High-resolution Solid-state NMR Spectra Research Articles

Amyloid fibril formation kinetics of low-pH denatured bovine PI3K-SH3 monitored by three different NMR techniques.

Introduction: Misfolding of amyloidogenic proteins is a molecular hallmark of neurodegenerative diseases in humans. A detailed understanding of the underlying molecular mechanisms is mandatory for developing innovative therapeutic approaches. The bovine PI3K-SH3 domain has been a model system for aggregation and fibril formation. Methods: We monitored the fibril formation kinetics of low pH-denatured recombinantly expressed [U-13C, 15N] labeled bovine PI3K-SH3 by a combination of solution NMR, high-resolution magic angle spinning (HR-MAS) NMR and solid-state NMR spectra. Solution NMR offers the highest sensitivity and, therefore, allows for the recording of two-dimensional NMR spectra with residue-specific resolution for individual time points of the time series. However, it can only follow the decay of the aggregating monomeric species. In solution NMR, aggregation occurs under quiescent experimental conditions. Solid-state NMR has lower sensitivity and allows only for the recording of one-dimensional spectra during the time series. Conversely, solid-state NMR is the only technique to detect disappearing monomers and aggregated species in the same sample by alternatingly recoding scalar coupling and dipolar coupling (CP)-based spectra. HR-MAS NMR is used here as a hybrid method bridging solution and solid-state NMR. In solid-state NMR and HR-MAS NMR the sample is agitated due to magic angle spinning. Results: Good agreement of the decay rate constants of monomeric SH3, measured by the three different NMR methods, is observed. Moderate MAS up to 8kHz seems to influence the aggregation kinetics of seeded fibril formation only slightly. Therefore, under sufficient seeding (1% seeds used here), quiescent conditions (solution NMR), and agitated conditions deliver similar results, arguing against primary nucleation induced by MAS as a major contributor. Using solid-state NMR, we find that the amount of disappeared monomer corresponds approximately to the amount of aggregated species under the applied experimental conditions (250µM PI3K-SH3, pH 2.5, 298K, 1% seeds) and within the experimental error range. Data can be fitted by simple mono-exponential conversion kinetics, with lifetimes τ in the 14-38h range. Atomic force microscopy confirms that fibrils substantially grew in length during the aggregation experiment. This argues for fibril elongation as the dominant growth mechanism in fibril mass (followed by the CP-based solid-state NMR signal). Conclusion: We suggest a combined approach employing both solution NMR and solid-state NMR, back-to-back, on two aliquots of the same sample under seeding conditions as an additional approach to follow monomer depletion and growth of fibril mass simultaneously. Atomic force microscopy images confirm fibril elongation as a major contributor to the increase in fibril mass.

Determining Local Magnetic Susceptibility Tensors in Paramagnetic Lanthanide Crystalline Powders from Solid-State NMR Chemical Shift Anisotropies

Exploring magnetic properties at the molecular level is a challenge that has been met by developing many experimental and theoretical solutions, such as polarized neutron diffraction (PND), muon-spin rotation (μ-SR), electron paramagnetic resonance (EPR), SQUID-based magnetometry measurements, and advanced modeling on open-shell systems and relativistic calculations. These methods are powerful tools that shed light on the local magnetic response in specifically designed magnetic materials such as contrast agents, for MRI, molecular magnets, magnetic tags for biological NMR, etc. All of these methods have their advantages and disadvantages. In order to complement the possibilities offered by these methods, we propose a new tool that implements a new approach combining simulation and fitting for high-resolution solid-state NMR spectra of lanthanide-based paramagnetic species. This method relies on a rigorous acquisition thanks to short high-power adiabatic pulses (SHAP) of high-resolution solid-state NMR isotropic and anisotropic data on a powdered magnetic material. It is also based on an efficient modeling of this data thanks to a semiempirical model based on a parametrization of the local magnetism and the crystal structure provided by diffraction methods. The efficiency of the calculation relies on a thorough simplification of the electron-nucleus interactions (point-dipole interaction, no Fermi contact) which is validated by experimental analysis. By taking advantage of the efficient calculation possibilities offered by our method, we can compare a great number of simulated spectra to experimental data and find the best-matching local magnetic susceptibility tensor. This method was applied to a series of isostructural lanthanide oxalates which are used as a benchmark system for many analytical methods. We present the results of thorough solid-state NMR and extensive modeling of the hyperfine interaction (including up to 400 paramagnetic centers) that yield local magnetic susceptibility tensor measurements that are self-consistent as well as consistent with bulk susceptibility measurements.

Probing the transformation paths from aluminum (oxy)hydroxides (boehmite, bayerite, and gibbsite) to metastable alumina: A view from high-resolution 27Al MAS NMR

Abstract Detailed knowledge of the structural evolution of diverse aluminum (oxy)hydroxides with varying temperatures up to ~300 °C provides insights into the dehydration processes involving transitions among metastable phases on Earth's surface and in the crust. Here, we report the high-resolution solid-state 27Al NMR spectra for three different types of aluminum (oxy)hydroxides (i.e., boehmite, bayerite, and gibbsite) with varying annealing temperatures up to 300 °C, revealing the effect of distinct precursor minerals on the stability of metastable alumina. 27Al MAS NMR results allow us to obtain the quantitative fractions and NMR parameters for each phase during transformation. The results demonstrate that each aluminum (oxy)hydroxide phase follows a unique transformation path to metastable alumina. The 27Al MAS and 3QMAS NMR spectra of boehmite show that a minor but observable [4]Al signal (~2%) is detected at ~50 °C, and the [4]Al fraction gradually increases up to 300 °C (~16%), indicating that the phase transformation from boehmite to γ-/η-Al2O3 occurs at a temperature as low as ~50 °C, significantly lower than earlier estimations based on XRD. Together with the [4]Al fraction, the [5]Al fraction increases from <1% at 50 °C to ~2.3% at 300 °C, whereas the NMR results of bayerite and gibbsite do not show the presence of [5]Al. In addition, the 27Al 3QMAS NMR spectra resolved the [6]Al site in boehmite and that in γ-/η-Al2O3, which could not be uniquely determined from 1D NMR spectra. The population of bayerite abruptly decreases from 100% (at 150 °C), through ~47% (at 200 °C), to 0% (at 250 °C), indicating that the phase transition from bayerite to boehmite + γ-/η-Al2O3 occurs within a narrow temperature range. As for gibbsite, while [4]Al is not observed in the spectra up to 200 °C, the [4]Al fraction of ~2% is observed in the spectra for gibbsite annealed at 250 °C, and the [4]Al fraction increases rapidly to ~15% as the annealing temperature increases to 300 °C, suggesting that the phase transformation into γ-/η-Al2O3 occurs at ~250 °C. The results confirm that the phase transformation paths (gradual vs. dramatic) depend on the type of precursor minerals. Particularly, the onset temperature of the phase transformation from boehmite to metastable alumina (~50 °C) is lower than those from other precursor minerals (>150 °C). Furthermore, the phase transformation from boehmite to γ-/η-Al2O3 occurs gradually within broad temperature ranges from ~50 °C. This is due to their configurational disorder as evidenced by the presence of [5]Al. The observed structural evolution in aluminum (oxy)hydroxides in the low-temperature range facilitates our understanding of the nature of phase transformation and dehydration of oxides and hydroxides in the Earth's surface environments.

Correlating the Structure and Activity of Y. pestis Ail in a Bacterial Cell Envelope

Understanding microbe-host interactions at the molecular level is a major goal of fundamental biology and therapeutic drug development. Structural biology strives to capture biomolecular structures in action, but the samples are often highly simplified versions of the complex native environment. Here, we present an Escherichia coli model system that allows us to probe the structure and function of Ail, the major surface protein of the deadly pathogen Yersinia pestis. We show that cell surface expression of Ail produces Y. pestis virulence phenotypes in E. coli, including resistance to human serum, cosedimentation of human vitronectin, and pellicle formation. Moreover, isolated bacterial cell envelopes, encompassing inner and outer membranes, yield high-resolution solid-state NMR spectra that reflect the structure of Ail and reveal Ail sites that are sensitive to the bacterial membrane environment and involved in the interactions with human serum components. The data capture the structure and function of Ail in a bacterial outer membrane and set the stage for probing its interactions with the complex milieu of immune response proteins present in human serum.

Membrane proteins in magnetically aligned phospholipid polymer discs for solid-state NMR spectroscopy

Well-hydrated phospholipid bilayers provide a near-native environment for membrane proteins. They enable the preparation of chemically-defined samples suitable for NMR and other spectroscopic experiments that reveal the structure, dynamics, and functional interactions of the proteins at atomic resolution. The synthetic polymer styrene maleic acid (SMA) can be used to prepare detergent-free samples that form macrodiscs with diameters greater than 30 nm at room temperature, and spontaneously align in the magnetic field of an NMR spectrometer at temperatures above 35 °C. Here we show that magnetically aligned macrodiscs are particularly well suited for solid-state NMR experiments of membrane proteins because the SMA-lipid assembly both immobilizes the embedded protein and provides uniaxial order for oriented sample (OS) solid-state NMR studies. We show that aligned macrodiscs incorporating four different membrane proteins with a wide range of sizes and topological complexity yield high-resolution OS solid-state NMR spectra. The work is dedicated to Michelle Auger who made key contributions to the field of membrane and membrane protein biophysics.

Hydrogen bonding in dicyclohexylmethane – or diphenylmethane based urea compounds and their polymer counterparts investigated by NMR spectroscopy: Interplay of electronic and geometrical factors

We investigated the H-bonding behavior of urea compounds with different electronic nature of substituents and different spatial organization. Variable-temperature 1H NMR spectroscopy studies were performed in polar solvents which are capable (DMSO‑d6 and DMF-d7) or not (CDCl3) to form the intermolecular hydrogen bonds and non-polar solvent (toluene-d8) at the temperature range from 25 to 70 °C. Our results indicates that in the investigated systems the geometrical factors dominated over electronic effects of substituents at formation of intra- and intermolecular H-bonds. By the reaction between isocyanate and heavy water was synthesized the polymers with selectively deuterated urea groups and recorded their high-resolution solid-state 2H NMR spectra and measured their 2H quadrupole interaction parameters (i.e., the quadrupole coupling constant χ and asymmetry parameter η). Quite unexpectedly, the intermolecular hydrogen bonds are weaker in phenyl based urea polymers than in their cyclohexane based counterparts.

Sensitivity improvement in 5QMAS NMR experiments using FAM-N pulses

The multiple-quantum magic-angle spinning (MQMAS) experiment is a popular choice for obtaining high-resolution solid-state NMR spectra of quadrupolar nuclei with half-integer spin quantum number. However, its inherently poor sensitivity limits its application in more challenging systems. In particular, the use of higher-order multiple-quantum coherences, which have the potential to provide higher resolution in the isotropic spectrum, results in a further decrease in sensitivity. Here we extend our recent work, which introduced an automated, high-throughput approach to generate amplitude-modulated composite pulses (termed FAM-N) to improve the efficiency of the conversion of three-quantum coherences, and explore the use of similar pulses in five-quantum MAS experiments. We consider three different approaches, and are able to demonstrate that all three provide good enhancements over single pulse conversion in all but the most extreme cases, and work well at a range of spinning rates. We show that FAM-N pulses are robust to variation in the quadrupolar coupling and rf nutation rate, demonstrating their applicability in multisite systems and systems where direct experimental optimisation of complex composite pulses is not feasible. This work will ease the implementation of higher-order MQMAS experiments and enable their application to materials and systems that were previously deemed too difficult to study.

High resolution solid-state NMR spectroscopy of the Yersinia pestis outer membrane protein Ail in lipid membranes.

The outer membrane protein Ail (Adhesion invasion locus) is one of the most abundant proteins on the cell surface of Yersinia pestis during human infection. Its functions are expressed through interactions with a variety of human host proteins, and are essential for microbial virulence. Structures of Ail have been determined by X-ray diffraction and solution NMR spectroscopy, but those samples contained detergents that interfere with functionality, thus, precluding analysis of the structural basis for Ail's biological activity. Here, we demonstrate that high-resolution solid-state NMR spectra can be obtained from samples of Ail in detergent-free phospholipid liposomes, prepared with a lipid to protein molar ratio of 100. The spectra, obtained with 13C or 1H detection, have very narrow line widths (0.40-0.60ppm for 13C, 0.11-0.15ppm for 1H, and 0.46-0.64ppm for 15N) that are consistent with a high level of sample homogeneity. The spectra enable resonance assignments to be obtained for N, CO, CA and CB atomic sites from 75 out of 156 residues in the sequence of Ail, including 80% of the transmembrane region. The 1H-detected solid-state NMR 1H/15N correlation spectra obtained for Ail in liposomes compare very favorably with the solution NMR 1H/15N TROSY spectra obtained for Ail in nanodiscs prepared with a similar lipid to protein molar ratio. These results set the stage for studies of the molecular basis of the functional interactions of Ail with its protein partners from human host cells, as well as the development of drugs targeting Ail.

Proton-Detected Solid-State NMR Spectroscopy of a Zinc Diffusion Facilitator Protein in Native Nanodiscs.

The structure, dynamics, and function of membrane proteins are intimately linked to the properties of the membrane environment in which the proteins are embedded. For structural and biophysical characterization, membrane proteins generally need to be extracted from the membrane and reconstituted in a suitable membrane-mimicking environment. Ensuring functional and structural integrity in these environments is often a major concern. The styrene/maleic acid co-polymer has recently been shown to be able to extract lipid/membrane protein patches directly from native membranes to form nanosize discoidal proteolipid particles, also referred to as native nanodiscs. In this work, we show that high-resolution solid-state NMR spectra can be obtained from an integral membrane protein in native nanodiscs, as exemplified by the 2×34 kDa bacterial cation diffusion facilitator CzcD.

Investigating FAM-N pulses for signal enhancement in MQMAS NMR of quadrupolar nuclei

Although a popular choice for obtaining high-resolution solid-state NMR spectra of quadrupolar nuclei, the inherently low sensitivity of the multiple-quantum magic-angle spinning (MQMAS) experiment has limited its application for nuclei with low receptivity or when the available sample volume is limited. A number of methods have been introduced in the literature to attempt to address this problem. Recently, we have introduced an alternative, automated approach, based on numerical simulations, for generating amplitude-modulated pulses (termed FAM-N pulses) to enhance the efficiency of the triple- to single-quantum conversion step within MQMAS. This results in efficient pulses that can be used without experimental reoptimisation, ensuring that this method is particularly suitable for challenging nuclei and systems. In this work, we investigate the applicability of FAM-N pulses to a wider variety of systems, and their robustness under more challenging experimental conditions. These include experiments performed under fast MAS, nuclei with higher spin quantum numbers, samples with multiple distinct sites, low-γ nuclei and nuclei subject to large quadrupolar interactions.

Comparison of celery (Apium graveolens L.) collenchyma and parenchyma cell wall polysaccharides enabled by solid-state 13C NMR

Collenchyma cells with their thickened walls are one of specific mechanical support tissues for plants, while parenchyma cells are thin walled and serve multiple functions. The parenchyma tissue is what you enjoy eating, while collenchyma, because of its fibrous nature, is not so attractive. Celery is a useful model for comparing the cell walls (CWs) of the two cell types such as collenchyma and parenchyma. However, to date, the structural characteristics of collenchyma and parenchyma cell walls from the same plant have not been compared. Monosaccharide composition suggested the collenchyma cell walls contained less pectin but more hemicellulose in comparison to parenchyma. High-resolution solid-state NMR spectra of highly mobile pectins revealed that the arabinan signals were more evident in the collenchyma spectrum, while galactan showed a much stronger resonance in the parenchyma spectrum. In addition, methyl esterified and non-esterified galacturonic acid signals were observed in parenchyma CWs, but only the latter one appeared in the collenchyma. The ratio of cellulose surface/interior obtained from CP/MAS spectra for collenchyma suggested the cellulose microfibrils were ~2.4 nm, while in the parenchyma, these were somewhat larger. X-ray diffraction indicated the size of the cellulose microfibrils were the same for both types of CWs.

Effect of iron content on the structure and disorder of iron-bearing sodium silicate glasses: A high-resolution 29Si and 17O solid-state NMR study

Despite its geochemical importance and implications for the properties of natural magmatic melts, understanding the detailed structure of iron-bearing silicate glasses remains among the outstanding problems in geochemistry. This is mainly because solid-state NMR techniques, one of the most versatile experimental methods to probe the structure of oxide glasses, cannot be fully utilized for exploring the structural details of iron-bearing glasses as the unpaired electrons in Fe induce strong local magnetic fields that mask the original spectroscopic features (i.e., paramagnetic effect). Here, we report high-resolution 29Si and 17O solid-state NMR spectra of iron-bearing sodium silicate glasses (Na2O–Fe2O3–SiO2, Fe3+/ΣFe=0.89±0.04, thus containing both ferric and ferrous iron) with varying XFe2O3 [=Fe2O3/(Na2O+Fe2O3)], containing up to 22.9wt% Fe2O3. This compositional series involves Fe–Na substitution at constant SiO2 contents of 66.7mol% in the glasses. For both nuclides, the NMR spectra exhibit a decrease in the signal intensities and an increase in the peak widths with increasing iron concentration partly because of the paramagnetic effect. Despite the intrinsic difficulties that result from the pronounced paramagnetic effect, the 29Si and 17O NMR results yield structural details regarding the effect of iron content on Q speciation, spatial distribution of iron, and the extent of polymerization in the iron-bearing silicate glasses. The 29Si NMR spectra show an apparent increase in highly polymerized Q species with increasing XFe2O3, suggesting an increase in the degree of melt polymerization. The 17O 3QMAS NMR spectra exhibit well-resolved non-bridging oxygen (NBO, Na–O–Si) and bridging oxygen (BO, Si–O–Si) peaks with varying iron concentration. By replacing Na2O with Fe2O3 (and thus with increasing iron content), the fraction of Na–O–Si decreases. Quantitative consideration of this effect confirms that the degree of polymerization is likely to increase with iron content and that Fe3+ is predominantly a network-former. The 17O NMR spectra suggest a moderate degree of preferential partitioning of iron between NBO and BO clusters. The present results bear strong promise for studying iron-bearing silicate glasses using solid-state NMR techniques, constraining the effect of iron content on the degree of polymerization. The observed changes in atomic structures of iron-bearing sodium silicate glasses will be helpful for unraveling atomic origins of the properties of natural silicate melts.

Probing Metal-Bridging Oxygen and Configurational Disorder in Amorphous Lead Silicates: Insights from 17O Solid-State Nuclear Magnetic Resonance

The detailed degree of mixing between framework Si and metal cations and the nature of oxygen that only links metal cations (metal-bridging oxygen) in archetypal metal oxide glasses have been among the unsolved problems in physical chemistry. Binary lead silicate (PbO-SiO2) glass is an ideal model system for exploring the extent of cation disorder and metal-bridging oxygen because of its peculiar glass-forming ability at high PbO concentration. Here we report the first high-resolution 17O solid-state NMR spectra for binary lead silicate glasses with varying composition (near that of orthosilicate, i.e., Pb/Si = 2), where peaks due to metal-bridging oxygen (Pb-O-Pb), as well as Pb-O-Si and Si-O-Si are clearly resolved. The spectra also reveals that the metal-bridging oxygen possesses a greater topological disorder due to structural complexity in Pb coordination environments. Together with a statistical thermodynamic model of oxygen speciation proposed to describe mixing behavior in oxide glasses with any o...

Solid-State NMR Spectra of Lipid-Anchored Proteins under Magic Angle Spinning

Solid-state NMR is a promising tool for elucidating membrane-related biological phenomena. We achieved the measurement of high-resolution solid-state NMR spectra for a lipid-anchored protein embedded in lipid bilayers under magic angle spinning (MAS). To date, solid-state NMR measurements of lipid-anchored proteins have not been accomplished due to the difficulty in supplying sufficient amount of stable isotope labeled samples in the overexpression of lipid-anchored proteins requiring complex posttranslational modification. We designed a pseudo lipid-anchored protein in which the protein component was expressed in E. coli and attached to a chemically synthesized lipid-anchor mimic. Using two types of membranes, liposomes and bicelles, we demonstrated different types of insertion procedures for lipid-anchored protein into membranes. In the liposome sample, we were able to observe the cross-polarization and the (13)C-(13)C chemical shift correlation spectra under MAS, indicating that the liposome sample can be used to analyze molecular interactions using dipolar-based NMR experiments. In contrast, the bicelle sample showed sufficient quality of spectra through scalar-based experiments. The relaxation times and protein-membrane interaction were capable of being analyzed in the bicelle sample. These results demonstrated the applicability of two types of sample system to elucidate the roles of lipid-anchors in regulating diverse biological phenomena.

Advances in Structural Studies on Alkylaluminum Species in the Solid State via Challenging 27Al–13C NMR Spectroscopy and X-ray Diffraction

Advanced multinuclear solid state NMR experiments were developed to probe the structure of two organometallic aluminum derivatives, Li[Al(CH3)3CH2Si(CH3)3] (1) and Li[Al(CH3)4] (2), which are relevant to olefin polymerization processes. For the first time, NMR observation of 27Al–13C covalent bonds in solids is performed with the natural abundance material 1. Unprecedented triple-resonance (1H–13C–27Al) and quadruple-resonance (1H–7Li–13C–27Al) heteronuclear correlation two-dimensional NMR experiments are also introduced to probe 27Al–13C and 13C–7Li proximities for 2. High-resolution solid-state NMR spectra thus obtained provide information on the local structure of these representative organometallic derivatives that proved to be most complementary and in full agreement with the structures obtained by X-ray diffraction.

Preparation of uniformly isotope labeled KcsA for solid state NMR: Expression, purification, reconstitution into liposomes and functional assay

We report the expression, purification, liposome reconstitution and functional validation of uniformly 13C and 15N isotope labeled KcsA, a bacterial potassium channel that has high homology with mammalian channels, for solid-state NMR studies. The expression and purification is optimized for an average yield of ∼35–40mg/L of M9 media in a time-efficient way. The protein purity is confirmed by gel electrophoresis and the protein concentration is quantified by UV–vis absorption spectroscopy. Protocols to efficiently reconstitute KcsA into liposomes are also presented. The presence of liposomes is confirmed by cryo-electron microscopy images and the effect of magic angle spinning on liposome packing is shown. High-resolution solid-state NMR spectra of uniformly isotope labeled KcsA in these liposomes reveal that our protocol yields to a very homogenous KcsA sample with high signal to noise and several well-resolved residues in NMR spectra. Electrophysiology of our samples before and after solid-state NMR show that channel function and selectivity remain intact after the solid-state NMR.

Quantifying the fraction of alanine residues in an α-helical conformation in hornet silk using solid-state NMR

Here, we demonstrate the first successful isotope labeling of Ala carbons in hornet silk produced by the larvae of Vespa (Vespinae, Vespidae) mandarinia. This labeled hornet silk was examined by high-resolution 13C solid-state NMR, and it was found that the fraction of Ala residues in α-helical conformations compared with Ala residues in the overall conformation of hornet silk can be quantitatively determined from Ala Cα NMR peaks. The value for this α-helical Ala fraction is close to that of the fraction of Ala residues in coiled-coil structures estimated in the four major hornet silk proteins by coiled-coil prediction analysis. This result indicates that most of the Ala residues in α-helices occur in those α-helices with a coiled-coil structure, and that the number of Ala residues in α-helices without a coiled-coil structure is small. Moreover, coiled-coil prediction analysis indicated that the potential coiled-coil domains are located only in the central portion of the protein chains of the major hornet silk proteins. From these results, we confirmed that the α-helical conformation mostly forms in the central portion of the hornet silk chains, whereas the ends of the protein chains are nearly devoid of α-helical structure. We deduce that the ends of the protein chains would preferentially adopt a β-sheet conformation. An isotope-labeled hornet (Vespa) cocoon in which the Ala carbons of silk proteins were substituted with 13C was successfully obtained by feeding a mixture of larval saliva with [13C3]Ala to mature larvae. The Ala fraction in the α-helix conformation of hornet silk was estimated by obtaining high-resolution 13C solid-state NMR spectra of the labeled cocoon, which also helped in understanding the molecular structure of hornet silk.

Obtaining accurate chemical shifts for all magnetic nuclei (1H, 13C, 17O, and 27Al) in tris(2,4-pentanedionato-O,O′)aluminium(III) — A solid-state NMR case study

We report a complete set of high-resolution solid-state NMR spectra for all magnetic nuclei (1H, 13C, 17O, and 27Al) in the α-form of tris(2,4-pentanedionato-O,O′)aluminium(III), α-Al(acac)3. These high-resolution NMR spectra were obtained by using a host of solid-state NMR techniques: standard cross-polarization under the magic-angle spinning (CPMAS) method for 13C, 1-D homonuclear decoupling using the windowed DUMBO sequence for 1H, double-rotation (DOR) for 17O and 27Al, and multiple-quantum MAS for 27Al. Some experiments were performed at multiple magnetic fields. We show that the isotropic chemical shifts obtained for 1H, 13C, 17O, and 27Al nuclei in α-Al(acac)3 are highly resolved and accurate, regardless of the nature of the targeted nuclear spins (i.e., spin-1/2 or quadrupolar) and, as such, can be treated equally in comparison with computational chemical shifts obtained from a gauge-including projector-augmented wave (GIPAW) plane-wave pseudopotential DFT method.

Variable Temperature High-Resolution19F MAS Solid-State NMR Characterization of Fluorocarbon Rubbers

Variable temperature high-resolution 19 F magic angle spinning (MAS) solid-state NMR spectroscopy was used to characterize fluorocarbon (FKM) rubbers. The high-resolution spectra of copolymers made from two monomers, vinylidene fluoride and hexafluoropropene, and terpolymers composed of vinylidene fluoride, hexafluoropropene, and tetrafluoroethylene, were obtained using MAS speeds of up to 18 kHz combined with high temperatures of up to 200 °C at a magnetic field strength of 9.4 Tesla. From these high resolution solidstate NMR spectra, we were able to assign the spectral peaks and differentiate the copolymer FKM from the terpolymer FKM. We also determined quantitatively the monomer compositions of each FKM rubber.

Preferential Clustering of Water Molecules During Hydration of the Ammonium Form of the Solid Acid Catalyst ZSM-5

Hydration of the ammonium form of the solid acid catalyst ZSM-5 is investigated by applying a technique developed recently for in situ solid-state NMR studies of adsorption processes. From high-resolution solid-state 1H NMR spectra recorded as a function of time during the hydration process, insights are established concerning the nature of the interaction between the adsorbed water molecules and the ammonium cations in the ZSM-5 host material. In particular, quantitative information is established regarding the clustering of water molecules during the hydration process, and indicates the preferential formation of clusters comprising two water molecules associated with each hydrated ammonium cation.