Magic Angle Spinning Experiments Research Articles

Enhancing MQMAS of low-γ nuclei by using a high B1 field balanced probe circuit

A balanced probe circuit is used to generate high B1 magnetic field for sensitivity enhancement of multiple-quantum magic-angle spinning (MQMAS) experiment applied to low-γ quadrupolar nuclei. Electrical balancing of the sample coil can cut the peak voltage by a half, therefore improving the power handling when generating a two-fold higher B1 field. Experimental results, illustrated here with 25Mg data for two layered double hydroxides, show that the MQMAS efficiency increases more than linearly with the B1 field strength. The multiplicative enhancements from high B0 and B1 fields and an optimized MQMAS pulse sequence provide the critically needed sensitivity for acquiring MQMAS spectra of low-γ quadrupolar nuclei such as 25Mg at natural abundance.

Optimized multiple quantum MAS lineshape simulations in solid state NMR

The majority of nuclei available for study in solid state Nuclear Magnetic Resonance have half-integer spin I > 1 / 2 , with corresponding electric quadrupole moment. As such, they may couple with a surrounding electric field gradient. This effect introduces anisotropic line broadening to spectra, arising from distinct chemical species within polycrystalline solids. In Multiple Quantum Magic Angle Spinning (MQMAS) experiments, a second frequency dimension is created, devoid of quadrupolar anisotropy. As a result, the center of gravity of peaks in the high resolution dimension is a function of isotropic second order quadrupole and chemical shift alone. However, for complex materials, these parameters take on a stochastic nature due in turn to structural and chemical disorder. Lineshapes may still overlap in the isotropic dimension, complicating the task of assignment and interpretation. A distributed computational approach is presented here which permits simulation of the two-dimensional MQMAS spectrum, generated by random variates from model distributions of isotropic chemical and quadrupole shifts. Owing to the non-convex nature of the residual sum of squares (RSS) function between experimental and simulated spectra, simulated annealing is used to optimize the simulation parameters. In this manner, local chemical environments for disordered materials may be characterized, and via a re-sampling approach, error estimates for parameters produced. Program summary Program title: mqmasOPT Catalogue identifier: AEEC_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEEC_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 3650 No. of bytes in distributed program, including test data, etc.: 73 853 Distribution format: tar.gz Programming language: C, OCTAVE Computer: UNIX/Linux Operating system: UNIX/Linux Has the code been vectorised or parallelized?: Yes RAM: Example: (1597 powder angles) × (200 Samples) × (81 F2 frequency pts) × (31 F1 frequency points) = 3.5M, SMP AMD opteron Classification: 2.3 External routines: OCTAVE ( http://www.gnu.org/software/octave/), GNU Scientific Library ( http://www.gnu.org/software/gsl/), OPENMP ( http://openmp.org/wp/) Nature of problem: The optimal simulation and modeling of multiple quantum magic angle spinning NMR spectra, for general systems, especially those with mild to significant disorder. The approach outlined and implemented in C and OCTAVE also produces model parameter error estimates. Solution method: A model for each distinct chemical site is first proposed, for the individual contribution of crystallite orientations to the spectrum. This model is averaged over all powder angles [1], as well as the (stochastic) parameters; isotropic chemical shift and quadrupole coupling constant. The latter is accomplished via sampling from a bi-variate Gaussian distribution, using the Box-Muller algorithm to transform Sobol (quasi) random numbers [2]. A simulated annealing optimization is performed, and finally the non-linear jackknife [3] is applied in developing model parameter error estimates. Additional comments: The distribution contains a script, mqmasOpt.m, which runs in the OCTAVE language workspace. Running time: Example: (1597 powder angles) × (200 Samples) × (81 F2 frequency pts) × (31 F1 frequency points) = 58.35 seconds, SMP AMD opteron.

Nuclear relaxation and critical fluctuations in membranes containing cholesterol

Nuclear resonance frequencies in bilayer membranes depend on lipid composition. Our calculations describe the combined effects of composition fluctuations and diffusion on nuclear relaxation near a miscibility critical point. Both tracer and gradient diffusion are included. The calculations involve correlation functions and a correlation length xi=xi(0)T/(T-T(c)), where T-T(c) is temperature above the critical temperature and xi(0) is a parameter of molecular length. Several correlation functions are examined, each of which is related in some degree to the Ising model correlation function. These correlation functions are used in the calculation of transverse deuterium relaxation rates in magic angle spinning and quadrupole echo experiments. The calculations are compared with experiments that report maxima in deuterium and proton nuclear relaxation rates at the critical temperature [Veatch et al., Proc. Nat. Acad. Sci. U.S.A. 104, 17650 (2007)]. One Ising-model-related correlation function yields a maximum 1/T(2) relaxation rate at the critical temperature for both magic angle spinning and quadrupole echo experiments. The calculated rates at the critical temperature are close to the experimental rates. The rate maxima involve relatively rapid tracer diffusion in a static composition gradient over distances of up to 10-100 nm.

NMR investigations of Li+ ion dynamics in the NASICON ionic conductors

NMR studies of 7Li and 31P nuclei are reported in the 150–900 K temperature range for the NASICON compounds with x = 0.8, 0.7, 0.6 and 0.3. Magic angle spinning (MAS mode) experiments were performed at room temperatureon the 7Li and 31P nuclei. The linewidth and the spin lattice relaxation times of these nuclei are studied versustemperature in the static mode. The spectra recorded in the MAS mode show that the 7Li ions occupy three chemical sites, the occupation of which being very sensitive to thex values but not sensitive to the coexistence of the two varieties and observed at room temperature in compounds withx≤0.5. On the otherhand, the 31P nucleus MAS spectra are very sensitive to lithium content but also to the variety coexistence.T1 measurements were performed in a static mode on the 7Li and 31P nuclei. In all thecompounds, the 7Li spin lattice relaxation time exhibits two branches with several minima, indicating the complexdynamics for this ion. One of these minima appears in the same temperature range as the minimum ofthe 31P nucleus T1, strongly suggesting a cross-relaxation process between these nuclei.T1ρ measurementson 7Li (static mode) allow us to show a slow motion different from the one probed by theT1. The analysisof the T1ρ behaviour with temperature and composition allows us to ascribe the motionprobed by this time to the oxygen ion motion which monitors the openingand closing of the lithium pathways. A qualitative interpretation of the 7Li T1 results is done; it takes into account the cross-relaxation phenomena between 31P and 7Li and quadrupolar fluctuations.

Characterization of local structures in amorphous and crystalline tris(8-hydroxyquinoline) aluminum(III) (Alq 3) by solid-state 27Al MQMAS NMR spectroscopy

The local structure of tris(8-hydroxyquinoline) aluminum(III) (Alq 3) in three different crystalline polymorphs, α-, γ-, and δ-Alq 3, and amorphous Alq 3, are investigated by 27Al NMR experiments, which include two-dimensional multi-quantum magic-angle spinning (MQMAS) experiments and one-dimensional variable B 0 field MAS experiments. The quadrupolar and chemical shift parameters of these Alq 3 samples are determined, including the distributions. These parameters reflect the local structure of respective samples and it is found that the γ- and δ-Alq 3 consist of the facial isomer with well-defined crystal structure, whereas the α- and amorphous Alq 3 consist of the meridional isomer with distributions of quadrupolar coupling constants. It is also found that two distinct components exist in α-Alq 3.

Solid-state NMR and molecular dynamics simulations reveal the oligomeric ion-channels of TM2-GABA A stabilized by intermolecular hydrogen bonding

The second transmembrane (TM2) domain of GABA A receptor forms the inner-lining surface of chloride ion-channel and plays important roles in the function of the receptor protein. In this study, we report the first structure of TM2 in lipid bilayers determined using solid-state NMR and MD simulations. The interatomic 13C– 15N distances measured from REDOR magic angle spinning experiments on multilamellar vesicles, containing a TM2 peptide site specifically labeled with 13C′ and 15N isotopes, were used to determine the secondary structure of the peptide. The 15N chemical shift and 1H– 15N dipolar coupling parameters measured from PISEMA experiments on mechanically aligned phospholipid bilayers, containing a TM2 peptide site specifically labeled with 15N isotopes, under static conditions were used to determine the membrane orientation of the peptide. Our results reveal that the TM2 peptide forms an alpha helical conformation with a tilted transmembrane orientation, which is unstable as a monomer but stable as pentameric oligomers as indicated by MD simulations. Even though the peptide consists of a number of hydrophilic residues, the transmembrane folding of the peptide is stabilized by intermolecular hydrogen bondings between the side chains of Ser and Thr residues as revealed by MD simulations. The results also suggest that peptide–peptide interactions in the tilted transmembrane orientation overcome the hydrophobic mismatch between the peptide and bilayer thickness.

Application of shaped adiabatic pulses to MQMAS NMR spectroscopy of spin 3/2 nuclei

Competition between nutation (r.f. driven) and adiabatic (rotor-driven) multi-quantum coherence transfer mechanisms in spin 3/2 systems results in diminished performance of rotation induced adiabatic coherence transfer (RIACT) in isotropic multiple-quantum magic-angle spinning (MQMAS) experiments for small e 2 qQ / h (<2 MHz) and high radio-frequency powers. We present a simple shaped RIACT pulse consisting of a truncated sine wave (spanning 0–0.8 π) that corrects the sensitivity losses, phase twist and relative intensity errors that can arise in MQMAS spectra utilizing constant-amplitude RIACT pulses. The shaped RIACT pulse may enhance the study of metals in biomolecules where quadrupole couplings of S = 3/2 nuclei such as 23Na tend to be small.

Solid‐state natural abundance 25Mg NMR studies of Na2MgEDTA·4H2O—a possible new reference compound for 25Mg NMR spectroscopy

Natural abundance solid-state (25)Mg NMR measurements were made of the disodium salt of magnesium ethylenediaminetetraacetate tetrahydrate (Na(2)MgEDTA x 4 H(2)O). Both magic angle spinning (MAS) and static experiments were employed to determine the quadrupole coupling constant (C(q)) and the asymmetry parameter (eta(q)) of the electric field gradient (EFG) tensor associated with (25)Mg in this compound, giving the values C(q) = 1.675(5) MHz and eta(q) = 0.15(1). The isotropic chemical shift was determined to be delta(iso) = 0.25(10) ppm (relative to 11 M MgCl(2) aqueous solution) and a small chemical shift anisotropy (CSA) contribution (approximately -13 ppm) was detected, one of the first CSA reports in (25)Mg NMR. This compound exhibited remarkably good (25)Mg NMR sensitivity, due to its fast spin-lattice relaxation and modest quadrupole coupling, which allowed its use as a secondary shift reference and as a test sample for the implementation and optimisation of signal-enhancement methods in (25)Mg NMR spectroscopy, such as double frequency sweeps (DFS) and the use of adiabatic hyperbolic secant (HS) and WURST pulses.

Analytical solutions to several magic-angle spinning NMR experiments

Using the Anderson–Weiss (AW) formalism, analytical expressions of the NMR signal are obtained for the following magic-angle spinning (MAS) experiments: total suppression of sidebands (TOSS); phase adjusted spinning sidebands (PASS); rotational-echo double-resonance (REDOR); rotor-encoded REDOR (REREDOR); cross-polarization magic-angle spinning (CPMAS); exchange induced sidebands (EIS); one-dimensional exchange spectroscopy by sideband alternation (ODESSA); time-reverse ODESSA (trODESSA); centerband-only detection of exchange (CODEX). In order to test the validity of the AW approach, the Gaussian powder approximation is compared with exact powder calculations. A quantitative study of the effect of molecular dynamics on the efficiency of the TOSS and REDOR pulse sequences is then presented.

Water chemical shift in 1H NMR of red cells: Effects of pH when transmembrane magnetic susceptibility differences are low

The (1)H magic angle spinning (MAS) NMR spectrum of water in erythrocyte suspensions shows peaks from each of the intracellular and extracellular water pools. The splitting is a true chemical shift and is brought about by the elimination of water exchange under MAS conditions due to physical separation of the two water populations. The size of the chemical shift difference is determined by the concentration of intracellular protein affecting the average extent of hydrogen bonding of water. We present here a model of the chemical shift behavior for water in erythrocytes under normal high-resolution NMR conditions based on results from MAS experiments on these cells exposed to different pH and osmotic conditions. The model accurately predicts the chemical shift of water for a static sample, and the results demonstrate that in high-resolution NMR experiments the chemical shift of water will appear to be invariant if differences in magnetic susceptibility across the cell membrane are minimal (<10% of the magnetic susceptibility of water). Thus, changes in the shape and chemical shift of the water resonance are not due to pH changes in the physiological range. The findings are fundamental to an interpretation of the mechanism of chemical shift effects on the water resonance that may occur in functional MRI.

Probing Local Structure in Zeolite Frameworks: Ultrahigh-Field NMR Measurements and Accurate First-Principles Calculations of Zeolite 29Si Magnetic Shielding Tensors

The principal components of zeolite 29Si magnetic shielding tensors have been accurately measured and calculated for the first time. The experiments were performed at an ultrahigh magnetic field of 21.1 T in order to observe the small anisotropies of the 29Si shielding interactions that arise for Si atoms in near-tetrahedral geometries. A robust two-dimensional (2D) chemical shift anisotropy (CSA) recoupling pulse sequence was employed that enables quasi-static powder patterns to be resolved according to the isotropic chemical shifts. For the zeolites Sigma-2 and ZSM-12, it is demonstrated that the 29Si chemical shift (CS) tensor components measured by the recoupling experiment are in excellent agreement with those determined from spinning sidebands in slow magic-angle spinning (MAS) experiments. For the zeolite ZSM-5, the principal components of the 29Si CS tensors of 15 of the 24 Si sites were measured using the 2D CSA recoupling experiment, a feat that would not be possible with a slow MAS experiment due to the complexity of the spectrum. A simple empirical relationship between the 29Si CS tensors and local structural parameters could not be established. However, the 29Si magnetic shielding tensors calculated using Hartree-Fock ab initio calculations on clusters derived from the crystal structures are in excellent agreement with the experimental results. The accuracy of the calculations is strongly dependent on the quality of the crystal structure used in the calculation, indicating that the 29Si magnetic shielding interaction is extremely sensitive to the local structure around each Si atom. It is anticipated that the measurement and calculation of 29Si shielding tensors could be incorporated into the "NMR crystallography" of zeolites and other related silicate materials, possibly being used for structure refinements that may lead to crystal structures with very accurate Si and O atomic coordinates.

A High-Resolution43Ca Solid-State NMR Study of the Calcium Sites of Hydroxyapatite

High resolution 43Ca solid-state NMR studies of hydroxyapatite (Ca10(PO4)6(OH)2) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using 1H-43Ca rotational echo double resonance and 1H-43Ca cross polarization magic angle spinning experiments.

Organosilicas based on purine–pyrimidinebase pair assemblies: a solid state NMR point of view

Organosilicas based on adenine (A) and thymine (T) assemblies have been synthesized. A surfactant-free route, based on specific molecular recognition between A and T entities, has been developed. The characterization of the H-bond networks, in both homo- and hetero-assemblies, has been emphasized by using 1H solid state NMR (nuclear magnetic resonance). The latest experimental developments were implemented (i.e. very fast MAS (magic angle spinning) experiments at 750 MHz and 33 kHz), in order to enhance drastically the spectral resolution. Moreover, 1H experiments at 67 kHz in 1.3 mm rotors were performed. Spatial connectivities between protons were established by using 1H–1H DQ (double quantum) MAS experiments, allowing the precise characterization of A/A, T/T and A/T associations.

An experimental study of decoupling sequences for multiple‐quantum and high‐resolution MAS experiments in solid‐state NMR

Recently, a sequence for heteronuclear dipolar decoupling in solid-state NMR, namely SWf-TPPM, was introduced by us. Under magic-angle spinning (MAS), the decoupling efficiency of the sequence was unaffected over a range of values for various experimental parameters such as the pulse length, pulse phase, and 1H resonance offset. We here demonstrate its use in multiple-quantum (MQ) and high-resolution (HR) MAS experiments. This sequence further improves the MQMAS spectra compared to the earlier reported decoupling sequences with improved immunity to any missets of the pulse length, pulse phase and decoupler offset. In contrast, for HRMAS, the simple CW scheme is as efficient as any of the decoupling schemes that were studied.

Cation Sublattice Disorder Induced by Swift Heavy Ions in MgAl2O4 and ZnAl2O4 Spinels: 27Al Solid-State NMR Study

MgAl2O4 and ZnAl2O4 spinels have been irradiated by swift heavy ions (86Kr and 36S) simulating the irradiation by fission products for applications in the transmutation targets. The structures of unirradiated and irradiated spinel samples have been studied by NMR spectroscopy, with 27Al magic angle spinning and multiple-quantum magic angle spinning experiments. The parameters of fluence and electronic stopping power have been compared. For 86Kr ions, the obtained spectra are modified by irradiation: we observe a rise of the amount of aluminum in tetrahedral sites and a widening of the lines associated with the different aluminum environments compared with those of the pristine samples. Site exchange in the cationic sublattice is then observed and can be quantified from NMR spectra, determining the inversion parameter. An inversion parameter of 0.77 is estimated for the MgAl2O4 spinel irradiated with 1013 Kr ions/cm2, for a value of 0.275 in the pristine samples. Moreover, a line attributed to aluminum in 5-fold coordination with oxygen is observed in irradiated spinel samples at the maximum fluence for krypton. These new aluminum environments can characterize a transition layer which could change toward an amorphous layer, increasing the electronic stopping power and/or the fluence.

NMR and IR characterization of the aluminium complexes of norfloxacin and ciprofloxacin fluoroquinolones

A set of new aluminium complexes of norfloxacin (NOR) and ciprofloxacin (CIP) that show an improvement in their pharmaceutical properties were studied using solution and solid-state nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. The complexes synthesized with two different methods were compared. One of these methods will allow formulation of the compounds at production scale. High-resolution (13)C spectra were obtained with the cross-polarization and magic angle spinning (CP-MAS) experiment. These spectra were assigned by comparing them with the solution data of the pure drug and by using a quaternary carbon edition technique. The carbon relaxation times in the rotating frame, T(1rhoC), were measured for all the complexes. A two-exponential decay evidences that the complexes are nonhomogeneous. The short T(1rhoC) values are in the range 320-1100 micros and the long values in the range 1.8-7 ms. (27)Al MAS NMR spectra revealed an octahedral coordination between the aluminium ion and oxygens of the pure drug, supporting a 3:1 ligand:metal stoichiometry in both CIP and NOR complexes. The stretching and deformation modes of carboxylic acid and carboxylate and keto groups were analyzed by IR. This technique shows that the same modes are present in the aluminium complexes obtained by the two methods and that the coordination of the fluoroquinolone to aluminium occurs through the 4-keto and 3-carboxylic groups.

Fullerene as a Standard Sample for Adjusting the NMR Spectral Resolution in Multiple-Quantum Magic-Angle-Spinning Experiments

A method using fullerene for adjusting the NMR spectral resolution for multiple quantum magic angle spinning (MQMAS) experiments is proposed. To observe its (13)C MAS signal, it is not necessary to apply (1)H decoupling, unavailable with single-resonance MQMAS probes. Although (13)C T(1) of fullerene is rather long, a recycle time of 5 s in shimming yields its signal with sufficiently high sensitivity if setting the appropriate Ernst angles corresponding to magnetic fields. It is demonstrated that so-adjusted high resolution is reflected in the (87)Rb MQMAS spectra of RbNO(3).

27Al NMR Study of the Structure of Lanthanum- and Yttrium-Based Aluminosilicate Glasses and Melts

We have investigated by (27)Al nuclear magnetic resonance spectroscopy some compositions in the Ln(2)O(3)-Al(2)O(3)-SiO2 (Ln = Y or La) ternary phase diagram containing more than 60 mol % of SiO2. One- and two-dimensional high-field (17.6 T) high-speed (30 kHz) magic angle spinning experiments have been performed along with simulations of the spectra to quantify the amount of penta-coordinated aluminum present in those glasses as a function of composition. Very high-temperature experiments have allowed to follow selected samples from 2200 degrees C down to 1700 degrees C and hence to characterize the aluminum coordination state and dynamics in those liquids. The present study re-enforces the current view that "minor" species such as penta-coordinated aluminum are actually present in a considerable amount in aluminosilicate glasses, and high-temperature liquids at and above the charge compensation join. The high-field strength of Y3+ and La3+ reveal, for the first time in glasses, a different mean electric field gradient perceived by the tetra- and penta-coordinated aluminum environments. The movements responsible for the NMR relaxation of aluminum in the high-temperature liquid are shown to be uncorrelated with the movements responsible for the macroscopic shear viscosity. Results obtained both on glasses and in situ at high-temperature suggest a preferential localization of Ln3+ nearby tetra-coordinated aluminum species, with possible formation of tricluster and/or Ln3+ coordination changes.

Investigation of Dynamics of Poly(dimethylsilane) in the Mesophase by Solid-State 29Si NMR: Evidence for Rotator Phase

The dynamics of poly(dimethylsilane) (PDMS) in the mesophase was investigated by DSC and 29Si solid-state NMR. A solid−solid transition is observed at 166 °C based on DSC measurements. Below the transition temperature, typical chemical shift anisotropy (CSA) spectra are obtained by the 29Si solid-state NMR with cross-polarization (CP) preparation for the static PDMS sample. In contrast, the intensities at the isotropic chemical shift are suppressed for the CSA spectra above the transition temperature. The intensity attenuation, termed “magic angle hole”, in the CSA spectra is theoretically derived for uniaxially rotating solids. Therefore, it was found that PDMS undergoes a uniaxial rotation motion above the transition temperature. Theoretically, both the direct polarization (DP) experiments without the CP process and ultraslow magic angle spinning (MAS) experiments retrieve standard CSA line shapes. Our DP and ultraslow MAS experiments confirm the theoretical consideration. The retrieved CSA spectra have...

NMR studies of the aluminum hydride phases and their stabilities

Multinuclear and multidimensional solid state NMR techniques including magic-angle-spinning (MAS) and multiple-quantum (MQ) MAS experiments have been used to characterize various AlH 3 samples. At least three distinct polymorphic AlH 3 phases have been prepared by desolvating the alane etherate product from its organometallic synthesis. MAS-NMR spectra for the 1H and 27Al nuclei have been obtained on a variety of AlH 3 samples that include the β- and γ-phases as well as the α-phase. 27Al MAS NMR was found to respond with high sensitivity for showing differences in spatial arrangements of AlH 6 octahedra in the three polymorphs studied. Based on the characteristic NMR signatures determined, phase transition of the γ-AlH 3 to the α-AlH 3 was studied at room and high temperatures. Direct decomposition of the γ-AlH 3 to aluminum metal at room temperature was also unambiguously confirmed by NMR studies.