Enhanced resolution in proton solid-state NMR with very-fast MAS experiments

Magnetic Susceptibility Modeling of Magic-Angle Spinning Modules for Part Per Billion Scale Field Homogeneity

Magic-angle spinning (MAS) solid-state NMR methods are crucial in many areas of biology and materials science. Conventional probe designs have often been specified with 0.1 part per million (ppm) or 100 part per billion (ppb) magnetic field resolution, which is a limitation for many modern scientific applications. Here we describe a novel 5-mm MAS module design that significantly improves the linewidth and line shape for solid samples by an improved understanding of the magnetic susceptibility of probe materials and geometrical symmetry considerations, optimized to minimize the overall perturbation to the applied magnetic field (B0). The improved spinning module requires only first and second order shimming adjustments to achieve a sub-Hz resolution of 13C resonances of adamantane at 150 MHz Larmor frequency (14.1Tesla magnetic field). Minimal use of third and higher order shims improves experimental reproducibility upon sample changes and the exact placement within the magnet. Furthermore, the shimming procedure is faster, and the required gradients smaller, thus minimizing thermal drift of the room temperature (RT) shims. We demonstrate these results with direct polarization (Bloch decay) and cross polarization experiments on adamantane over a range of sample geometries and with multiple superconducting magnet systems. For a direct polarization experiment utilizing the entire active sample volume of a 5-mm rotor (90 µl), we achieved full width at half maximum (FWHM) of 0.76 Hz (5 ppb) and baseline resolved the 13C satellite peaks for adamantane as a consequent of the 7.31 Hz (59 ppb) width at 2% intensity. We expect these approaches to be increasingly pivotal for high-resolution solid-state NMR spectroscopy at and above 1 GHz 1H frequencies.

Optimisation of 1H PMLG homonuclear decoupling at 60 kHz MAS to enable 15N-1H through-bond heteronuclear correlation solid-state NMR spectroscopy

The Lee–Goldburg condition for homonuclear decoupling in ¹H magic-angle spinning (MAS) solid-state NMR sets the angle θ, corresponding to arctan of the ratio of the rf nutation frequency, ν₁, to the rf offset, to be the magic angle, θ_m, equal to tan⁻¹(√2) = 54.7°. At 60 kHz MAS, we report enhanced decoupling compared to MAS alone in a ¹H spectrum of ¹⁵N-glycine with at θ = 30° for a ν₁ of ∼100 kHz at a ¹H Larmor frequency, ν₀, of 500 MHz and 1 GHz, corresponding to a high chemical shift scaling factor (λ_CS) of 0.82. At 1 GHz, we also demonstrate enhanced decoupling compared to 60 kHz MAS alone for a lower ν₁ of 51 kHz, i.e., a case where the nutation frequency is less than the MAS frequency, with θ = 18°, λ_CS = 0.92. The ratio of the rotor period to the decoupling cycle time, Ψ = τ_r/τ_c, is in the range 0.53 to 0.61. Windowed decoupling using the optimised parameters for a ν₁ of ∼100 kHz also gives good performance in a ¹H spin-echo experiment, enabling implementation in a ¹H-detected ¹⁵N–¹H cross polarisation (CP)-refocused INEPT heteronuclear correlation NMR experiment. Specifically, initial ¹⁵N transverse magnetisation as generated by ¹H–¹⁵N CP is transferred back to ¹H using a refocused INEPT pulse sequence employing windowed ¹H decoupling. Such an approach ensures the observation of through-bond N–H connectivities. For ¹⁵N-glycine, while the CP-refocused INEPT experiment has a lower sensitivity (∼50%) as compared to a double CP experiment (with a 200 μs ¹⁵N to ¹H CP contact time), there is selectivity for the directly bonded NH₃⁺ moiety, while intensity is observed for the CH₂¹H resonances in the double CP experiment. Two-dimensional ¹⁵N–¹H correlation MAS NMR spectra are presented for the dipeptide β-AspAla and the pharmaceutical cimetidine at 60 kHz MAS, both at natural isotopic abundance. For the dipeptide β-AspAla, different build-up dependence on the first spin-echo duration is observed for the NH and NH₃⁺ moieties demonstrating that the experiment could be used to distinguish resonances for different NH_x groups.

¹⁵N–¹H heteronuclear NMR correlation at natural abundance in the solid state via J couplings is enabled by optimisation of phase-modulated Lee–Goldburg (PMLG) ¹H homonuclear decoupling during the spin echoes, far from the ideal magic-angle condition.

1H line width dependence on MAS speed in solid state NMR - Comparison of experiment and simulation

Recent developments in magic angle spinning (MAS) technology permit spinning frequencies of ≥100 kHz. We examine the effect of such fast MAS rates upon nuclear magnetic resonance proton line widths in the multi-spin system of β-Asp-Ala crystal. We perform powder pattern simulations employing Fokker-Plank approach with periodic boundary conditions and ¹H-chemical shift tensors calculated using the bond polarization theory. The theoretical predictions mirror well the experimental results. Both approaches demonstrate that homogeneous broadening has a linear-quadratic dependency on the inverse of the MAS spinning frequency and that, at the faster end of the spinning frequencies, the residual spectral line broadening becomes dominated by chemical shift distributions and susceptibility effects even for crystalline systems.

Minimizing the t1-noise when using an indirect 1H high-resolution detection of unlabeled samples

The most utilized through-space correlation ¹H-{X} methods with proton indirect detection use two consecutive transfers, ¹H → X and then X → ¹H, with the evolution time t₁ in the middle. When the X isotope is not 100% naturally abundant (NA), only the signal of the protons close to these isotopes is modulated by the ¹H-X dipolar interactions. This signal is theoretically disentangled with phase-cycling from the un-modulated one. However, this separation is never perfect and it may lead to t₁-noise in case of isotopes with very small NA, such as ¹³C or even worse ¹⁵N. One way to reduce this t₁-noise is to minimize, 'purge', during t₁ the un-modulated ¹H magnetization before trying to suppress it with phase-cycling. We analyze experimentally several sequences following the HORROR condition, which allow purging the ¹H transverse magnetization. The comparison is made at three spinning speeds, including very fast ones for ¹H resolution: 27.75, 55.5 and 111 kHz. We show (i) that the efficiency of this purging process increases with the spinning speed, and (ii) that the best recoupling sequences are the two simplest ones: XY and S₁ = SR2¹₂. We then compare the S/N that can be achieved with the two most used ¹H-{X} 2D methods, called D-HMQC and CP-CP. The only difference in between these two methods is that the transfers are done with either two π/2-pulses on X channel (D-HMQC), or two Cross-Polarization (CP) transfers (CP-CP). The first method, D-HMQC, is very robust and should be preferred when indirectly detecting nuclei with high NA. The second method, CP-CP, (i) requires experimental precautions to limit the t₁-noise, and (ii) is difficult to use with quadrupolar nuclei because the two CP transfers are then not efficient nor robust. However, CP-CP is presently the best method to indirectly detect isotopes with small NA, such as ¹³C and ¹⁵N.

Five decades of homonuclear dipolar decoupling in solid-state NMR: Status and outlook

It has been slightly more than fifty years since the first homonuclear spin decoupling scheme, Lee-Goldburg decoupling, was proposed for removing homonuclear dipolar interactions in solid-state nuclear magnetic resonance. A family of such schemes has made observation of high-resolution NMR spectra of abundant spins possible in various applications in solid state. This review outlines the strategies used in this field and the future prospects of homonuclear spin decoupling in solid-state NMR.

Proton-detected solid-state NMR spectroscopy of fully protonated proteins at slow to moderate magic-angle spinning frequencies

(1)H-detection offers a substitute to the sensitivity-starved experiments often used to characterize biomolecular samples using magic-angle spinning solid-state NMR spectroscopy (MAS-ssNMR). To mitigate the effects of the strong (1)H-(1)H dipolar coupled network that would otherwise severely broaden resonances, high MAS frequencies (>40kHz) are often employed. Here, we have explored the alternative of stroboscopic (1)H-detection at moderate MAS frequencies of 5-30kHz using windowed version of supercycled-phase-modulated Lee-Goldburg homonuclear decoupling. We show that improved resolution in the (1)H dimension, comparable to that obtainable at high spinning frequencies of 40-60kHz without homonuclear decoupling, can be obtained in these experiments for fully protonated proteins. Along with detailed analysis of the performance of the method on the standard tri-peptide f-MLF, experiments on micro-crystalline GB1 and amyloid-β aggregates are used to demonstrate the applicability of these pulse-sequences to challenging biomolecular systems. With only two parameters to optimize, broadbanded performance of the homonuclear decoupling sequence, linear dependence of the chemical-shift scaling factor on resonance offset and a straightforward implementation under experimental conditions currently used for many biomolecular studies (viz. spinning frequencies and radio-frequency amplitudes), we expect these experiments to complement the current (13)C-detection based methods in assignments and characterization through chemical-shift mapping.

Minimizing the effects of RF inhomogeneity and phase transients allows resolution of two peaks in the (1)H CRAMPS NMR spectrum of adamantane

One of the limiting factors to achieving highly resolved (1)H NMR spectra with (1)H homonuclear decoupling sequences is imperfections in the applied radiofrequency (RF) pulses, most notably phase transients and RF inhomogeneity. Through a series of simulations and solid-state NMR experiments, it is demonstrated that the combined effects of phase transients and RF inhomogeneity can be minimized by a combination of (i) restricting the sample to small volume of the rotor, (ii) by employing a super-cycled version of the DUMBO decoupling sequence, and (iii) by carefully adjusting the probe tuning such that the asymmetric component of phase transients is minimized. Under these optimal conditions, it was possible to clearly resolve two signals in the (1)H CRAMPS NMR spectrum of adamantane arising from the CH and CH2 protons in the molecule. It is proposed that adamantane could be a very useful setup sample for (1)H CRAMPS NMR as the two peaks are only resolved when the effects of RF inhomogeneity and phase transients are minimized.

Selective excitation enables assignment of proton resonances and (1)H-(1)H distance measurement in ultrafast magic angle spinning solid state NMR spectroscopy

Remarkable developments in ultrafast magic angle spinning (MAS) solid-state NMR spectroscopy enabled proton-based high-resolution multidimensional experiments on solids. To fully utilize the benefits rendered by proton-based ultrafast MAS experiments, assignment of (1)H resonances becomes absolutely necessary. Herein, we propose an approach to identify different proton peaks by using dipolar-coupled heteronuclei such as (13)C or (15)N. In this method, after the initial preparation of proton magnetization and cross-polarization to (13)C nuclei, transverse magnetization of desired (13)C nuclei is selectively prepared by using DANTE (Delays Alternating with Nutations for Tailored Excitation) sequence and then, it is transferred to bonded protons with a short-contact-time cross polarization. Our experimental results demonstrate that protons bonded to specific (13)C atoms can be identified and overlapping proton peaks can also be assigned. In contrast to the regular 2D HETCOR experiment, only a few 1D experiments are required for the complete assignment of peaks in the proton spectrum. Furthermore, the finite-pulse radio frequency driven recoupling sequence could be incorporated right after the selection of specific proton signals to monitor the intensity buildup for other proton signals. This enables the extraction of (1)H-(1)H distances between different pairs of protons. Therefore, we believe that the proposed method will greatly aid in fast assignment of peaks in proton spectra and will be useful in the development of proton-based multi-dimensional solid-state NMR experiments to study atomic-level resolution structure and dynamics of solids.

Improving the resolution in proton-detected through-space heteronuclear multiple quantum correlation NMR spectroscopy

Connectivities and proximities between protons and low-gamma nuclei can be probed in solid-state NMR spectroscopy using two-dimensional (2D) proton-detected heteronuclear correlation, through Heteronuclear Multiple Quantum Correlation (HMQC) pulse sequence. The indirect detection via protons dramatically enhances the sensitivity. However, the spectra are often broadened along the indirect F1 dimension by the decay of heteronuclear multiple-quantum coherences under the strong (1)H-(1)H dipolar couplings. This work presents a systematic comparison of the performances of various decoupling schemes during the indirect t1 evolution period of dipolar-mediated HMQC (D-HMQC) experiment. We demonstrate that (1)H-(1)H dipolar decoupling sequences during t1, such as symmetry-based schemes, phase-modulated Lee-Goldburg (PMLG) and Decoupling Using Mind-Boggling Optimization (DUMBO), provide better resolution than continuous wave (1)H irradiation. We also report that high resolution requires the preservation of (1)H isotropic chemical shifts during the decoupling sequences. When observing indirectly broad spectra presenting numerous spinning sidebands, the D-HMQC sequence must be fully rotor-synchronized owing to the rotor-synchronized indirect sampling and dipolar recoupling sequence employed. In this case, we propose a solution to reduce artefact sidebands caused by the modulation of window delays before and after the decoupling application during the t1 period. Moreover, we show that (1)H-(1)H dipolar decoupling sequence using Smooth Amplitude Modulation (SAM) minimizes the t1-noise. The performances of the various decoupling schemes are assessed via numerical simulations and compared to 2D (1)H-{(13)C} D-HMQC experiments on [U-(13)C]-L-histidine⋅HCl⋅H2O at various magnetic fields and Magic Angle spinning (MAS) frequencies. Great resolution and sensitivity enhancements resulting from decoupling during t1 period enable the detection of heteronuclear correlation between aliphatic protons and ammonium (14)N sites in L-histidine⋅HCl⋅H2O.

High-resolution proton CRAMPS NMR using narrowband analog filters and postponed data acquisition

Proton linewidths decrease with increasing magic-angle spinning (MAS) rates. However, without spin dilution by deuteration, even with the fastest MAS rates available today, the narrowest proton linewidths are obtained by using the combined rotation and multiple pulse spectroscopy (CRAMPS) method. Direct observation under windowed CRAMPS typically introduces several tens of times more noise, partly because wideband analog filters (e.g. 5 MHz) must be used or sometimes even bypassed. Here we report that it is possible to keep using narrowband analog filters (about 50 kHz cutoff frequency) in CRAMPS by taking advantage of the time delay caused by the filters, which is inversely proportional to the cutoff frequency. This delay coincides with typical CRAMPS cycle times, enabling acquisition of the data point in the next detection window. The noise of such CRAMPS spectra is only about 5 times larger than MAS-only spectra. This new method allows CRAMPS to be performed on systems that lack wideline hardware (wideband filters and fast ADCs), for example, older spectrometers originally intended for solution NMR.

Spectral editing in 13C solid-state NMR at high magnetic field using fast MAS and spin-echo dephasing

A simple method is proposed for separating NMR resonances from protonated and non-protonated aromatic carbons in solids under fast magic angle spinning (MAS). The approach uses a MAS-synchronized spin-echo to exploit the differences in rotational recoupling of the dipolar interactions while fully refocusing the isotropic chemical shifts. This strategy extends the relevant time scale of spin evolution to milliseconds and circumvents the limitation of the traditional dipolar dephasing method, which in fast rotating solids is disrupted by rotational refocusing. The proposed approach can be used for quantitative measurement of carbon aromaticities in complex solids with poorly resolved spectra, as demonstrated for model compounds.

Detailed analysis of the TIMES and TIMES0 high-resolution MAS methods for high-resolution proton NMR

We analyze and compare the specifications of TIMES and TIMES(0) proton high-resolution NMR methods for solid-state samples. This comparison is performed in terms of resolution versus magic-angle spinning (MAS) spinning speed, ν(R), rf-field amplitude, ν(1), and tilt-angle for the effective rf-field, θ(p). The chemical-shift and homo-nuclear dipolar scaling factors are calculated for both methods. For all MAS speeds, the best resolution is always observed with rf-field of ν(1)≈120-130 kHz. At slow MAS speed (ν(R)≤10 kHz), the best resolution is observed for a tilt-angle of θ(P)≈90°. At moderate spinning speed (15≤ν(R)≤35 kHz), θ(P)≈55° gives the best resolution. At higher MAS speed (ν(R)≥60 kHz), with TIMES and TIMES(0) the best resolution is obtained for θ(P)≤40°; but we then recommend TIMES(0), owing to its simpler set-up. We also show that in addition to the usual high rf-field regime (ν(1)≈120-130 kHz), another low rf-regime (ν(1)≈40-50 kHz) exists at MAS speed higher than ν(R)≥60 kHz, which also gives a good (1)H resolution. This low rf-regime should be useful for multi-dimensional analyses of bio-molecules with (1)H detection under high-resolution, in order to limit the heating of the sample.

Practical choice of ¹H-¹H decoupling schemes in through-bond ¹H-{X} HMQC experiments at ultra-fast MAS

Three (1)H-(1)H homonuclear dipolar decoupling schemes for (1)H indirect detection measurements at very fast MAS are compared. The sequences require the following conditions: (i) being operable at very fast MAS, (ii) a long T(2)(') value, (iii) a large scaling factor, (iv) a small number of adjustable parameters, (v) an acquisition window, (vi) a low rf-power requirement, and (vii) a z-rotation feature. To satisfy these conditions a modified sequence named TIlted Magic-Echo Sandwich with zero degree sandwich pulse (TIMES(0)) is introduced. The basic elements of TIMES(0) consist of one sampling window and two phase-ramped irradiations, which realize alternating positive and negative 360° rotations of (1)H magnetization around an effective field tilted with an angle θ from the B(0) axis. The TIMES(0) sequence benefits from very large chemical shift scaling factors at ultra-fast MAS that reach κ(cs)=0.90 for θ=25° at ν(r)=80kHz MAS and only four adjustable parameters, resulting in easy setup. Long κ(cs)T(2)(') values, where T(2)(') is a irreversible proton transverse relaxation time, greatly enhance the sensitivity in (1)H-{(13)C} through-bond J-HMQC (Heteronuclear Multiple-Quantum Coherence) measurements with (1)H-(1)H decoupling during magnetization transfer periods. Although similar sensitivity can be obtained with through-space D-HMQC sequences, in which (13)C-(1)H dipolar interactions are recoupled, J-HMQC experiments incorporating (1)H-(1)H decoupling benefit from lower t(1)-noise, more uniform excitation of both CH, CH(2) and CH(3) moieties, and easier identification of through-bond connectivities.

Rotor-synchronized dipolar-filter sequence at fast MAS in solid-state NMR

Dipolar filters are of considerable importance for eliminating the (1)H NMR signal of the rigid components of heterogeneous compounds while selecting the signal of their mobile parts. On the basis of such filters, structural and dynamical information of these compounds can often be acquired through further manipulations (e.g. spin diffusion) on the spin systems. To overcome the destructive interferences between the magic angle spinning (MAS) speed and the cycle-time of the widely-used Rotor-Asynchronized Dipolar Filter (RADF) sequence, we introduce a new method called Rotor-Synchronized Dipolar Filter (RSDF). This communication shows that this sequence does not present any interference with the spinning speed and is more compatible than RADF with high MAS frequencies (ν(R)>12 kHz). This new pulse sequence will potentially contribute to future researches on heterogeneous materials, such as multiphase polymer and membrane systems.

Progress in correlation spectroscopy at ultra-fast magic-angle spinning: basic building blocks and complex experiments for the study of protein structure and dynamics

Recent progress in multi-dimensional solid-state NMR correlation spectroscopy at high static magnetic fields and ultra-fast magic-angle spinning is discussed. A focus of the review is on applications to protein resonance assignment and structure determination as well as on the characterization of protein dynamics in the solid state. First, the consequences of ultra-fast spinning on sensitivity and sample heating are considered. Recoupling and decoupling techniques at ultra-fast MAS are then presented, as well as more complex experiments assembled from these basic building blocks. Furthermore, we discuss new avenues in biomolecular solid-state NMR spectroscopy that become feasible in the ultra-fast spinning regime, such as sensitivity enhancement based on paramagnetic doping, and the prospect of direct proton detection.

A scaling factor theorem for homonuclear dipolar decoupling in solid-state NMR spectroscopy

A relationship between the dipolar and the chemical-shift scaling factors of cyclic radio-frequency irradiation schemes is introduced. This scaling factor theorem is derived analytically using Average Hamiltonian Theory, and its validity is illustrated numerically with homonuclear dipolar decoupling sequences generated randomly, and with the analysis of existing sequences. While derived for a static sample, the theorem provides insight into homonuclear dipolar decoupling schemes that combine radio-frequency irradiation with fast rotation of the sample at the magic-angle with respect to the static magnetic field.

Improving the 1H indirect dimension resolution of 2D CRAMPS NMR spectra: a simulation and experimental investigation

An experimental and simulation investigation regarding the effect of various CRAMPS parameters (delays interleaved with the decoupling shape, z-rotation symmetry and offset frequency) on the DUMBO decoupling performance is presented using two model compounds, glycine and tyrosine·HCl. Although the conclusions of this work may be extended to the other homonuclear-decoupling schemes, this work focuses essentially on the effect of adding interleaved delays to DUMBO decoupling during the indirect dimension using different 2D pulse-scheme architectures. While the simulations revealed an increasing loss of the spectral resolution with longer delays (from 0 to 6 μs), the experimental 2D (1)H DUMBO NMR correlation spectra revealed that the inclusion of delays, during the indirect dimension, improves the (1)H resolution whether the z-rotation symmetry was used or not. The best experimental spectra are obtained when z-rotation symmetry and windows are combined.

Reduction of spin diffusion artifacts from 2D zfr-INADEQUATE MAS NMR spectra

The primary shortcoming of the z-filtered refocused INADEQUATE MAS NMR pulse sequence is the possibility of artifacts introduced during the z-filter due to spin diffusion where by extra peaks in the single-quantum dimension (from other sites in the molecule) appear correlated with a given double-quantum frequency. This is a problem when the spinning speeds are too slow (less than 15 kHz) to sufficiently average the proton-proton homonuclear dipolar couplings. This would be especially important when working with large volume rotors that are difficult to spin fast enough to completely average the homonuclear couplings. In our experiments we used the frequency-switched Lee-Goldberg (FSLG) method of homonuclear decoupling during the z-filter to remove the artifact peaks. This method has the advantage of being quite easy to setup and implement on most modern NMR spectrometers.

1H Homonuclear dipolar decoupling using symmetry-based pulse sequences at ultra fast magic-angle spinning frequencies

We demonstrate here the application of symmetry-based pulse sequences for homonuclear dipolar decoupling in solid-state NMR at magic-angle spinning (MAS) frequencies up to 65 kHz using moderate radiofrequency (RF) amplitudes. Theoretical arguments favouring the requirement of low RF amplitudes at high MAS frequencies are given for these sequences. A comparison with wPMLGmmxx¯ is given at 65 kHz of MAS frequency to emphasise that the symmetry-based pulse sequences have a lower RF amplitude requirement at high MAS frequencies.

1H homonuclear dipolar decoupling using rotor-synchronised pulse sequences: towards pure absorption phase spectra

We demonstrate a pulse sequence using symmetry-based rotor-synchronised RN(n)(nu) sequences for homonuclear dipolar decoupling that achieves pure absorption phase high-resolution (1)H spectra in solid-state NMR. This sequence is compared with the phase-modulated Lee-Goldburg scheme. Experimental results are shown for samples of glycine and L-histidine.HCl.H(2)O for magic-angle-spinning frequencies in the range of 14-30 kHz and at two different magnetic fields.

Homonuclear dipolar recoupling under ultra-fast magic-angle spinning: probing 19F-19F proximities by solid-state NMR

We describe dipolar recoupling methods that accomplish, at high magic-angle spinning (MAS) frequencies, the excitation of double-quantum (DQ) coherences between spin-1/2 nuclei. We employ rotor-synchronized symmetry-based pulse sequences which are either gamma-encoded or non-gamma-encoded. The sensitivity and the robustness to both chemical-shift anisotropy and offset are examined. We also compare different techniques to avoid signal folding in the indirect dimension of two-dimensional double-quantum<-->single-quantum (DQ-SQ) spectra. This comprehensive analysis results in the identification of satisfactory conditions for dipolar (19)F-(19)F recoupling at high magnetic fields and high MAS frequencies. The utility of these recoupling methods is demonstrated with high-resolution DQ-SQ NMR spectra, which allow probing (19)F-(19)F proximities in powered fluoroaluminates.

Homonuclear dipolar decoupling under fast MAS: resolution patterns and simple optimization strategy

A simple method is shown for optimization of (1)H homonuclear dipolar decoupling at MAS rates exceeding 10 kHz. By monitoring the intensity of a spin-echo under the decoupling conditions, it is possible to optimize the amplitude of the RF magnetic field, the cycle time of the decoupling sequence and the resonance offset within minutes. As a result, the decoupling efficiency can be quickly and reliably fine-tuned without using a reference sample. The utility of this method has been confirmed by studying the resolution patterns for the supercycled PMLG scheme, which were found to be in excellent agreement with earlier theoretical predictions and verified in high-resolution 2D (1)H-(1)H experiments.

Indirect detection via spin-1/2 nuclei in solid state NMR spectroscopy: application to the observation of proximities between protons and quadrupolar nuclei

We present a comprehensive comparison of through-space heteronuclear correlation techniques for solid state NMR, combining indirect detection and single-channel recoupling method. These techniques, named D-HMQC and D-HSQC, do not suffer from dipolar truncation and can be employed to correlate quadrupolar nuclei with spin-1/2 nuclei. The heteronuclear dipolar couplings are restored under magic-angle spinning by applying supercycled symmetry-based pulse sequences (SR412) or simultaneous frequency and amplitude modulation (SFAM). The average Hamiltonian theory (AHT) of these recoupling methods is developed. These results are applied to analyze the performances of D-HMQC and D-HSQC sequences. It is shown that, whatever the magnitude of spin interations, D-HMQC experiment offers larger efficiency and higher robustness than D-HSQC. Furthermore, the spectral resolution in both dimensions of proton detected two-dimensional D-HMQC and D-HSQC spectra can be enhanced by applying recently introduced symmetry-based homonuclear dipolar decoupling schemes that cause a z-rotation of the spins. This is demonstrated by 1H-13C and 1H-23Na correlation experiments on l-histidine and NaH2PO4, respectively. The two-dimensional heteronuclear 1H-23Na correlation spectrum yields the assignment of 23Na resonances of NaH2PO4. This assignment is corroborated by first-principles calculations.

Line-narrowing in proton-detected nitrogen-14 NMR

In solids spinning at the magic angle, the indirect detection of single-quantum (SQ) and double-quantum (DQ) (14)N spectra (I=1) via spy nuclei S=1/2 such as protons can be achieved in the manner of heteronuclear single- or multiple-quantum correlation (HSQC or HMQC) spectroscopy. The HMQC method relies on the excitation of two-spin coherences of the type T(11)(I)T(11)(S) and T(21)(I)T(11)(S) at the beginning of the evolution interval t(1). The spectra obtained by Fourier transformation from t(1) to omega(1) may be broadened by the homogenous decay of the transverse terms of the spy nuclei S. This broadening is mostly due to homonuclear dipolar S-S' interactions between the proton spy nuclei. In this work we have investigated the possibility of inserting rotor-synchronized symmetry-based C or R sequences and decoupling schemes such as Phase-Modulated Lee-Goldburg (PMLG) sequences in the evolution period. These schemes reduce the homonuclear proton-proton interactions and lead to an enhancement of the resolution of both SQ and DQ proton-detected (14)N HMQC spectra. In addition, we have investigated the combination of HSQC with symmetry-based sequences and PMLG and shown that the highest resolution in the (14)N dimension is achieved by using HSQC in combination with symmetry-based sequences of the R-type. We show improvements in resolution in samples of l-alanine and the tripeptide ala-ala-gly (AAG). In particular, for l-alanine the width of the (14)N SQ peak is reduced from 2 to 1.2 kHz, in agreement with simulations. We report accurate measurements of quadrupolar coupling constants and asymmetry parameters for amide (14)N in AAG peptide bonds.

Directly and indirectly detected through-bond heteronuclear correlation solid-state NMR spectroscopy under fast MAS

Two-dimensional through-bond (1)H{(13)C} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse (1)H decoupling are presented. Remarkable efficiency of polarization transfer can be achieved at MAS rates exceeding 40 kHz, which is instrumental in these measurements. Schemes utilizing direct and indirect detection of heteronuclei are compared in terms of resolution and sensitivity. A simple procedure for optimization of (1)H homonuclear decoupling sequences under these conditions is proposed. The capabilities of these techniques were confirmed on two naturally abundant solids, tripeptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (f-MLF-OH) and brown coal.

Why does PMLG proton decoupling work at 65kHz MAS?

Schemes such as phase-modulated Lee-Goldburg (PMLG) for homonuclear dipolar decoupling have been shown to yield high-resolution (1)H spectra at high magic-angle spinning (MAS) frequencies of 50-70kHz. This is at variance to the commonly held notion that these methods require MAS frequencies not comparable to the cycle frequencies of the pulse schemes. Here, a theoretical argument, based on bimodal Floquet theory, is presented to explain this aspect together with conditions where PMLG type of schemes may be successful at high MAS frequencies.

Determining relative proton-proton proximities from the build-up of two-dimensional correlation peaks in 1H double-quantum MAS NMR: insight from multi-spin density-matrix simulations

The build-up of intensity-as a function of the number, n(rcpl), of POST-C7 elements used for the excitation and reconversion of double-quantum (DQ) coherence (DQC)-is analysed for the fifteen distinct DQ correlation peaks that are observed experimentally for the eight separate (1)H resonances in a (1)H (500 MHz) DQ CRAMPS solid-state (12.5 kHz MAS) NMR spectrum of the dipeptide beta-AspAla (S. P. Brown, A. Lesage, B. Elena, and L. Emsley, J. Am. Chem. Soc., 2004, 126, 13230). The simulation in SPINEVOLUTION (M. Veshtort and R. G. Griffin, J. Magn. Reson., 2006, 178, 248) of t(1) ((1)H DQ evolution) FIDs for clusters of eight dipolar-coupled protons gives separate simulated (1)H DQ build-up curves for the CH(2)(a), CH(2)(b), CH(Asp), CH(Ala), NH and OH (1)H single-quantum (SQ) (1)H resonances. An analysis of both the simulated and experimental (1)H DQ build-up leads to the following general observations: (i) considering the build-up of (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the DQC corresponding to the shortest H-H distance; (ii) for the maximum intensity (1)H DQ peak at a particular SQ frequency, the recoupling time for the observed maximum intensity depends on the corresponding H-H distance, e.g., maximum intensity for the CH(2)(a)-CH(2)(b) (H-H distance = 1.55 A) and OH-CH(Asp) (H-H distance = 2.49 A) DQ peaks is observed at n(rcpl) = 2 and 3, respectively; (iii) for DQ peaks involving a CH(2) proton at a non-CH(2) SQ frequency, there is much reduced intensity and a maximum intensity at a short recoupling time; (iv) for the other lower intensity (1)H DQ peaks at a particular SQ frequency, maximum intensity is observed for the same (or close to the same) recoupling time, but the relative intensity of the DQ peaks is a reliable indicator of the relative H-H distance-the ratio of the maximum intensities for the peaks at the CH(Ala) SQ frequency due to the two DQCs with the NH and OH protons are found to be approximately in the ratio of the squares of the corresponding dipolar coupling constants. While the simulated (1)H DQ build-up curves reproduce most of the features of the experimental curves, maximum intensity is often observed at a longer recoupling time in simulations. In this respect, simulations for two to eight spins show a trend towards a faster decay for an increasing number of considered spins. Finally, simulations show that increasing either the Larmor frequency (to 1 GHz) or the MAS frequency (to 125 kHz) does not lead to changes in the marked differences between the (1)H DQ build-up curves at the CH(Asp) SQ frequency for DQCs to the CH(2)(a) and OH protons that correspond to similar H-H distances (2.39 A and 2.49 A, respectively).

Robust and efficient spin-locked symmetry-based double-quantum homonuclear dipolar recoupling for probing (1)H-(1)H proximity in the solid-state

We report a novel symmetry-based method, using inversion elements bracketed by spin locks, for exciting double-quantum (DQ) coherences between spin-1/2 nuclei, such as protons. Compared to previous DQ-recoupling techniques, this new pulse sequence requires moderate rf field, even at ultra-fast MAS speeds. Furthermore, it is easy to implement and it displays higher robustness to both chemical shift anisotropy and to spreads in resonance frequencies. These advances greatly facilitate the observation of (1)H-(1)H proximities at high fields and high MAS frequencies.

Assessing the performance of windowed 1H CRAMPS methods, on biological solids, at high-field and MAS up to 35 kHz

The performance of various high-resolution 1D (1)H CRAMPS pulse schemes at moderate and high static magnetic fields (400 MHz and 800 (1)H Larmor frequencies) and spinning rates up to 35 kHz, using state-of-the-art electronics is compared. The performance of the (1)H windowed acquisition decoupling schemes, wDUMBO, wPMLG3 and wSAM3 is investigated using their effective z-rotation variants on glycine and other small biological molecules, tripeptide reduced glutathione and nucleoside uridine. (1)H CRAMPS spectra, recorded with windowed (1)H-(1)H decoupling methods and fast MAS (35 kHz) and high-field are reported for the first time. (1)H spectra exhibiting outstanding resolution and completely free from any artifact are also shown. The effect on spectra quality of the decoupling rf cycle and rotor periods ratio (tau(C)/tau(R)) and the power requirements needed for each windowed (1)H CRAMPS methods are discussed.

High-resolution solid-state NMR spectroscopy of protons with homonuclear dipolar decoupling schemes under magic-angle spinning

High-resolution NMR spectroscopy of (1)H spins in the solid state is normally rendered difficult due to the strong homonuclear (1)H-(1)H dipolar couplings. Even under very high-speed magic-angle spinning (MAS) at ca. 60-70kHz, these couplings are not completely removed. An appropriate radiofrequency pulse scheme is required to average out the homonuclear dipolar interactions in combination with MAS to get high-resolution (1)H NMR spectrum in solid state. Several schemes have been introduced in the recent past with a variety of applications also envisaged. Development of some of these schemes has been made possible with a clear understanding of the underlying spin physics based on bimodal Floquet theory. The utility of these high-resolution pulse schemes in combination with MAS has been demonstrated for spinning speeds of 10-65kHz in a range of (1)H Larmor frequencies from 300 to 800MHz.

Homonuclear dipolar decoupling schemes for fast MAS

We present two one-dimensional methods that allow direct observation of highly resolved (1)H spectra in solid-state NMR. The methods, which work mainly at 30-35kHz MAS speeds, are very robust and efficient. In addition, they present very few false resonances. The windowed smooth amplitude-modulated (wSAM) version has a potential to work at ultra-fast MAS provided the electronics is capable to define the pulse-shape in a smooth way, without any pulse transients.

Correlation NMR spectroscopy involving quadrupolar nuclei

We review the recent developments proposed for integer or half-integer quadrupolar nuclei, focussing on the methods to observe them under high-resolution and to analyze their through-space and through-bond connectivities.

1H-1H double-quantum CRAMPS NMR at very-fast MAS (nuR=35 kHz): a resolution enhancement method to probe 1H-1H proximities in solids

A High-resolution two-dimensional (2D) (1)H double-quantum (DQ) homonuclear recoupling experiments, combined with smooth amplitude-modulation (SAM) homonuclear decoupling is presented. The experiment affords highly resolved and clean (1)H DQ-SQ 2D spectra at very-fast MAS rates (nu(R)=35 kHz). The method is well suited to probe (1)H-(1)H distances in powdered solids and demonstrations are applied on a NaH(2)PO(4) powdered sample, an inorganic compound having hydrogen bonding networks.

High-resolution 14N-edited 1H-13C correlation NMR experiment to study biological solids

It was recently shown that nuclear magnetic resonance (NMR) spectra of nitrogen-14 (spin I=1) can be obtained by indirect detection via spin S=1/2 nuclei in powders spinning at the magic angle. An increased number of solid-state NMR methods are now available to tailor sequences for specific purposes, e.g., hetero-nuclear dipolar recoupling or homo-nuclear dipolar decoupling schemes. Here, we combine the latest recoupling and decoupling techniques to obtain high-resolution (1)H-(13)C through-space correlation spectra, where only the correlation peaks of those carbons close to nitrogen nuclei are observed. The experiment is demonstrated on a (13)C enriched l-histidine. HCl x H(2)O powder sample.

3D 1H-13C-14N correlation solid-state NMR spectrum

Nitrogen-14 (spin I=1) has always been a nucleus difficult to observe in solid-state NMR and until recently its observation was restricted to one-dimensional (1D) spectra. We present here the first 3D 1H-13C-14N NMR correlation spectrum. This spectrum was acquired on a test sample L-histidine.HCl.H2O using a recently developed technique, which consists in indirectly observing 14N nuclei via dipolar recoupling with an HMQC-type experiment.