBu)] to = 0.9 for [(SiO)Mo(NAr)(CHtBu)(R′) with R′ = CH2tBu, NPh2, NC4H4. The data also show that the motion is not isotropic and could be either a jump between two sites or more likely restricted librational motion. The dynamics are discussed in terms of the molecular structure of the surface organometallic complexes, and the orientation of the CSAs tensor at the alkylidene carbon is shown to be directly related to the magnitude of the α-alkylidene CH agostic interation.

J. Am. Chem. Soc., 130 (17), 5798–5807, 2008.

Magic Angle Spinning NMR Experiments for Structural Studies of Differentially Enriched Protein Interfaces and Protein Assemblies
Jun Yang, Maria Luisa Tasayco, and Tatyana Polenova

Protein–protein interactions play vital roles in numerous biological processes. These interactions often result in formation of insoluble and noncrystalline protein assemblies. Solid-state NMR spectroscopy is rapidly emerging as a premier method for structural analysis of such systems. We introduce a family of two-dimensional magic angle spinning (MAS) NMR experiments for structural studies of differentially isotopically enriched protein assemblies. Using 1–73(13C,15N)/74–108(15N) labeled thioredoxin reassembly, we demonstrate that dipolar dephasing followed by proton-assisted heteronuclear magnetization transfer yields long-range 15N−13C correlations arising exclusively from the interfaces formed by the pair of differentially enriched complementary fragments of thioredoxin. Incorporation of dipolar dephasing into the 15N proton-driven spin diffusion and into the 1H−15N FSLG−HETCOR sequences permits 1H and 15N resonance assignments of the 74–108(15N) enriched C-terminal fragment of thioredoxin alone. The differential isotopic labeling scheme and the NMR experiments demonstrated here allow for structural analysis of both the interface and each interacting protein. Isotope editing of the magnetization transfers results in spectral simplification, and therefore larger protein assemblies are expected to be amenable to these experiments.

Thursday, May 08, 2008

Physical Review B

Phys. Rev. B 77, 144404 (2008)
63,65Cu NMR and NQR evidence for an unusual spin dynamics in PrCu2 below 100 K
A. Sacchetti, M. Weller, J. L. Gavilano, R. Mudliar, B. Pedrini, K. Magishi, H. R. Ott, R. Monnier, B. Delley, and Y. Onuki
We report the results of a 63,65Cu NMR/NQR study probing the intermetallic compound PrCu2. The previously claimed onset of magnetic order at 65 K, indicated in a muon spin resonance study, is not confirmed. Based on our data, we discuss different possible reasons for this apparent discrepancy, including a non-negligible influence of the implanted muons on their environment. Competing dipolar and quadrupolar interactions lead to unusual features of the magnetic-ion/conduction-electron system, different from those of common intermetallics exhibiting structural or magnetic instabilities.

Phys. Rev. B 77, 144419 (2008)
7Li NMR study of heavy-fermion LiV2O4 containing magnetic defects
X. Zong, S. Das, F. Borsa, M. D. Vannette, R. Prozorov, J. Schmalian, and D. C. Johnston
We present a systematic study of the variations of the 7Li NMR properties versus magnetic defect concentration ndefect within the spinel structure of polycrystalline powder samples (ndefect=0.21, 0.49, and 0.83 mol %) and a collection of small single crystals (ndefect=0.38 mol %) of LiV2O4 in the temperature range from 0.5 to 4.2 K. We also report static magnetization measurements and ac magnetic susceptibility measurements at 14 MHz on the samples at low temperatures. Both the 7Li NMR spectrum and nuclear spin-lattice relaxation rate are inhomogeneous in the presence of the magnetic defects. The 7Li NMR data for the powders are well explained by assuming that (i) there is a random distribution of magnetic point defects, (ii) the same heavy Fermi liquid is present in the samples containing the magnetic defects as in magnetically pure LiV2O4, and (iii) the influences of the magnetic defects and of the Fermi liquid on the magnetization and NMR properties are separable. In the single crystals, somewhat different behaviors are observed, which are possibly due to a modification of the heavy Fermi liquid, to a lack of separability of the relaxation effects due to the Fermi liquid and the magnetic defects, to non-Fermi liquid behavior of the conduction electrons, and/or to quantum fluctuations of finite-size magnetic defects (magnetic droplets). Remarkably, the magnetic defects in the powder samples show evidence of spin freezing below T1.0 K, whereas in the single crystals with similar magnetic defect concentration, no spin freezing was found down to T=0.5 K. Thus, different types of magnetic defects and/or interactions between them appear to arise in the powders versus the crystals, which are possibly due to the substantially different synthesis conditions of the powders and crystals.

Journal of Magnetic Resonance - Vol 192 Issue 1

Through-space MP-CPMAS experiments between spin-1/2 and half-integer quadrupolar nuclei in solid-state NMR
B. Hu
, J.P. Amoureux, J. Trébosc and S. Hafner
We present a new CPMAS method that allows the acquisition of through-space 2D HETCOR spectra between spin-1/2 nuclei and half-integer quadrupolar nuclei in the solid state. It uses rotor-synchronized selective pulses on the quadrupolar nucleus and continuous-wave RF irradiation on the spin-1/2 nucleus to create hetero-nuclear dipolar coherences. The method is more robust, more efficient, and easier to set up than the standard CPMAS transfer.

Spin state selective coherence transfer: A method for discrimination and complete analyses of the overlapped and unresolved 1H NMR spectra of enantiomers
Bikash Baishya
, Uday Ramesh Prabhu and N. Suryaprakash
In general, the proton NMR spectra of chiral molecules aligned in the chiral liquid crystalline media are broad and featureless. The analyses of such intricate NMR spectra and their routine use for spectral discrimination of R and S optical enantiomers are hindered. A method is developed in the present study which involves spin state selective two dimensional correlation of higher quantum coherence to its single quantum coherence of a chemically isolated group of coupled protons. This enables the spin state selective detection of proton single quantum transitions based on the spin states of the passive nuclei. The technique provides the relative signs and magnitudes of the couplings by overcoming the problems of enantiomer discrimination, spectral complexity and poor resolution, permitting the complete analyses of the otherwise broad and featureless spectra. A non-selective 180° pulse in the middle of MQ dimension retains all the remote passive couplings. This accompanied by spin selective MQ–SQ conversion leads to spin state selective coherence transfer. The removal of field inhomogeneity contributes to dramatically enhanced resolution. The difference in the cumulative additive values of chemical shift anisotropies and the passive couplings, between the enantiomers, achieved by detecting Nth quantum coherence of N magnetically equivalent spins provides enhanced separation of enantiomer peaks. The developed methodology has been demonstrated on four different chiral molecules with varied number of interacting spins, each having a chiral centre.

Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC
B. Hu
, J. Trébosc and J.P. Amoureux
We compare several hetero-nuclear dipolar recoupling sequences available for HMQC or HSQC experiments applied to spin-1/2 and quadrupolar nuclei. These sequences, which are applied to a single channel, are based either on the rotary resonance recoupling (R3) irradiation, or on two continuous rotor-synchronized modulations (SFAM1 and SFAM2), or on four symmetry-based sequences
, or on the REDOR scheme. We analyze systems exhibiting purely hetero-nuclear dipolar interactions as well as systems where homo-nuclear dipolar interactions need to be canceled. A special attention is given to the behavior of these sequences at very fast MAS. It is shown that R3 methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are and SFAM (SFAM1 or SFAM2 if homo-nuclear interactions are not negligible). REDOR can also recouple efficiently hetero-nuclear dipolar interactions, provided the sequence is sent on the non-observed channel and homo-nuclear dipolar interactions are negligible. We anticipate that at ultra-fast spinning speed, SFAM1 and SFAM2 will be the most efficient methods.

A simple, small and low cost permanent magnet design to produce homogeneous magnetic fields
B. Manz
, M. Benecke and F. Volke
A new portable, pocket-size NMR probe based on a novel permanent magnet arrangement is presented. It is based on a Halbach-type magnet design which mimics the field of a spherical dipole by using cylindrical bar and ring magnets. The magnet system is made up of only three individual magnets, and most field calculations and optimisations can be performed analytically. A prototype system has been built using a set of small, off the shelf commercially available permanent magnets. Proton linewidths of 50 ppm FWHM could be achieved at a field strength of 1 T. Calculations show that with custom-sized permanent magnets, linewidths of less than 1 ppm can be achieved over sample volumes of up to 1 mm3, which would in theory enable chemical shift resolved proton spectroscopy on mass-limited samples. But even with the achieved linewidth of 50 ppm, this can be a useful portable sensor for small amounts of liquid samples with restricted molecular mobility, like gels, polymers or high viscosity liquids.

High-performance solvent suppression for proton detected solid-state NMR
Donghua H. Zhou
and Chad M. Rienstra
High-sensitivity proton detected experiments in solid-state NMR have been recently demonstrated in proton diluted proteins as well as fully protonated samples under fast magic-angle spinning. One key element for performing successful proton detection is effective solvent suppression achieved by pulsed field gradients (PFG) and/or saturation pulses. Here we report a high-performance solvent suppression method that attenuates multiple solvent signals simultaneously by more than a factor of 10,000, achieved by an optimized combination of homospoil gradients and supercycled saturation pulses. This method, which we call Multiple Intense Solvent Suppression Intended for Sensitive Spectroscopic Investigation of Protonated Proteins, Instantly (MISSISSIPPI), can be applied without a PFG probe. It opens up new opportunities for two-dimensional heteronuclear correlation spectroscopy of hydrated proteins at natural abundance as well as high-sensitivity and multi-dimensional experimental investigation of protein–solvent interactions.

Wednesday, May 07, 2008

Am. Chem. Soc., 130 (15), 5014–5015, 2008.

Hadamard Amino-Acid-Type Edited NMR Experiment for Fast Protein Resonance Assignment
Ewen Lescop, Rodolfo Rasia, and Bernhard Brutscher

An original Hadamard-encoding scheme allows discrimination among seven amino acid types in a single two-dimensional NMR experiment. Combined with hyperdimensional NMR techniques, this presents a promising new method for fast, automated backbone resonance assignment

Chem. Mater., 20 (7), 2583–2591, 2008.

Dispersion of Silicate in Tricalcium Phosphate Elucidated by Solid-State NMR
A. Rawal,† X. Wei,‡ M. Akinc,‡ and K. Schmidt-Rohr*†

The dispersion of silicate in tricalcium phosphate, a resorbable bioceramics for bone replacement, has been investigated by various solid-state nuclear magnetic resonance (NMR) methods. In samples prepared with 5 and 10 mol % of both 29SiO2 and ZnO, three types of silicate have been detected: (i) SiO44− (Q0 sites) with long longitudinal (T1,Si) relaxation times (~10 000 s), which substitute for ~1% of PO43−; (ii) silicate nanoinclusions containing Q2, Q1, and Q0 sites with T1,Si ~ 100 s, which account for most of the silicon; and (iii) crystalline Q4 (SiO2) with long T1,Si. Sensitivity was enhanced >100-fold by 29Si enrichment and refocused detection. The inclusions in both samples have a diameter of ~8 nm, as proved by 29Si{31P} REDOR dephasing on a 30-ms time scale, which was simulated using a multispin approach specifically suited for nanoparticles. 29Si CODEX NMR with 30-s 29Si spin diffusion confirms that an inclusion contains >10 Si (consistent with the REDOR result of >100 Si per inclusion). Overlapping signals of silicate Q2, Q1, and Q0 sites were spectrally edited based on their J-couplings, using double-quantum filtering. The large inhomogeneous broadening of the Q2, Q1, and Q0 29Si subspectra indicates that the nanoinclusions are amorphous.

Monday, May 05, 2008

Phys. Rev. Lett. 100, 133002 (2008)

Nuclear Magnetic Resonance Chemical Shift in an Arbitrary Electronic Spin StateTeemu O. Pennanen and Juha Vaara

We present a general and systematic electronic structure theory of the nuclear magnetic resonance shielding tensor and the associated chemical shift for paramagnetic atoms, molecules, and nonmetallic solids. The approach is for the first time rigorous for an arbitrary spin state as well as arbitrary spatial symmetry and is formulated without reference to spin susceptibility. The leading-order magnetic-field dependence of shielding is derived. The theory is demonstrated by first principles calculations of organometallic molecules.

Friday, May 02, 2008

SSNMR up to March 2008 issue - IUPAC Chemical shift recommendations

SSNMR up to March 2008 issue

Solid State Nuclear Magnetic Resonance
Volume 33, Issue 3, March 2008, Pages 41-56
doi:10.1016/j.ssnmr.2008.02.004 How to Cite or Link Using DOI (Opens New Window)

Further conventions for NMR shielding and chemical shifts IUPAC recommendations 2008star, open
Prepared for publication by, Robin K. Harris, Edwin D. Becker, Sonia M. Cabral De Menezes, Pierre Granger, Roy E. Hoffman and Kurt W. Zilm
Available online 20 February 2008.

IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS).

This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice.

Keywords: Nuclear magnetic resonance; Recommendations; Chemical shifts; Conventions; IUPAC physical and biophysical chemistry division; Shielding tensors