Tuesday, June 30, 2009

J. Am. Chem. Soc., Article ASAP

Chemical Shift Anisotropy of Imino 15N Nuclei in Watson−Crick Base Pairs from Magic Angle Spinning Liquid Crystal NMR and Nuclear Spin Relaxation
Alexander Grishaev†, Lishan Yao†, Jinfa Ying†, Arthur Pardi‡ and Ad Bax*†

Knowledge of 15N chemical shift anisotropy is prerequisite both for quantitative interpretation of nuclear spin relaxation rates in terms of local dynamics and for the use of residual chemical shift anisotropy (RCSA) as a constraint in structure determination. Accurate measurement of the very small RCSA from the difference in 15N chemical shift under isotropic and weakly aligning liquid crystalline conditions is very sensitive to minute differences in sample conditions, such as pH or ionic strength. For this reason, chemical shifts were measured for the same solution, under static liquid crystalline alignment, and under magic angle spinning conditions where alignment relative to the magnetic field is removed. Measurements were made for 14 well-resolved G−N1 and 6 U−N3 15N nuclei in a sample of tRNAVal. Fitting these RCSA data together with 15N−1H dipole-CSA cross-correlated relaxation measurements to the recently refined structural model of tRNAVal yields the magnitude, asymmetry, and orientation of the 15N CSA tensors.

J. Am. Chem. Soc., Article ASAP

Solvation and Hydrogen Bonding in Alanine- and Glycine-Containing Dipeptides Probed Using Solution- and Solid-State NMR Spectroscopy
Manasi P. Bhate, Jaie C. Woodard and Manish A. Mehta*

The NMR chemical shift is a sensitive reporter of peptide secondary structure and its solvation environment, and it is potentially rich with information about both backbone dihedral angles and hydrogen bonding. We report results from solution- and solid-state 13C and 15N NMR studies of four zwitterionic model dipeptides, l-alanyl-l-alanine, l-alanyl-glycine, glycyl-l-alanine, and glycyl-glycine, in which we attempt to isolate structural and environmental contributions to the chemical shift. We have mapped hydrogen-bonding patterns in the crystalline states of these dipeptides using the published crystal structures and correlated them with 13C and 15N magic angle spinning chemical shift data. To aid in the interpretation of the solvated chemical shifts, we performed ab initio quantum chemical calculations to determine the low-energy conformers and their chemical shifts. Assuming low energy barriers to interconversion between thermally accessible conformers, we compare the Boltzmann-averaged chemical shifts with the experimentally determined solvated-state shifts. The results allow us to correlate the observed differences in chemical shifts between the crystalline and solvated states to changes in conformation and hydrogen bonding that occur upon solvation.

J. Am. Chem. Soc., Article ASAP

Characterization of Dispersed Heteropoly Acid on Mesoporous Zeolite Using Solid-State 31P NMR Spin−Lattice Relaxation
Kake Zhu, Jianzhi Hu, Xiaoyan She, Jun Liu*, Zimin Nie, Yong Wang, Charles H. F. Peden and Ja Hun Kwak

Dispersion and quantitative characterization of supported catalysts is a grand challenge in catalytic science. In this paper, heteropoly acid H3PW12O40 (HPA) is dispersed on mesoporous zeolite silicalite-1 derived from hydrothermal synthesis using carbon black nanoparticle templates, and the catalytic activity is studied for 1-butene isomerization. The HPAs supported on conventional zeolite and on mesoporous zeolite exhibit very different activities and thus provide good model systems to investigate the structure dependence of the catalytic properties. The HPA on mesoporous silicalite-1 shows enhanced catalytic activity for 1-butene isomerization, while HPA on conventional silicalite-1 exhibits low activity. To elucidate the structural difference, supported HPA catalysts are characterized using a variety of techniques, including 31P magic angle spinning nuclear magnetic resonance, and are shown to contain a range of species on both mesoporous and conventional zeolites. However, contrary to studies reported in the literature, conventional NMR techniques and chemical shifts alone do not provide sufficient information to distinguish the dispersed and aggregated surface species. The dispersed phase and the nondispersed phase can only be unambiguously and quantitatively characterized using spin−lattice relaxation NMR techniques. The HPA supported on mesoporous zeolite contains a fast relaxation component related to the dispersed catalyst, giving a much higher activity, while the HPA supported on conventional zeolite has essentially only the slow relaxation component with very low activity. The results obtained from this work demonstrate that the combination of spinning sideband fitting and spin−lattice relaxation techniques can provide detailed structural information on not only the Keggin structure for HPA but also the degree of dispersion on the support.

Friday, June 19, 2009

Chem. Mater., 2009, 21 (12), pp 2518–2524

A One-Step Mechanochemical Route to Core−Shell Ca2SnO4 Nanoparticles Followed by 119Sn MAS NMR and 119Sn Mssbauer Spectroscopy

Vladimir epelk*† and Klaus Dieter BeckerChem

Calcium stannate (Ca2SnO4) nanoparticles with an average size of about 15 nm were synthesized via single-step mechanochemical processing of binary oxide precursors at room temperature. High-resolution TEM studies revealed a nonuniform structure of mechanosynthesized Ca2SnO4 nanoparticles consisting of an ordered core surrounded by a disordered surface shell region. The inner core of a Ca2SnO4 nanoparticle possesses a fully ordered orthorhombic structure, and the surface shell exhibits the thickness of about 1.5 nm. The volume fraction of surface shell regions in the nanostructured mechanosynthesized stannate is estimated to be about 50%. Because of the ability of both solid-state 119Sn MAS NMR and 119Sn Mssbauer spectroscopies to probe the local environment of Sn nuclei, valuable complementary insight into the local structural disorder in mechanosynthesized Ca2SnO4 was obtained. It was concluded that the near-surface layers of stannate nanoparticles are disordered because of broadly distorted geometry of SnO6 octahedra. The octahedra are deformed in such a way that they become more regular.

J. Am. Chem. Soc., 2009, 131 (24), pp 8587–8594

NMR Detected Hydrogen−Deuterium Exchange Reveals Differential Dynamics of Antibiotic-and Nucleotide-Bound Aminoglycoside Phosphotransferase 3′-IIIa
Adrianne L. Norris† and Engin H. Serpersu*†‡§

In this work, hydrogen−deuterium exchange detected by NMR spectroscopy is used to determine the dynamic properties of the aminoglycoside phosphotransferase 3′-IIIa (APH), a protein of intense interest due to its involvement in conferring antibiotic resistance to both Gram negative and Gram positive microorganisms. This represents the first characterization of dynamic properties of an aminoglycoside-modifying enzyme. Herein we describe in vitro dynamics of apo, binary, and ternary complexes of APH with kanamycin A, neomycin B, and metal−nucleotide. Regions of APH in different complexes that are superimposable in crystal structures show remarkably different dynamic behavior. A complete exchange of backbone amides is observed within the first 15 h of exposure to D2O in the apo form of this 31 kDa protein. Binding of aminoglycosides to the enzyme induces significant protection against exchange, and 30% of the amides remain unexchanged up to 95 h after exposure to D2O. Our data also indicate that neomycin creates greater solvent protection and overall enhanced structural stability to APH than kanamycin. Surprisingly, nucleotide binding to the enzyme−aminoglycoside complex increases solvent accessibility of a number of amides and is responsible for destabilization of a nearby β-sheet, thus providing a rational explanation for previously observed global thermodynamic parameters. Our data also provide a molecular basis for broad substrate selectivity of APH.

Tuesday, June 16, 2009

Macromolecules, v42, issue 10-12

Chain Folding and Diffusion in Monodisperse Long n-Alkanes by Solid-State NMR

Giuseppe Grasso* and Jeremy J. Titman
Macromolecules, 2009, 42 (12), pp 4175–4180
DOI: 10.1021/ma801049j

Abstract: The size of the folded segment of the alkane chain in the integer-folded F2 form of C246H494 has been measured directly by NMR and shown to contain an average of 8 ± 2 carbon atoms at 293 K. This tight fold is in contrast to the looser fold obtained indirectly for similar samples in earlier NMR work but in good agreement with the picture provided by SAXS and LAM Raman spectroscopy studies. The recovery after saturation of the carbon-13 magnetization associated with the all-trans carbon-13 peak has also been measured for recovery times as long as 4000 s. The behavior as a function of recovery time has been compared with simple simulations which model the combined effects of chain diffusion and spin−lattice relaxation. These results suggest that at short times the mechanism for the magnetization recovery involves the alkane chain diffusing around the fold with the methyl groups constrained from entering the crystal through the opposite surface of the lamella. Hence, only small fluctuations in the length of the fold segment occur, and the two linked stems move in a loosely co-operative fashion. Despite the tight fold, the rate of magnetization recovery is compatible with a frequency for the underlying chain jump process similar to that responsible for the α-relaxation in polyethylene.

J. Phys Chem B and C, vol. v113, i24

1H Photo-CIDNP Enhancements in Heteronuclear Correlation NMR Spectroscopy

Ashok Sekhar and Silvia Cavagnero*
J. Phys. Chem. B, 2009, 113 (24), pp 8310–8318

Abstract:Photochemically induced dynamic nuclear polarization (photo-CIDNP) is usually employed as a probe of solvent exposure in biomolecular NMR. The potential of the photo-CIDNP effect for sensitivity enhancement, however, remains poorly explored. Here, we introduce 1H-photo-CIDNP in heteronuclear correlation spectroscopy at low laser irradiation power (1 W), and compare the sensitivity of various 1H-photo-CIDNP-enhanced- (HPE) 1H−15N heteronuclear correlation pulse sequences, including HSQC, HMQC, and SOFAST-HMQC, in terms of their ability to detect the Trp indole Hε1 resonance. Both Trp and the Trp-containing protein apoHmpH were analyzed using flavin mononucleotide as photosensitizer in aqueous solutions either containing or lacking urea. We find that 1H−15N photo-CIDNP-SOFAST-HMQC, denoted here as HPE-SOFAST-HMQC, yields a 2-fold higher signal-to-noise per unit time than the parent SOFAST-HMQC, for the solvent-exposed Trp of urea-unfolded apoHmpH. Thus, HPE-SOFAST-HMQC is the most sensitive heteronuclear correlation pulse sequence for the detection of solvent-exposed Trp.

Solid-State NMR Study of Nanodiamonds Produced by the Detonation Technique
Marc Dubois
*, Katia Gurin, Elodie Petit, Nicolas Batisse, Andr Hamwi, Naoki Komatsu, Jrme Giraudet§, Pascal Pirotte§ and Francis Masin*§
J. Phys. Chem. C, 2009, 113 (24), pp 10371–10378

Abstract:Nanodiamonds obtained by the detonation method have been investigated by means of solid-state magnetic nuclear resonance (NMR) and electron paramagnetic resonance. 13C and 1H magic angle spinning (MAS) NMR and 13C MAS NMR with 1H to 13C cross-polarization allow the determination of surface-hydrogenated groups (CH, CH2, and COH) and the quasi-absence of an sp2 carbon fullerene-like shell on the diamond surface to be underlined. The 1H and 13C spin−lattice relaxation time (T1) and second moment measurements are presented as a function of the temperature. Relaxation is shown to be mainly caused by paramagnetic centers in the case of 13C nuclei, whereas the presence of a molecular motion with an activation energy of 11.15 kJ·mol−1 is involved for 1H nuclei.

Identification of Mixed Valence Vanadium in ETS-10 Using Electron Paramagnetic Resonance, 51V Solid-State Nuclear Magnetic Resonance, and Density Functional Theory Studies

Kristopher Ooms and Tatyana Polenova
Michael J. Nash
and Raul F. Lobo*
J. Phys. Chem. C, 2009, 113 (24), pp 10477–10484

Abstract: Microporous vanadium-substituted titanosilicate ETS-10 solids are promising photocatalysts for decomposition of organic molecules. The dopant vanadium metal modulates the electronic environment of the titanosilicate matrix and plays a major role in the enhancement of the photocatalytic activity. However, the local electronic and geometric structure of the vanadium sites in these materials is a subject of controversy. Using vanadium electron paramagnetic resonance (EPR) and 51V nuclear magnetic resonance (NMR) spectroscopy, we have characterized the local environments of the vanadium sites in vanadium-substituted ETS-10 samples with different vanadium loadings. The measurements reveal clearly the presence of V(IV) and V(V) oxidation states. The EPR results suggest that V(IV) is in octahedral sites and, therefore, must substitute for Ti in the framework. 51V NMR studies indicate that the V(V) species are adjacent to the V(IV) species in most cases on the basis of significant electron−nuclear dipolar interaction between the V(V) nuclei and the unpaired electron on V(IV). The NMR chemical shift and electric field gradient parameters estimated from the NMR spectra are used in conjunction with density functional theory calculations to propose a model where the V(V) species preferentially occupy sites at the ends of the octahedral chains.

Long-Time Scale Ionic Dynamics in Dense Clay Sediments Measured by the Frequency Variation of the 7Li Multiple-Quantum NMR Relaxation Rates in Relation with a Multiscale Modeling
Patrice Porion
*, Anne Marie Faugre and Alfred Delville*
J. Phys. Chem. C, 2009, 113 (24), pp 10580–10597

Abstract: 7Li NMR relaxation measurements under spin-locking conditions are used to probe the dynamical properties of the lithium counterions within dense dispersions of charged anisotropic nanoplatelets. By simultaneously measuring the T1ρ and T2ρ relaxation times in addition to triple-quantum filtered relaxation times under the same spin-locking conditions, it is possible to separately quantify the contributions from the quadrupolar and the heterogeneous dipolar relaxation mechanisms. Thanks to the contribution from the residual quadrupolar coupling felt by the condensed lithium counterions, that procedure allows a broad dynamical range to be probed by performing spin-locking relaxation measurements using a limited number of irradiation powers. As illustrated by a multiscale modeling of the lithium diffusion and relaxation within such heterogeneous system, the frequency variation of the spectral densities characterizing the decorrelation of the quadrupolar coupling is a sensitive probe of the ionic mobility and the structure of the colloidal dispersion.

Proton Dynamics in Layered Double Hydroxides: A 1H T1 Relaxation and Line Width Investigation

Marc X. Reinholdt*, Panakkattu K. Babu§ and R. James Kirkpatrick
J. Phys. Chem. C, 2009, 113 (24), pp 10623–10631

Abstract:The investigation of the dynamics of water and organic species confined in minerals or adsorbed at their surface is of significant geochemical, environmental, catalytic, biomedicine, and life’s growth interests but is poorly understood on the molecular scale. This work explores the behavior of water molecules and glutamate species adsorbed on and between the double hydroxide layers of hydrotalcite [HT; (Mg2Al)(OH)6A−·nH2O, where A− is a counteranion which may bear different charges] and compares the results to those for HT containing small inorganic anions. The relative humidity (RH) dependence of the 1H T1 relaxation rates for all samples reveals the existence of two separate spin systems with 1/T1 relaxation rates differing by a factor of approximately 2 × 103. The static 1H spectral line widths allow assigning the fast relaxing protons to the fixed “static” interlayer and adsorbed species—i.e. bound water, bound organic species, and most of the structural hydroxyl groups (-OH)—and the slow ones to the “mobile” species—i.e. free water and solvated organic molecules and some of the structural -OH groups.

Transformation of AlPO-53 to JDF-2: Reversible Dehydration of a Templated Aluminophosphate Studied by MAS NMR and Diffraction

Sharon E. Ashbrook
*, Marica Cutajar, John M. Griffin, Zoe A. D. Lethbridge§, Richard I. Walton*§ and Stephen Wimperis*
J. Phys. Chem. C, 2009, 113 (24), pp 10780–10789

Abstract:We describe a detailed study of the aluminum phosphate AlPO-53 in both its as-made and calcined forms. In its as-made state, AlPO-53(A), the material is templated by methylammonium cations and contains occluded water molecules and also hydroxide ions that bridge pairs of aluminum atoms, increasing their coordination number to 5. Solid-state NMR experiments confirm the local environment of the aluminum and phosphorus atoms proposed in a previous structural model from powder X-ray diffraction. 31P NMR shows the presence of four distinct resonances with an intensity ratio of 1:1:2:2, consistent with the expected six crystallographic P sites. 27Al triple-quantum MAS NMR resolves five aluminum peaks, two with NMR parameters characteristic of four-coordinate Al and three of five-coordinate Al. One of these latter signals has a greater intensity than that of the others, consistent with the presence of two overlapping signals from two distinct crystallographic Al sites. First-principles calculations of NMR parameters provide a complete spectral assignment and confirm our interpretation of unresolved spectra. AlPO-53(A) is found to convert easily into a second crystalline phase on moderate heating (upon spinning in the NMR rotor for an extended period, for example), and variable-temperature powder X-ray experiments, together with TGA, suggest that this is a dehydration process yielding a second aluminophosphate, JDF-2. This is confirmed using both 31P and 27Al NMR, with the spectral assignment of JDF-2 supported by first-principles calculations. Calcination of AlPO-53(A) or of the dehydrated material, JDF-2, at 300 °C yields the microporous open-framework material AlPO-53(B), a tetrahedral network with three Al and three P sites, as confirmed by NMR and first-principles calculations. In addition to demonstrating the power of the combined use of NMR, first-principles calculations, and diffraction for detailed structural investigations, we show that the possibility of a reversible dehydration in as-made AlPO-53 and similar systems is an important consideration in structural studies and provides evidence that the published structural model for AlPO-53(A) may be incomplete.

Friday, June 12, 2009

J. Chem. Phys. 130, 225103 (2009)

Multiple-oscillating-field techniques for accurate distance measurements by solid-state NMR
J. Chem. Phys. 130, 225103 (2009)
Lasse Arnt Straasø, Morten Bjerring, Navin Khaneja, and Niels Chr. Nielsen
Dipolar truncation prevents accurate measurement of long-range internuclear distances between nuclei of the same spin species, e.g., within 13C–13C spin pairs in uniformly 13C-isotope-labeled proteins, using magic-angle spinning solid-state NMR spectroscopy. Accordingly, one of the richest sources of accurate structure information is at present not exploited fully, leaving the bulk part of the experimentally derived structural constraints to less accurate long-range 13C–13C dipolar couplings estimated from methods based on spin diffusion through proton spins in the close environment. In this paper, we extend our previous triple-oscillating field technique [N. Khaneja and N. C. Nielsen, J. Chem. Phys. 128, 015103 (2008)] for dipolar recoupling without dipolar truncation in homonuclear spin systems to a more advanced rf modulation with four independent oscillations and rotations involving nonorthogonal axes. This provides important new degrees of freedom, which are used to improve the scaling factor of the recoupled dipole-dipole couplings by a factor of 2.5 relative to the triple-oscillating field approach. This significant improvement, obtained by refocusing of otherwise defocused parts of the residual dipolar coupling Hamiltonian, may be exploited to measure much weaker 13C–13C dipolar couplings (and thereby longer distances) with much higher accuracy. We present a detailed theoretical description of multiple-field oscillating recoupling experiments, along with numerical simulations and experimental results on U–13C, 15N-L-threonine and U–13C,15N-ubiquitin.

Thursday, June 11, 2009

J. Am. Chem. Soc., 2009, 131 (23), pp 8108–8120

Paramagnetic Ions Enable Tuning of Nuclear Relaxation Rates and Provide Long-Range Structural Restraints in Solid-State NMR of Proteins
Philippe S. Nadaud, Jonathan J. Helmus, Stefanie L. Kall and Christopher P. Jaroniec

Magic-angle-spinning solid-state nuclear magnetic resonance (SSNMR) studies of natively diamagnetic uniformly 13C,15N-enriched proteins, intentionally modified with side chains containing paramagnetic ions, are presented, with the aim of using the concomitant nuclear paramagnetic relaxation enhancements (PREs) as a source of long-range structural information. The paramagnetic ions are incorporated at selected sites in the protein as EDTA−metal complexes by introducing a solvent-exposed cysteine residue using site-directed mutagenesis, followed by modification with a thiol-specific reagent, N-[S-(2-pyridylthio)cysteaminyl]EDTA-metal. Here, this approach is demonstrated for the K28C and T53C mutants of B1 immunoglobulin-binding domain of protein G (GB1), modified with EDTA-Mn2+ and EDTA-Cu2+ side chains. It is shown that incorporation of paramagnetic moieties, exhibiting different relaxation times and spin quantum numbers, facilitates the convenient modulation of longitudinal (R1) and transverse (R2, R1ρ) relaxation rates of the protein 1H, 13C, and 15N nuclei. Specifically, the EDTA-Mn2+ side chain generates large distance-dependent transverse relaxation enhancements, analogous to those observed previously in the presence of nitroxide spin labels, while this phenomenon is significantly attenuated for the Cu2+ center. Both Mn2+ and Cu2+ ions cause considerable longitudinal nuclear PREs. The combination of negligible transverse and substantial longitudinal relaxation enhancements obtained with the EDTA-Cu2+ side chain is especially advantageous, because it enables structural restraints for most sites in the protein to be readily accessed via quantitative, site-resolved measurements of nuclear R1 rate constants by multidimensional SSNMR methods. This is demonstrated here for backbone amide 15N nuclei, using methods based on 2D 15N−13C chemical shift correlation spectroscopy. The measured longitudinal PREs are found to be highly correlated with the proximity of the Cu2+ ion to 15N spins, with significant effects observed for nuclei up to 20 Å away, thereby providing important information about protein structure on length scales that are inaccessible to traditional SSNMR techniques.

Wednesday, June 10, 2009

J. Am. Chem. Soc., 2009, 131 (23), pp 8271–8279

Application of Solid-State 209Bi NMR to the Structural Characterization of Bismuth-Containing Materials

Hiyam Hamaed†, Michael W. Laschuk†, Victor V. Terskikh‡ and Robert W. Schurko*†

Herein, we report the first detailed study of 209Bi solid-state NMR (SSNMR) spectroscopy of extremely broad central transition powder patterns. 209Bi ultrawideline SSNMR spectra of several bismuth-containing materials (bismuth oxyhalides, bismuth nitrate pentahydrate, nonaaquabismuth triflate, and bismuth acetate) were acquired at field strengths of 9.4 and 21.1 T using frequency-stepped techniques. The 209Bi SSNMR experiments at 9.4 T yield powder patterns with breadths ranging from 0.9 to 14.6 MHz, from which quadrupolar coupling constants, CQ(209Bi), between 78 and 256 MHz, were extracted via analytical simulations. The breadths of the quadrupolar-dominated spectra and overall experimental times are greatly reduced for experiments conducted at 21.1 T, which yield high signal-to-noise spectra in which the smaller effects of bismuth chemical shift anisotropy can be clearly observed. The 209Bi electric field gradient (EFG) and chemical shift (CS) tensor parameters extracted from these spectra are correlated to the molecular structures at the bismuth sites, via first principles calculations of 209Bi EFG and CS tensors performed using CASTEP for periodic solids and Gaussian 03 for molecular clusters. The rapidity with which 209Bi SSNMR spectra can be acquired at ultrahigh fields, the sensitivity of the 209Bi NMR parameters to the bismuth environment, and the predictive power of theoretically calculated NMR interaction tensors suggest that 209Bi SSNMR may be useful for the characterization of a variety of Bi-containing materials and compounds.

Tuesday, June 09, 2009

Cryst. Growth Des., Article ASAP

Guest Molecules Confined in Amphipathic Crystals as Revealed by X-ray Diffraction and MAS NMR†
Angiolina Comotti*‡, Silvia Bracco‡, Piero Sozzani‡, Samuel M. Hawxwell#, Chunhua Hu# and Michael D. Ward*#

Multinuclear 1H−13C heterocorrelated (HETCOR) NMR spectroscopy combined with X-ray diffraction revealed the unusual dual properties of identical guest molecules confined in two crystallographically distinct host cavities within single crystals based on hexagonal frameworks comprising guanidinium ions and organomonosulfonates. The environments of the two cavities differ substantially, as one is lined by the highly polar quasihexagonal guanidinium-sulfonate network, while the other is defined by walls consisting of nonpolar aromatic groups. The effect of these different environments on the NMR properties of the guest molecules is evident from chemical shift data and two-dimensional HETCOR spectra. The large magnetic susceptibility effect due to ring currents of the aromatic hosts enables determination of the host−guest distances and suggests intermolecular CH···π interactions. The 13C relaxation times reveal the molecular dynamics of the guests in two nanoscale environments that differ with respect to shape and dimensionality.

J. Am. Chem. Soc., Article ASAP

Longitudinal-Relaxation-Enhanced NMR Experiments for the Study of Nucleic Acids in Solution
Jonathan Farjon†, Jrme Boisbouvier†, Paul Schanda§, Arthur Pardi‡, Jean-Pierre Simorre† and Bernhard Brutscher*†

Atomic-resolution information on the structure and dynamics of nucleic acids is essential for a better understanding of the mechanistic basis of many cellular processes. NMR spectroscopy is a powerful method for studying the structure and dynamics of nucleic acids; however, solution NMR studies are currently limited to relatively small nucleic acids at high concentrations. Thus, technological and methodological improvements that increase the experimental sensitivity and spectral resolution of NMR spectroscopy are required for studies of larger nucleic acids or protein−nucleic acid complexes. Here we introduce a series of imino-proton-detected NMR experiments that yield an over 2-fold increase in sensitivity compared to conventional pulse schemes. These methods can be applied to the detection of base pair interactions, RNA−ligand titration experiments, measurement of residual dipolar 15N−1H couplings, and direct measurements of conformational transitions. These NMR experiments employ longitudinal spin relaxation enhancement techniques that have proven useful in protein NMR spectroscopy. The performance of these new experiments is demonstrated for a 10 kDa TAR-TAR*GA RNA kissing complex and a 26 kDa tRNA.

J. Am. Chem. Soc., Article ASAP

Simultaneous measurement of residual dipolar couplings for proteins in complex using the isotopically discriminated NMR approach

Wolfgang Bermel†, Elena N. Tkach‡, Alexander G. Sobol‡ and Alexander P. Golovanov*§

One-bond residual dipolar couplings (RDCs) measured for the amide groups of proteins partially aligned in a magnetic field provide valuable information regarding the relative orientation of protein units. In order for RDCs obtained for individual proteins to be useful in the structure determination of heterodimer complexes, they should be measured for exactly the same alignment of the complex. Here, an isotopically discriminated IDIS-RDC-TROSY NMR experiment is proposed, which enables the measurement of HN RDCs for two proteins simultaneously and independently, but in the same sample, while they are part of the same complex. The signals for both proteins, one of which should be labeled with 15N and the other with 15N and 13C, are observed in different subspectra, thus reducing spectral overlap. The approach uniquely ensures that RDCs measured for both proteins relate to exactly the same alignment tensor, allowing accurate measurement of the relative angle between the two proteins. The method is also applicable for complexes containing three or more protein components. The experiment can speed up and lead to automation of protein−protein docking on the basis of angular restraints.

J. Am. Chem. Soc., Article ASAP

Broadband “Infinite-Speed” Magic-Angle Spinning NMR Spectroscopy

Yan-Yan Hu†, E. M. Levin‡ and Klaus Schmidt-Rohr*†

High-resolution magic-angle spinning NMR of high-Z spin-1/2 nuclei such as 125Te, 207Pb, 119Sn, 113Cd, and 195Pt is often hampered by large (>1000 ppm) chemical-shift anisotropies, which result in strong spinning sidebands that can obscure the centerbands of interest. In various tellurides with applications as thermoelectrics and as phase-change materials for data storage, even 22-kHz magic-angle spinning cannot resolve the center- and sidebands broadened by chemical-shift dispersion, which precludes peak identification or quantification. For sideband suppression over the necessary wide spectral range (up to 200 kHz), radio frequency pulse sequences with few, short pulses are required. We have identified Gan’s two-dimensional magic-angle-turning (MAT) experiment with five 90° pulses as a promising broadband technique for obtaining spectra without sidebands. We have adapted it to broad spectra and fast magic-angle spinning by accounting for long pulses (comparable to the dwell time in t1) and short rotation periods. Spectral distortions are small and residual sidebands negligible even for spectra with signals covering a range of 1.5 γB1, due to a favorable disposition of the narrow ranges containing the signals of interest in the spectral plane. The method is demonstrated on various technologically interesting tellurides with spectra spanning up to 170 kHz, at 22 kHz MAS.

Friday, June 05, 2009

Organometallics, v28, Issues 10 to 11

73Ge NMR Spectral Investigations of Singly Bonded Oligogermanes

Monika L. Amadoruge§, Claude H. Yoder, Julia Hope Conneywerdy, Katie Heroux, Arnold L. Rheingold and Charles S. Weinert*§

Organometallics, 2009, 28 (10), pp 3067–3073
DOI: 10.1021/om900035r
Abstract:The synthesis of the two digermanes Bus3GeGePh3 and PhMe2GeGePh3, as well as the branched tetragermane PhGe(GeBun3)3, was achieved using the hydrogermolysis reaction. These species were fully characterized by NMR (1H, 13C) spectroscopy and elemental analysis, and the crystal structure of PhMe2GeGePh3 was determined. These three species, along with 11 other oligogermanes, were also characterized by 73Ge NMR spectroscopy. Chemical shifts of the 73Ge NMR resonances for these oligogermanes have been correlated with the substitution pattern at germanium and also with the number of germanium−germanium bonds at the individual Ge centers. Germanium centers having only one attached germanium atom result in resonances appearing in the range δ −30 to −65 ppm, while those having two or three bonded germanium atoms exhibit resonances in the respective ranges δ −100 to −120 and δ −195 to −210 ppm. Chemical shifts of resonances for germanium centers bearing phenyl substituents appear upfield from those having alkyl substituents.

J. Phys. Chem. C, vol. 113, Issues 20 to 23

MgCl2·4(CH3)2CHOH: A New Molecular Adduct and Super Active Polymerization Catalyst Support

K. S. Thushara
, Renny Mathew, T. G. Ajithkumar, P. R. Rajamohanan, Sumit Bhaduri*§ and Chinnakonda S. Gopinath*

J. Phys. Chem. C, 2009, 113 (20), pp 8556–8559
DOI: 10.1021/jp9026546
Abstract: A new molecular adduct, MgCl2·4(CH3)2CHOH, has been synthesized and characterized for structural aspects and demonstrated for super active ethylene polymerization activity with TiCl4 to ultrahigh molecular weight polyethylene in high yield.

Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS 13C NMR Investigations

Niki Baccile
*, Guillaume Laurent, Florence Babonneau, Franck Fayon§, Maria-Magdalena Titirici and Markus Antonietti

J. Phys. Chem. C, 2009, 113 (22), pp 9644–9654
DOI: 10.1021/jp901582x
Abstract: The local structure of carbon spheres obtained via the hydrothermal carbonization process is characterized by using a combination of advanced solid-state 13C NMR techniques. Glucose was chosen as the starting product because it offers the possibility of 13C isotopic enrichment and is regarded as a model compound for more complex polysaccharides and biomass, as reported in recent studies. A number of 13C solid-state MAS NMR techniques (single-pulse, cross-polarization, inversion recovery cross-polarization, INEPT, 13C−13C proton-driven magnetization exchange, and 13C−13C double-quantum−single-quantum correlation experiments) were combined to retrieve information about binding motifs and C−C closest neighbor relations. We found that the core of the carbonaceous scaffold is composed of furan rings cross-linked by domains containing short keto-aliphatic chains instead of otherwise expected graphene-type sheets, as mainly reported either for hydrothermal carbon spheres or for biomass-related carbons obtained by low-temperature (<350>

Optimized Synthesis and Structural Characterization of the Borosilicate MCM-70

Dan Xie
, Lynne B. McCusker*, Christian Baerlocher, Lisa Gibson#, Allen W. Burton and Son-Jong Hwang§
J. Phys. Chem. C, 2009, 113 (22), pp 9845–9850
DOI: 10.1021/jp903500q

Abstract:A structure analysis of the borosilicate zeolite MCM-70, whose synthesis had been patented in 2003, was reported in 2005. Unfortunately, that structure analysis was somewhat ambiguous. Anisotropic line broadening made it difficult to model the peak shape, some peaks in the electron density map could not be interpreted satisfactorily, the framework geometry was distorted, and MAS NMR results were partially contradictory. In an attempt to resolve some of these points, an optimization of the synthesis was undertaken, and the structure was reinvestigated. The structure was solved from synchrotron powder diffraction data collected on an as-synthesized sample (Pmn21, a = 13.3167(1) Å, b = 4.6604(1) Å, c = 8.7000(1) Å) using a powder charge-flipping algorithm. The framework topology, with a 1-dimensional, 10-ring channel system, is identical to the one previously reported. However, the B in this new sample was found to be ordered in the framework, fully occupying one of the four tetrahedral sites. Two extra-framework K+ ion positions, each coordinated to five framework O atoms and one water molecule, were also found. The solid state 29Si, 11B and 1H NMR results are fully consistent with this ordered structure.

LiSc(BH4)4 as a Hydrogen Storage Material: Multinuclear High-Resolution Solid-State NMR and First-Principles Density Functional Theory Studies

Chul Kim
, Son-Jong Hwang*, Robert C. Bowman, Jr., Joseph W. Reiter, Jason A. Zan, James G. Kulleck, Houria Kabbour§#, E. H. Majzoub and V. Ozolins

J. Phys. Chem. C, 2009, 113 (22), pp 9956–9968
DOI: 10.1021/jp9011685
Abstract: A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH4)4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH4 and ScCl3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH4)4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH4)3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I (#82) is predicted to be the lowest energy state for LiSc(BH4)4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB2 and [B12H12]−2 anion containing (e.g., Li2B12H12) along with residual LiBH4. Reaction of a desorbed LiSc(BH4)4 + 4LiCl mixture (from 4LiBH4/ScCl3 sample) with hydrogen gas at 70 bar resulted only in an increase in the contents of Li2B12H12 and LiBH4. Full reversibility to reform the LiSc(BH4)4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications.

A 47/49Ti Solid-State NMR Study of Layered Titanium Phosphates at Ultrahigh Magnetic Field

Jianfeng Zhu
, Nick Trefiak, Tom K. Woo and Yining Huang*

J. Phys. Chem. C, 2009, 113 (23), pp 10029–10037
DOI: 10.1021/jp901235w
Abstract: Layered titanium phosphates (TiPs) have many potentially important applications in ion exchange, catalysis, intercalation, and sorption. Characterization of metal local environments by solid-state 47/49Ti NMR has been difficult due to many unfavorable 47/49Ti NMR properties. In this work, we have directly characterized the local structures around Ti in several representative layered TiPs, including α-, β-, and γ-TiP, by examining the 47/49Ti static NMR spectra of these materials at an ultrahigh magnetic field of 21.1 T. The 47/49Ti chemical shielding and electric field gradient (EFG) tensors have been extracted from spectral analysis. The observed 47/49Ti spectra are mainly determined by the second-order quadrupolar interactions. The quadrupole coupling constants (CQ) are sensitive to the distortion of the TiO6 octahedron in this series of layered TiPs. Quantum mechanical calculations have been performed on several model clusters as well as periodic systems. The results indicate that, in addition to the oxygens in the first coordination sphere of Ti, the atoms in the second and third coordination spheres and beyond also have significant effects on the EFG at the metal center, and this long-range effect contributes substantially to the CQ. A relationship between observed CQ and the Ti−O bond length distortion parameter appears to exist, and this empirical correlation is also confirmed by theoretical calculations. Using sodium-exchanged α-TiP (α-Na-TiP) with an unknown structure as an example, we show that the 47/49Ti NMR spectra can provide partial information on the local environment of the metal center. For this material, the ion exchange does not affect the Ti local environment significantly. It appears that the layer in α-TiP is more robust compared to that of the zirconium analogue.

Electron−Nuclear Spin Dynamics in a Bacterial Photosynthetic Reaction Center

Eugenio Daviso
, A. Alia, Shipra Prakash, Anna Diller, Peter Gast, Johan Lugtenburg, Jrg Matysik* and Gunnar Jeschke§

J. Phys. Chem. C, 2009, 113 (23), pp 10269–10278
DOI: 10.1021/jp900286q
Abstract: The solid-state photo-CIDNP effect is known to occur in natural photosynthetic reaction centers (RCs) where it can be observed by magic-angle spinning (MAS) NMR as strong modification of signal intensities under illumination compared to experiments performed in the dark. The origin of the effect has been debated. In this paper, we report time-resolved photo-CIDNP MAS NMR data of reaction centers of quinone depleted Rhodobacter sphaeroides. It is demonstrated that the build-up of nuclear polarization on the primary donor and the bacteriopheophytin acceptor depends on the presence and lifetimes of the molecular triplet states of the donor and carotenoid. Analysis of the data proves that up to three electron−nuclear spin-coupling mechanisms and two transient effects are working concomitantly in the spin-chemical machinery of the reaction center

Thursday, June 04, 2009

J. Am. Chem. Soc., 2009, 131 (22), pp 7806–7816

Accurate Measurement of Methyl 13C Chemical Shifts by Solid-State NMR for the Determination of Protein Side Chain Conformation: The Influenza A M2 Transmembrane Peptide as an Example
Mei Hong*, Tatiana V. Mishanina and Sarah D. Cady

The use of side chain methyl 13C chemical shifts for the determination of the rotameric conformation of Val and Leu residues in proteins by solid-state NMR spectroscopy is described. Examination of the solution NMR stereospecifically assigned methyl groups shows significant correlation between the difference in the two methyl carbons’ chemical shifts and the side chain conformation. It is found that α-helical and β-sheet backbones cause different side chain methyl chemical shift trends. In α-helical Leu’s, a relatively large absolute methyl 13C shift difference of 2.89 ppm is found for the most populated mt rotamer (χ1 = −60°, χ2 = 180°), while a much smaller value of 0.73 ppm is found for the next populated tp rotamer (χ1 = 180°, χ2 = 60°). For α-helical Val residues, the dominant t rotamer (χ1 = 180°) has more downfield Cγ2 chemical shifts than Cγ1 by 1.71 ppm, while the next populated m rotamer (χ1 = −60°) shows the opposite trend of more downfield Cγ1 chemical shift by 1.23 ppm. These significantly different methyl 13C chemical shifts exist despite the likelihood of partial rotameric averaging at ambient temperature. We show that these conformation-dependent methyl 13C chemical shifts can be utilized for side chain structure determination once the methyl 13C resonances are accurately measured by double-quantum (DQ) filtered 2D correlation experiments, most notably the dipolar DQ to single-quantum (SQ) correlation technique. The advantage of the DQ−SQ correlation experiment over simple 2D SQ−SQ correlation experiments is demonstrated on the transmembrane peptide of the influenza A M2 proton channel. The methyl chemical shifts led to predictions of the side chain rotameric states for several Val and Leu residues in this tetrameric helical bundle. The predicted Val rotamers were further verified by dipolar correlation experiments that directly measure the χ1 torsion angles. It was found that the chemical-shift-predicted side chain conformations are fully consistent with the direct torsion angle results; moreover, the methyl 13C chemical shifts are sensitive to 5° changes in the χ1 torsion angle due to drug binding.

J. Am. Chem. Soc., 2009, 131 (22), pp 7641–7653

Acid-Induced Amino Side-Chain Interactions and Secondary Structure of Solid Poly-l-lysine Probed by 15N and 13C Solid State NMR and ab Initio Model Calculations
Alexandra Dos‡, Volkmar Schimming‡, Monique Chan Huot‡ and Hans-Heinrich Limbach*‡

The acid−base and base−base interactions of the 15N-labeled side-chain amino groups of dry solid poly-l-lysine (PLL) and the consequences for the secondary structure have been studied using high-resolution solid state 15N and 13C CPMAS NMR spectroscopy. In a previous study we had shown that at acid/base ratios of 1 per amino group the halogen acids HI, HCl and HBr form PLL salts in the β-pleated sheet but not in the α-helical structure, whereas HF and various oxygen acids form 1:1 acid−base hydrogen-bonded complexes in both secondary structures. In the present study we performed NMR experiments at reduced acid/base ratios in order to elucidate whether also 1:2 and 1:3 acid−base complexes are formed under these conditions. Generally, the PLL samples containing HF, HBr, HCl, HI, CH3COOH, H3PO4, H2SO4, or HNO3 were obtained by lyophilization at different pH. For comparison, samples were also obtained by letting dry acid-free PLL interact with gaseous HCl. In a theoretical section we first study the probability of the different acid−base complexes as a function of the acid/base ratio and the equilibrium constants of the complex formation. Using this information, the 15N NMR spectra of acid doped PLL obtained were analyzed and assigned. Indeed, evidence for the formation of 1:2 and 1:3 acid−base complexes at lower acid/base ratios could be obtained. Moreover, the salt structures of the halides of PLL are already destroyed at acid/base ratios of about 0.8. By contrast, when acid-free poly-l-lysine is exposed to HCl gas, a biexponential conversion of amino groups into ammonium groups is observed without formation of 1:2 and 1:3 complexes. 13C NMR reveals that the β-pleated sheet environments of acid-free PLL react rapidly with HCl, whereas the α-helices first have to be converted in a slow reaction to β-pleated sheets before they can react. Interestingly, after partial doping with HCl, exposure to gaseous H2O catalyzes the interconversion of the ammonium and amino groups into a mixture of 1:1, 1:2 and 1:3 complexes. Finally, the 15N NMR assignments were assisted by DFT calculations on methylamine−acid model complexes.

J. Am. Chem. Soc., 2009, 131 (22), pp 7633–7640

Versatile Coordination of 2-Pyridinetetramethyldisilazane at Ruthenium: Ru(II) vs Ru(IV) As Evidenced by NMR, X-ray, Neutron, and DFT Studies
Mary Grellier*†, Tahra Ayed‡, Jean-Claude Barthelat‡, Alberto Albinati§, Sax Mason, Laure Vendier†, Yannick Coppel† and Sylviane Sabo-Etienne*†

The novel disilazane compound 2-pyridinetetramethyldisilazane (1) has been synthesized. The competition between N-pyridine coordination and Si−H bond activation was studied through its reactivity with two ruthenium complexes. The reaction between 1 and RuH2(H2)2(PCy3)2 led to the isolation of the new complex RuH2{(η2-HSiMe2)N(κN-C5H4N)(SiMe2H)}(PCy3)2 (2) resulting from the loss of two dihydrogen ligands and coordination of 1 to the ruthenium center via a κ2N,(η2-Si−H) mode. Complex 2 has been characterized by multinuclear NMR experiments (1H, 31P, 13C, 29Si), X-ray diffraction and DFT studies. In particular, the HMBC 29Si−1H spectrum supports the presence of two different silicon environments: one Si−H bond is dangling, whereas the other one is η2-coordinated to the ruthenium with a JSiH value of 50 Hz. DFT calculations (B3PW91) were also carried out to evaluate the stability of the agostic species versus a formulation corresponding to a bis(σ-Si−H) isomer and confirmed that N-coordination overcomes any stabilization that could be gained by the establishment of SISHA interactions. There is no exchange between the two Si−H bonds present in 2, as demonstrated by deuterium-labeling experiments. Heating 2 at 70 °C under vacuum for 24 h, leads to the formal loss of one equivalent of H2 from 2 and formation of the 16-electron complex RuH{(SiMe2)N(κN-C5H4N)(SiMe2H)}(PCy3)2 (3) formulated as a hydrido(silyl) species on the basis of multinuclear NMR experiments. The dehydrogenation reaction is fully reversible under dihydrogen atmosphere. Reaction of Ru(COD)(COT) with 3 equiv of 1 under a H2 pressure led to the isolation of the new complex RuH{(SiMe2)N(κN-C5H4N)(SiMe2H)}3 (4) characterized as a hydridotrisilyl complex by multinuclear NMR techniques, X-ray and neutron diffractions, as well as DFT calculations. The 29Si HMBC experiments confirm the presence of two different silicon atoms in 4, with a signal at −14.64 ppm for three dangling Si-Me2H fragments and a signal at 64.94 ppm (correlating with the hydride signal) assigned to three Si-Me2N groups bound to Ru. Comparison of DFT and neutron parameters involving the hydride clearly indicates an excellent correlation. The Si−H distance of 2.15 Å is much shorter than the sum of the van der Waals radii and typically in the range of a significant interaction between a silicon and a hydrogen atom (SISHA interactions). In 4, three dangling Si−H groups remain accessible for further functionalization.

J. Am. Chem. Soc., 2009, 131 (22), pp 7500–7501

Robust NMR Screening for Lead Compounds Using Tryptophan-Containing Proteins
Michal Bista†, Kaja Kowalska, Weronika Janczyk†, Alexander Dmling‡ and Tad A. Holak*†

NMR-based drug screening methods provide the most reliable characterization of binding propensities of ligands to their target proteins. Unique to NMR is its capability to detect weak μM−mM bindings. NMR assays are, however, one of the least effective methods in terms of the amount of protein required and the time needed for acquiring NMR experiments. We have recently described a time efficient 1D proton NMR assay for studying the effect of antagonists on protein−protein interactions. The method, named AIDA-NMR (for Antagonist Induced Dissociation Assay-NMR), can provide information on whether an antagonist of a protein−protein interaction is strong enough to dissociate the complex and, in addition, whether its interaction is through denaturation, precipitation, or release of a protein in its functional folded state. AIDA requires a large protein fragment (larger than ca. 30 kDa) to bind to a small reporter protein (smaller than ca.12 kDa). Here, we present an extension of this method, named SEI AIDA (SEI, for Selective Excitation-Inversion). The SEI AIDA uses tryptophan-bearing proteins, and by selectively exciting only the proton NMR signals of the NH indole side chains of tryptophans, the acquisition time of the AIDA experiment can be reduced by an order of magnitude relative to the corresponding 1D AIDA that uses hard pulses. Thus, at 600 MHz, the NH signal of a 35 μM protein complex can be acquired in only 2.5 min, making the SEI AIDA suitable for high-throughput screening pipelines in drug discovery.

Wednesday, June 03, 2009

SSNMR update: up to Volume 35, Issue 4, July 2009, Pages 208-213

SSNMR update: up to Volume 35, Issue 4, July 2009, Pages 208-213
Highlights include:
- deconvolution of CP/MAS NMR data sets
- using polyethylene glycol to prevent sample decomposition during MAS
- Ram's review of antimicrobial peptides
- some new 25Mg NMR of metals from T.J. Bastow
- QUADFIT, a new program from M.E. Smith's group
- 13C MAS NMR of surgical sutures
- an XML program for designing cogwheel phase cycles

Solid State Nuclear Magnetic Resonance
Volume 35, Issue 4, July 2009, Pages 208-213

Spectral deconvolution of NMR cross polarization data sets

Niklas Hedin, Jovice B.S. Ng and Peter Stilbs

The COmponent-REsolved (CORE) strategy has been employed, for the first time to solid state NMR spectroscopy. CORE was used to extract two time-dependent spectral components in 24 29Si{1H} NMR spectra, recorded on a meso-structured silica material under conditions of cross polarization evolution. No prior assumptions were made about the component bandshapes, which were both found to be skewed to higher chemical shifts. For the silica fragments close to protons this skewness could be rationalized by a distribution of the degree of condensation in the silica network; however, for the other component the non-Gaussian shape was unexpected. We expect that the same strategy could be applied to a range of experiments in solid-state NMR spectroscopy, where spectral distributions or kinetic parameters need to be accurately extracted.

Keywords: Multivariate analysis; Cross polarization; Deconvolution; Si-29; Mesoporous


Solid State Nuclear Magnetic Resonance
Volume 35, Issue 4, July 2009, Pages 214-216

Zhen Wu and Shangwu Ding

A simple solution is proposed to prevent a solid state polycrystalline sample from deterioration during long time high speed spinning experiments in solid state NMR. It is found that if a certain percentage (not, vert, similar40% volume) of polyethylene glycol (PEG, (HO–CH2–(CH2–O–CH2–)n–CH2–OH)n) is mixed with the sample that are subject to deterioration, the quality of the sample can be maintained for a long time under high speed spinning for a few days or longer, sufficient for multi-dimensional and/or low-sensitivity experiments. Both 1D and 2D experimental results are shown to support this idea.

Keywords: Magic angle spinning; Sample deterioration; Quadrupolar nuclei; Multiple quantum magic angle spinning; Polyethylene glycol


Solid State Nuclear Magnetic Resonance
Volume 35, Issue 4, July 2009, Pages 201-207

Beyond NMR spectra of antimicrobial peptides: Dynamical images at atomic resolution and functional insights

Ayyalusamy Ramamoorthy

There is a considerable current interest in understanding the function of antimicrobial peptides for the development of potent novel antibiotic compounds with a very high selectivity. Since their interaction with the cell membrane is the major driving force for their function, solid-state NMR spectroscopy is the unique method of choice to study these insoluble, non-crystalline, membrane-peptide complexes. Here I discuss solid-state NMR studies of antimicrobial peptides that have reported high-resolution structure, dynamics, orientation, and oligomeric states of antimicrobial peptides in a membrane environment, and also address important questions about the mechanism of action at atomic-level resolution. Increasing number of solid-state NMR applications to antimicrobial peptides are expected in the near future, as these compounds are promising candidates to overcome ever-increasing antibiotic resistance problem and are well suited for the development and applications of solid-state NMR techniques.

Keywords: Antimicrobial peptides; Membrane; Bilayers; Solid-state NMR


Solid State Nuclear Magnetic Resonance
Volume 35, Issue 4, July 2009, Pages 217-222

25Mg NMR site analysis in metals and intermetallics

T.J. Bastow and S. Celotto

Nuclear magnetic resonance (NMR) of the low abundance and low gyromagnetic ratio isotope 25Mg, I=5/2, 2.606 MHz/T, 10% abundant, is shown here to provide an informative probe for phase identification, site symmetry and site multiplicity of the intermetallic compounds which occur as strengthening precipitate phases in lightweight alloys. The intermetallics discussed here, Mg17Al12, MgZn2, Mg2Al3 and Al2CuMg, are the final equilibrium precipitate phases in a number of Mg- and Al-based heat-treatable alloys. The 25Mg spectra of Mg in Al–10 at%Mg alloy show the progressive precipitation of Mg2Al3 from Mg in solid solution as a function of annealing time at 150 °C. Also reported are 25Mg spectra for CuMg2, Mg44Al15Zn41 and Mg2Sn, along with the counter atom 67Zn and 63Cu NMR spectra for MgZn2 and CuMg2. All spectra are simulated to determine nuclear interaction parameters and confirm site occupancy.

Keywords: A. Metals; C. Crystal structure and symmetry; E. Nuclear magnetic resonance; E. Hyperfine interactions


Solid State Nuclear Magnetic Resonance
Volume 35, Issue 4, July 2009, Pages 223-229

Determination of NMR cogwheel phase cycle with XML

Yannick Millot, Redouane Hajjar and Pascal P. Man

The selection of correct coherence transfer pathways is an essential component of an NMR pulse sequence. This article describes a new method based on the use of web tools (eXtensible Markup Language and eXtensible Stylesheet Language Transformation) to generate a cogwheel phase cycle for selecting coherence transfer pathways. We illustrate this method with the three-pulse phase-modulated shifted-echo or split-t1 MQMAS sequences for triple-quantum spin-3/2 systems. After generalization to the different half-integer quadrupole spins, we use the SIMPSON program to confirm our results. Finally, we apply our method to the case of the z-filter 3QMAS sequence for I=3/2 systems.

Keywords: NMR; Cogwheel phase cycling; Coherence transfer pathways; XML; XSLT; SIMPSON; MQMAS; z-filter

Solid State Nuclear Magnetic Resonance
Volume 35, Issue 4, July 2009, Pages 230-234

Kinetics of solid-state NMR cross-polarization from protons to carbon-13 in surgical sutures

Marcin Sobczak, Tadeusz Chreptowicz, Joanna Kolmas and Waclaw Kolodziejski

Commercial Dexon surgical sutures, made of polyglycolide (PGA), were examined using 13C CP/MAS NMR. The study shows that detailed analysis of the cross-polarization (CP) process is useful in the peak assignments and in the assessment of molecular mobility in the polymer domains. Crystallinity of PGA in the sutures was estimated at ca. 55%.

Keywords: Biodegradable polymers; Polyglicolide; Surgical sutures; 13C NMR; Cross-polarization


Solid State Nuclear Magnetic Resonance
Volume 35, Issue 4, July 2009, Pages 243-252

QuadFit—A new cross-platform computer program for simulation of NMR line shapes from solids with distributions of interaction parameters

T.F. Kemp and M.E. Smith

A new Java computer program called QuadFit has been written to simulate NMR line shapes from solid materials. The program takes into account the major interactions, with a key feature that distributions of isotropic chemical shift and quadrupolar interaction parameters can be calculated, which are often encountered in amorphous and disordered materials. The quadrupolar interaction can be simulated for all the transitions for both half-integer and integer spins. The utility of the program is demonstrated with examples of 27Al (nuclear spin View the MathML source) in an atomically disordered aluminoborate mullite, 65Cu (View the MathML source) in CuInSe2 and 10B (I=3) in amorphous B2O3. The program has good cross-platform compatibility and is written for high stability. The program has been designed with an easy to use graphical interface. It can be run efficiently on any reasonably powerful PC and is freely available from the Warwick website (http://go.warwick.ac.uk/quadfit).

Keywords: NMR line shape simulation; Computer program; Interaction distribution; Amorphous materials