Progress in Nuclear Magnetic Resonance Spectroscopy最新文献

Solid-state NMR spectroscopy (ssNMR) can provide details about the structure, host–guest/guest–guest interactions and dynamic behavior of materials at atomic length scales. A crucial use of ssNMR is for the characterization of zeolite catalysts that are extensively employed in industrial catalytic processes. This review aims to spotlight the recent advancements in ssNMR spectroscopy and its application to zeolite chemistry. We first review the current ssNMR methods and techniques that are relevant to characterize zeolite catalysts, including advanced multinuclear and multidimensional experiments, in situ NMR techniques and hyperpolarization methods. Of these, the methodology development on half-integer quadrupolar nuclei is emphasized, which represent about two-thirds of stable NMR-active nuclei and are widely present in catalytic materials. Subsequently, we introduce the recent progress in understanding zeolite chemistry with the aid of these ssNMR methods and techniques, with a specific focus on the investigation of zeolite framework structures, zeolite crystallization mechanisms, surface active/acidic sites, host–guest/guest–guest interactions, and catalytic reaction mechanisms.

Extension of magnetic resonance imaging (MRI) techniques to the single micron scale has been the goal of research in multiple laboratories over several decades. It has proven difficult to achieve isotropic spatial resolution better than 3.0 μm in inductively-detected MRI near 300 K, even with well-behaved test samples, microcoils, and optimized MRI pulse sequences. This article examines the factors that limit spatial resolution in MRI, especially the inherently low signal-to-noise ratio of nuclear magnetic resonance (NMR), and explains how these limiting factors can be overcome in principle, by acquiring MRI data at low temperatures and using dynamic nuclear polarization (DNP) to enhance signal amplitudes. Recent efforts directed at micron-scale MRI enabled by low-temperature DNP, culminating in images with 1.7 μm isotropic resolution obtained at 5 K, are reviewed. The article concludes with a discussion of areas in which further developments are likely to lead to further improvements in resolution, eventually to 1.0 μm or better.

This review focuses on metabolomics from an NMR point of view. It attempts to cover the broad scope of metabolomics and describes the NMR experiments that are most suitable for each sample type. It is addressed not only to NMR specialists, but to all researchers who wish to approach metabolomics with a clear idea of what they wish to achieve but not necessarily with a deep knowledge of NMR. For this reason, some technical parts may seem a bit naïve to the experts. The review starts by describing standard metabolomics procedures, which imply the use of a dedicated 600 MHz instrument and of four properly standardized 1D experiments. Standardization is a must if one wants to directly compare NMR results obtained in different labs. A brief mention is also made of standardized pre-analytical procedures, which are even more essential. Attention is paid to the distinction between fingerprinting and profiling, and the advantages and disadvantages of fingerprinting are clarified. This aspect is often not fully appreciated. Then profiling, and the associated problems of signal assignment and quantitation, are discussed. We also describe less conventional approaches, such as the use of different magnetic fields, the use of signal enhancement techniques to increase sensitivity, and the potential of field-shuttling NMR. A few examples of biomedical applications are also given, again with the focus on NMR techniques that are most suitable to achieve each particular goal, including a description of the most common heteronuclear experiments. Finally, the growing applications of metabolomics to foodstuffs are described.

Over the last two decades magic angle spinning dynamic nuclear polarization (MAS DNP) has revolutionized NMR for materials characterization, tackling its main limitation of intrinsically low sensitivity. Progress in theoretical understanding, instrumentation, and sample formulation expanded the range of materials applications and research questions that can benefit from MAS DNP. Currently the most common approach for hyperpolarization under MAS consists in impregnating the sample of interest with a solution containing nitroxide radicals, which upon microwave irradiation serve as exogenous polarizing agents. On the other hand, in metal ion based (MI)-DNP, inorganic materials are doped with paramagnetic metal centres, which then can be used as endogenous polarizing agents. In this work we give an overview of the electron paramagnetic resonance (EPR) concepts required to characterize the metal ions and discuss the expected changes in the NMR response due to the presence of paramagnetic species. We highlight which properties of the electron spins are beneficial for applications as polarizing agents in DNP and how to recognize them, both from the EPR and NMR data. A theoretical description of the main DNP mechanisms is given, employing a quantum mechanical formalism, and these concepts are used to explain the spin dynamics observed in the DNP experiment. In addition, we highlight the main differences between MI-DNP and the more common approaches in MAS DNP, which use organic radicals as exogenous polarizing source. Finally, we review some applications of metal ions as polarizing agents in general and then focus particularly on research questions in materials science that can benefit from MI-DNP.

The present review is focused on experimental and theoretical methods together with applications of helium NMR in chemistry and biochemistry. It comprises two main sections, the first dealing with standardization and instrumentation for ³He NMR spectroscopy and the second dealing with its practical applications, mainly those in general and organic chemistry with a special emphasis on the rapidly developing and exciting area of fullerenes encapsulating helium atoms. Several general applications of ³He NMR spectroscopy in physical chemistry and biomedicine are also briefly discussed.

In this review, we describe the application of shaped pulses for EPR spectroscopy. Pulses generated by fast arbitrary waveform generators are mostly used in the field of EPR spectroscopy for broadband (200 MHz-1 GHz) excitation of paramagnetic species. The implementation and optimization of such broadband pulses in existing EPR spectrometers, often designed and optimized for short rectangular microwave pulses, is demanding. Therefore, a major part of this review will describe in detail the implementation, testing and optimization of shaped pulses in existing EPR spectrometers. Additionally, we review applications using such pulses for broadband inversion of longitudinal magnetization as well as for the creation and manipulation of transverse magnetization in the field of dipolar and hyperfine EPR spectroscopy. They demonstrate the great potential of shaped pulses to improve the performance of pulsed EPR experiments. We give a brief theoretical description of shaped pulses and their limitations, especially for adiabatic pulses, most often used in EPR. We believe that this review can on the one hand be of practical use to EPR groups starting to work with such pulses, and on the other hand give readers an overview of the state of the art of shaped pulse applications in EPR spectroscopy.

The scalar couplings that result in the splitting of the signals in the NMR spectrum arise due to the interaction of the nuclear spins, whereby the spin polarization is transmitted through chemical bonds. The interaction strengths depend inter alia on the number of consecutive chemical bonds intervening between the two interacting spins and on the molecular conformation. The pairwise interaction of many spins in a molecule resulting in a complex spectrum poses a severe challenge to analyse the spectrum and hence the determination of magnitudes and signs of homo- and heteronuclear couplings. The problem is more severe in the analysis of ¹H spectra than the spectra of most of the other nuclei due to the often very small chemical shift dispersion. As a consequence, the straightforward analysis and the accurate extraction of the coupling constants from the ¹H spectrum of a complex spin system continues to remain a challenge, and often may be a formidable task. Over the years, the several pure shift-based one‐dimensional and two‐dimensional methodologies have been developed by workers in the field, which provide broadband homonuclear decoupling of proton spectra, removing the complexity but at the cost of the very informative scalar couplings. To circumvent this problem, several one‐dimensional and two‐dimensional NMR experiments have been developed for the determination of homonuclear and heteronuclear couplings (ⁿJ_HX, where n = 1,2,3) while retaining the high resolution obtained by implementing pure shift strategies. This review attempts to summarize the extensive work reported by a large number of researchers over the years for the accurate determination of homo- and heteronuclear scalar couplings.

NMR spectroscopy is currently extensively used in binding assays for hit identification, but its use in dissociation constant determination is more limited when compared to other biophysical techniques, in particular for tight binders. Although NMR is quite suitable for measuring the binding strength of weak to medium affinity ligands with dissociation constant K_D > 1 μM, it has some limitations in the determination of the binding strength of tight binders (K_D < 1 μM). A theoretical analysis of the binding affinity determination of strong ligands using different types of NMR experiments is provided and practical guidelines are given for overcoming the limitations and for the proper set-up of the experiments. Some approaches require reagents with unique properties or highly specialized equipment, while others can be applied quite generally. We describe all approaches in detail, but give higher emphasis to the more general methods, like competition experiments, where we include actual experimental data and discuss the practical aspects.

Sodium is an essential ion that plays a central role in many physiological processes including the transmembrane electrochemical gradient and the maintenance of the body’s homeostasis. Due to the crucial role of sodium in the human body, the sodium nucleus is a promising candidate for non-invasively assessing (patho-)physiological changes. Almost 10 years ago, Madelin et al. provided a comprehensive review of methods and applications of sodium (²³Na) MRI (Madelin et al., 2014) [1]. More recent review articles have focused mainly on specific applications of ²³Na MRI. For example, several articles covered ²³Na MRI applications for diseases such as osteoarthritis (Zbyn et al., 2016, Zaric et al., 2020) [[2], [3]], multiple sclerosis (Petracca et al., 2016, Huhn et al., 2019) [[4], [5]] and brain tumors (Schepkin, 2016) [6], or for imaging certain organs such as the kidneys (Zollner et al., 2016) [7], the brain (Shah et al., 2016, Thulborn et al., 2018) [[8], [9]], and the heart (Bottomley, 2016) [10]. Other articles have reviewed technical developments such as radiofrequency (RF) coils for ²³Na MRI (Wiggins et al., 2016, Bangerter et al., 2016) [[11], [12]], pulse sequences (Konstandin et al., 2014) [13], image reconstruction methods (Chen et al., 2021) [14], and interleaved/simultaneous imaging techniques (Lopez Kolkovsky et al., 2022) [15]. In addition, ²³Na MRI topics have been covered in review articles with broader topics such as multinuclear MRI or ultra-high-field MRI (Niesporek et al., 2019, Hu et al., 2019, Ladd et al., 2018) [[16], [17], [18]].

During the past decade, various research groups have continued working on technical improvements to sodium MRI and have investigated its potential to serve as a diagnostic and prognostic tool. Clinical research applications of ²³Na MRI have covered a broad spectrum of diseases, mainly focusing on the brain, cartilage, and skeletal muscle (see Fig. 1). In this article, we aim to provide a comprehensive summary of methodological and hardware developments, as well as a review of various clinical research applications of sodium (²³Na) MRI in the last decade (i.e., published from the beginning of 2013 to the end of 2022).

Macromolecular protein assemblies are of fundamental importance for many processes inside the cell, as they perform complex functions and constitute central hubs where reactions occur. Generally, these assemblies undergo large conformational changes and cycle through different states that ultimately are connected to specific functions further regulated by additional small ligands or proteins. Unveiling the 3D structural details of these assemblies at atomic resolution, identifying the flexible parts of the complexes, and monitoring with high temporal resolution the dynamic interplay between different protein regions under physiological conditions is key to fully understanding their properties and to fostering biomedical applications.

In the last decade, we have seen remarkable advances in cryo-electron microscopy (EM) techniques, which deeply transformed our vision of structural biology, especially in the field of macromolecular assemblies. With cryo-EM, detailed 3D models of large macromolecular complexes in different conformational states became readily available at atomic resolution. Concomitantly, nuclear magnetic resonance (NMR) and electron paramagnetic resonance spectroscopy (EPR) have benefited from methodological innovations which also improved the quality of the information that can be achieved. Such enhanced sensitivity widened their applicability to macromolecular complexes in environments close to physiological conditions and opened a path towards in-cell applications.

In this review we will focus on the advantages and challenges of EPR techniques with an integrative approach towards a complete understanding of macromolecular structures and functions.