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Improving the spectral sensitivity and resolution of biological solids is one of the long-standing problems in nuclear magnetic resonance (NMR) spectroscopy. In this report, we introduce low-power supercycled variants of two-pulse phase-modulated (TPPM) sequence for heteronuclear decoupling. The utility of the sequence is shown by improvements in the transverse relaxation time of observed nuclei (with 1H decoupling) with its application to different samples (uniformly 13C, 15N, 2H-labeled GB1 back-exchanged with 25% H 2 O and 75% D 2 O, uniformly 13C, 15N, 2H-labeled human derived Asyn fibril back-exchanged with 100% H 2 O and uniformly 13C, 15N -labeled human derived Asyn fibril) at fast MAS using low radiofrequency (RF) fields. To understand the effect of spinning speed, the transverse relaxation time is monitored under different spinning frequencies. In comparison to existing heteronuclear decoupling sequences, the supercycled TPPM (sTPPM) sequence significantly improves the spectral sensitivity and resolution and is robust towards B 1 inhomogeneity and decoupler offset. [Display omitted] • Low power supercycled TPPM sequence for heteronuclear decoupling in solid-state NMR. • Improved robustness towards B 1 inhomogeneity and chemical shift offset. • Robust performance over a large range of parameters. • Improved protocol for 1H detection in solid-state NMR. [ABSTRACT FROM AUTHOR]

• A (S + 1/2) loss in many correlation and polarization-transfer experiments of quadrupole nuclei S. • Level-crossings of satellite transition and rf frequency under MAS complicate the spin dynamics of quadrupole nuclei. • NMR method development, high magnetic fields, efficient and high-power probes are important to quadrupole nuclei. High magnetic field can dramatically increase the spectral resolution and sensitivity of quadrupole nuclei S > 1/2 by the reduction of the second-order quadrupole broadening. A brief overview and outlook on spectral acquisition, the importance of high magnetic field, inter-nuclei distance measurement, various 2D separation and correlation methods of quadrupole nuclei are presented. The complications and consequences of spin dynamics under rf irradiation for the (2S + 1) level system and level-crossing with the satellite transition frequencies under magic-angle spinning are discussed. There is a scaling down of (S + 1/2) to the efficiency of many experiments in comparison with a spin-1/2 due to the fact that only two central transition spin states out of the (2S + 1) levels contribute to polarization transfer and spin correlation. [ABSTRACT FROM AUTHOR]

We characterize a new generation of MAS probes, designed for 1H detection in solid and viscous structures. High top speed (currently 170 kHz), existence of a wide speed range and quick acceleration enable numerous new experiment categories. Most notably, massive biomolecular structures become amenable to a detailed structural and dynamics studies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Instrumental development to achieve sustainable, cost-effective cryogenic Helium sample spinning. • Solid-state NMR and DNP at ultra-low temperature. • Closed-loop helium system composed of a powerful heat exchanger, a cryocooler and a single Helium compressor. • 30 K at the sample with a single cryocooler. • Cryogenic sample exchange system. We report here instrumental developments to achieve sustainable, cost-effective cryogenic Helium sample spinning in order to conduct dynamic nuclear polarisation (DNP) and solid-state NMR (ssNMR) at ultra-low temperatures (<30 K). More specifically, we describe an efficient closed-loop helium system composed of a powerful heat exchanger (95% efficient), a single cryocooler, and a single helium compressor to power the sample spinning and cooling. The system is integrated with a newly designed triple-channel NMR probe that minimizes thermal losses without compromising the radio frequency (RF) performance and spinning stability (±0.05%). The probe is equipped with an innovative cryogenic sample exchange system that allows swapping samples in minutes without introducing impurities in the closeloop system. We report that significant gain in sensitivity can be obtained at 30–40 K on large micro-crystalline molecules with unfavorable relaxation timescales, making them difficult or impossible to polarize at 100 K. We also report rotor-synchronized 2D experiments to demonstrate the stability of the system. [ABSTRACT FROM AUTHOR]

An improved implementation of single-crystal magic-angle-spinning (MAS) NMR is presented which gives access to chemical shift tensors both in orientation (relative to the crystal axis system) and principal axis values. For mounting arbitrary crystals inside ordinary MAS rotors, a mounting tool is described which allows to relate the crystal orientation determined by diffraction techniques to the rotor coordinate system. The crystal is finally mounted into a MAS rotor equipped with a special insert which allows a defined reorientation of the single-crystal by 90°. The approach is based on the idea that the dispersive spectra, which are obtained when applying read-pulses at specific rotor-phases, not only yield the size of the eigenvalues but also encode the orientation of the different chemical shift (rank-2) tensors. For this purpose two 2D-data sets with orthogonal crystal orientation are fitted simultaneously. The presented analysis for chemical shift tensors is supported by an analytical formula which allows fast calculation of phase and amplitude of individual spinning side-bands and by a protocol which solves the problem of finding the correct reference phase of the spectrum. Different rotor-synchronized pulse-sequences are introduced for the same reason. Experiments are performed on L -alanine and O-phosphorylethanolamine and the observed errors are analyzed in detail. The experimental data are opposed to DFT-computed chemical shift tensors which have been obtained by the extended embedded ion method. [ABSTRACT FROM AUTHOR]

[Display omitted] • Various recoupling techniques for probing 14N-1H interactions are compared. • Similarity of phase-inverted rotary-resonance recoupling and the symmetry based SR4 1 2 method is explored. • Offset dependence of TRAPDOR for application to nitrogen sites with different quadrupolar coupling constants. 1H-detected 14N heteronuclear multiple-quantum coherence (HMQC) magic-angle-spinning (MAS) NMR experiments performed at fast magic-angle spinning (≥50 kHz) are finding increasing application, e.g., to pharmaceuticals. Of importance to the efficacy of these techniques is the recoupling technique applied to reintroduce the 1H-14N dipolar coupling. In this paper, we compare, by experiment and 2-spin density matrix simulations, two classes of recoupling scheme: first, those based on n = 2 rotary resonance, namely R3 and spin-polarisation inversion SPI-R3, and the symmetry based SR4 1 2 method and, second, the TRAPDOR method. Both classes require optimisation depending on the magnitude of the quadrupolar interaction, and thus there is a compromise choice for samples with more than one nitrogen site, as is the case for the studied dipeptide β-AspAla that contains two nitrogen sites with a small and large quadrupolar coupling constant. Considering this, we observe better sensitivity for the TRAPDOR method, though noting the marked sensitivity of TRAPDOR to the 14N transmitter offset, with both SPI-R3 and SR4 1 2 giving similar recoupling performance. [ABSTRACT FROM AUTHOR]

The paper describes a family of novel recoupling pulse sequences in magic angle spinning (MAS) solid state NMR, called four pulse recoupling. These pulse sequences can be employed for both homonuclear and heteronuclear recoupling experiments and are robust to dispersion in chemical shifts and rf-inhomogeneity. The homonuclear pulse sequence consists of a building block π 2 0 ° 3 π 2 ϕ ° π 2 180 ° + ϕ ° 3 π 2 180 ° where ϕ = π n ϕ ° = 180 ° n , and n is number of blocks in a two rotor period. The heteronuclear recoupling pulse sequence consists of a building block π 2 0 ° 3 π 2 ϕ 1 ° π 2 180 ° + ϕ 1 ° 3 π 2 180 ° and π 2 0 ° 3 π 2 ϕ 2 ° π 2 180 ° + ϕ 2 ° 3 π 2 180 ° on channel I and S , where ϕ 1 = 3 π 2 n , ϕ 2 = π 2 n and n is number of blocks in a two rotor period. The recoupling pulse sequences mix the y magnetization. We show that four pulse recoupling is more broadband compared to three pulse recoupling [1] . Experimental quantification of this method is shown for 13 C α - 13 CO, homonuclear recoupling in a sample of Glycine and 15 N- 13 C α , heteronuclear recoupling in Alanine. Application of this method is demonstrated on a sample of tripeptide N-formyl-[U- 13 C, 15 N]-Met-Leu-Phe-OH (MLF). [ABSTRACT FROM AUTHOR]

The paper describes a family of novel recoupling pulse sequences, called three pulse recoupling. These pulse sequences can be employed for both homonuclear and heteronuclear recoupling experiments and are robust to dispersion in chemical shifts and rf-inhomogeneity. These recoupling pulse sequences can be used in design of two-dimensional solid state NMR experiments that use powdered dephased antiphase coherence ( γ preparation) to encode chemical shifts in the indirect dimension. Both components of this chemical shift encoded gamma-prepared states can be refocused into inphase coherence by a recoupling element. This helps to achieve sensitivity enhancement in 2D NMR experiments by quadrature detection. [ABSTRACT FROM AUTHOR]

Proton NMR spin-diffusion experiments are often combined with magic-angle spinning (MAS) to achieve higher spectral resolution of solid samples. Here we show that local proton spin diffusion can indeed become faster at low (<10 kHz) spinning rates as compared to static conditions. Spin diffusion under static conditions can thus be slower than the often referred value of 0.8 nm 2 /ms, which was determined using slow MAS (Clauss et al., 1993). The enhancement of spin diffusion by slow MAS relies on the modulation of the orientation-dependent dipolar couplings during sample rotation and goes along with transient level crossings in combination with dipolar truncation. The experimental finding and its explanation is supported by density matrix simulations, and also emphasizes the sensitivity of spin diffusion to the local coupling topology. The amplification of spin diffusion by slow MAS cannot be explained by any model based on independent spin pairs; at least three spins have to be considered. [ABSTRACT FROM AUTHOR]

A new method for precise setting of the spinning angle to the magic angle by using a saddle coil is described. The coil, which is referred to as an X 0 shim coil, is wound to produce a uniform static magnetic field B x perpendicular to the main magnetic field B 0 . The magnetic field felt by a sample is a vector sum of the main field B 0 and the transverse field B x produced by the X 0 shim coil. Hence the angle between the spinner axis and the effective magnetic field can be controlled by current I supplied to the X 0 shim coil, leading to precise angle adjustment without backlash accompanied with a mechanical system conventionally used. It is shown that the angle range achieved is ± 0.05 ° for I = ± 5 A at B 0 = 7 T. [ABSTRACT FROM AUTHOR]

Magic-angle spinning (MAS) is a technique that is a prerequisite for high-resolution solid-state NMR spectroscopy of proteins and other biomolecules. Recently, the 100 kHz limit for the rotation frequency has been broken, arguably making MAS rotors the man-made objects with the highest rotation frequency. This development is expected to have a significant impact on biomolecular NMR as it facilitates proton detection, which allows to partially compensate the loss in overall sensitivity associated with the small sample amounts that fit into MAS rotors with less than 1 mm outer diameter. Under these conditions, the mass-normalized sensitivity of a small rotor becomes much higher than that of larger-volume rotor. [ABSTRACT FROM AUTHOR]

The negative global impact of the AIDS pandemic is well known. In this perspective article, the utility of magic angle spinning (MAS) NMR spectroscopy to answer pressing questions related to the structure and dynamics of HIV-1 protein assemblies is examined. In recent years, MAS NMR has undergone major technological developments enabling studies of large viral assemblies. We discuss some of these evolving methods and technologies and provide a perspective on the current state of MAS NMR as applied to the investigations into structure and dynamics of HIV-1 assemblies of CA capsid protein and of Gag maturation intermediates. [ABSTRACT FROM AUTHOR]

Understanding the structure of membrane-active peptides faces many challenges associated with the development of appropriate model membrane systems as the peptide structure depends strongly on the lipid environment. This perspective provides a brief overview of the approach taken to study antimicrobial and amyloid peptides in phospholipid bilayers using oriented bilayers and magic angle spinning techniques. In particular, Boltzmann statistics REDOR and maximum entropy analysis of spinning side bands are used to analyse systems where multiple states of peptide or lipid molecules may co-exist. We propose that in future, rather than model membranes, structural studies in whole cells are feasible. [ABSTRACT FROM AUTHOR]

Two 3D experiments, capable of producing enhanced resolution two-spin double-quantum (DQ) homonuclear correlations for half-integer quadrupolar nuclei, are described. The first uses a split-t 1 MQMAS sequence followed by a sandwiched oR 3 symmetry-based dipolar recoupling sequence to directly excite DQ coherences. In this case an isotropic single-quantum (SQ) coherence starts the homonuclear DQ excitation. In the second experiment a single strong pulse is used to create triple quantum (TQ) coherence followed by a further single pulse conversion to zero-order before a non-sandwiched oR 3 DQ sequence. The first experiment is demonstrated using 87 Rb in RbNO 3 , with three Rb sites in a ∼5 ppm range, and the second to 11 B in caesium triborate, CsB 3 O 5 , with two three-coordinated sites separated by ∼2 ppm and one four-coordinated boron site. In both cases, all sites are clearly resolved and their connections observed. The second experiment has higher sensitivity and a good signal to noise is obtained in a reasonable time despite the long T 1 relaxation time of 11 B in this material. [ABSTRACT FROM AUTHOR]

Highlights: [•] Correct angular restraints are obtained for rotationally aligned Vpu TM. [•] Pulse sequences for a dipolar assisted assignment protocol are described. [•] Pulse sequences that measure chemical shifts and dipolar frequencies are described. [•] Chemical shift and dipolar frequency correlation provides high resolution. [Copyright &y& Elsevier]

Highlights: [•] REDOR with CPMG speeds up distance measurement in solids by one order of magnitude. [•] CPMG with XY-8 phase cycling maintains long lasting echo while compensating imperfect refocusing pulses. [•] Full echo acquisition has a √2 gain in S/N over half-echo signal. [ABSTRACT FROM AUTHOR]

Highlights: [•] DANTE 1D excitation in solids works better under ultra-fast MAS. [•] Paramagnetic samples can be excited with weak rf-field. [•] Integer-spin quadrupolar nuclei, such as 14N, can be manipulated with DANTE. [•] Overtone DANTE can be used for 90° excitation, not for 180° inversion. [•] Overtone DANTE works well for excitation of large quadrupole interaction. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fokker–Planck equations can be solved directly for powder-average NMR spectra. [•] Hamiltonian is time-independent and spherical quadrature grids are avoided completely. [•] The proposed formalism contains Floquet theory as a special case. [•] Large matrices are handled with state space restriction and tensor train techniques. [•] Low correlation order basis sets appear to produce accurate answers for powders. [ABSTRACT FROM AUTHOR]

Abstract: We demonstrate here that the principle of frequency-selective spin-echoes can be extended to the measurements of long-range homonuclear scalar J-couplings in the solid-state. Singly or doubly frequency-selective pulses were used to generate either a J-modulated experiment (S) or a reference experiment (S 0). The combination of these two distinct experiments provides experimental data that, in favorable cases, are insensitive to incoherent relaxation effects, and which can be used to estimate long-range homonuclear J-couplings in multiple spin-systems. The concept is illustrated in the case of a uniformly 13C and 15N labeled sample of l-histidine, where the absolute value of homonuclear J-couplings between two spins separated by one, two or three covalent bonds are measured. Moreover, we show that a 2 J(15N–C–15N) coupling as small as 0.9Hz can be precisely measured with the method presented here. [Copyright &y& Elsevier]

Abstract: High pressure magic angle spinning (MAS) nuclear magnetic resonance (NMR) with a sample spinning rate exceeding 2.1kHz and pressure greater than 165bar has never been realized. In this work, a new sample cell design is reported, suitable for constructing cells of different sizes. Using a 7.5mm high pressure MAS rotor as an example, internal pressure as high as 200bar at a sample spinning rate of 6kHz is achieved. The new high pressure MAS rotor is re-usable and compatible with most commercial NMR set-ups, exhibiting low 1H and 13C NMR background and offering maximal NMR sensitivity. As an example of its many possible applications, this new capability is applied to determine reaction products associated with the carbonation reaction of a natural mineral, antigorite ((Mg,Fe2+)3Si2O5(OH)4), in contact with liquid water in water-saturated supercritical CO2 (scCO2) at 150bar and 50°C. This mineral is relevant to the deep geologic disposal of CO2, but its iron content results in too many sample spinning sidebands at low spinning rate. Hence, this chemical system is a good case study to demonstrate the utility of the higher sample spinning rates that can be achieved by our new rotor design. We expect this new capability will be useful for exploring solid-state, including interfacial, chemistry at new levels of high-pressure in a wide variety of fields. [Copyright &y& Elsevier]

Abstract: A sensitivity enhancement scheme aiming at selective adiabatic inversion of a single set of satellite transition sidebands under magic angle spinning has been employed on samples of albite containing a single moderately distorted 27Al site and zoisite containing two highly distorted octahedral 27Al sites. Overall enhancements of ∼2.5 for albite and ∼3 for the two AlO6 sites of zoisite are reported by applying this scheme at different spinning speeds reflecting the versatility of this enhancement scheme which achieves significant signal-to-noise enhancements for the systems with moderately high quadrupolar coupling and high quadrupolar coupling. Repeating the sensitivity enhancement scheme and signal readout several times without allowing for spin-lattice relaxation leads to sensitivity enhancements of factors of ∼4 for albite and ∼5 for zoisite which substantially increases the detectability of the quadrupolar sites. The effectiveness of this scheme at high magnetic field under very fast magic angle spinning has been demonstrated. Finally, the possibility of performing spectral editing by selective enhancement of one of the quadrupolar sites in zoisite whilst keeping the other site unaffected has been explored. [Copyright &y& Elsevier]

Abstract: The use of susceptibility matching to minimize spectral distortion of biphasic samples layered in a standard 5mm NMR tube is described. The approach uses magic angle spinning (MAS) to first extract chemical shift differences by suppressing bulk magnetization. Then, using biphasic coaxial samples, magnetic susceptibilities are matched by titration with a paramagnetic salt. The matched phases are then layered in a standard NMR tube where they can be shimmed and examined. Linewidths of two distinct spectral lines, selected to characterize homogeneity in each phase, are simultaneously optimized. Two-dimensional distortion-free, slice-resolved spectra of an octanol/water system illustrate the method. These data are obtained using a 2D stepped-gradient pulse sequence devised for this application. Advantages of this sequence over slice-selective methods are that acquisition efficiency is increased and processing requires only conventional software. [Copyright &y& Elsevier]

Abstract: We present a time-shared Third Spin Assisted Recoupling (TSTSAR) experiment that allows for simultaneous acquisition of homonuclear (13C–13C) and heteronuclear (15N–13C) long-distance contacts in biomolecular solids under magic angle spinning. TSTSAR leads to substantial time savings and increases the information content of 2D correlation spectra. [Copyright &y& Elsevier]

Abstract: Heterogeneous samples, such as soils, sediments, plants, tissues, foods and organisms, often contain liquid-, gel- and solid-like phases and it is the synergism between these phases that determine their environmental and biological properties. Studying each phase separately can perturb the sample, removing important structural information such as chemical interactions at the gel-solid interface, kinetics across boundaries and conformation in the natural state. In order to overcome these limitations a Comprehensive Multiphase-Nuclear Magnetic Resonance (CMP-NMR) probe has been developed, and is introduced here, that permits all bonds in all phases to be studied and differentiated in whole unaltered natural samples. The CMP-NMR probe is built with high power circuitry, Magic Angle Spinning (MAS), is fitted with a lock channel, pulse field gradients, and is fully susceptibility matched. Consequently, this novel NMR probe has to cover all HR-MAS aspects without compromising power handling to permit the full range of solution-, gel- and solid-state experiments available today. Using this technology, both structures and interactions can be studied independently in each phase as well as transfer/interactions between phases within a heterogeneous sample. This paper outlines some basic experimental approaches using a model heterogeneous multiphase sample containing liquid-, gel- and solid-like components in water, yielding separate 1H and 13C spectra for the different phases. In addition, 19F performance is also addressed. To illustrate the capability of 19F NMR soil samples, containing two different contaminants, are used, demonstrating a preliminary, but real-world application of this technology. This novel NMR approach possesses a great potential for the in situ study of natural samples in their native state. [Copyright &y& Elsevier]

Abstract: A simple method to automatically shim NMR samples spinning at the magic angle is introduced based on the gradient shimming approach. The field inhomogeneity along the spinning axis is measured and automatically corrected. The combination of a normal magic angle spinning (MAS) probe, a conventional homospoil gradient, and a set of properly chosen standard room-temperature shims are used to perform the gradient shimming of samples spinning at the magic angle. The resulting 13C NMR adamantane linewidth is less than 1Hz (0.0078ppm at 11.7T). [Copyright &y& Elsevier]

Abstract: We have developed a novel variant of REDOR which is applicable to multiple-spin systems without proton decoupling. The pulse sequence is constructed based on a systematic time displacement of the pi pulses of the conventional REDOR sequence. This so-called time displacement REDOR (td-REDOR) is insensitive to the effect of homonuclear dipole–dipole interaction when the higher order effects are negligible. The validity of td-REDOR has been verified experimentally by the P-31{C-13} measurements on glyphosate at a spinning frequency of 25kHz. The experimental dephasing curve is in favorable agreement with the simulation data without considering the homonuclear dipole–dipole interactions. [Copyright &y& Elsevier]

Abstract: A two-dimensional experiment for measuring chemical shift anisotropy (CSA) under fast magic-angle spinning (MAS) is presented. The chemical shift anisotropy evolution is amplified by a sequence of π-pulses that repetitively interrupt MAS averaging. The amplification generates spinning sideband manifolds in the indirect dimension separated by the isotropic shift along the direct dimension. The basic unit of the pulse sequence is designed based on the magic-angle turning experiment and can be concatenated for larger amplification factors. [Copyright &y& Elsevier]

Abstract: We present a Floquet theory approach for the analysis of homonuclear recoupling assisted by radio frequency (RF) irradiation of surrounding heteronuclear spins. This description covers a broad range of systems from fully protonated to deuterated proteins, focusing in detail on recoupling via protons and deuterons separately as well as simultaneously by the double nucleus enhanced recoupling (DONER) scheme. The theoretical description, supported by numerical simulations and compared to experimental results from a partially deuterated model compound, indicates that in perdeuterated systems setting the RF amplitude equal to the magic angle spinning (MAS) frequency is not necessarily optimal for recoupling via 1H and/or 2H nuclei and modified recoupling conditions are identified. [Copyright &y& Elsevier]

Abstract: The underlying relation between the magic-angle turning (MAT) and phase-adjusted spinning sidebands (PASS) experiments is examined. The MAT experiment satisfies the PASS conditions for separating spinning sidebands with a non-constant total evolution time and only requires linear increments of up to one rotor period. The time-domain data of the two experiments are related by a shearing transformation. A combination of the linear evolution-time increments of MAT and simple data processing of PASS are particularly attractive for the implementation of MAT for measuring chemical shift anisotropy. [Copyright &y& Elsevier]

Abstract: In this article, we show the potential for utilizing proton-detected heteronuclear single quantum correlation (HSQC) NMR in rigid solids under ultra-fast magic angle spinning (MAS) conditions. The indirect detection of carbon-13 from coupled neighboring hydrogen nuclei provides a sensitivity enhancement of 3- to 4-fold in crystalline amino acids over direct-detected versions. Furthermore, the sensitivity enhancement is shown to be significantly larger for disordered solids that display inhomogeneously broadened carbon-13 spectra. Latrodectus hesperus (Black Widow) dragline silk is given as an example where the sample is mass-limited and the sensitivity enhancement for the proton-detected experiment is 8- to 13-fold. The ultra-fast MAS proton-detected HSQC solid-state NMR technique has the added advantage that no proton homonuclear decoupling is applied during the experiment. Further, well-resolved, indirectly observed carbon-13 spectra can be obtained in some cases without heteronuclear proton decoupling. [Copyright &y& Elsevier]

Abstract: Accurate determination of 1H NMR signal intensities is useful for quantitative analysis of the hydrogen content and also to determine the relative peak intensity ratios in different application scenarios. To this end we have investigated the reliability and sources of intensity errors in 1H solid-state MAS NMR. If sufficient resolution can be achieved by very high spinning speeds and high magnetic fields, quantification is straight forward. However, for poorly resolved spectra we show that small phase errors add a considerable amount of uncertainty. An analytical expression for the phase induced intensity-errors allowed us to suggest a robust and reliable recipe which is based on a combination of the spin-echo experiment, an extrapolation technique and a deconvolution algorithm which includes fitting of the signal phase. It significantly reduces errors caused by phase distortions, homonuclear dipolar dephasing, the receiver dead time delay and baseline rolling. The method was validated experimentally on samples with strong homonuclear dipolar interactions. [Copyright &y& Elsevier]

Abstract: A protocol is presented for correcting the effect of non-specific cross-polarization in CHHC solid-state MAS NMR experiments, thus allowing the recovery of the 1H–1H magnetization exchange functions from the mixing-time dependent buildup of experimental CHHC peak intensity. The presented protocol also incorporates a scaling procedure to take into account the effect of multiplicity of a CH2 or CH3 moiety. Experimental CHHC buildup curves are presented for l-tyrosine·HCl samples where either all or only one in 10 molecules are U–13C labeled. Good agreement between experiment and 11-spin SPINEVOLUTION simulation (including only isotropic 1H chemical shifts) is demonstrated for the initial buildup (t mix <100μs) of CHHC peak intensity corresponding to an intramolecular close (2.5Å) H–H proximity. Differences in the initial CHHC buildup are observed between the one in 10 dilute and 100% samples for cases where there is a close intermolecular H–H proximity in addition to a close intramolecular H–H proximity. For the dilute sample, CHHC cross-peak intensities tended to significantly lower values for long mixing times (500μs) as compared to the 100% sample. This difference is explained as being due to the dependence of the limiting total magnetization on the ratio N obs/N tot between the number of protons that are directly attached to a 13C nucleus and hence contribute significantly to the observed 13C CHHC NMR signal, and the total number of 1H spins into the system. 1H–1H magnetization exchange curves extracted from CHHC spectra for the 100% l-tyrosine·HCl sample exhibit a clear sensitivity to the root sum squared dipolar coupling, with fast buildup being observed for the shortest intramolecular distances (2.5Å) and slower, yet observable buildup for the longer intermolecular distances (up to 5Å). [Copyright &y& Elsevier]

Abstract: An isotropic–anisotropic shift 2D correlation spectroscopy is introduced that combines the advantages of both magic angle turning (MAT) and magic angle hopping (MAH) technologies. In this new approach, denoted DMAT for “discrete magic angle turning”, the sample rotates clockwise followed by an anticlockwise rotation of exactly the same amount with each rotation less or equal than 360° but greater than 240°, with the rotation speed being constant only for times related to the evolution dimension. This back and forth rotation is repeated and synchronized with a special radio frequency (RF) pulse sequence to produce an isotropic–anisotropic shift 2D correlation spectrum. For any spin-interaction of rank-2 such as chemical shift anisotropy, isotropic magnetic susceptibility interaction, and residual homo-nuclear dipolar interaction in biological fluid samples, the projection along the isotropic dimension is a high resolution spectrum. Since a less than 360° sample rotation is involved, the design potentially allows for in situ control over physical parameters such as pressure, flow conditions, feed compositions, and temperature so that true in situ NMR investigations can be carried out. [Copyright &y& Elsevier]

Abstract: We show that the C-REDOR sequence can be modified according to the framework of rotary-REDOR to realize heteronuclear recoupling in multiple-spin systems. Because of the inherent favorable properties of C-REDOR with respect to rf inhomogeneity, our approach is anticipated to find useful applications in biological research. Furthermore, active rotor synchronization of the windowless C-REDOR is implemented by an ad hoc shortening of the pulse elements. Numerical simulations and experiments measured for [U-13C,15N]-l-alanine are used to validate our approach. [Copyright &y& Elsevier]

Abstract: We compare the performance of the windowed phase-modulated Lee–Goldburg (wPMLG) and the windowed decoupling using mind boggling optimisation (wDUMBO) sequences at various magic-angle spinning rates and nutation frequencies of the pulses. Additionally, we introduce a supercycled version of wDUMBO and compare its efficiency with that of the non-supercycled implementation of wDUMBO. The efficiency of the supercycled version of wPMLG, denoted wPMLG-S2, is compared with a new supercycled version of wPMLG that we notate as wPMLG-S3. The interaction between the supercycled homonuclear dipolar decoupling sequences and the sample rotation is analysed using symmetry-based selection rules. [Copyright &y& Elsevier]

Abstract: Pulsed field gradient (PFG) NMR and magic-angle spinning (MAS) NMR have been combined in order to measure the diffusion coefficients of liquid crystals in confined geometry. Combination of MAS NMR with PFG NMR has a higher spectroscopic resolution in comparison with conventional PFG NMR and improves the application of NMR diffusometry to liquid crystals. It is found that the confinement of the liquid crystal 5CB in porous glasses with mean pore diameters of 30 and 200nm does not notably change its diffusion behavior in comparison with the bulk state. [Copyright &y& Elsevier]

Abstract: A remarkable enhancement of sensitivity can be often achieved in 29Si solid-state NMR by applying the well-known Carr–Purcell–Meiboom–Gill (CPMG) train of rotor-synchronized π pulses during the detection of silicon magnetization. Here, several one- and two-dimensional (1D and 2D) techniques are used to demonstrate the capabilities of this approach. Examples include 1D 29Si{X} CPMAS spectra and 2D 29Si{X} HETCOR spectra of mesoporous silicas, zeolites and minerals, where X= 1H or 27Al. Data processing methods, experimental strategies and sensitivity limits are discussed and illustrated by experiments. The mechanisms of transverse dephasing of 29Si nuclei in solids are analyzed. Fast magic angle spinning, at rates between 25 and 40kHz, is instrumental in achieving the highest sensitivity gain in some of these experiments. In the case of 29Si–29Si double-quantum techniques, CPMG detection can be exploited to measure homonuclear J-couplings. [Copyright &y& Elsevier]

Abstract: A combination of techniques, including rational number synchronisation and pre-diagonalisation of the time-dependent periodic Hamiltonian, are described which allow the efficient simulation of NMR experiments involving both magic-angle spinning (MAS) and RF irradiation, particularly in the important special case of phase-modulated decoupling sequences. Chebyshev and conventional diagonalisation approaches to calculating propagators under MAS are also compared, with Chebyshev methods offering significant advantages in cases where the Hamiltonian is large and time-dependent but not block-diagonal (as is the case for problems involving combined MAS and RF). The ability to simulate extended coupled spin systems efficiently allows 1H spectra under homonuclear decoupling to be calculated directly and compared to experimental results. Reasonable agreement is found for the conditions under which homonuclear decoupling is typically applied for rigid solids (although the increasing deviation of experimental results from the predictions of theory and simulation at higher RF powers is still unexplained). Numerical simulations are used to explore three features of these experiments: the interaction between the magic-angle spinning and RF decoupling, the effects of tilt pulses in acquisition windows and the effects of “phase propagation delays” on tilted axis precession. In each case, the results reveal features that are not readily anticipated by previous analytical studies and shed light on previous empirical observations. [Copyright &y& Elsevier]

Abstract: A 13C-detected N–H dipolar coupling technique is introduced for uniaxially mobile membrane proteins for orientation determination using unoriented samples. For proteins undergoing rigid-body uniaxial rotation in the lipid bilayer, the intrinsic equality between the dipolar coupling constants measured in unoriented samples and the anisotropic coupling measured in static oriented samples has been shown recently. Here, we demonstrate that the orientation-sensitive backbone N–H dipolar couplings can be measured with 13C detection using 2D and 3D MAS correlation experiments, so that maximal site resolution can be achieved and multiple orientational constraints can be extracted from each experiment. We demonstrate this technique on the M2 transmembrane peptide of the influenza A virus, where the N–H dipolar couplings of various residues fit to a dipolar wave for a helical tilt angle of 37°, in excellent agreement with data obtained from singly 15N-labeled samples. [Copyright &y& Elsevier]

Abstract: Resistive and resistive-superconducting hybrid magnets can generate dc magnetic fields much higher than conventional superconducting NMR magnets but the field spatial homogeneity and temporal stability are usually not sufficient for high-resolution NMR experiments. Hardware and technique development addressing these issues are presented for high-resolution NMR at magnetic fields up to 40T. Passive ferromagnetic shimming and magic-angle spinning are used effectively to reduce the broadening from inhomogeneous magnetic field. A phase correction technique based on simultaneous heteronuclear detection is developed to compensate magnetic field fluctuations to achieve high spectral resolution. [Copyright &y& Elsevier]

Abstract: Numerical simulations and experiments were used to examine the possibility of employing strong spin-lock fields for recoupling of homonuclear dipolar interactions between spin-3/2 quadrupolar nuclei and to compare it to the rotary-resonance recoupling at weak spin-lock fields. It was shown that strong spin-lock pulses under MAS conditions can lead to recoupling, provided that the electric-field gradient principal axes systems of the coupled nuclei are aligned and that their quadrupolar coupling constants are approximately the same. The phenomenon is based on the fact that strong spin-lock pulses induce adiabatic transfer of magnetization between the central-transition coherence and the triple-quantum coherence with equal periodicity as is the periodicity of the time-dependent dipolar coupling. Because of the synchronous variation of the state of the spin system and of the dipolar interaction, the effect of the latter on the central-transition coherence and on the triple-quantum coherence is not averaged out by sample rotation. The approach is, however, very sensitive to the relative orientation of the electric-field gradient principal axes systems and therefore less robust than the approach based on weak spin-lock pulses that satisfy rotary-resonance condition. [Copyright &y& Elsevier]

Abstract: The effect of nitrogen-15 proton-driven spin diffusion on quantitative 15N T 1 measurements in solid proteins is investigated, and the impact on the measurement of dynamic parameters is assessed. A simple model of exchange between neighboring nitrogens is used to reproduce the evolution of 15N spin systems whose longitudinal relaxation rates and exchange rates are compatible with experimental measurements. We show that the induced error in the measured T 1 and its effect on the determination of dynamics parameters is likely to be less than the current experimental error. The use of deuterated protein samples is shown to have a small but sometimes visible effect, and may also considerably slow down or even suppress the exchange of magnetization due to spin diffusion. [Copyright &y& Elsevier]

Abstract: A two-dimensional 13C/14N heteronuclear multiple quantum correlation (HMQC) experiment using dipolar recoupling under magic-angle spinning (MAS) is described. The experiment is an extension of the recent indirect 13C detection scheme for measuring 14N quadrupolar coupling under MAS. The recoupling allows the direct use of the much larger dipolar interaction instead of the small J and residual dipolar couplings for establishing 13C/14N correlations. Two recoupling methods are incorporated into the HMQC sequence, both applying rf only to the observed 13C spin. The first one uses the REDOR sequence with two π-pulses per rotor cycle. The second one uses a cw rf field matching the spinning frequency, known as rotary resonance. The effects of CSA, signal loss, MAS frequency and stability and t 1-noise are compared and discussed. [Copyright &y& Elsevier]

Abstract: Hetero-nuclear coherence transfers in HMQC and refocused-INEPT experiments involving half-integer quadrupolar nuclei in solids are analyzed. 1D and 2D schemes are considered under MAS for the general case of multi-spin systems SI n (n ⩽4), where S is an observed nucleus. These results are also discussed in the context of high-resolution schemes featuring MQMAS or STMAS. The theoretical predictions are verified experimentally in a series of 1D and 2D experiments performed at 9.4 and 18.8T. [Copyright &y& Elsevier]

Abstract: In this paper, we introduce a class of dipolar recoupling experiments under magic angle spinning (MAS), which use gamma dependent antiphase polarization during the t 1 evolution period. We show that this helps us to design dipolar recoupling experiments that transfer both components of the transverse magnetization of spin S to a coupled spin I in the mixing step of a 2D NMR experiment. We show that it is possible to design such transfer schemes and make them insensitive to the orientation dependency of the couplings in powders. This helps us to develop sensitivity enhanced 2D NMR experiments of powder samples under MAS. [Copyright &y& Elsevier]

Abstract: A rotary resonance echo double resonance (R-REDOR) experiment is described for measuring heteronuclear dipolar coupling under magic-angle spinning. Rotary resonance reintroduces both dipolar coupling and chemical shift anisotropy with an rf field matching the spinning frequency. The resonance effect from chemical shift anisotropy can be refocused with a rotary resonance echo. The R-REDOR experiment thus measures the dephasing of the rotary resonance echo from the heteronuclear dipolar coupling to determine the dipolar coupling constant. The rotary resonance experiment is suitable for measuring dipolar coupling with quadrupolar nuclei because it applies the recoupling rf only to the observed spin-1/2. The rotary resonance scheme has the advantages of a long and susceptible to spinning frequency fluctuation. [Copyright &y& Elsevier]

Abstract: In this paper we describe the construction and performance of high pressure magic angle inserts made from the polymer PEEK. The inserts were designed to fit inside standard commercial 7mm magic angle spinning rotors and spin at the maximum frequency of the probe. The sample volume of the inserts was 100μL. A gas loading chamber that operates at room temperature is described. The performance of the inserts is discussed for a number of gases in terms of resolution as a function of spinning speed and leakage of the gas due to permeation through the polymer. Finally, some preliminary results are shown in relation to complex food materials. [Copyright &y& Elsevier]

Abstract: Four different coil designs for use with MAS in triple-resonance multi-nuclear experiments at high fields are compared, using a combination of finite element analysis (FEA) software and NMR experiments, with respect to RF field strength per unit power and relative sample heating, as governed by mean E/B 1 within the sample region. A commercial FEA package, Microwave Studio 5.1 by Computer Simulation Technology (CST) is shown to obtain remarkably accurate agreement with the experiments in Q L, L, B, E, and mode frequencies in all cases. A simplified treatment of RF heating in NMR MAS samples is derived and shown to agree with the NMR experimental results within about 10% for two representative stator designs. The coil types studied include: (1) a variable-pitch solenoid outside a ceramic coilform, (2) a conventional solenoid very closely spaced to the MAS rotor, (3) a scroll coil, and (4) a segmented saddle cross coil (XC) for 1H with an additional solenoid over it for the two lower-frequency channels. The XC/solenoid is shown to offer substantial advantages in reduced decoupler heating, improved S/N, and improved compatibility with multinuclear tuning and high-power decoupling. This seems largely because the division of labor between two orthogonal coils allows them each, and their associated circuitry, to be separately optimized for their respective regimes. [Copyright &y& Elsevier]

Abstract: We have performed magic-angle-spinning solid-state NMR experiments in which protons are recoupled to oxygen-17 nuclei by applying a symmetry-based recoupling sequence at the proton Larmor frequency. Two-dimensional quadrupole-dipole correlation spectra are produced, in which the second-order quadrupolar shift of the oxygen-17 central transition is correlated with the recoupled heteronuclear dipole–dipole interaction. These spectra are sensitive to the relative orientation of the electric field gradient at the site of the oxygen-17 nucleus and the O–H internuclear vector. We also demonstrate experiments in which polarization is transferred from protons to oxygen-17, and show that oxygen-17 signals may be selected according to the protonation state of the oxygen site. We discuss the small observed value of the heteronuclear dipolar splitting in the central-transition oxygen-17 spectra. [Copyright &y& Elsevier]

Abstract: The popular PISEMA experiment is highly sensitive to the 1H chemical shift dispersion and the choice of the 1H carrier frequency. This is due to the off-resonance 1H irradiation in the FSLG-CP sequence employed during the dipolar evolution period. In the modified approach described in this work, the interfering frequency offset terms are suppressed. In the new pulse schemes, conventional FSLG-CP is intercalated with 180° pulses applied simultaneously to both frequency channels, and with phases set orthogonal to those of the spin-lock fields. The technique is demonstrated on a nematic liquid-crystalline sample. Extensions to amplitude-modulated FSLG-CP recoupling under MAS are also presented. [Copyright &y& Elsevier]