Category Archives: Review

Challenges and perspectives in quantitative NMR

Giraudeau, Patrick. “Challenges and Perspectives in Quantitative NMR.” Magn. Reson. Chem., 2016, 9.

This perspective article summarizes, from the author’s point of view at the beginning of 2016, the major challenges and perspectives in the field of quantitative NMR. The key concepts in quantitative NMRare first summarized; then, themost recent evolutions in terms of resolution and sensitivity are discussed, as well as some potential future research directions in this field. A particular focus is made on methodologies capable of boosting the resolution and sensitivity of quantitative NMR, which could open application perspectives in fields where the sample complexity and the analyte concentrations are particularly challenging. These include multi-dimensional quantitative NMR and hyperpolarization techniques such as para-hydrogen-induced polarization or dynamic nuclear polarization. Because quantitative NMR cannot be dissociated from the key concepts of analytical chemistry, i.e. trueness and precision, the methodological developments are systematically described together with their level of analytical performance.

Magnets for Small-Scale and Portable NMR

Blümich, Bernhard, Christian Rehorn, and Wasif Zia. “Magnets for Small-Scale and Portable NMR.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 1–20. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.

Nuclear magnetic resonance (NMR) exploits the resonance of the precessing motion of nuclear magnetization in magnetic fields. From the measurement methodology, three groups of common techniques of probing resonance can be assigned: those employing forced oscillations, free oscillations, and interferometric principles. In either case, the sensitivity depends on the strength of the nuclear magnetic polarization, which, in thermodynamic equilibrium at temperatures higher than few degrees above absolute zero, is in good approximation proportional to the strength of the magnetic field. In recognition of this fact, one guideline in the development of NMR magnets has always been to reach high field strength.The highest field strength of temporally stable magnetic fields today is achieved with superconducting electromagnets. This is why most standard NMR instruments used for NMR spectroscopy in chemical analysis and magnetic resonance imaging (MRI) in medical diagnostics employ superconducting magnets cooled to the low temperature of boiling helium with cryogenic technology.

Small Molecules, Non-Covalent Interactions, and Confinement #DNPNMR

Buntkowsky, Gerd, and Michael Vogel. “Small Molecules, Non-Covalent Interactions, and Confinement.” Molecules 25, no. 14 (July 21, 2020): 3311.

This review gives an overview of current trends in the investigation of small guest molecules, confined in neat and functionalized mesoporous silica materials by a combination of solid-state NMR and relaxometry with other physico-chemical techniques. The reported guest molecules are water, small alcohols, and carbonic acids, small aromatic and heteroaromatic molecules, ionic liquids, and surfactants. They are taken as characteristic role-models, which are representatives for the typical classes of organic molecules. It is shown that this combination delivers unique insights into the structure, arrangement, dynamics, guest-host interactions, and the binding sites in these confined systems, and is probably the most powerful analytical technique to probe these systems.

Microcoils for Broadband Multinuclei Detection

Anders, Jens, and Aldrik H. Velders. “Microcoils for Broadband Multinuclei Detection.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 265–96. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.

NMR techniques are among the most influential analytical tools developed in the past century and widely used in various disciplines from oil well drilling to medicine. To date, two major hurdles inhibit an even more widespread use of NMR spectroscopy in science and society: first, NMR’s relatively low sensitivity severely constrains applications of mass- and volume-limited samples including lab-on-chip integration, in-cell analysis, and bioanalyte detection. Typical NMR samples contain micromole quantities of material in a relatively large sample volume of about 0.5ml; this large sample volume in turn imposes stringent requirements on the magnetic field – both for the generation but also on the susceptibility of the materials utilized in the probe head – which has to be homogenous in the whole sample volume with ppb resolution. Second, NMR equipment is very complex and costly. A major contribution to the high price of NMR equipment is constituted by the (cryogenic) superconducting magnets used to generate the static magnetic field.This problem will hopefully be tackled by the introduction of new magnet-manufacturing techniques and materials, for example, high-temperature superconductors, and the development of miniaturized spectrometers. Another complex and costly aspect concerns the heart of spectrometers consisting of intricate multifrequency probes, with coils integrated in sophisticated tuning–matching circuits connected to complex RF transceiver circuits. In viewof these limitations of currentNMRsystems, to make NMR more versatile and affordable, a key challenge is improving sensitivity and, at the same time, reducing cost and complexity of NMR probes and electronics.

Platforms for Stable Carbon‐Centered Radicals #DNPNMR

Kato, Kenichi, and Atsuhiro Osuka. “Platforms for Stable Carbon‐Centered Radicals.” Angewandte Chemie International Edition 58, no. 27 (July 2019): 8978–86.

Organic radicals can play important roles potentially in diverse functional materials owing to an unpaired electron but are usually highly reactive and difficult to use. Therefore, stabilization of organic radicals is crucially important. Among organic radicals, carbon-centered radicals are promising because of their trivalent nature that enables structural diversity and elaborate designs but they show less stabilities because of reactivities toward carboncarbon bond formation and atmospheric oxygen. Recently, stable carbon-centered radicals have been increasingly explored on the basis of diverse molecular platforms. This minireview highlights these newly explored stable carbon-centered radicals with a particular focus on porphyrinoid-stabilized radicals owing to their remarkable spin delocalization abilities.

Field Guide to Terahertz Sources, Detectors, and Optics

I recently came across this very handy and useful publication. It is helpful to anyone who wants to understand the design of quasi-optical elements, which are for example used in many high-field EPR spectrometers.

O’Sullivan, Créidhe M., and J. Anthony Murphy. Field Guide to Terahertz Sources, Detectors, and Optics. SPIE, 2012.

The region of the electromagnetic spectrum between microwaves and infrared radiation has come to be known as the “THz gap,” mainly due to the lack of readily available laboratory sources and detectors. For many years technology development was driven by astronomers and planetary scientists, but other potential uses, particularly in medical and security applications, have led to increased activity by the mainstream physics and engineering community in recent times. Because diffraction is important at these frequencies, THz systems cannot be successfully designed using traditional optical techniques alone.

The primary objective of this Field Guide is to provide the reader with a concise description of the quasi-optical techniques used at THz frequencies, as well as the basic principles of operation of the most common THz system components in use today. More detailed accounts of specific devices can be found in the bibliography and references therein.

Wave Guides for Micromagnetic Resonance

Yilmaz, Ali, and Marcel Utz. “Wave Guides for Micromagnetic Resonance.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 75–108. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.

In nuclear magnetic resonance, a system of nuclear spins exposed to a static magnetic interacts with an oscillatory field, usually in the radio frequency range. In most NMR setups, including all commercially available NMR spectrometers, coherent transitions between spin states are detected by a voltage induced into a conductor surrounding the sample. Whereas other detection techniques have their advantages in certain cases, inductive detection has proven to be both robust and easy to implement.

Microscale Hyperpolarization #DNPNMR

Kiss, Sebastian, Lorenzo Bordonali, Jan G. Korvink, and Neil MacKinnon. “Microscale Hyperpolarization.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 297–351. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.

Magnetic resonance (MR) is a tremendously powerful technique for obtaining both structural and dynamical information non-invasively and with atomic resolution. The primary limitation of MR is sensitivity, with the detected resonant exchange of energy dependent on population differences on the order of tens of parts per million as dictated by Boltzmann statistics. The MR community has implemented various strategies to overcome this inherent limitation, including maximizing the static polarizing magnetic field and cooling the probe electronics. As discussed throughout this book, an alternative strategy is to miniaturize the MR detector in order to maximize resonant energy exchange efficiency between the sample and the instrument electronics. In this chapter, we discuss approaches that aim to overcome Boltzmann population statistics. These hyperpolarization techniques rely on the transfer of a large polarization source to the target nuclear spin system, or the preparation of pure spin states that are transferred into the target spin system. The archetypal example of the former case is dynamic nuclear polarization (DNP), whereas in the latter case para-hydrogen and optically pumped 3He or 129Xe are examples.

Use of dissolved hyperpolarized species in NMR: Practical considerations #DNPNMR

Berthault, Patrick, Céline Boutin, Charlotte Martineau-Corcos, and Guillaume Carret. “Use of Dissolved Hyperpolarized Species in NMR: Practical Considerations.” Progress in Nuclear Magnetic Resonance Spectroscopy 118–119 (June 2020): 74–90.

Hyperpolarization techniques that can transiently boost nuclear spin polarization are generally carried out at low temperature – as in the case of dynamic nuclear polarization – or at high temperature in the gaseous state – as in the case of optically pumped noble gases. This review aims at describing the various issues and challenges that have been encountered during dissolution of hyperpolarized species, and solutions to these problems that have been or are currently proposed in the literature. During the transport of molecules from the polarizer to the NMR detection region, and when the hyperpolarized species or a precursor of hyperpolarization (e.g. parahydrogen) is introduced into the solution of interest, several obstacles need to be overcome to keep a high level of final magnetization. The choice of the magnetic field, the design of the dissolution setup, and ways to isolate hyperpolarized compounds from relaxation agents will be presented. Due to the non-equilibrium character of the hyperpolarization, new NMR pulse sequences that perform better than the classical ones will be described. Finally, three applications in the field of biology will be briefly mentioned.

Overhauser Dynamic Nuclear Polarization: A Tool for Building Maps of Hydration Water #DNPNMR #ODNP #Review

Franck, John M. “Overhauser Dynamic Nuclear Polarization: A Tool for Building Maps of Hydration Water.” Biophysical Journal 118, no. 3, Supplement 1 (February 7, 2020): 487a.

Coating the surface of every macromolecule or macromolecular assembly, one finds a hydration layer composed of water molecules that move typically between 3× and 10× slower than water molecules in bulk water. The interaction between the water molecules in the hydration layer and the macromolecules contributes to the structural stability and sometimes the function of, e.g., proteins and lipid bilayers. Overhauser Dynamic Nuclear Polarization (ODNP) is an emerging electron-spin nuclear-spin (EPR-NMR) double-resonance tool that has demonstrated a capability of measuring the translational dynamics of water in the hydration layer. Here we discuss our efforts on two fronts: First, we design a scheme for measuring the thickness of the hydration layer and the effect of confinement on translational dynamics, as measured by ODNP, with controlled, appropriately labeled reverse micelle systems. Second, we describe the development of an a priori technique for converting ODNP measurements into a 3D “map” of hydration layer properties in dynamic room temperature samples that explore an ensemble of structures. This latter effort focuses on transmembrane model systems and utilizes the modern structure-prediction tool Rosetta in a fashion analogous to successful efforts to predict NMR order parameters. Particular focus is given to improving the quality and automation of the ODNP measurement, as well as validating predicted ensemble structures against both continuous wave EPR and NMR Paramagnetic Relaxation Enhancement (PRE) data.

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