Wednesday, December 01, 2010

J. Phys. Chem. B and C, volume 114, Issues 43 - 45 + November ASAPs

Quantum Oscillations and Polarization of Nuclear Spins in Photoexcited Triplet States†

Gerd Kothe*‡, Tomoaki Yago‡, Jrg-Ulrich Weidner‡, Gerhard Link‡, Michail Lukaschek‡, and Tien-Sung Lin§

J. Phys. Chem. B, 2010, 114 (45), pp 14755–14762
DOI: 10.1021/jp103508t
Publication Date (Web): July 28, 2010
Copyright © 2010 American Chemical Society

Abstract: The unique physical properties of photoexcited triplet states have been explored in numerous spectroscopic studies employing electron paramagnetic resonance (EPR). So far, however, no quantum interference effects were found in these systems in the presence of a magnetic field. In this study, we report the successful EPR detection of nuclear quantum oscillations in an organic triplet state subject to an external magnetic field. The observed quantum coherences can be rationalized using an analytical theory. Analysis suggests that the nuclear spins are actively involved in the intersystem crossing process. The novel mechanism also acts as a source of oscillatory nuclear spin polarization that gives rise to large signal enhancement in nuclear magnetic resonance (NMR). This opens new perspectives for the analysis of chemically induced dynamic nuclear polarization in mechanistic studies of photoactive proteins.

W/Mo-Oxide Nanomaterials: Structure−Property Relationships and Ammonia-Sensing Studies†

Ying Zhou‡, Kaibo Zheng§, Jan-Dierk Grunwaldt, Thomas Fox‡, Leilei Gu§, Xiaoliang Mo§, Guorong Chen§, and Greta R. Patzke*‡

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp106439n
Publication Date (Web): November 30, 2010
Copyright © 2010 American Chemical Society

Abstract:W/Mo-oxides of the hexagonal tungsten bronze (HTB) type have been investigated by X-ray absorption spectroscopy to obtain detailed insight into the substitution process of W by Mo that leads to mixed HTB frameworks. Both the morphology of the nanostructured W/Mo-HTBs as well as the oxidation state of Mo are significantly influenced through the incorporation of different alkali cations into the hexagonal channels of this open structure. A variety of complementary analytical methods, including TG, in situ and ex situ XRD, SEM, and solid-state NMR analyses, were applied to determine the thermal stability of the obtained W/Mo-HTB materials with respect to their alkali cation and NH4+ contents. A strong correlation between composition and stability was found with the Rb-W/Mo-HTBs exhibiting the highest structural and morphological resistance among the series (up to 580 °C). The NH3-sensing properties of selected W/Mo-oxides in test atmospheres furthermore point to promising features of the Rb-stabilized hexagonal framework materials

Hydrogen Physisorption in a Cu(II) Metallacycle

Tanja Pietraβ*†, Itza Cruz-Campa‡, Justine Kombarakkaran†, Suman Sirimulla§, Atta M. Arif§, and Juan C. Noveron*‡
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp104544r
Publication Date (Web): November 19, 2010
Copyright © 2010 American Chemical Society

Abstract: The interaction of molecular hydrogen with a novel microporous dinuclear Cu(II) complex, [bis-μ-di(4-pyridyl)methanol-1,4,7-triazacyclononane copper(II)] triflate (1), and its derivatives formed from oxidation and solvent removal was studied with 2H NMR and density functional theory (DFT). The Cu-complex 1 was characterized with X-ray diffraction methods and consists of a dinuclear macrocycle that forms one-dimensional channels of 9.55 Å in diameter. 2H NMR studies of deuterium gas adsorption by 1 suggest that physisorption condensation of D2 occurs within two distinct microenvironments: in the interior and in-between the microtubular structures. The assignment of NMR resonances to specific adsorption sites is supported by spectral decomposition and analysis of the line widths and integrated signal intensities of the components. The dynamics of the system are probed by spin−lattice relaxation time measurements and spectral hole-burn experiments as a function of temperature and pressure. NMR and DFT calculations suggest that hydrogen uptake is mediated through interactions with the Cu(II) centers via dipole−ion interactions.

Influence of Structure on the Spectroscopic Properties of the Polymorphs of Piroxicam

Wei Liu†, Wei David Wang†, Wei Wang†, Shi Bai*†‡, and Cecil Dybowski‡

J. Phys. Chem. B, ASAP
DOI: 10.1021/jp1084444
Publication Date (Web): November 18, 2010
Copyright © 2010 American Chemical Society

Abstract: The complete 13C NMR chemical-shift tensors for the carbon sites of the two polymorphic forms (PI and PII) and the monohydrate form (PM) of the analgesic drug, piroxicam, are reported. The NMR parameters (isotropic chemical shifts, chemical-shielding anisotropies and asymmetries, and dipolar couplings), X-ray powder diffraction, and density functional calculations of piroxicam are analyzed in terms of hydrogen bonding and structure. The integration of all the data gives an improved model of the local solid-state structures of the polymorphs. In particular, the solid-state NMR spectra demonstrate that the asymmetric unit of the monohydrate, PM, contains two zwitterionic piroxicam molecules.

Heterogeneities in Gelatin Film Formation Using Single-Sided NMR

Sushanta Ghoshal*, Carlos Mattea, Paul Denner, and Siegfried Stapf

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp1068363
Publication Date (Web): November 18, 2010
Copyright © 2010 American Chemical Society

Abstract: Gelatin solutions were prepared in D2O. The drying process of cast solutions was followed with a single-sided nuclear magnetic resonance (NMR) scanner until complete solidification occurred. Spin−spin relaxation times (T2) were measured at different layers with microscopic resolution and were correlated with the drying process during film formation. Additionally, the evaporation of the gelatin solution was observed optically from the reduction of the sample thickness, revealing that at the macroscopic level, the rate of evaporation is not uniform throughout the experiment. A crossover in the spatial evolution of the drying process is observed from the NMR results. At the early stages, the gel appears to be drier in the upper layers near the evaporation front, while this tendency is inverted at the later stages, when drying is faster from the bottom. XRD (X-ray diffraction) data showed that a structural heterogeneity persists in the final film.

Understanding the Properties of the Coagel and Gel Phases: A 2H and 13C NMR Study of Amphiphilic Ascorbic Acid Derivatives

Silvia Borsacchi†, Moira Ambrosi‡, Pierandrea Lo Nostro‡, and Marco Geppi*†

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp107324e
Publication Date (Web): November 15, 2010
Copyright © 2010 American Chemical Society

Abstract: The coagel and gel phases formed by the d and l diastereoisomers of ascorbyl-dodecanoate (ASC12) in deuterated water were studied through solid-state NMR techniques. In particular, the dynamic properties of water and surfactant chains were investigated by 2H and 13C NMR static spectra, respectively. Two fractions of water with very different dynamics were found in the coagel phases, one solidlike and one liquidlike, assigned to water strongly bound to the surfactant polar heads and bulk water, respectively. Only one kind of “intermediate” water was instead detected in the gel phase suggesting that the merging of the two types of water in the interlayers between the surfactant lamellae occurs at the coagel-to-gel transition. Moreover, the surfactant chains, very rigid in the coagel phase, give rise to fast trans−gauche interconformational jumps in the gel phase, where almost isotropic reorientations of the whole aggregates also occur. A different dynamic behavior was found for the two diastereoisomers in particular for what concerns the surfactant molecules in the gel phase and the water molecules in the coagel presumably ascribable to different inter- and intramolecular interactions that involve the polar heads

The “Alkyl” and “Carbenium” Pathways of Methane Activation on Ga-Modified Zeolite BEA: 13C Solid-State NMR and GC-MS Study of Methane Aromatization in the Presence of Higher Alkane

Mikhail V. Luzgin, Anton A. Gabrienko, Vladimir A. Rogov, Alexander V. Toktarev, Valentin N. Parmon, and Alexander G. Stepanov*
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1078899
Publication Date (Web): November 11, 2010
Copyright © 2010 American Chemical Society

Abstract:By using 13C solid-state NMR spectroscopy and GC-MS analysis, the activation of methane and coaromatization of methane and propane have been monitored on gallium-modified zeolite BEA at 573−823 K. A noticeable degree involvement of the 13C-label from methane-13C into the aromatic reaction products (benzene, toluene) has been demonstrated. The major intermediate of the methane activation represents gallium-methyl species, which are formed by methane dissociative adsorption on Ga2O3 species of the zeolite. The minor species of methane activation, Ga-methoxy groups, provide the involvement of methane into aromatics by the methylation of aromatic molecules, which are generated exclusively from propane, by the mechanism of electrophilic substitution. Ga-methyl species can serve as methylating nucleophilic agent for the reaction of nucleophilic substitution with participation of aromatic molecules, which contain the electron-withdrawing substitutes.

Slow Exchange Model of Nonrigid Rotational Motion in RNA for Combined Solid-State and Solution NMR Studies

Prashant S. Emani†, Gregory L. Olsen‡, Dorothy C. Echodu‡, Gabriele Varani‡§, and Gary P. Drobny*‡

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp107193z
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: Functional RNA molecules are conformationally dynamic and sample a multitude of dynamic modes over a wide range of frequencies. Thus, a comprehensive description of RNA dynamics requires the inclusion of a broad range of motions across multiple dynamic rates which must be derived from multiple spectroscopies. Here we describe a slow conformational exchange theoretical approach to combining the description of local motions in RNA that occur in the nanosecond to microsecond window and are detected by solid-state NMR with nonrigid rotational motion of the HIV-1 transactivation response element (TAR) RNA in solution as observed by solution NMR. This theoretical model unifies the experimental results generated by solution and solid-state NMR and provides a comprehensive view of the dynamics of HIV-1 TAR RNA, a well-known paradigm of an RNA where function requires extensive conformational rearrangements. This methodology provides a quantitative atomic level view of the amplitudes and rates of the local and collective displacements of the TAR RNA molecule and provides directly motional parameters for the conformational capture hypothesis of this classical RNA−ligand interaction.

Molecular Dynamics of Amorphous Gentiobiose Studied by Solid-State NMR

Teresa G. Nunes*†, Hermnio P. Diogo†, Susana S. Pinto†, and Joaquim J. Moura Ramos‡

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp106371w
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: A solid-state NMR (SSNMR) study is reported on the effect of temperature on the molecular mobility of amorphous gentiobiose, which is complemented with data obtained from crystalline samples. 13C cross-polarization/magic-angle-spinning (CPMAS) spectra and 1H MAS spectra were obtained for gentiobiose at natural abundance, in the amorphous state, from 293 K up to the glass transformation region (Tg = 359 K). Two well-defined molecular mobility regimes were observed, corresponding to different motional modes. NMR results on molecular dynamics are discussed and compared with those obtained by thermally stimulated depolarization currents (TSDC) and dielectric relaxation spectroscopy (DRS). SSNMR spectra presented evidence for a new polymorphic form of gentiobiose, not yet reported in the literature, which is obtained by slow heating of the amorphous solid up to 364 K inside the NMR zirconia rotor.

73Ge Solid-State NMR of Germanium Oxide Materials: Experimental and Theoretical Studies

Vladimir K. Michaelis and Scott Kroeker*

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1071082
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: A comprehensive series of crystalline germanates has been studied by ultrahigh-field 73Ge NMR and quantum chemical calculations. Despite its low gyromagnetic ratio, low natural abundance and large quadrupole moment, interpretable spectra were obtained in almost all cases, demonstrating that 73Ge is an accessible NMR nucleus. The spectra yield a wide range of quadrupole coupling constants (CQ = 9 to 35 MHz), with calculations indicating a range twice that, which are rationalized principally in terms of the variation in Ge−O bond lengths. The isotropic chemical shifts appear to fall into distinct regions for four-, five-, and six-coordinate Ge, with increasing coordination number corresponding to lower frequencies. Both CASTEP and WIEN2k consistently underestimate the CQs, suggesting that the exchange-correlation functional is poorly optimized for these systems. 73Ge NMR spectra of alkali germanate glasses are broad and featureless, rendering them difficult to interpret in terms of specific structural elements, even with the well understood NMR parameters from the crystalline systems. This study represents the first systematic 73Ge NMR investigation of solids, and shows that valuable structural information can be obtained in favorable cases.

NMR Study of LiBH4 with C60

David T. Shane*†, Robert L. Corey‡, Laura H. Rayhel†, Matthew Wellons§, Joseph A. Teprovich, Jr.§, Ragaiy Zidan§, Son-Jong Hwang, Robert C. Bowman, Jr., and Mark S. Conradi†

J. Phys. Chem. C, 2010, 114 (46), pp 19862–19866
DOI: 10.1021/jp107911u
Publication Date (Web): November 3, 2010
Copyright © 2010 American Chemical Society

Abstract:LiBH4 doped with 1.6 mol % well-dispersed C60 is studied with solid-state nuclear magnetic resonance (NMR). Variable-temperature hydrogen NMR shows large changes between the data upon first heating and after exposure to 300 °C. After heating, a large fraction on the order of 50% of the hydrogen signal appears in a motionally narrowed peak, similar to a previous report of LiBH4 in a porous carbon aerogel nanoscaffold. Magic-angle spinning (MAS) NMR of 13C in a 13C-enriched sample finds the C60 has reacted already in the as-mixed (unheated) material. Dehydriding and rehydriding result in further 13C spectral changes, with nearly all intensity being found in a broad peak corresponding to aromatic carbons. It thus appears that the previously reported improved dehydriding and rehydriding kinetics of this material at least partially result from in situ formation of a carbon framework. The method may offer a new route to dispersal of hydrides in carbon support structures.

Investigation of Si Atom Migration in the Framework of MSE-Type Zeolite YNU-2

Takuji Ikeda*†, Satoshi Inagaki‡, Taka-aki Hanaoka†, and Yoshihiro Kubota‡

J. Phys. Chem. C, 2010, 114 (46), pp 19641–19648
DOI: 10.1021/jp1079586
Publication Date (Web): November 2, 2010
Copyright © 2010 American Chemical Society

Abstract: The change in distribution of Si atom defects in the framework of zeolite YNU-2 by steam treatment was investigated using powder X-ray diffraction and solid-state NMR spectroscopy. The precursor of zeolite YNU-2 (abbreviated to YNU-2P) with a three-dimensional pore system has a large number of Si atom defects (more than 10% of all T sites in a unit cell) around the supercage. These defect sites were confirmed by observation of a Q3 ((−SiO)3Si−OH) resonance peak by 29Si magic angle spinning NMR spectroscopy. We have shown that steam treatment of YNU-2P at 523 K for 24 h significantly decreases the relative intensity ratio of the observed Q3 resonance peak for the Q4((−SiO)4Si) peak. The Rietveld analysis of steam-treated YNU-2P (YNU-2PST) shows a marked increase in the site occupancies of the defective Si sites. Furthermore, the Si atom defects in YNU-2PST almost disappeared after calcination, yielding siliceous zeolite YNU-2. These results indicate that the defective framework structure was almost restored by steam treatment. Experimental results suggest that Si atom migration in the framework of YNU-2P takes place during steam treatment. The migrated Si atom fragment fills defect sites and is connected with adjacent silanol groups. In addition, the quantity of the half amount of structure-directing agent molecules was removed from the micropores in YNU-2PST by steam treatment.

Ryan M. Ravenelle†, Florian Schüβler‡, Andrew D’Amico†, Nadiya Danilina§, Jeroen A. van Bokhoven§, Johannes A. Lercher‡, Christopher W. Jones†, and Carsten Sievers*†

J. Phys. Chem. C, 2010, 114 (46), pp 19582–19595
DOI: 10.1021/jp104639e
Publication Date (Web): November 2, 2010
Copyright © 2010 American Chemical Society

Abstract: Zeolites Y and ZSM-5 with varying Si/Al ratios are treated in liquid water at 150 and 200 °C under autogenic pressure to assess their hydrothermal stability. The changes in the structure are characterized by atomic absorption spectroscopy, X-ray diffraction, scanning electron microscopy, argon physisorption, 27Al and 29Si MAS NMR spectroscopy, temperature-programmed desorption of ammonia, and pyridine adsorption followed by IR spectroscopy. During treatment in hot water, zeolite Y with a Si/Al ratio of 14 or higher is transformed into an amorphous material, and the rate of this degradation increases with increasing Si/Al ratio. In contrast, ZSM-5 is not modified under the same conditions. The main degradation mechanism is suggested to be hydrolysis of the siloxane bonds (Si−O−Si) as opposed to dealumination, which dominates under steaming conditions. In the resulting amorphous phase, Al remains tetrahedrally coordinated, but the micropore volume and concentration of accessible acid sites is reduced dramatically. The results demonstrate that potential structural changes of zeolites have to be considered when these materials are used as catalysts for aqueous phase conversion of biomass.

Structure and Characterization of KSc(BH4)4

Radovan ern*†, Dorthe B. Ravnsbæk‡, Godwin Severa§, Yaroslav Filinchuk, Vincenza D’ Anna, Hans Hagemann, Drthe Haase#, Jørgen Skibsted‡, Craig M. Jensen*§, and Torben R. Jensen*‡

J. Phys. Chem. C, 2010, 114 (45), pp 19540–19549
DOI: 10.1021/jp106280v
Publication Date (Web): October 25, 2010
Copyright © 2010 American Chemical Society

Abstract: A new potassium scandium borohydride, KSc(BH4)4, is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH4)4, forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl3 together with a new ternary chloride K3ScCl6, which is also structurally characterized. This indicates that the formation of KSc(BH4)4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (31 mol %) can be obtained with a reactant ratio KBH4:ScCl3 of 2:1. KSc(BH4)4 crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) Å, V = 936.4(8) Å3 at RT, with the space group symmetry Pnma. KSc(BH4)4 has a BaSO4 type structure where the BH4 tetrahedra take the oxygen positions. Regarding the packing of cations, K+, and complex anions, [Sc(BH4)4]−, the structure of KSc(BH4)4 can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH4)4 in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH4)4 with A = Li, Na. KSc(BH4)4 is stable from RT up to 405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460−500 K and 510−590 K. The hydrogen release involves the formation of KBH4, which reacts with K3ScCl6 and forms a solid solution, K(BH4)1−xClx. The ternary potassium scandium chloride K3ScCl6 observed in all samples has a monoclinic structure at room temperature, P21/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) Å, β = 109.22(2)°, V = 1135.6(4) Å3, which is isostructural to K3MoCl6. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 Å (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K3YCl6.

Phase Behavior and 13C NMR Spectroscopic Analysis of the Mixed Methane + Ethane + Propane Hydrates in Mesoporous Silica Gels

Seungmin Lee, Inuk Cha, and Yongwon Seo*

J. Phys. Chem. B, 2010, 114 (46), pp 15079–15084
DOI: 10.1021/jp108037m
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: In this study, the phase behavior and quantitative determination of hydrate composition and cage occupancy for the mixed CH4 + C2H6 + C3H8 hydrates were closely investigated through the experimental measurement of three-phase hydrate (H)−water-rich liquid (LW)−vapor (V) equilibria and 13C NMR spectra. To examine the effect of pore size and salinity, we measured hydrate phase equilibria for the quaternary CH4 (90%) + C2H6 (7%) + C3H8 (3%) + water mixtures in silica gel pores of nominal diameters of 6.0, 15.0, and 30.0 nm and for the quinary CH4 (90%) + C2H6 (7%) + C3H8 (3%) + NaCl + water mixtures of two different NaCl concentrations (3 and 10 wt %) in silica gel pores of a nominal 30.0 nm diameter. The value of hydrate−water interfacial tension for the CH4 (90%) + C2H6 (7%) + C3H8 (3%) hydrate was found to be 47 ± 4 mJ/m2 from the relation of the dissociation temperature depression with the pore size of silica gels at a given pressure. At a specified temperature, three-phase H−LW−V equilibrium curves of pore hydrates were shifted to higher pressure regions depending on pore sizes and NaCl concentrations. From the cage-dependent 13C NMR chemical shifts of enclathrated guest molecules, the mixed CH4 (90%) + C2H6 (7%) + C3H8 (3%) gas hydrate was confirmed to be structure II. The cage occupancies of each guest molecule and the hydration number of the mixed gas hydrates were also estimated from the 13C NMR spectra.

Biomimetic Apatite Mineralization Mechanisms of Mesoporous Bioactive Glasses as Probed by Multinuclear 31P, 29Si, 23Na and 13C Solid-State NMR

Philips N. Gunawidjaja†, Andy Y. H. Lo†, Isabel Izquierdo-Barba‡§, Ana Garca‡§, Daniel Arcos‡§, Baltzar Stevensson†, Jekabs Grins, Mara Vallet-Reg‡§, and Mattias Edn*†

J. Phys. Chem. C, 2010, 114 (45), pp 19345–19356
DOI: 10.1021/jp105408c
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: An array of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy experiments is applied to explore the surface reactions of a mesoporous bioactive glass (MBG) of composition Ca0.10Si0.85P0.04O1.90 when subjected to a simulated body fluid (SBF) for variable intervals. Powder X-ray diffraction and 31P NMR techniques are employed to quantitatively monitor the formation of an initially amorphous calcium phosphate surface layer and its subsequent crystallization into hydroxycarbonate apatite (HCA). Prior to the onset of HCA formation, 1H → 29Si cross-polarization (CP) NMR evidence dissolution of calcium ions; a slightly increased connectivity of the speciation of silicate ions is observed at the MBG surface over 1 week of SBF exposure. The incorporation of carbonate and sodium ions into the bioactive orthophosphate surface layer is explored by 1H → 13C CPMAS and 23Na NMR, respectively. We discuss similarities and distinctions in composition−bioactivity relationships established for traditional melt-prepared bioglasses compared to MBGs. The high bioactivity of phosphorus-bearing MBGs is rationalized to stem from an acceleration of their surface reactions due to presence of amorphous calcium orthophosphate clusters of the MBG pore wall.

Analysis of the 7Li NMR signals in the Monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3 Phases
A. Castets, D. Carlier*, K. Trad, C. Delmas, and M. Mntrier

J. Phys. Chem. C, 2010, 114 (44), pp 19141–19150
DOI: 10.1021/jp106871z
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: The monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3 phosphates are materials for positive electrodes in Li-ion batteries. They also have interesting structures to test and improve the understanding of Li NMR signals in paramagnetic compounds. The position of such signals is governed by the transfer of electron spin density from the transition metal ion to the Li nucleus. These mechanisms are based on delocalization and polarization effects which induce positive and negative Fermi contact shifts, respectively. We have characterized Li3Fe2(PO4)3 by Li NMR. To understand the signals observed, we have analyzed the electron spin density transfer mechanisms (i) by considering the different Li environments, (ii) by using DFT calculations. We compare our analysis to the one very recently reported by Davis et al. These analyses have been extended to Li3V2(PO4)3 studied by NMR by Cahill et al.

Mechanically, Magnetically, and “Rotationally Aligned” Membrane Proteins in Phospholipid Bilayers Give Equivalent Angular Constraints for NMR Structure Determination

Sang Ho Park, Bibhuti B. Das, Anna A. De Angelis, Mario Scrima, and Stanley J. Opella*

J. Phys. Chem. B, 2010, 114 (44), pp 13995–14003
DOI: 10.1021/jp106043w
Publication Date (Web): October 20, 2010
Copyright © 2010 American Chemical Society

Abstract: The native environment for membrane proteins is the highly asymmetric phospholipid bilayer, and this has a large effect on both their structure and dynamics. Reproducing this environment in samples suitable for spectroscopic and diffraction experiments is a key issue, and flexibility in sample preparation is essential to accommodate the diverse size, shape, and other physical properties of membrane proteins. In most cases, to ensure that the biological activities are maintained, this means reconstituting the proteins in fully hydrated planar phospholipid bilayers. The asymmetric character of protein-containing bilayers means that it is possible to prepare either oriented or unoriented (powder) samples. Here we demonstrate the equivalence of mechanical, magnetic, and what we refer to as “rotational alignment” of membrane proteins in phospholipid bilayer samples for solid-state NMR spectroscopy. The trans-membrane domain of virus protein “u” (Vpu) from human immunodeficiency virus (HIV-1) and the full-length membrane-bound form of fd bacteriophage coat protein in phospholipid bilayers are used as examples. The equivalence of structural constraints from oriented and unoriented (powder) samples of membrane proteins is based on two concepts: (1) their alignment is defined by the direction of the bilayer normal relative to the magnetic field and (2) they undergo rapid rotational diffusion about the same bilayer normal in liquid crystalline membranes. The measurement of angular constraints relative to a common external axis system defined by the bilayer normal for all sites in the protein is an essential element of oriented sample (OS) solid-state NMR.

Controlled Interactions between Anhydrous Keggin-Type Heteropolyacids and Silica Support: Preparation and Characterization of Well-Defined Silica-Supported Polyoxometalate Species

Eva Grinenval†, Xavier Rozanska§, Anne Baudouin†, Elise Berrier‡, Franoise Delbecq§, Philippe Sautet§, Jean-Marie Basset†, and Frdric Lefebvre*†

J. Phys. Chem. C, 2010, 114 (44), pp 19024–19034
DOI: 10.1021/jp107317s
Publication Date (Web): October 20, 2010
Copyright © 2010 American Chemical Society

Abstract:Anhydrous Keggin-type phosphorus heteropolyacids were deposited on partially dehydroxylated silica by using the surface organometallic chemistry (SOMC) strategy. The resulting solids were characterized by a combination of physicochemical methods including IR, Raman, 1D and 2D 1H, and 31P MAS NMR, electron microscopy experiments and density functional theory (DFT) calculations. It is shown that the main surface species is [≡Si(OH...H+)]2[H+]1[PM12O403−] where the polyoxometalate is linked to the support by proton interaction with two silanols. Two other minor species (10% each) are formed by coordination of the polyoxometalate to the surface via the interaction between all three protons with three silanol groups or via three covalent bonds formed by dehydroxylation of the above species. Comparison of the reactivity of these solids and of compounds prepared by a classical way shows that the samples prepared by the SOMC approach contain ca. 7 times more acid sites

Thermal Spreading As an Alternative for the Wet Impregnation Method: Advantages and Downsides in the Preparation of MoO3/SiO2−Al2O3 Metathesis Catalysts

Damien P. Debecker*†, Mariana Stoyanova‡, Uwe Rodemerck‡, Pierre Eloy†, Alexandre Lonard§, Bao-Lian Su§, and Eric M. Gaigneaux*†

J. Phys. Chem. C, 2010, 114 (43), pp 18664–18673
DOI: 10.1021/jp1074994
Publication Date (Web): October 14, 2010
Copyright © 2010 American Chemical Society

Abstract:Silica−alumina-supported MoO3 catalysts are classically prepared via impregnation of the support with a molybdenum salt solution, usually ammonium heptamolybdate, and subsequent drying and calcination (three steps). The downsides of such a route for the synthesis of heterogeneous metathesis catalysts are linked to the limited control on the nature of the MoOx stabilized at the surface, to the uneven distribution of the deposit in the pores of the support, and to the build up of inactive species that find their origin in the wet step of the preparation. In opposition, the direct thermal spreading of molybdenum oxide onto the support is a straightforward (one step) method involving no wet stage. It allows the conversion of bulk MoO3 crystals to amorphous molybdate species dispersed at the surface of the silica−alumina support. This contribution shows that the catalysts obtained via both methods exhibit similar performances in the self-metathesis of propene to butene and ethene. However, based on XRD, XPS, Raman spectroscopy, ICP-AES, N2 physisorption, TEM, and MAS-NMR spectroscopy, it is shown that the origin of active and inactive species in the two systems is different. Whereas the activity of wet-made catalysts is limited by the formation of bulky MoO3 crystals and of aluminum molybdate, the performances of dry-made catalysts are limited by the incomplete spreading of MoO3 nanocrystallites.

On the Performance of Spin Diffusion NMR Techniques in Oriented Solids: Prospects for Resonance Assignments and Distance Measurements from Separated Local Field Experiments

Nathaniel J. Traaseth†, T. Gopinath†, and Gianluigi Veglia*†‡

J. Phys. Chem. B, 2010, 114 (43), pp 13872–13880
DOI: 10.1021/jp105718r
Publication Date (Web): October 11, 2010
Copyright © 2010 American Chemical Society

Abstract: NMR spin diffusion experiments have the potential to provide both resonance assignment and internuclear distances for protein structure determination in oriented solid-state NMR. In this paper, we compared the efficiencies of three spin diffusion experiments: proton-driven spin diffusion (PDSD), cross-relaxation-driven spin diffusion (CRDSD), and proton-mediated proton transfer (PMPT). As model systems for oriented proteins, we used single crystals of N-acetyl-L-15N-leucine (NAL) and N-acetyl-L-15N-valyl-L-15N-leucine (NAVL) to probe long and short distances, respectively. We demonstrate that, for short 15N/15N distances such as those found in NAVL (3.3 Å), the PDSD mechanism gives the most intense cross-peaks, while, for longer distances (>6.5 Å), the CRDSD and PMPT experiments are more efficient. The PDSD was highly inefficient for transferring magnetization across distances greater than 6.5 Å (NAL crystal sample), due to small 15N/15N dipolar couplings (<4.5>

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