Radiation damping in modern NMR experiments: progress and challenges

Radiation damping at clinical field strength: Characterization and compensation in quantitative measurements

PURPOSE

In any MR experiment, the bulk magnetization acts on itself, caused by the induced current in the RF receiver circuit that generates an oscillating damping field. This effect, known as "radiation damping" (RD), is usually weak and, therefore, unconsidered in MRI, but can affect quantitative studies performed with dedicated coils that provide a high SNR. The current work examined RD in a setup for investigations of small tissue specimens including a quantitative characterization of the spin-coil system.

THEORY AND METHODS

A custom-made Helmholtz coil (radius and spacing 16 mm) was interfaced to a transmit-receive (Tx/Rx) switch with integrated passive feedback for modulation or suppression of RD similar to preamplifier decoupling. Pulse sequences included pulse-width arrays to demonstrate the absence/ presence of RD and difference techniques employing gradient pulses or composite RF pulses to quantify RD effects during free precession and transmission, respectively. Experiments were performed at 3T in small samples of MnCl2 solution.

RESULTS

Significant RD effects may impact RF pulse application and evolution periods. Effective damping time constants were comparable to typical T2 * times or echo spacings in multi-echo sequences. Measurements of the phase relation showed that deviations from the commonly assumed 90° angle between the damping field and the transverse magnetization may occur.

CONCLUSION

Radiation damping may affect the accuracy of quantitative MR measurements performed with dedicated RF coils. Efficient mitigation can be achieved hardware-based or by appropriate consideration in the pulse sequence.

Optimal 13C NMR investigation of intrinsically disordered proteins at 1.2 GHz

Nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for characterizing biomolecules such as proteins and nucleic acids at atomic resolution. Increased magnetic field strengths drive progress in biomolecular NMR applications, leading to improved performance, e.g., higher resolution. A new class of NMR spectrometers with a 28.2 T magnetic field (1.2 GHz 1H frequency) has been commercially available since the end of 2019. The availability of ultra-high-field NMR instrumentation makes it possible to investigate more complex systems using NMR. This is especially true for highly flexible intrinsically disordered proteins (IDPs) and highly flexible regions (IDRs) of complex multidomain proteins. Indeed, the investigation of these proteins is frequently hampered by the crowding of NMR spectra. The advantages, however, are accompanied by challenges that the user must overcome when conducting experiments at such a high field (e.g., large spectral widths, radio frequency bandwidth, performance of decoupling schemes). This protocol presents strategies and tricks for optimising high-field NMR experiments for IDPs/IDRs based on the analysis of the relaxation properties of the investigated protein. The protocol, tested on three IDPs of different molecular weight and structural complexity, focuses on 13C-detected NMR at 1.2 GHz. A set of experiments, including some multiple receiver experiments, and tips to implement versions tailored for IDPs/IDRs are described. However, the general approach and most considerations can also be applied to experiments that acquire 1H or 15N nuclei and to experiments performed at lower field strengths.

Rapid and Simple 13C-Hyperpolarization by 1H Dissolution Dynamic Nuclear Polarization Followed by an Inline Magnetic Field Inversion

Dissolution dynamic nuclear polarization (dDNP) is a method of choice for preparing hyperpolarized 13C metabolites such as 1-13C-pyruvate used for in vivo applications, including the real-time monitoring of cancer cell metabolism in human patients. The approach consists of transferring the high polarization of electron spins to nuclear spins via microwave irradiation at low temperatures (1.0-1.5 K) and moderate magnetic fields (3.3-7 T). The solid sample is then dissolved and transferred to an NMR spectrometer or MRI scanner for detection in the liquid state. Common dDNP protocols use direct hyperpolarization of 13C spins reaching polarizations of >50% in ∼1-2 h. Alternatively, 1H spins are polarized before transferring their polarization to 13C spins using cross-polarization, reaching polarization levels similar to those of direct DNP in only ∼20 min. However, it relies on more complex instrumentation, requiring highly skilled personnel. Here, we explore an alternative route using 1H dDNP followed by inline adiabatic magnetic field inversion in the liquid state during the transfer. 1H polarizations of >70% in the solid state are obtained in ∼5-10 min. As the hyperpolarized sample travels from the dDNP polarizer to the NMR spectrometer, it goes through a field inversion chamber, which causes the 1H → 13C polarization transfer. This transfer is made possible by the J-coupling between the heteronuclei, which mixes the Zeeman states at zero-field and causes an antilevel crossing. We report liquid-state 13C polarization up to ∼17% for 3-13C-pyruvate and 13C-formate. The instrumentation needed to perform this experiment in addition to a conventional dDNP polarizer is simple and readily assembled.

HR-MAS DREAMTIME NMR for Slow Spinning ex Vivo and in Vivo Samples

HR-MAS NMR is a powerful tool, capable of monitoring molecular changes in intact heterogeneous samples. However, one of the biggest limitations of 1H NMR is its narrow spectral width which leads to considerable overlap in complex natural samples. DREAMTIME NMR is a highly selective technique that allows users to isolate suites of metabolites from congested spectra. This permits targeted metabolomics by NMR and is ideal for monitoring specific processes. To date, DREAMTIME has only been employed in solution-state NMR, here it is adapted for HR-MAS applications. At high spinning speeds (>5 kHz), DREAMTIME works with minimal modifications. However, spinning over 3-4 kHz leads to cell lysis, and if maintaining sample integrity is necessary, slower spinning (<2.5 kHz) is required. Very slow spinning (≤500 Hz) is advantageous for in vivo analysis to increase organism survival; however, sidebands from water pose a problem. To address this, a version of DREAMTIME, termed DREAMTIME-SLOWMAS, is introduced. Both techniques are compared at 2500, 500, and 50 Hz, using ex vivo worm tissue. Following this, DREAMTIME-SLOWMAS is applied to monitor key metabolites of anoxic stress in living shrimp at 500 Hz. Thus, standard DREAMTIME works well under MAS conditions and is recommended for samples reswollen in D2O or spun >2500 Hz. For slow spinning in vivo or intact tissue samples, DREAMTIME-SLOWMAS provides an excellent way to target process-specific metabolites while maintaining sample integrity. Overall, DREAMTIME should find widespread application wherever targeted molecular information is required from complex samples with a high degree of spectral overlap.

Water irradiation devoid pulses enhance the sensitivity of 1H,1H nuclear Overhauser effects

The nuclear Overhauser effect (NOE) is one of NMR spectroscopy's most important and versatile parameters. NOE is routinely utilized to determine the structures of medium-to-large size biomolecules and characterize protein-protein, protein-RNA, protein-DNA, and protein-ligand interactions in aqueous solutions. Typical [¹H,¹H] NOESY pulse sequences incorporate water suppression schemes to reduce the water signal that dominates ¹H-detected spectra and minimize NOE intensity losses due to unwanted polarization exchange between water and labile protons. However, at high- and ultra-high magnetic fields, the excitation of the water signal during the execution of the NOESY pulse sequences may cause significant attenuation of NOE cross-peak intensities. Using an evolutionary algorithm coupled with artificial intelligence, we recently designed high-fidelity pulses [Water irrAdiation DEvoid (WADE) pulses] that elude water excitation and irradiate broader bandwidths relative to commonly used pulses. Here, we demonstrate that WADE pulses, implemented into the 2D [¹H,¹H] NOESY experiments, increase the intensity of the NOE cross-peaks for labile and, to a lesser extent, non-exchangeable protons. We applied the new 2D [¹H,¹H] WADE-NOESY pulse sequence to two well-folded, medium-size proteins, i.e., the K48C mutant of ubiquitin and the Raf kinase inhibitor protein. We observed a net increase of the NOE intensities varying from 30 to 170% compared to the commonly used NOESY experiments. The new WADE pulses can be easily engineered into 2D and 3D homo- and hetero-nuclear NOESY pulse sequences to boost their sensitivity.

Spin Hyperpolarization in Modern Magnetic Resonance

Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in various practical applications, with medical magnetic resonance imaging being the most widely known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the rapidly developing field of spin hyperpolarization. Hyperpolarization techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity. This provides new impetus for existing applications of magnetic resonance and opens the gates to exciting new possibilities. In this review, we provide a unified picture of the many methods and techniques that fall under the umbrella term "hyperpolarization" but are currently seldom perceived as integral parts of the same field. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization, to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target spins. We then give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues, and possible future directions. We are seeing a continuous growth of activity in the field of spin hyperpolarization, and we expect the field to flourish as new and improved hyperpolarization techniques are implemented. Some key areas for development are in prolonging polarization lifetimes, making hyperpolarization techniques more generally applicable to chemical/biological systems, reducing the technical and equipment requirements, and creating more efficient excitation and detection schemes. We hope this review will facilitate the sharing of knowledge between subfields within the broad topic of hyperpolarization, to help overcome existing challenges in magnetic resonance and enable novel applications.

Parahydrogen-induced polarization and spin order transfer in ethyl pyruvate at high magnetic fields

Nuclear magnetic resonance has experienced great advances in developing and translating hyperpolarization methods into procedures for fundamental and clinical studies. Here, we propose the use of a wide-bore NMR for large-scale (volume- and concentration-wise) production of hyperpolarized media using parahydrogen-induced polarization. We discuss the benefits of radio frequency-induced parahydrogen spin order transfer, we show that 100% polarization is theoretically expected for homogeneous B₀ and B₁ magnetic fields for a three-spin system. Moreover, we estimated that the efficiency of spin order transfer is not significantly reduced when the B₁ inhomogeneity is below ± 5%; recommendations for the sample size and RF coils are also given. With the latest breakthrough in the high-yield synthesis of 1-¹³C-vinyl pyruvate and its deuterated isotopologues, the high-field PHIP-SAH will gain increased attention. Some remaining challenges will be addressed shortly.

Parahydrogen-induced polarization and spin order transfer in ethyl pyruvate at high magnetic fields

Nuclear magnetic resonance has experienced great advances in developing and translating hyperpolarization methods into procedures for fundamental and clinical studies. Here, we propose the use of a wide-bore NMR for large-scale (volume- and concentration-wise) production of hyperpolarized media using parahydrogen-induced polarization. We discuss the benefits of radio frequency-induced parahydrogen spin order transfer, we show that 100% polarization is theoretically expected for homogeneous B₀ and B₁ magnetic fields for a three-spin system. Moreover, we estimated that the efficiency of spin order transfer is not significantly reduced when the B₁ inhomogeneity is below ± 5%; recommendations for the sample size and RF coils are also given. With the latest breakthrough in the high-yield synthesis of 1-¹³C-vinyl pyruvate and its deuterated isotopologues, the high-field PHIP-SAH will gain increased attention. Some remaining challenges will be addressed shortly.

A solution NMR view of Lipidic Cubic Phases: Structure, dynamics, and beyond

Nuclear magnetic resonance (NMR) spectroscopy is well-established nowadays for the elucidation of the 3D structures of proteins and protein complexes, the evaluation of biomolecular dynamics with atomistic resolution across a range of time scales, the screening of drug candidates with site specificity, and for the quantitation of molecular translational diffusion. Lyotropic lipidic cubic phases (LCPs) are lipid bilayer-based materials with a complex geometry, formed via the spontaneous self-assembly of certain lipids in an aqueous environment at specific temperature ranges. LCPs have been successfully applied to the in meso crystallization of membrane proteins for structural studies by X-ray crystallography, and have also shown promising potential for serving as matrices for drug and nutrient delivery/release in vivo. The characterization of the structural and dynamics properties of LCPs is of significant interest for the application of these materials. Here we present a systematic review detailing the characterization of LCPs by solution NMR. Using LCPs formed by monoolein (MO) as an example, various aspects of LCPs readily accessible by solution NMR are covered, including spectral perturbation in the presence of additives, quantification of hydration levels, ¹³C relaxation-based measurements for studying atom-specific dynamics along the MO hydrocarbon chain, PGSE NMR measurement of translational diffusion and its correlation with release profiles, and the encapsulation of soluble proteins in LCPs. A brief discussion of future perspectives for the characterization of LCPs by solution NMR is also presented.

Radio-frequency pulse calibration using the MISSTEC sequence

NMR sequences are composed of multiple radio-frequency pulses. Probe adjustment, sample concentration and solvent influence the loading factor, therefore these parameters also impact the validity of flip angles. The commonly used method to calibrate RF pulses is to measure a nutation curve by varying the pulse duration. However, this method is impacted by off-resonance effects, radiation damping and B₁ and B₀ inhomogeneities. Furthermore, it is important to avoid partial saturation. In this work, the MISSTEC sequence is proposed for pulse calibration. This sequence takes only 8 s or 2 min for ¹H or ¹³C calibration, respectively. High accuracy (with an error below 1%) was obtained for both nuclei. Therefore, the calibrations can be done rapidly and accurately. Furthermore, the MISSTEC measurement could be performed on each sample - in an automated way- before acquisitions, after which the calibration found could be automatically used.

Measurement of 1Hα transverse relaxation rates in proteins: application to solvent PREs

It has recently been demonstrated that accurate near surface electrostatic potentials can be calculated for proteins from solvent paramagnetic relaxation enhancements (PREs) of amide protons measured using spin labels of similar structures but different charges (Yu et al. in Proc Natl Acad Sci 118(25):e2104020118, 2021). Here we develop methodology for extending such measurements to intrinsically disordered proteins at neutral pH where amide spectra are of very poor quality. Under these conditions it is shown that accurate PRE values can be measured using the haCONHA experiment that has been modified for recording ¹H^α transverse relaxation rates. The optimal pulse scheme includes a spin-lock relaxation element for suppression of homonuclear scalar coupled evolution for all ¹H^α protons, except those derived from Ser and Thr residues, and minimizes the radiation damping field from water magnetization that would otherwise increase measured relaxation rates. The robustness of the experiment is verified by developing a second approach using a band selective adiabatic decoupling scheme for suppression of scalar coupling modulations during ¹H^α relaxation and showing that the measured PRE values from the two methods are in excellent agreement. The near surface electrostatic potential of a 103-residue construct comprising the C-terminal intrinsically disordered region of the RNA-binding protein CAPRIN1 is obtained at pH 5.5 using both ¹H^N and ¹H^α-based relaxation rates, and at pH 7.4 where only ¹H^α rates can be quantified, with very good agreement between potentials obtained under all experimental conditions.

RASER MRI: Magnetic resonance images formed spontaneously exploiting cooperative nonlinear interaction

The spatial resolution of magnetic resonance imaging (MRI) is limited by the width of Lorentzian point spread functions associated with the transverse relaxation rate 1/T2*. Here, we show a different contrast mechanism in MRI by establishing RASER (radio-frequency amplification by stimulated emission of radiation) in imaged media. RASER imaging bursts emerge out of noise and without applying radio-frequency pulses when placing spins with sufficient population inversion in a weak magnetic field gradient. Small local differences in initial population inversion density can create stronger image contrast than conventional MRI. This different contrast mechanism is based on the cooperative nonlinear interaction between all slices. On the other hand, the cooperative nonlinear interaction gives rise to imaging artifacts, such as amplitude distortions and side lobes outside of the imaging domain. Contrast mechanism and artifacts are explored experimentally and predicted by simulations on the basis of a proposed RASER MRI theory.

Recent advances in NMR spectroscopy of ionic liquids

This review presents recent developments in the application of NMR spectroscopic techniques in the study of ionic liquids. NMR has been the primary tool not only for the structural characterization of ionic liquids, but also for the study of dynamics. The presence of a host of NMR active nuclei in ionic liquids permits widespread use of multinuclear NMR experiments. Chemical shifts and multinuclear coupling constants are used routinely for the structure elucidation of ionic liquids and of products formed by their covalent interactions with other materials. Also, the availability of a multitude of NMR techniques has facilitated the study of dynamical processes in them. These include the use of NOESY to study inter-ionic interactions, pulsed-field gradient techniques for probing transport properties, and relaxation measurements to elucidate rotational dynamics. This review will focus on the application of each of these techniques to investigate ionic liquids.

Absolute Carbon Stable Isotope Ratio in the Vienna Peedee Belemnite Isotope Reference Determined by 1H NMR Spectroscopy

The Vienna Peedee Belemnite (VPDB) isotope reference defines the zero point of the carbon stable isotope scale that is used to describe the relative abundance of ¹³C and ¹²C. An accurate and precise characterization of this isotope reference is valuable for interlaboratory comparisons and conducting robust carbon stable isotope analyses in a vast array of fields, such as chemical forensics, (bio)geochemistry, ecology, or (astro)biology. Here, we report an absolute ¹³C/¹²C ratio for VPDB that has been obtained, for the first time, using proton nuclear magnetic resonance spectroscopy (¹H NMR). Four different NMR instruments were used to determine ¹³C/¹²C ratios in a set of glycine reference materials from the US Geological Survey (USGS64, USGS65, and USGS66) and a set of formate samples that were characterized by isotope ratios mass spectrometry (IRMS). Intercalibration of the NMR-derived ¹³C/¹²C ratios with relative abundance (δ¹³C_VPDB) measurements from IRMS yields a value of 0.011100 for the absolute ¹³C/¹²C ratio in VPDB, with an expanded uncertainty of ±0.000026 (2σ, n = 114). This is significantly different from the value of 0.011180 that is commonly used but falls within the range of values recently revised using IRMS and infrared absorption measurements. ¹H NMR was found to be an effective method for measuring absolute ¹³C/¹²C ratios due to its ability to simultaneously detect signals associated with ¹²C and ¹³C. Results provide a new and independent measure of the carbon isotope composition of VPDB, improving our understanding of this important isotope reference.

Frozen water NMR lineshape analysis enables absolute polarization quantification

Typical magnetic resonance experiments are routinely limited by weak signal responses. In some cases, the low intrinsic sensitivity can be alleviated by the implementation of hyperpolarization technologies. Dissolution-dynamic nuclear polarization offers a means of hyperpolarizing small molecules. Hyperpolarized water is employed in several dynamic nuclear polarization studies, and hence accurate and rapid quantification of the ¹H polarization level is of utmost importance. The solid-state nuclear magnetic resonance spectrum of water acquired under dissolution-dynamic nuclear polarization conditions has revealed lineshapes which become asymmetric at high levels of ¹H polarization, which is an interesting fundamental problem in itself, but also complicates data interpretation and can prevent correct estimations of polarization levels achieved. In previous studies, attempts to simulate the ¹H spectral lineshape of water as a function of the ¹H polarization led to significant disagreement with the experimental results. Here we propose and demonstrate that such simulations, and therefore polarization quantification, can be implemented accurately, in particular by taking into account the detector dead time during ¹H signal acquisition that can lead to severe spectral distortions. Based on these findings, we employed an echo-based radiofrequency pulse sequence to achieve distortion-free ¹H spectra of hyperpolarized water, and adequate simulations of these echo-based spectra were implemented to extract the absolute ¹H polarization level from the hyperpolarized water signal only, thus alleviating the need for lengthy and insensitive measurements of thermal equilibrium signals.

Radiation damping strongly perturbs remote resonances in the presence of homonuclear mixing

In this work, it is experimentally shown that the weak oscillating magnetic field (known as the "radiation damping" field) caused by the inductive coupling between the transverse magnetization of nuclei and the radio frequency circuit perturbs remote resonances when homonuclear total correlation mixing is applied. Numerical simulations are used to rationalize this effect.

Protonation tuned dipolar order mediated 1H→13C cross-polarization for dissolution-dynamic nuclear polarization experiments

A strategy of dipolar order mediated nuclear spin polarization transfer has recently been combined with dissolution-dynamic nuclear polarization (dDNP) and improved by employing optimized shaped radiofrequency pulses and suitable molecular modifications. In the context of dDNP experiments, this offers a promising means of transferring polarization from high-gamma ¹H spins to insensitive ¹³C spins with lower peak power and lower energy compared with state-of-the-art cross-polarization schemes. The role of local molecular groups and the glassing matrix protonation level are both postulated to play a key role in the polarization transfer pathway via an intermediary reservoir of dipolar spin order. To gain appreciation of the mechanisms involved in the dipolar order mediated polarization transfer under dDNP conditions, we investigate herein the influence of the pivotal characteristics of the sample makeup: (i) revising the protonation level for the constituents of the DNP glass; and (ii) utilizing deuterated molecular derivatives. Experimental demonstrations are presented for the case of [1-¹³C]sodium acetate. We find that the proton sample molarity has a large impact on both the optimal parameters and the performance of the dipolar order mediated cross-polarization sequence, with the ¹³C signal build-up time drastically shortened in the case of high solvent protonation levels. In the case of a deuterated molecular derivative, we observe that the nearby ²H substituted methyl group is deleterious to the ¹H→¹³C transfer phenomenon (particularly at low levels of sample protonation). Overall, increased solvent protonation makes the dipolar order governed polarization transfer significantly faster and more efficient. This study sheds light on the influential sample formulation traits which govern the dipolar order-controlled transfer of polarization and indicates that the polarization transfer efficiencies of deuterated molecules can be boosted and reach high performances simply by adequate solvent protonation.

Spin noise gradient echoes

Nuclear spin noise spectroscopy in the absence of radio frequency pulses was studied under the influence of pulsed field gradients (PFGs) on pure and mixed liquids. Under conditions where the radiation-damping-induced line broadening is smaller than the gradient-dependent inhomogeneous broadening, echo responses can be observed in difference spectra between experiments employing pulsed field gradient pairs of the same and opposite signs. These observed spin noise gradient echoes (SNGEs) were analyzed through a simple model to describe the effects of transient phenomena. Experiments performed on high-resolution nuclear magnetic resonance (NMR) probes demonstrate how refocused spin noise behaves and how it can be exploited to determine sample properties. In bulk liquids and their mixtures, transverse relaxation times and translational diffusion constants can be determined from SNGE spectra recorded following tailored sequences of magnetic field gradient pulses.

Practical dissolution dynamic nuclear polarization

This review article intends to provide insightful advice for dissolution-dynamic nuclear polarization in the form of a practical handbook. The goal is to aid research groups to effectively perform such experiments in their own laboratories. Previous review articles on this subject have covered a large number of useful topics including instrumentation, experimentation, theory, etc. The topics to be addressed here will include tips for sample preparation and for checking sample health; a checklist to correctly diagnose system faults and perform general maintenance; the necessary mechanical requirements regarding sample dissolution; and aids for accurate, fast and reliable polarization quantification. Herein, the challenges and limitations of each stage of a typical dissolution-dynamic nuclear polarization experiment are presented, with the focus being on how to quickly and simply overcome some of the limitations often encountered in the laboratory.

Solid-state 1H spin polarimetry by 13CH3 nuclear magnetic resonance

Dissolution dynamic nuclear polarization is used to prepare nuclear spin polarizations approaching unity. At present, 1 H polarization quantification in the solid state remains fastidious due to the requirement of measuring thermal equilibrium signals. Line shape polarimetry of solid-state nuclear magnetic resonance spectra is used to determine several useful properties regarding the spin system under investigation. In the case of highly polarized nuclear spins, such as those prepared under the conditions of dissolution dynamic nuclear polarization experiments, the absolute polarization of a particular isotopic species within the sample may be directly inferred from the characteristics of the corresponding resonance line shape. In situations where direct measurements of polarization are complicated by deleterious phenomena, indirect estimates of polarization using coupled heteronuclear spins prove informative. We present a simple analysis of the 13 C spectral line shape of [2-13 C]sodium acetate based on the normalized deviation of the centre of gravity of the 13 C peaks, which can be used to indirectly evaluate the proton polarization of the methyl group moiety and very likely the entire sample in the case of rapid and homogeneous 1 H-1 H spin diffusion. For the case of positive microwave irradiation, 1 H polarization was found to increase with an increasing normalized centre of gravity deviation. These results suggest that, as a dopant, [2-13 C]sodium acetate could be used to indirectly gauge 1 H polarizations in standard sample formulations, which is potentially advantageous for (i) samples polarized in commercial dissolution dynamic nuclear polarization devices that lack 1 H radiofrequency hardware, (ii) measurements that are deleteriously influenced by radiation damping or complicated by the presence of large background signals and (iii) situations where the acquisition of a thermal equilibrium spectrum is not feasible.

A Magic Angle Spinning Activated 17O DNP Raser

In the raser effect, a sample spontaneously emits continuous radiofrequency radiation, allowing exceptionally narrow NMR line widths to be recorded without applying pulses. To achieve this phenomenon, a large negative magnetization must be induced, which we show here can be achieved for the ¹⁷O magnetization of isotopically labeled Gd-doped CeO₂ using solid effect dynamic nuclear polarization (DNP), at high field and 110 K. This allows a 2 mHz line width to be measured, which is limited only by the magnetic field stability. The raser effect can be reversibly activated and deactivated by magic angle spinning (MAS), which modulates the nuclear spin coherence lifetime. The use of MAS DNP to enable the raser effect should be further applicable to other systems and nuclei.

A Nuclear Magnetic Resonance (NMR) Platform for Real-Time Metabolic Monitoring of Bioprocesses

We present a Nuclear Magnetic Resonance (NMR) compatible platform for the automated real-time monitoring of biochemical reactions using a flow shuttling configuration. This platform requires a working sample volume of ∼11 mL and it can circulate samples with a flow rate of 28 mL/min, which makes it suitable to be used for real-time monitoring of biochemical reactions. Another advantage of the proposed low-cost platform is the high spectral resolution. As a proof of concept, we acquire 1H NMR spectra of waste orange peel, bioprocessed using Trichoderma reesei fungus, and demonstrate the real-time measurement capability of the platform. The measurement is performed over more than 60 h, with a spectrum acquired every 7 min, such that over 510 data points are collected without user intervention. The designed system offers high resolution, automation, low user intervention, and, therefore, time-efficient measurement per sample.

Gradient-based pulse sequences for benchtop NMR spectroscopy

Benchtop NMR spectroscopy has been on the rise for the last decade, by bringing high-resolution NMR in environments that are not easily compatible with high-field NMR. Benchtop spectrometers are accessible, low cost and show an impressive performance in terms of sensitivity with respect to the relatively low associated magnetic field (40-100 MHz). However, their application is limited by the strong and ubiquitous peak overlaps arising from the complex mixtures which are often targeted, often characterized by a great diversity of concentrations and by strong signals from non-deuterated solvents. Such limitations can be addressed by pulse sequences making clever use of magnetic field gradient pulses, capable of performing efficient coherence selection or encoding chemical shift or diffusion information. Gradients pulses are well-known ingredients of high-field pulse sequence recipes, but were only recently made available on benchtop spectrometers, thanks to the introduction of gradient coils in 2015. This article reviews the recent methodological advances making use of gradient pulses on benchtop spectrometers and the applications stemming from these developments. Particular focus is made on solvent suppression schemes, diffusion-encoded, and spatially-encoded experiments, while discussing both methodological advances and subsequent applications. We eventually discuss the exciting development and application perspectives that result from such advances.

Intramolecular distribution of 13C/12C isotopes in amino acids of diverse origins

Carbon stable isotope analysis can provide information about the origin and synthetic pathways that produce organic molecules, with applications in chemical, medical and (bio)geochemical sciences. The ¹³C/¹²C isotope ratios of organics such as amino acids are most commonly obtained as whole molecule averages. In this study, we apply proton nuclear magnetic resonance spectroscopy to conduct position-specific carbon isotope analyses of L-/D-alanine, L-threonine and L-histidine from different sources, in addition to molecule average stable isotope analyses obtained via mass spectrometry. Our results demonstrate that carbon isotope ratios can vary significantly between the individual carbon positions within an amino acid. For example, the β- and γ- carbons of L-threonine can differ in ¹³C/¹²C ratio by > 20 ‰. Comparisons of the position-specific and whole molecule average stable isotope abundances show that whole molecule analyses can mask the intramolecular isotope variation. These results provide the first experimentally measured position-specific isotope ratios for alpha and side chain carbons of alanine, threonine and histidine. Comparison with previous ab initio calculations of intramolecular equilibrium fractionation shows that the carbon isotope distributions are not at equilibrium, thus kinetic isotope effects play a significant role in amino acid synthesis. We hypothesize that position-specific ¹³C/¹²C isotope ratios provide an "isotopic fingerprint" that can give insight into the origin or synthesis pathway that formed an amino acid, and that this emerging analytical field will be a valuable addition to traditional stable isotope analysis.

Insights into Interfacial Water Structuring at the Nafion Surface by T1-Weighted Magnetic Resonance Imaging

T₁-weighted magnetic resonance images of water in the surroundings of a Nafion surface allowed the identification of the presence of a low-mobility zone (LMZ), 60 μm thick, consisting of water molecules structured in a hydrogen-bonding network, promoted by the presence of the acidic protons on the surface of the sulphonated polymer. In parallel, the exclusion zone (EZ) was assessed by observing in optical microscopy the distribution of microspheres suspended in the medium in contact with the Nafion membrane. It was found that the LMZ and the EZ do not correspond: in fact, the former is thinner and more stable over time than the latter and they behave differently when ions are present in the medium in which Nafion is immersed.

Geometrically-differential NMR in a stripline front-end

A major challenge facing the development of portable, low-cost NMR is the development of robust yet sensitive transceivers, for which several trade-offs in scalability, performance and complexity are usually necessary. Here we report on a stripline-based NMR detector that overcomes previous limitations. It features a sensitivity of 5.7×10^-4 TA^-1Ω^-0.5 over a sample volume of 10 mm × 10 mm × 3 mm, an exceptionally high B₁ homogeneity of A₄₅₀/A₉₀=98.4%, intrinsic electromagnetic shielding of 27 dB from environmental influences, and a total signal gain of 68 dB in the presence of a noise factor of 1.28, without any exterior shielding. The new dual-coil arrangement offers a downscalable geometry optimised for gap magnets, and it is voltage-tunable and plug-in compatible with commercial software-defined radio spectrometer boards. Exceptionally, it features both common-mode and novel differential-mode NMR measurement abilities.

Laser- and cryogenic probe-assisted NMR enables hypersensitive analysis of biomolecules at submicromolar concentration

Solution-state NMR typically requires 100 μM to 1 mM samples. This limitation prevents applications to mass-limited and aggregation-prone target molecules. Photochemically induced dynamic nuclear polarization was adapted to data collection on low-concentration samples by radiofrequency gating, enabling rapid 1D NMR spectral acquisition on aromatic amino acids and proteins bearing aromatic residues at nanomolar concentration, i.e., a full order of magnitude below other hyperpolarization techniques in liquids. Both backbone H¹-C¹³ and side-chain resonances were enhanced, enabling secondary and tertiary structure analysis of proteins with remarkable spectral editing, via the ¹³C PREPRINT pulse sequence. Laser-enhanced 2D NMR spectra of 5 μM proteins at 600 MHz display 30-fold better S/N than conventional 2D data collected at 900 MHz. Sensitivity enhancements achieved with this technology, denoted as low-concentration photo-CIDNP (LC-photo-CIDNP), depend only weakly on laser intensity, highlighting the opportunity of safer and more cost-effective hypersensitive NMR applications employing low-power laser sources.

"Radiation Damping" in gas spin comagnetometers

We report a new kind of interaction between overlapping Rb-Xe spin ensembles polarized by spin-exchange optical pumping. The Rb acts as both a medium to optically polarize the Xe spins and as a magnetometer to probe the precession of Xe spins. When Xe spins precess, they result in the precession of Rb spins. Like the radiation damping effect caused by the coil in conventional NMR systems, the precessing Rb spins lead to damping and a frequency-shift for the precessing Xe spins. When Xe spins are operated in a free-induction decay mode, the transverse relaxation time and oscillating frequency of Xe spins change due to the "radiation damping" effect of Rb spins. When Xe spins are operated in the self-oscillating mode, its transverse relaxation time and oscillating frequency will also be changed. These effects will influence the accuracy of NMR probes, which are widely used in the search for CPT- and Lorentz-invariance violations, the fifth force, etc. If this problem is solved or compensated for, the limit of the aforementioned search may be improved.

Production of highly concentrated and hyperpolarized metabolites within seconds in high and low magnetic fields

We introduce two experiments that allow for the rapid production of hyperpolarized metabolites. More than 50% ¹³C polarization in 50 mM concentrations is achieved. This can be translated to portable low field NMR devices.

Optimized slice-selective 1H NMR experiments combined with highly accurate quantitative 13C NMR using an internal reference method

Isotope ratio monitoring by ¹³C NMR spectrometry (irm-¹³C NMR) provides the complete ¹³C intramolecular position-specific composition at natural abundance. It represents a powerful tool to track the (bio)chemical pathway which has led to the synthesis of targeted molecules, since it allows Position-specific Isotope Analysis (PSIA). Due to the very small composition range (which represents the range of variation of the isotopic composition of a given nuclei) of ¹³C natural abundance values (50‰), irm-¹³C NMR requires a 1‰ accuracy and thus highly quantitative analysis by ¹³C NMR. Until now, the conventional strategy to determine the position-specific abundance x_i relies on the combination of irm-MS (isotopic ratio monitoring Mass Spectrometry) and ¹³C quantitative NMR. However this approach presents a serious drawback since it relies on two different techniques and requires to measure separately the signal of all the carbons of the analyzed compound, which is not always possible. To circumvent this constraint, we recently proposed a new methodology to perform ¹³C isotopic analysis using an internal reference method and relying on NMR only. The method combines a highly quantitative ¹H NMR pulse sequence (named DWET) with a ¹³C isotopic NMR measurement. However, the recently published DWET sequence is unsuited for samples with short T₁, which forms a serious limitation for irm-¹³C NMR experiments where a relaxing agent is added. In this context, we suggest two variants of the DWET called Multi-WET and Profiled-WET, developed and optimized to reach the same accuracy of 1‰ with a better immunity towards T₁ variations. Their performance is evaluated on the determination of the ¹³C isotopic profile of vanillin. Both pulse sequences show a 1‰ accuracy with an increased robustness to pulse miscalibrations compared to the initial DWET method. This constitutes a major advance in the context of irm-¹³C NMR since it is now possible to perform isotopic analysis with high relaxing agent concentrations, leading to a strong reduction of the overall experiment time.

Modeling of Polarization Transfer Kinetics in Protein Hydration Using Hyperpolarized Water

Water-protein interactions play a central role in protein structure, dynamics, and function. These interactions, traditionally, have been studied using nuclear magnetic resonance (NMR) by measuring chemical exchange and nuclear Overhauser effect (NOE). Polarization transferred from hyperpolarized water can result in substantial transient signal enhancements of protein resonances due to these processes. Here, we use dissolution dynamic nuclear polarization and flow-NMR for measuring the pH dependence of transferred signals to the protein trypsin. A maximum enhancement of 20 is visible in the amide proton region of the spectrum at pH 6.0, and of 47 at pH 7.5. The aliphatic region is enhanced up to 2.3 times at pH 6.0 and up to 2.5 times at pH 7.5. The time dependence of these observed signals can be modeled quantitatively using rate equations incorporating chemical exchange to amide sites and, optionally, intramolecular NOE to aliphatic protons. On the basis of these two- and three-site models, average exchange (k_ex) and cross-relaxation rates (σ) obtained were k_ex = 12 s^-1, σ = -0.33 s^-1 for pH 7.5 and k_ex = 1.8 s^-1, σ = -0.72 s^-1 for pH 6.0 at a temperature of 304 K. These values were validated using conventional EXSY and NOESY measurements. In general, a rapid measurement of exchange and cross-relaxation rates may be of interest for the study of structural changes of the protein occurring on the same time scale. Besides protein-water interactions, interactions with cosolvent or solutes can further be investigated using the same methods.

The new face of isotopic NMR at natural abundance

The most widely used method for isotope analysis at natural abundance is isotope ratio monitoring by Mass Spectrometry (irm-MS) which provides bulk isotopic composition in ² H, ¹³ C, ¹⁵ N, ¹⁸ O or ³⁴ S. However, in the 1980s, the direct access to Site-specific Natural Isotope Fractionation by Nuclear Magnetic Resonance (SNIF-NMR^TM ) was immediately recognized as a powerful technique to authenticate the origin of natural or synthetic products. The initial - and still most popular - application consisted in detecting the chaptalization of wines by irm-² H NMR. The approach has been extended to a wide range of methodologies over the last decade, paving the way to a wide range of applications, not only in the field of authentication but also to study metabolism. In particular, the emerging irm-¹³ C NMR approach delivers direct access to position-specific ¹³ C isotope content at natural abundance. After highlighting the application scope of irm-NMR (² H and ¹³ C), this article describes the major improvements which made possible to reach the required accuracy of 1‰ (0.1%) in irm-¹³ C NMR. The last part of the manuscript summarizes the different steps to perform isotope analysis as a function of the sample properties (concentration, peak overlap) and the kind of targeted isotopic information (authentication, affiliation). Copyright © 2016 John Wiley & Sons, Ltd.

Gradient-based solvent suppression methods on a benchtop spectrometer

Benchtop NMR emerges as an appealing alternative to widely extend the scope of NMR spectroscopy in harsh environments and for on-line monitoring. Obviously, the use of low-field magnets induces a dramatic reduction of the spectral resolution leading to frequent peak overlaps. This issue is even more serious because applications such as chemical process monitoring involve the use of non-deuterated solvents, leading to intense and broad peaks overlapping with the signals of interest. In this article, we highlight the need for efficient suppression methods compatible with flowing samples, which is not the case of the common pre-saturation approaches. Thanks to a gradient coil included in our benchtop spectrometer, we were able to implement modern and efficient solvent suppression blocks such as WET or excitation sculpting to deliver quantitative spectra in the conditions of the on-line monitoring. While these methods are commonly used at high field, this is the first time that they are investigated on a benchtop setting. Their analytical performance is evaluated and compared under static and on-flow conditions. The results demonstrate the superiority of gradient-based methods, thus highlighting the relevance of implementing this device on benchtop spectrometers. The comparison of major solvent suppression methods reveals an optimum performance for the WET-180-NOESY experiment, both under static and on-flow conditions. Copyright © 2016 John Wiley & Sons, Ltd.

Large dose hyperpolarized water with dissolution-DNP at high magnetic field

We demonstrate a method for the preparation of hyperpolarized water by dissolution Dynamic Nuclear Polarization at high magnetic field. Protons were polarized at 6.7T and 1.1K to >70% with frequency modulated microwave irradiation at 188GHz. 97.2±0.7% of the radical was extracted from the sample in the dissolution in a two-phase system. 16±1mL of 5.0M ¹H in D₂O with a polarization of 13.0±0.9% in the liquid state was obtained, corresponding to an enhancement factor of 4000±300 compared to the thermal equilibrium at 9.4T and 293K. A longitudinal relaxation time constant of 16±1s was measured. The sample was polarized and dissolved in a fluid path compatible with clinical polarizers. The volume of hyperpolarized water produced by this method enables angiography and perfusion measurements in large animals, as well as NMR experiments for studies of e.g. proton exchange and polarization transfer to other nuclei.

Acquiring and processing ultrafast biomolecular 2D NMR experiments using a referenced-based correction

Thanks to their special spatiotemporal encoding/decoding scheme, ultrafast (UF) NMR sequences can deliver arbitrary 2D spectra following a single excitation. Regardless of their nature, these sequences have in common their tracing of a path in the [Formula: see text]-[Formula: see text] plane, that will deliver the spectrum being sought after a 1D Fourier transformation versus [Formula: see text]. This need to simultaneously digitize two domains, tends to impose bandwidth limitations along all spectral axes. Along the [Formula: see text]/[Formula: see text] dimension this problem is exacerbated by the fact that odd and even time points are not equispaced, and by additional artifacts such as time shifts between time points sampled while under the action of positive and negative decoding gradients. As a result, odd and even [Formula: see text] points are typically Fourier transformed separately, halving the potential spectral width along this dimension. While this halving of the [Formula: see text] span can be overcome by an interlaced Fourier transform, this post-processing is seldom used because of its sensitivity to hardware inaccuracies requiring even finer corrections of the even/odd [Formula: see text] data points. These corrections have so far been done manually, but are challenging to implement when dealing with low signal-to-noise ratio signals like those associated with biomolecular NMR experiments. This study introduces an algorithm for an automatic correction of all even/odd ultrafast NMR inconsistencies, based on the acquisition of a reference scan on the solvent. This algorithm was verified experimentally using an [Formula: see text]-[Formula: see text] UF-HSQC variant on ubiquitin at 600 MHz. Features of this method as well as of the interlaced Fourier transformation in general, are discussed.

EXponentially Converging Eradication Pulse Train (EXCEPT) for solvent-signal suppression in investigations with variable T(1) times

Selective presaturation is a common technique for suppressing excessive solvent signals during proton NMR analysis of dilute samples in protic solvents. When the solvent T1 relaxation time constant varies within a series of samples, parameters for the presaturation sequence must often be re-adjusted for each sample. The EXCEPT (EXponentially Converging Eradication Pulse Train) presaturation pulse sequence was developed to eliminate time consuming pulse-parameter re-optimization as long as the variation in the solvent's T1 remains within an order of magnitude. EXCEPT consists of frequency-selective inversion pulses with progressively decreasing interpulse delays. The interpulse delays were optimized to encompass T1 relaxation times ranging from 1 to 10s, but they can be easily adjusted by a single factor for other ranges that fall within an order of magnitude with respect to T1. Sequences with different numbers of inversion pulses were tested to maximize suppression while minimizing the number of pulses and thus the total time needed for suppression. The EXCEPT-16 experiment, where 16 denotes the number of inversion pulses, was found satisfactory for many standard applications. Experimental results demonstrate that EXCEPT provides effective T1-insensitive solvent suppression as predicted by the theory. The robustness of EXCEPT with respect to changes in solvent T1 allows NMR investigations to be carried out for a series of samples without the need for pulse-parameter re-optimization for each sample.

Suppression of radiation damping for high precision quantitative NMR

True quantitative analysis of concentrated samples by (1)H NMR is made very difficult by Radiation Damping. A novel NMR sequence (inspired by the WET NMR sequence and by Outer Volume Saturation methods) is therefore proposed to suppress this phenomenon by reducing the spatial area and consequently the number of spins contributing to the signal detected. The size of the detected volume can be easily chosen in a large range and line shape distortions are avoided thanks to a uniform signal suppression of the outer volume. Composition of a mixture can as a result be determined with very high accuracy (precision and trueness) at the per mille level whatever the concentrations and without hardware modification.

Ultrafast multidimensional Laplace NMR for a rapid and sensitive chemical analysis

Traditional nuclear magnetic resonance (NMR) spectroscopy relies on the versatile chemical information conveyed by spectra. To complement conventional NMR, Laplace NMR explores diffusion and relaxation phenomena to reveal details on molecular motions. Under a broad concept of ultrafast multidimensional Laplace NMR, here we introduce an ultrafast diffusion-relaxation correlation experiment enhancing the resolution and information content of corresponding 1D experiments as well as reducing the experiment time by one to two orders of magnitude or more as compared with its conventional 2D counterpart. We demonstrate that the method allows one to distinguish identical molecules in different physical environments and provides chemical resolution missing in NMR spectra. Although the sensitivity of the new method is reduced due to spatial encoding, the single-scan approach enables one to use hyperpolarized substances to boost the sensitivity by several orders of magnitude, significantly enhancing the overall sensitivity of multidimensional Laplace NMR.

Laplace NMR provides complementary information to traditional NMR, such as details of molecular motion. Here, the authors report a correlation experiment capable of providing information on the physical environment of molecules while enhancing the chemical resolution and greatly reducing the experiment times.

Effects of radiation damping for biomolecular NMR experiments in solution: a hemisphere concept for water suppression

Abundant solvent nuclear spins, such as water protons in aqueous solution, cause radiation damping in NMR experiments. It is important to know how the effect of radiation damping appears in high-resolution protein NMR because macromolecular studies always require very high magnetic field strengths with a highly sensitive NMR probe that can easily cause radiation damping. Here, we show the behavior of water magnetization after a pulsed-field gradient (PFG) using nutation experiments at 900 MHz with a cryogenic probe: when water magnetization is located in the upper hemisphere (having +Z component, parallel to the external magnetic field), dephasing of the magnetization by a PFG effectively suppresses residual water magnetization in the transverse plane. In contrast, when magnetization is located in the lower hemisphere (having -Z component), the small residual transverse component remaining after a PFG is still sufficient to induce radiation damping. Based on this observation, we designed ¹H-¹⁵N HSQC experiments in which water magnetization is maintained in the upper hemisphere, but not necessarily along Z, and compared them with the conventional experiments, in which water magnetization is inverted during the t₁ period. The result demonstrates moderate gain of signal-to-noise ratio, 0-28%. Designing the experiments such that water magnetization is maintained in the upper hemisphere allows shorter pulses to be used compared to the complete water flip-back and, thereby, is useful as a building block of protein NMR pulse programs in solution.

Internal Referencing for ¹³C Position-Specific Isotope Analysis Measured by NMR Spectrometry

The intramolecular (13)C composition of a molecule retains evidence relevant to its (bio)synthetic history and can provide valuable information in numerous fields ranging from biochemistry to environmental sciences. Isotope ratio monitoring by (13)C NMR spectrometry (irm-(13)C NMR) is a generic method that offers the potential to conduct (13)C position-specific isotope analysis with a precision better than 1‰. Until now, determining absolute values also required measurement of the global (or bulk) (13)C composition (δ(13)Cg) by mass spectrometry. In a radical new approach, it is shown that an internal isotopic chemical reference for irm-(13)C NMR can be used instead. The strategy uses (1)H NMR to quantify both the number of moles of the reference and of the studied compound present in the NMR tube. Thus, the sample preparation protocol is greatly simplified, bypassing the previous requirement for precise purity and mass determination. The key to accurate results is suppressing the effect of radiation damping in (1)H NMR which produces signal distortion and alters quantification. The methodology, applied to vanillin with dimethylsulfone as an internal standard, has an equivalent accuracy (<1‰) to that of the conventional approach. Hence, it was possible to clearly identify vanillin from different origins based on the (13)C isotopic profiles.