Magic Angle Spinning NMR Measurements Research Articles

The effects of daptomycin on cell wall biosynthesis in Enterococcal faecalis

Daptomycin is a cyclic lipodepsipeptide antibiotic reserved for the treatment of serious infections by multidrug-resistant Gram-positive pathogens. Its mode of action is considered to be multifaceted, encompassing the targeting and depolarization of bacterial cell membranes, alongside the inhibition of cell wall biosynthesis. To characterize the daptomycin mode of action, 15N cross-polarization at magic-angle spinning NMR measurements were performed on intact whole cells of Staphylococcus aureus grown in the presence of a sub-inhibitory concentration of daptomycin in a chemically defined media containing l-[ϵ-15N]Lys. Daptomycin-treated cells showed a reduction in the lysyl-ε-amide intensity that was consistent with cell wall thinning. However, the reduced lysyl-ε-amine intensity at 10 ppm indicated that the daptomycin-treated cells did not accumulate in Park’s nucleotide, the cytoplasmic peptidoglycan (PG) precursor. Consequently, daptomycin did not inhibit the transglycosylation step of PG biosynthesis. To further elucidate the daptomycin mode of action, the PG composition of daptomycin-susceptible Enterococcus faecalis grown in the presence of daptomycin was analyzed using liquid chromatography-mass spectrometry. Sixty-nine muropeptide ions correspond to PG with varying degrees of modifications including crosslinking, acetylation, alanylation, and 1,6-anhydrous ring formation at MurNAc were quantified. Analysis showed that the cell walls of daptomycin-treated E. faecalis had a significant reduction in PG crosslinking which was accompanied by an increase in lytic transglycosylase activities and a decrease in PG-stem modifications by the carboxypeptidases. The changes in PG composition suggest that daptomycin inhibits cell wall biosynthesis by impeding the incorporation of nascent PG into the cell walls by transpeptidases and maturation by carboxypeptidases. As a result, the newly formed cell walls become highly susceptible to degradation by the autolysins, resulting in thinning of the cell wall.

Nanostructure and Molecular-Level Characterization of Aminoalkyl Methacrylate Copolymer and the Impact on Drug Solubilization Ability.

The effects of pH changes and saccharin (SAC) addition on the nanostructure and mobility of the cationic aminoalkyl methacrylate copolymer Eudragit E PO (EUD-E) and its drug solubilization ability were investigated. Small-angle X-ray scattering performed using synchrotron radiation and atomic force microscopy showed that the EUD-E nanostructure, which has a size of approximately several nanometers, changed from a random coil structure at low pH (pH 4.0-5.0) to a partially folded structure at high pH (pH 5.5-6.5). The EUD-E also formed a partially folded structure in a wide pH range of 4.5-6.5 when SAC was present, and the coil-to-globule transition was moderate with pH increase, compared with that when SAC was absent. The equilibrium solubility of the neutral drug naringenin (NAR) was enhanced in the EUD-E solution and further increased as the pH increased. The enlargement of the hydrophobic region of EUD-E in association with the coil-to-globule transition led to efficient solubilization of NAR. The interaction with SAC enhanced the mobility of the EUD-E chains in the hydrophobic region of EUD-E, resulting in changes in the drug-solubilizing ability. 1H high-resolution magic-angle spinning NMR measurements revealed that the solubilized NAR in the partially folded structure of EUD-E showed higher molecular mobility in the presence of SAC than in the absence of SAC. This study highlighted that solution pH and the presence of SAC significantly changed the drug solubilization ability of EUD-E, followed by changes in the EUD-E nanostructure, including its hydrophobic region.

Transport Properties of Flexible Composite Electrolytes Composed of Li1.5Al0.5Ti1.5(PO4)3 and a Poly(vinylidene fluoride-co-hexafluoropropylene) Gel Containing a Highly Concentrated Li[N(SO2CF3)2]/Sulfolane Electrolyte.

Flexible solid-state electrolyte membranes are beneficial for feasible construction of solid-state batteries. In this study, a flexible composite electrolyte was prepared by combining a Li+-ion-conducting solid electrolyte Li1.5Al0.5Ti1.5(PO4)3 (LATP) and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF–HFP) gel containing a highly concentrated electrolyte of Li[N(SO2CF3)2] (LiTFSA)/sulfolane using a solution casting method. We successfully demonstrated the operation of Li/LiCoO2 cells with the composite electrolyte; however, the rate capability of the cell degraded with increasing LATP content. We investigated the Li-ion transport properties of the composite electrolyte and found that the gel formed a continuous phase in the composite electrolyte and Li-ion conduction mainly occurred in the gel phase. Solid-state 6Li magic-angle spinning NMR measurements for LATP treated with the 6LiTFSA/sulfolane electrolyte suggested that the Li+-ion exchange occurred at the interface between LATP and 6LiTFSA/sulfolane. However, the kinetics of Li+ transfer at the interface between LATP and the PVDF–HFP gel was relatively slow. The interfacial resistance of LATP/gel was evaluated to be 67 Ω·cm2 at 30 °C, and the activation energy for interfacial Li+ transfer was 39 kJ mol–1. The large interfacial resistance caused the less contribution of LATP particles to the Li-ion conduction in the composite electrolyte.

Molecular-Level to Cell-Level Understanding of the High-Rate Cycling Capability of Rechargeable Aluminum-Graphite Batteries

Rechargeable aluminum-graphite batteries using chloroaluminate-containing ionic liquid electrolytes are a promising beyond-lithium technology because they utilize electrodes that are globally abundant, low-cost, and inherently safe.1,2 Notably, these batteries have demonstrated ultrafast charge/discharge rates, which are critical for advancing high-power and rapid-charging applications such as electric transportation. Since Lin et al.’s pioneering report in 2015, several groups have engineered graphene-like materials to further improve the high-rate capability and capacity retention to as high as 120 mAh/g @ 400 A/g, on the cell-level.3 Recently, we elucidated the relationship between mesoscale graphite structure and macroscopic electrochemical properties such as high-rate performance.4 However, molecular-level understanding of the electrochemical charge storage mechanism upon cycling, which involves the intercalation of sterically bulky, molecular AlCl4 - anions into graphite, remains incomplete. To better understand what enables such rapid molecular-level ionic diffusion and consequently high-rate cell-level cycling, we combined electrochemical studies with solid-state 27Al magic-angle-spinning (MAS) NMR and density functional theory (DFT) to understand the molecular-level environments and ion transport properties of intercalated AlCl4 - anions.First, to directly observe local environments of chloroaluminate species intercalated within the graphite electrodes, solid-state 27Al MAS NMR measurements were acquired on intercalated graphite electrodes at various states-of-charge. DFT calculations were performed to simulate different molecular geometries of intercalated AlCl4 - anions. Correlation of experimental NMR and DFT results established that the high extents of local disorder observed in the experimental solid-state 27Al NMR spectra can be attributed to distributions of intercalated chloroaluminate anions in different molecular configurations. Second, to address the technological objective of further improving the high-rate capacity retention of graphite electrodes, we employed liquid-phase exfoliation and centrifugation to modify the c-axis thickness of the highly ordered pristine graphites. In full-cell electrochemical tests of the exfoliated-graphite electrodes, variable-rate galvanostatic cycling revealed increased capacity retention, while variable-rate cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT) experiments revealed modified effective ionic diffusivities and ohmic resistances. Collectively, this multi-scale study of chloroaluminate intercalation into graphitic electrodes has revealed new insights that are key contributing factors to their high-rate capability, which can be adapted to engineer Al-graphite batteries with enhanced performance.(1) Lin, M.-C.; Gong, M.; Lu, B.; Wu, Y.; Wang, D.-Y.; Guan, M.; Angell, M.; Chen, C.; Yang, J.; Hwang, B.-J.; Dai, H. An Ultrafast Rechargeable Aluminium-Ion Battery. Nature 2015, 520, Pages: 324–328.(2) Sun, H.; Wang, W.; Yu, Z.; Yuan, Y.; Wang, S.; Jiao, S. A New Aluminium-Ion Battery with High Voltage, High Safety and Low Cost. Chem. Commun. 2015, 51, 11892–11895.(3) Chen, H.; Xu, H.; Wang, S.; Huang, T.; Xi, J.; Cai, S.; Guo, F.; Xu, Z.; Gao, W.; Gao, C. Ultrafast All-Climate Aluminum-Graphene Battery with Quarter-Million Cycle Life. Sci. Adv. 2017, 3.(4) Xu, J. H.; Turney, D. E.; Jadhav, A. L.; Messinger, R. J. Effects of Graphite Structure and Ion Transport on the Electrochemical Properties of Rechargeable Aluminum-Graphite Batteries. ACS Appl. Energy Mater. 2019, 2, 7799–7810.

Efficient CO2 Sequestration by a Solid–Gas Reaction Enabled by Mechanochemistry: The Case of l-Lysine

This study describes an efficient and solvent-free method for the regioselective production of l-lysine ammonium ε-carbamate by ball-milling l-lysine under a CO2 atmosphere. The regioselective formation of l-lysine ammonium ε-carbamate via this mechanochemical approach was confirmed by a complete analytical study, including 1H → 13C and 1H → 15N cross polarization magic-angle spinning NMR measurements as well as liquid NMR analyses, powder X-ray diffraction measurements, thermal analyses, and elemental analyses. The time of milling, rotational speed, milling material, and liquid assistants were screened, while the reversibility of the process was assessed. The milling approach was compared with the synthesis in aqueous solutions and in the solid state without agitation. In addition to being regioselective, the synthesis by solvent-free mechanochemistry was found to be much faster than the other approaches while also producing less waste.

Elucidating Aluminum-Ion Intercalation and Its Effects on the Local Crystalline Framework Using Solid-State NMR Spectroscopy

Rechargeable multivalent-ion batteries are safe and low-cost alternatives to traditional lithium-ion batteries. However, higher Coulombic charge density of the multivalent ions results into sluggish diffusion of their respective cations within crystalline transition metal compounds. The Chevrel phases Mo6X8, where X is a chalcogen atom (e.g., S or Se) are among the few materials known to reversibly and electrochemically intercalate both divalent (e.g. Mg2+, Zn 2+, etc.) and trivalent (Al3+) ions. Thus, understanding the ion intercalation and charge transfer mechanisms within these unique compounds may provide insights into the molecular-level design of new intercalation electrodes for rechargeable multivalent ion batteries. Here, we use multi-nuclear solid-state magic-angle-spinning (MAS) NMR spectroscopy to reveal the ion intercalation and charge transfer mechanisms of zinc and aluminum intercalation in Chevrel electrodes Mo6S8 and Mo6Se8. Ex situ single pulse solid-state 27Al MAS NMR measurements were performed on the thio-chevrel electrodes at different states-of-charge to observe changes in the environments and to quantify the local populations of aluminum ions within the host crystal structure, revealing both intercalated ions and additional aluminum species associated with surface layers and/or decomposition products. Al-Mo6Se8 batteries were made for the first time and their electrochemical properties were compared to the thio-chevrel Mo6S8. Ex situ 77Se and 95Mo measurements on the seleno-chevrel Mo6Se8 at charged and discharged states revealed that the electrons are transferred to the anionic chalcogen framework the electron charge transfer mechanism and its effects on the local crystalline framework environments upon aluminum ion intercalation. These results were then generalized for zinc intercalation into chevrel phase electrodes.Overall, the solid-state NMR measurements combined with the electrochemical measurements yield molecular-level insights into the coupled aluminum ion intercalation and electron charge transfer processes that occur in a model crystalline transition metal electrode. Unique reversible anionic redox reactions were observed for multivalent ion intercalation into chevrel, which is very different that typically observed for lithium-ion intercalation into transition metal oxides, where the electrons are stored into the d-orbitals of the transition metal. The results suggest materials design principles aimed at designing multivalent-ion intercalation electrodes with improved electrochemical properties. Figure 1

Aromatic Ring Dynamics, Thermal Activation, and Transient Conformations of a 468 kDa Enzyme by Specific 1H-13C Labeling and Fast Magic-Angle Spinning NMR.

Aromatic residues are located at structurally important sites of many proteins. Probing their interactions and dynamics can provide important functional insight but is challenging in large proteins. Here, we introduce approaches to characterize the dynamics of phenylalanine residues using 1H-detected fast magic-angle spinning (MAS) NMR combined with a tailored isotope-labeling scheme. Our approach yields isolated two-spin systems that are ideally suited for artifact-free dynamics measurements, and allows probing motions effectively without molecular weight limitations. The application to the TET2 enzyme assembly of ∼0.5 MDa size, the currently largest protein assigned by MAS NMR, provides insights into motions occurring on a wide range of time scales (picoseconds to milliseconds). We quantitatively probe ring-flip motions and show the temperature dependence by MAS NMR measurements down to 100 K. Interestingly, favorable line widths are observed down to 100 K, with potential implications for DNP NMR. Furthermore, we report the first 13C R1ρ MAS NMR relaxation-dispersion measurements and detect structural excursions occurring on a microsecond time scale in the entry pore to the catalytic chamber and at a trimer interface that was proposed as the exit pore. We show that the labeling scheme with deuteration at ca. 50 kHz MAS provides superior resolution compared to 100 kHz MAS experiments with protonated, uniformly 13C-labeled samples.

Dynamic Nuclear Polarization Magic-Angle Spinning Nuclear Magnetic Resonance Combined with Molecular Dynamics Simulations Permits Detection of Order and Disorder in Viral Assemblies

We report dynamic nuclear polarization (DNP)-enhanced magic-angle spinning (MAS) NMR spectroscopy in viral capsids from HIV-1 and bacteriophage AP205. Viruses regulate their life cycles and infectivity through modulation of their structures and dynamics. While static structures of capsids from several viruses are now accessible with near-atomic-level resolution, atomic-level understanding of functionally important motions in assembled capsids is lacking. We observed up to 64-fold signal enhancements by DNP, which permitted in-depth analysis of these assemblies. For the HIV-1 CA assemblies, a remarkably high spectral resolution in the 3D and 2D heteronuclear data sets permitted the assignment of a significant fraction of backbone and side-chain resonances. Using an integrated DNP MAS NMR and molecular dynamics (MD) simulation approach, the conformational space sampled by the assembled capsid at cryogenic temperatures was mapped. Qualitatively, a remarkable agreement was observed for the experimental 13C/15N chemical shift distributions and those calculated from substructures along the MD trajectory. Residues that are mobile at physiological temperatures are frozen out in multiple conformers at cryogenic conditions, resulting in broad experimental and calculated chemical shift distributions. Overall, our results suggest that DNP MAS NMR measurements in combination with MD simulations facilitate a thorough understanding of the dynamic signatures of viral capsids.

Correcting for magnetic field drift in magic-angle spinning NMR datasets.

Magnetic field drift during magic angle spinning (MAS) NMR measurements is detrimental to the spectra, causing broadening of lines and distortion of lineshapes, especially in high-quality samples with linewidths of less than 0.1 ppm. We report that a simple linear correction for magnet drift can be used to improve the quality of proton detected MAS NMR measurements. Despite the fact that the magnetic field of superconducting magnets changes in a non-linear fashion, we find that when data acquisition is sufficiently short, a linear correction is a good approximation to the actual field drift. We used a script written in the C programming language for linear drift correction of multidimensional datasets (2D, 3D, 4D), which can be executed directly from Bruker Topspin. A second script allows datasets to be subdivided into arbitrarily short measurements, individually corrected, and concatenated before processing.

Synthesis and Characterization of the Lithium-Rich Phosphidosilicates Li10Si2P6 and Li3Si3P7.

The lithium phosphidosilicates Li10Si2P6 and Li3Si3P7 are obtained by high-temperature reactions of the elements or including binary Li-P precursors. Li10Si2P6 (P21/n, Z = 2, a = 7.2051(4) Å, b = 6.5808(4) Å, c = 11.6405(7) Å, β = 90.580(4)°) features edge-sharing SiP4 double tetrahedra forming [Si2P6]10- units with a crystal structure isotypic to Na10Si2P6 and Na10Ge2P6. Li3Si3P7 (P21/m, Z = 2, a = 6.3356(4) Å, b = 7.2198(4) Å, c = 10.6176(6) Å, β = 102.941(6)°) crystallizes in a new structure type, wherein SiP4 tetrahedra are linked via common vertices and which are further connected by polyphosphide chains to form unique ∞2[Si3P7]3- double layers. The two-dimensional Si-P slabs that are separated by Li atoms can be regarded as three covalently linked atoms layers: a defect α-arsenic type layer of P atoms sandwiched between two defect wurzite-type Si3P4 layers. The single crystal and powder X-ray structure solutions are supported by solid-state 7Li, 29Si, and 31P magic-angle spinning NMR measurements.

Solid-state characterization of sertraline base–β-cyclodextrin inclusion complex

Sertraline is one of the serotonin-specific reuptake inhibitors that is effective in treating several disorders such as major depression, obsessive–compulsive disorder, panic disorder, and social phobia. It is marketed in the form of its hydrochloride salt, which exhibits better solubility in water than its free base form. However, the absorption of sertraline through biological membranes could be improved by enhancing the solubility of its base because it is more hydrophobic than sertraline hydrochloride. To clarify the mechanism for the interaction of sertraline base with β-CD, it is important to study the basic interaction between the β-CD ring and sertraline base. Therefore, in this study, the currently used hydrochloride salt form was converted into the free base and β-CD was used as a model for β-CD derivatives to evaluate the interaction between β-CD and the sertraline base. The solid-state physicochemical characteristics of the sertraline–β-CD complex were investigated by the phase solubility method, differential scanning calorimetry, Fourier transform IR spectroscopy, FT-Raman spectroscopy, powder X-ray diffraction, and 13C cross-polarization magic-angle spinning NMR measurements. The results showed that sertraline base and β-CD form an inclusion complex, and the stoichiometric ratio of the solid-state sertraline base–β-CD complex is 1:1, which was estimated by the 1H NMR measurements of the complex dissolved in DMSO-d6.

NMR studies of DNA microcapsules prepared using sonochemical methods.

DNA molecules were recently converted using ultrasonic irradiation into microcapsules that can trap hydrophobic molecules in aqueous solution. These DNA microcapsules are capable of penetrating prokaryotic and eukaryotic cells, delivering drugs and transferring genetic information e.g. for protein expression into the host cells. DNA molecules of different sizes and structures can be assembled into spherical capsules, but to date, the interactions that hold them together in these large structural constructs are unknown. In the current study, capsules prepared from a 12 base double helix DNA were investigated using NMR spectroscopy. Solution NMR studies of the DNA emulsion reveal DNA molecules with a perturbed structure with a size similar to the precursor DNA based on diffusion NMR measurements. 2D NMR correlation measurements and chemical shift perturbation analysis show partial unzipping of AT base pairs in the centre of the modified duplex, freeing nucleoside bases to interact with other bases on other precursor molecules thereby facilitating aggregation. Slow tumbling of the microspheres renders them invisible in solution NMR spectra; therefore magic angle spinning NMR measurements are performed which provide limited evidence of the DNA in the microcapsule state.

Sequential structural changes in rhodopsin occurring upon photoactivation.

We describe the use of solid-state magic angle spinning NMR spectroscopy for characterizing the structure and dynamics of dark, inactive rhodopsin and the active metarhodopsin II intermediate. Solid-state NMR spectroscopy is well suited for structural measurements in both detergent micelles and membrane bilayer environments. We first outline the methods for large-scale production of stable, functional rhodopsin containing (13)C- and (15)N-labeled amino acids. The expression methods make use of eukaryotic HEK293S cell lines that produce correctly folded, fully functional receptors. We subsequently describe the basic methods used for solid-state magic angle spinning NMR measurements of chemical shifts and dipolar couplings, which provide information on rhodopsin structure and dynamics, and describe the use of low-temperature methods to trap the active metarhodopsin II intermediate.

Magnetic Field Dependence of the Solid-State Photo-CIDNP Effect Observed in Phototropin LOV1-C57S

Here we present the magnetic field dependence of the solid-state Photo-CIDNP effect observed in phototropin LOV1-C57S using 13 C magic-angle spinning (MAS) NMR spectroscopy. Both dark and light spectra were measured at 4.7 T (i.e., 200 MHz 1 H frequency) using a spinning frequency of 8 kHz. An Avance 200 MHz spectrometer equipped with 4-mm MAS probe (Bruker, Karlsruhe, Germany) was used for the 13 C MAS NMR experiments. The sample was packed into a 4-mm sapphire rotor and inserted into the MAS probe. For a homogeneous sample distribution against the rotor wall, the sample was frozen at a very low spinning frequency of 500 Hz. Freezing was monitored on the proton tuning channel as a shift of ca. 0.1 MHz. The variable temperature unit on the spectrometer was set to 235 K. The spinning frequency was in- creased to 8 kHz after the sample is completely frozen. This frequency and set temperature were used for all 13 C MAS NMR measurements. A simple Hahn-echo pulse sequence with two-pulse phase modulation proton-decoupling was used. Continu- ous illumination was supplied by a 1 kW xenon lamp. The cycle delay was 2 s, and the measurement time was about 12 h. In the spectrum obtained in the dark, standard broad protein responses appear. Under illumination, several strong additional light-induced signals appeared. In contrast to the entirely emissive (negative) peaks in the photo-CIDNP MAS NMR spectra observed at 2.3 T (i.e., 100 MHz 1 H frequency), the first observation of this effect in a nonphotosynthetic system, the light induced 13 C NMR peaks at 4.7 T show mixed absorptive/emissive enhancement pattern. This pattern is reminiscent of the spectra observed by liquid state photo-CIDNP of a LOV2 sample. The observed solid-state photo-CIDNP effect is strongly magnetic field dependent, and this field-dependence is well distinguished for the various nuclei. This large difference in magnetic field dependence reflects the large variety of hyperfine factors found in this comparable small-sized and asymmetric radical pair. Keywords photo-CIDNP; MAS; NMR; phototropin; LOV

Formation Process of [B<SUB>12</SUB>H<SUB>12</SUB>]<SUP>2−</SUP> from [BH<SUB>4</SUB>]<SUP>−</SUP> during the Dehydrogenation Reaction of Mg(BH<SUB>4</SUB>)<SUB>2</SUB>

The existence of Mg(B12H12) as an intermediate compound during the dehydrogenation process of Mg(BH4)2 has been verified. To elucidate the formation process of Mg(B12H12), the dehydrogenation products of Mg(BH4)2 at different temperatures were characterized via X-ray diffraction and 11B magic angle spinning NMR measurements. The experimental results indicate that several new intermediate compounds such as Mg(B2H6) and Mg(B5H9) or Mg(B5H8)2 are expected to be formed prior to the formation of Mg(B12H12). Thus, it is suggested that the formation of [B12H12]2− from [BH4]− occurs via the gradual evolution of the B-H complex anion, i.e., [BH4]−→[B2H6]2−→[B5H9]2− or [B5H8]−→[B12H12]2−.

Solid-State NMR Structural Studies of Alzheimer's Disease Aβ(1-42) And Aβ(1-40) In Phospholipid Bilayers

Amyloid-β peptides, Aβ(1-42) and Aβ(1-40) are believed to cause loss of nerve cell function in individuals suffering from Alzheimer's disease, where evidence suggests that interaction with the cell membrane correlates strongly with cytotoxicity. Previous studies report a range of different but plausible structures, which depend on the molecular environment of the peptide. This sensitivity to sample preparation suggests that the structure relevant to disease is found in lipid bilayer membranes. While model membrane vesicles are too large to be studied by conventional solution NMR, solid-state NMR is one of the few technologies available to study such systems. We present recent magic angle spinning NMR measurements which suggest a novel peptide structure in a lipid bilayer environment: (i) rotational-echo double-resonance (REDOR) distance measurements between selectively enriched 13C-carbonyl and 15N-amide positions constrain dihedral angles of intervening residues and suggest that at least part of the Aβ(1-42) peptide folds into a β-sheet like conformation, in contrast to the helical and coiled structures in previous reports; and (ii) double quantum filtered DRAWS measurements indicate that the extended strand does not assemble into an in-register parallel sheet as reported for amyloid fibrils. Additional static NMR experiments (sensitivity enhance PISEMA) of 15N labelled Aβ(1-40) in mechanically aligned phospholipid bilayers and magnetically oriented bicelles indicate that the peptide has multiple conformations in model membranes.

Two Kinds of Framework Al Sites Studied in BEA Zeolite by X-ray Diffraction, Fourier Transform Infrared Spectroscopy, NMR Techniques, and V Probe

The dealumination of BEA zeolite by treatment with concentrated nitric acid is evidenced by X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. Two-dimensional 27Al 3Q and 5Q magic-angle-spinning (MAS) NMR allow the detection of two kinds of tetrahedral Al atoms whose relative amounts depend on the Si/Al ratio and which correspond to two specific T-sites. 29Si MAS NMR and 1H MAS NMR measurements confirm these results. 29Si MAS NMR spectra evidence two resonances at around −114 and −111 ppm ascribed to Si sites of the Si(OSi)4 environment of two different crystallographic sites. Moreover, the presence of Si atoms associated with hydroxyl groups is confirmed by a resonance at −102 ppm when 1H−29Si CP is applied. The Bronsted and Lewis acidic sites in dealuminated BEA zeolites are evidenced by FTIR spectra of adsorbed pyridine which show two kinds of bridging hydroxyl groups (Si−O(H)−Al) of different acid strength. 51V MAS NMR confirms the incorporation of vanadium atoms into vacant T...

Acidic Properties of SSZ-33 and SSZ-35 Novel Zeolites: a Complex Infrared and MAS NMR Study

Two novel zeolites SSZ-33 and SSZ-35 were investigated with respect to their acidic properties using different probe molecules to characterize the accessibility and acid strength of Lewis and Brønsted acid sites. Ammonia, pyridine, pivalonitrile, and acetonitrile-d3 were used as probe molecules, and the results were correlated with 27Al and 1H magic angle spinning (MAS) NMR. Both SSZ-33 and SSZ-35 zeolites were found to possess bridging Si−OH−Al groups of virtually uniform and high acid strength. For both SSZ-33 and SSZ-35, there is the typical presence of highly disturbed OH groups (IR band around 3500 cm-1), which amounts to almost half of the overall Brønsted acidity. It was found that almost all bridging Si−OH−Al groups in SSZ-33 are located in the 12-MR rings. Both acetonitrile-d3 and pyridine sorptions suggest the presence of two types of Lewis sites in SSZ-35, differing in acid strength and electron-acceptor properties, whereas in the SSZ-33 zeolite only one type is present. The relative strength of these sites is higher than that of the Brønsted type for SSZ-35 and is of comparable strength for SSZ-33. 1H and 27Al MAS NMR measurements during thermal treatment allowed the assignment of NMR peaks to different surface OH species and the establishment of their relation to IR bands. NMR spectroscopy enabled the quantitative analysis of both free and hydrogen-bonded OH groups separately, showing that for both zeolites the amount of disturbed sites is higher than the number of free OH groups.

Anion diffusivity in highly conductive nanocrystalline BaF2:CaF2 composites prepared by high-energy ball milling

We report on the fluorine diffusivity in nanocrystalline BaF2 and CaF2 as well as in BaF2:CaF2 composites prepared by high-energy ball milling. Mechanical treatment of polycrystalline BaF2 together with CaF2 results in a nanocrystalline composite with an unexpectedly high dc conductivity of about 0.1 mS cm−1 at 450 K. X-ray diffraction and 19F magic angle spinning NMR measurements at extraordinary high spinning speeds reveal that this can be explained inter alia by the interplay of structural disorder and the formation of a mixed (Ba,Ca)F2 phase.

Structural constraints on the transmembrane and juxtamembrane regions of the phospholamban pentamer in membrane bilayers: Gln29 and Leu52

The Ca 2+-ATPase of cardiac muscle cells transports Ca 2+ ions against a concentration gradient into the sarcoplasmic reticulum and is regulated by phospholamban, a 52-residue integral membrane protein. It is known that phospholamban inhibits the Ca 2+ pump during muscle contraction and that inhibition is removed by phosphorylation of the protein during muscle relaxation. Phospholamban forms a pentameric complex with a central pore. The solid-state magic angle spinning (MAS) NMR measurements presented here address the structure of the phospholamban pentamer in the region of Gln22–Gln29. Rotational echo double resonance (REDOR) NMR measurements show that the side chain amide groups of Gln29 are in close proximity, consistent with a hydrogen-bonded network within the central pore. 13C MAS NMR measurements are also presented on phospholamban that is 1- 13C-labeled at Leu52, the last residue of the protein. pH titration of the C-terminal carboxyl group suggests that it forms a ring of negative charge on the lumenal side of the sarcoplasmic reticulum membrane. The structural constraints on the phospholamban pentamer described in this study are discussed in the context of a multifaceted mechanism for Ca 2+ regulation that may involve phospholamban as both an inhibitor of the Ca 2+ ATPase and as an ion channel.