Magnetic Resonance in Chemistry

Becker, Johanna; Luy, Burkhard

doi: 10.1002/mrc.4276pmid: 26137959

Fast measurement of heteronuclear one‐bond couplings, a class of NMR parameters valuable for structure elucidation, is highly desirable, especially if samples undergo chemical reactions or dynamic processes are observed. Methods presented so far face severe limitations in terms of resolution, accessible bandwidth, and sensitivity. We present the CLean InPhase‐Acceleration by Sharing Adjacent Polarization‐HSQC (CLIP‐ASAP‐HSQC) pulse sequence that allows fast acquisition of spectra with clean inphase multiplets in about 25 s. The performance in terms of accurate extraction of one‐bond couplings is demonstrated on three test samples including partially aligned molecules. Copyright © 2015 John Wiley & Sons, Ltd.

Koos, Martin R. M.; Feyrer, Hannes; Luy, Burkhard

doi: 10.1002/mrc.4297pmid: 26259565

Pulse sequences in NMR spectroscopy sometimes require the adjustment of effective flip angles with respect to experiment‐specific or sample‐specific parameters. Here, we present a quality factor for efficient optimization of offset‐compensated broadband excitation pulses with RF amplitude‐dependent effective flip angles (RADFA). After proof of principle, physical limits of RF amplitude‐restricted and RF power‐restricted broadband RADFA pulses are explored and corresponding pulse shapes and performances characterized in detail. Copyright © 2015 John Wiley & Sons, Ltd.

Foroozandeh, Mohammadali; Jeannerat, Damien

doi: 10.1002/mrc.4283pmid: 26289946

Resolution enhancement is a long‐sought goal in NMR spectroscopy. In conventional multidimensional NMR experiments, such as the 1H‐13C HSQC, the resolution in the indirect dimensions is typically 100 times lower as in 1D spectra because it is limited by the experimental time. Reducing the spectral window can significantly increase the resolution but at the cost of ambiguities in frequencies as a result of spectral aliasing. Fortunately, this information is not completely lost and can be retrieved using methods in which chemical shifts are encoded in the aliased spectra and decoded after processing to reconstruct high‐resolution 1H‐13C HSQC spectrum with full spectral width and a resolution similar to that of 1D spectra. We applied a new reconstruction method, RHUMBA (reconstruction of high‐resolution using multiplet built on aliased spectra), to spectra obtained from the differential evolution for non‐ambiguous aliasing‐HSQC and the new AMNA (additional modulation for non‐ambiguous aliasing)‐HSQC experiments. The reconstructed spectra significantly facilitate both manual and automated spectral analyses and structure elucidation based on heteronuclear 2D experiments. The resolution is enhanced by two orders of magnitudes without the usual complications due to spectral aliasing. Copyright © 2015 John Wiley & Sons, Ltd.

Ramírez‐Gualito, Karla; Jeannerat, Damien

doi: 10.1002/mrc.4301pmid: 26288958

Taking advantage of the phase of nuclear magnetic resonance (NMR) signals to encode NMR information is not easy because of their low precision and their sensitivity to nearby signals. We nevertheless demonstrated that the phase in indirect dimension of 1H–13C heteronuclear single quantum coherence (HSQC) signals could provide carbon chemical shifts at low, but sufficient precision to resolve the ambiguities of the chemical shifts in aliased spectra. This approach, we called phase‐encoding of the aliasing order Na (PHANA), only requires inserting a constant delay during the t1 evolution time to obtain spectra where signals with mixed phases can be decoded at the processing to reconstruct full spectra with a 15‐fold increase in resolution. Copyright © 2015 John Wiley & Sons, Ltd.

Stern, Alan S.; Hoch, Jeffrey C.

doi: 10.1002/mrc.4287pmid: 26256110

Compressed sensing (CS) has attracted a great deal of recent interest as an approach for spectrum analysis of nonuniformly sampled NMR data. Although theoretical justification for the method is abundant, it suffers from several weaknesses, among them poor convergence of some algorithms, and it remains an open question whether NMR spectra satisfy the sparsity requirements of CS theorems. The versions of CS used in NMR involve minimizing the l1 norm of the spectrum. They bear similarity to maximum entropy (MaxEnt) reconstruction, but critical comparison of the methods can be difficult. Here we describe a formalism that places CS and MaxEnt reconstruction on equal footing, enabling critical comparison of the two methods. We also describe a new algorithm for CS that restricts the computation of the l1 norm to the real channel for complex spectra and ensures causality. Preliminary 1D results demonstrate that this approach ameliorates some artifacts that can occur when using the l1 norm of the complex spectrum. Copyright © 2015 John Wiley & Sons, Ltd.

Le Guennec, Adrien; Dumez, Jean‐Nicolas; Giraudeau, Patrick; Caldarelli, Stefano

doi: 10.1002/mrc.4258pmid: 26053155

NMR is a powerful tool for the analysis of complex mixtures and the identification of individual components. Two‐dimensional (2D) NMR potentially offers a wealth of information, but resolution is often sacrificed in order to contain experimental times. We explore the use of non‐uniform sampling (NUS) to increase substantially the resolution of 2D NMR spectra of complex mixtures of small molecules, with no increase in experimental time. Two common pulse sequences for metabolomics applications are analysed, HSQC and TOCSY. Specific attention is paid to sensitivity in resolution‐enhanced NUS spectra, using the signal‐to‐maximum‐noise ratio as a metric. With a careful choice of sampling schedule and reconstruction algorithm, resolution in the 13C dimension for HSQC is increased by a factor of at least 32, with no loss in sensitivity and no spurious peaks. For TOCSY, multiplets can be resolved in the indirect dimension in a reasonable experimental time. These properties should increase the usefulness of 2D NMR for metabolomics applications by, for example, increasing the chances of metabolite identification. Copyright © 2015 John Wiley & Sons, Ltd.

Kazimierczuk, Krzysztof; Orekhov, Vladislav

doi: 10.1002/mrc.4284pmid: 26290057

The invention of multidimensional techniques in the 1970s revolutionized NMR, making it the general tool of structural analysis of molecules and materials. In the most straightforward approach, the signal sampling in the indirect dimensions of a multidimensional experiment is performed in the same manner as in the direct dimension, i.e. with a grid of equally spaced points. This results in lengthy experiments with a resolution often far from optimum. To circumvent this problem, numerous sparse‐sampling techniques have been developed in the last three decades, including two traditionally distinct approaches: the radial sampling and non‐uniform sampling. This mini review discusses the sparse signal sampling and reconstruction techniques from the point of view of an underdetermined linear algebra problem that arises when a full, equally spaced set of sampled points is replaced with sparse sampling. Additional assumptions that are introduced to solve the problem, as well as the shape of the undersampled Fourier transform operator (visualized as so‐called point spread function), are shown to be the main differences between various sparse‐sampling methods. Copyright © 2015 John Wiley & Sons, Ltd.

Lesot, Philippe; Kazimierczuk, Krzysztof; Trébosc, Julien; Amoureux, Jean‐Paul; Lafon, Olivier

doi: 10.1002/mrc.4290pmid: 26332109

Unique information about the atom‐level structure and dynamics of solids and mesophases can be obtained by the use of multidimensional nuclear magnetic resonance (NMR) experiments. Nevertheless, the acquisition of these experiments often requires long acquisition times. We review here alternative sampling methods, which have been proposed to circumvent this issue in the case of solids and mesophases. Compared to the spectra of solutions, those of solids and mesophases present some specificities because they usually display lower signal‐to‐noise ratios, non‐Lorentzian line shapes, lower spectral resolutions and wider spectral widths. We highlight herein the advantages and limitations of these alternative sampling methods. A first route to accelerate the acquisition time of multidimensional NMR spectra consists in the use of sparse sampling schemes, such as truncated, radial or random sampling ones. These sparsely sampled datasets are generally processed by reconstruction methods differing from the Discrete Fourier Transform (DFT). A host of non‐DFT methods have been applied for solids and mesophases, including the G‐matrix Fourier transform, the linear least‐square procedures, the covariance transform, the maximum entropy and the compressed sensing. A second class of alternative sampling consists in departing from the Jeener paradigm for multidimensional NMR experiments. These non‐Jeener methods include Hadamard spectroscopy as well as spatial or orientational encoding of the evolution frequencies. The increasing number of high field NMR magnets and the development of techniques to enhance NMR sensitivity will contribute to widen the use of these alternative sampling methods for the study of solids and mesophases in the coming years. Copyright © 2015 John Wiley & Sons, Ltd.