Perspectives on DNP-enhanced NMR spectroscopy in solutions

Overhauser enhanced liquid state nuclear magnetic resonance spectroscopy in one and two dimensions

Nuclear magnetic resonance (NMR) is fundamental in the natural sciences, from chemical analysis and structural biology, to medicine and physics. Despite its enormous achievements, one of its most severe limitations is the low sensitivity, which arises from the small population difference of nuclear spin states. Methods such as dissolution dynamic nuclear polarization and parahydrogen induced hyperpolarization can enhance the NMR signal by several orders of magnitude, however, their intrinsic limitations render multidimensional hyperpolarized liquid-state NMR a challenge. Here, we report an instrumental design for 9.4 Tesla liquid-state dynamic nuclear polarization that enabled enhanced high-resolution NMR spectra in one and two-dimensions for small molecules, including drugs and metabolites. Achieved enhancements of up to two orders of magnitude translate to signal acquisition gains up to a factor of 10,000. We show that hyperpolarization can be transferred between nuclei, allowing DNP-enhanced two-dimensional 13C-13C correlation experiments at 13C natural abundance. The enhanced sensitivity opens up perspectives for structural determination of natural products or characterization of drugs, available in small quantities. The results provide a starting point for a broader implementation of DNP in liquid-state NMR.

Possibilities and limitations of solution-state NMR spectroscopy to analyze the ligand shell of ultrasmall metal nanoparticles

Ultrasmall nanoparticles have a diameter between 1 and 3 nm at the border between nanoparticles and large molecules. Usually, their core consists of a metal, and the shell of a capping ligand with sulfur or phosphorus as binding atoms. While the core structure can be probed by electron microscopy, electron and powder diffraction, and single-crystal structure analysis for atom-sharp clusters, it is more difficult to analyze the ligand shell. In contrast to larger nanoparticles, ultrasmall nanoparticles cause only a moderate distortion of the NMR signal, making NMR spectroscopy a qualitative as well as a quantitative probe to assess the nature of the ligand shell. The application of isotope-labelled ligands and of two-dimensional NMR techniques can give deeper insight into ligand-nanoparticle interactions. Applications of one- and two-dimensional NMR spectroscopy to analyze ultrasmall nanoparticles are presented with suitable examples, including a critical discussion of the limitations of NMR spectroscopy on nanoparticles.

Unified understanding of the breakdown of thermal mixing dynamic nuclear polarization: The role of temperature and radical concentration

We reveal an interplay between temperature and radical concentration necessary to establish thermal mixing (TM) as an efficient dynamic nuclear polarization (DNP) mechanism. We conducted DNP experiments by hyperpolarizing widely used DNP samples, i.e., sodium pyruvate-1-13C in water/glycerol mixtures at varying nitroxide radical (TEMPOL) concentrations and microwave irradiation frequencies, measuring proton and carbon-13 spin temperatures. Using a cryogen consumption-free prototype-DNP apparatus, we could probe cryogenic temperatures between 1.5 and 6.5 K, i.e., below and above the boiling point of liquid helium. We identify two mechanisms for the breakdown of TM: (i) Anderson type of quantum localization for low radical concentration, or (ii) quantum Zeno localization occurring at high temperature. This observation allowed us to reconcile the recent diverging observations regarding the relevance of TM as a DNP mechanism by proposing a unifying picture and, consequently, to find a trade-off between radical concentration and electron relaxation times, which offers a pathway to improve experimental DNP performance based on TM.

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.

Metabolomics and NMR

The purpose of this manuscript will be to convince the reader to dive deeper into NMR spectroscopy and prevent the technique from being just another "black-box" in the lab. We will try to concisely highlight interesting topics and supply additional references for further exploration at each stage. The advantages of delving into the technique will be shown. The secondary objective, i.e., avoiding common problems before starting, will hopefully then become clear. Lastly, we will emphasize the spectrometer information needed for manuscript reporting to allow reproduction of results and confirm findings.

Hyperpolarized water as universal sensitivity booster in biomolecular NMR

NMR spectroscopy is the only method to access the structural dynamics of biomolecules at high (atomistic) resolution in their native solution state. However, this method's low sensitivity has two important consequences: (i) typically experiments have to be performed at high concentrations that increase sensitivity but are not physiological, and (ii) signals have to be accumulated over long periods, complicating the determination of interaction kinetics on the order of seconds and impeding studies of unstable systems. Both limitations are of equal, fundamental relevance: non-native conditions are of limited pharmacological relevance, and the function of proteins, enzymes and nucleic acids often relies on their interaction kinetics. To overcome these limitations, we have developed applications that involve 'hyperpolarized water' to boost signal intensities in NMR of proteins and nucleic acids. The technique includes four stages: (i) preparation of the biomolecule in partially deuterated buffers, (ii) preparation of 'hyperpolarized' water featuring enhanced ¹H NMR signals via cryogenic dynamic nuclear polarization, (iii) sudden melting of the cryogenic pellet and dissolution of the protein or nucleic acid in the hyperpolarized water (enabling spontaneous exchanges of protons between water and target) and (iv) recording signal-amplified NMR spectra targeting either labile ¹H or neighboring ¹⁵N/¹³C nuclei in the biomolecule. Water in the ensuing experiments is used as a universal 'hyperpolarization' agent, rendering the approach versatile and applicable to any biomolecule possessing labile hydrogens. Thus, questions can be addressed, ranging from protein and RNA folding problems to resolving structure-function relationships of intrinsically disordered proteins to investigating membrane interactions.

Synergies between Hyperpolarized NMR and Microfluidics: A Review

Hyperpolarized nuclear magnetic resonance and lab-on-a-chip microfluidics are two dynamic, but until recently quite distinct, fields of research. Recent developments in both areas increased their synergistic overlap. By microfluidic integration, many complex experimental steps can be brought together onto a single platform. Microfluidic devices are therefore increasingly finding applications in medical diagnostics, forensic analysis, and biomedical research. In particular, they provide novel and powerful ways to culture cells, cell aggregates, and even functional models of entire organs. Nuclear magnetic resonance is a non-invasive, high-resolution spectroscopic technique which allows real-time process monitoring with chemical specificity. It is ideally suited for observing metabolic and other biological and chemical processes in microfluidic systems. However, its intrinsically low sensitivity has limited its application. Recent advances in nuclear hyperpolarization techniques may change this: under special circumstances, it is possible to enhance NMR signals by up to 5 orders of magnitude, which dramatically extends the utility of NMR in the context of microfluidic systems. Hyperpolarization requires complex chemical and/or physical manipulations, which in turn may benefit from microfluidic implementation. In fact, many hyperpolarization methodologies rely on processes that are more efficient at the micro-scale, such as molecular diffusion, penetration of electromagnetic radiation into a sample, or restricted molecular mobility on a surface. In this review we examine the confluence between the fields of hyperpolarization-enhanced NMR and microfluidics, and assess how these areas of research have mutually benefited one another, and will continue to do so.

A cryogen-free, semi-automated apparatus for bullet-dynamic nuclear polarization with improved resolution

In dissolution-dynamic nuclear polarization, a hyperpolarized solid is dissolved with a jet of hot solvent. The solution is then transferred to a secondary magnet, where spectra can be recorded with improved sensitivity. In bullet-dynamic nuclear polarization this order is reversed. Pressurized gas is used to rapidly transfer the hyperpolarized solid to the secondary magnet, and the hyperpolarized solid is dissolved only upon arrival. A potential advantage of this approach is that it may avoid excessive dilution and the associated signal loss, in particular for small sample quantities. Previously, we have shown that liquid-state NMR spectra with polarization levels of up to 30 % may be recorded within less than 1 s after the departure of the hyperpolarized solid from the polarizing magnet. The resolution of the recorded spectra however was limited. The system consumed significant amounts of liquid helium, and substantial manual work was required in between experiments to prepare for the next shot. Here, we present a new bullet-DNP (dynamic nuclear polarization) system that addresses these limitations.

NMR spectroscopy of wastewater: A review, case study, and future potential

NMR spectroscopy is arguably the most powerful tool for the study of molecular structures and interactions, and is increasingly being applied to environmental research, such as the study of wastewater. With over 97% of the planet's water being saltwater, and two thirds of freshwater being frozen in the ice caps and glaciers, there is a significant need to maintain and reuse the remaining 1%, which is a precious resource, critical to the sustainability of most life on Earth. Sanitation and reutilization of wastewater is an important method of water conservation, especially in arid regions, making the understanding of wastewater itself, and of its treatment processes, a highly relevant area of environmental research. Here, the benefits, challenges and subtleties of using NMR spectroscopy for the analysis of wastewater are considered. First, the techniques available to overcome the specific challenges arising from the nature of wastewater (which is a complex and dilute matrix), including an examination of sample preparation and NMR techniques (such as solvent suppression), in both the solid and solution states, are discussed. Then, the arsenal of available NMR techniques for both structure elucidation (e.g., heteronuclear, multidimensional NMR, homonuclear scalar coupling-based experiments) and the study of intermolecular interactions (e.g., diffusion, nuclear Overhauser and saturation transfer-based techniques) in wastewater are examined. Examples of wastewater NMR studies from the literature are reviewed and potential areas for future research are identified. Organized by nucleus, this review includes the common heteronuclei (¹³C, ¹⁵N, ¹⁹F, ³¹P, ²⁹Si) as well as other environmentally relevant nuclei and metals such as ²⁷Al, ⁵¹V, ²⁰⁷Pb and ¹¹³Cd, among others. Further, the potential of additional NMR methods such as comprehensive multiphase NMR, NMR microscopy and hyphenated techniques (for example, LC-SPE-NMR-MS) for advancing the current understanding of wastewater are discussed. In addition, a case study that combines natural abundance (i.e. non-concentrated), targeted and non-targeted NMR to characterize wastewater, along with in vivo based NMR to understand its toxicity, is included. The study demonstrates that, when applied comprehensively, NMR can provide unique insights into not just the structure, but also potential impacts, of wastewater and wastewater treatment processes. Finally, low-field NMR, which holds considerable future potential for on-site wastewater monitoring, is briefly discussed. In summary, NMR spectroscopy is one of the most versatile tools in modern science, with abilities to study all phases (gases, liquids, gels and solids), chemical structures, interactions, interfaces, toxicity and much more. The authors hope this review will inspire more scientists to embrace NMR, given its huge potential for both wastewater analysis in particular and environmental research in general.

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.

Challenges and Advances in the Application of Dynamic Nuclear Polarization to Liquid-State NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy is a powerful method to study the molecular structure and dynamics of materials. The inherently low sensitivity of NMR spectroscopy is a consequence of low spin polarization. Hyperpolarization of a spin ensemble is defined as a population difference between spin states that far exceeds what is expected from the Boltzmann distribution for a given temperature. Dynamic nuclear polarization (DNP) can overcome the relatively low sensitivity of NMR spectroscopy by using a paramagnetic matrix to hyperpolarize a nuclear spin ensemble. Application of DNP to NMR can result in sensitivity gains of up to four orders of magnitude compared to NMR without DNP. Although DNP NMR is now more routinely utilized for solid-state (ss) NMR spectroscopy, it has not been exploited to the same degree for liquid-state samples. This Review will consider challenges and advances in the application of DNP NMR to liquid-state samples. The Review is organized into four sections: (i) mechanisms of DNP NMR relevant to hyperpolarization of liquid samples; (ii) applications of liquid-state DNP NMR; (iii) available detection schemes for liquid-state samples; and (iv) instrumental challenges and outlook for liquid-state DNP NMR.

Viscosity effects on optically generated electron and nuclear spin hyperpolarization

Spin hyperpolarization can dramatically increase signal intensities in magnetic resonance experiments, providing either improved bulk sensitivity or additional spectroscopic detail through selective enhancements. While typical hyperpolarization approaches have utilized microwave irradiation, one emerging route is the use of optically generated triplet states. We report an investigation into the effects of solution viscosity on radical-triplet pair interactions, propose a new standard for quantification of the hyperpolarization in EPR experiments, and demonstrate a significant increase in the optically generated 1H NMR signal enhancement upon addition of glycerol to aqueous solutions.

High-resolution Overhauser dynamic nuclear polarization enhanced proton NMR spectroscopy at low magnetic fields

Dynamic nuclear polarization (DNP) has gained large interest due to its ability to increase signal intensities in nuclear magnetic resonance (NMR) experiments by several orders of magnitude. Currently, DNP is typically used to enhance high-field, solid-state NMR experiments. However, the method is also capable of dramatically increasing the observed signal intensities in solution-state NMR spectroscopy. In this work, we demonstrate the application of Overhauser dynamic nuclear polarization (ODNP) spectroscopy at an NMR frequency of 14.5 MHz (0.35 T) to observe DNP-enhanced high-resolution NMR spectra of small molecules in solutions. Using a compact hybrid magnet with integrated shim coils to improve the magnetic field homogeneity we are able to routinely obtain proton linewidths of less than 4 Hz and enhancement factors >30. The excellent field resolution allows us to perform chemical-shift resolved ODNP experiments on ethyl crotonate to observe proton J-coupling. Furthermore, recording high-resolution ODNP-enhanced NMR spectra of ethylene glycol allows us to characterize the microwave induced sample heating in-situ, by measuring the separation of the OH and CH2 proton peaks.

Nitroxide Derivatives for Dynamic Nuclear Polarization in Liquids: The Role of Rotational Diffusion

Polarization transfer efficiency in liquid-state dynamic nuclear polarization (DNP) depends on the interaction between polarizing agents (PAs) and target nuclei modulated by molecular motions. We show how translational and rotational diffusion differently affect the DNP efficiency. These contributions were disentangled by measuring ¹H-DNP enhancements of toluene and chloroform doped with nitroxide derivatives at 0.34 T as a function of either the temperature or the size of the PA. The results were employed to analyze ¹³C-DNP data at higher fields, where the polarization transfer is also driven by the Fermi contact interaction. In this case, bulky nitroxide PAs perform better than the small TEMPONE radical due to structural fluctuations of the ring conformation. These findings will help in designing PAs with features specifically optimized for liquid-state DNP at various magnetic fields.

High-Field Dynamic Nuclear Polarization

Dynamic nuclear polarization (DNP) is one of the most prominent methods of sensitivity enhancement in nuclear magnetic resonance (NMR). Even though solid-state DNP under magic-angle spinning (MAS) has left the proof-of-concept phase and has become an important tool for structural investigations of biomolecules as well as materials, it is still far from mainstream applicability because of the potentially overwhelming combination of unique instrumentation, complex sample preparation, and a multitude of different mechanisms and methods available. In this review, I introduce the diverse field and history of DNP, combining aspects of NMR and electron paramagnetic resonance. I then explain the general concepts and detailed mechanisms relevant at high magnetic field, including solution-state methods based on Overhauser DNP but with a greater focus on the more established MAS DNP methods. Finally, I review practical considerations and fields of application and discuss future developments.

Spatially resolved NMR spectroscopy of heterogeneous gas phase hydrogenation of 1,3-butadiene with parahydrogen

Glass tube reactors with Pd, Pt, Rh or Ir nanoparticles dispersed on a thin layer of TiO₂, CeO₂, SiO₂ or Al₂O₃ provided mechanistic insight into the hydrogenation of 1,3-butadiene using parahydrogen.

In Situ SABRE Hyperpolarization with Earth's Field NMR Detection

Hyperpolarization methods, which increase the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), have the potential to expand the range of applications of these powerful analytical techniques and to enable the use of smaller and cheaper devices. The signal amplification by reversible exchange (SABRE) method is of particular interest because it is relatively low-cost, straight-forward to implement, produces high-levels of renewable signal enhancement, and can be interfaced with low-cost and portable NMR detectors. In this work, we demonstrate an in situ approach to SABRE hyperpolarization that can be achieved using a simple, commercially-available Earth’s field NMR detector to provide ¹H polarization levels of up to 3.3%. This corresponds to a signal enhancement over the Earth’s magnetic field by a factor of ε > 2 × 10⁸. The key benefit of our approach is that it can be used to directly probe the polarization transfer process at the heart of the SABRE technique. In particular, we demonstrate the use of in situ hyperpolarization to observe the activation of the SABRE catalyst, the build-up of signal in the polarization transfer field (PTF), the dependence of the hyperpolarization level on the strength of the PTF, and the rate of decay of the hyperpolarization in the ultra-low-field regime.

Dehydration of fructose over thiol- and sulfonic- modified alumina in a continuous reactor for 5-HMF production: Study of catalyst stability by NMR

In the present study, the synthesis of an organic group-modified alumina by the sol-gel method is proposed. This material has shown to have an enhanced catalytic performance with grafted organic groups and showed an improved stability. The prepared material has shown to have several O - H groups and an enhanced surface acidity. The alumina acidity was improved by incorporating thiol groups by grafting method, which promotes the tautomerization of fructose to its furanose form. Furthermore, the grafting of sulfonic groups catalyzes its dehydration. The modified alumina was thermally treated up to 200 °C to improve the functional groups stability. After, this modified material was packed into a continuous reactor system, designed and built by this group, to obtain 5-hydroxymethylfurfural (5-HMF) from fructose dissolved in a single-phase solution of tetrahydrofuran (THF) and H2O (4:1 w/w). The catalytic activity of this material was evaluated by the reaction of fructose dehydration at different reaction temperatures (60, 70, 80 and 90 °C). Fructose conversion and selectivity toward 5-HMF were determined by high performance liquid chromatography (HPLC), obtaining 95% and 73% respectively for the highest temperature. The catalyst showed an efficient stability after 24 hours in continuous flow at 70 °C. The loss of sulfur content was 15%, but the fructose conversion yield and the selectivity to 5-HMF after 24 hours of continuous reaction did not undergo significant changes (less than 5%). The nuclear magnetic resonance (NMR) tests confirmed the presence of the thiol and sulfonic groups before and after 24 hours of reaction, as well as the conservation of the same structure, demonstrating the efficient catalytic performance of the material. The catalysts were characterized by nitrogen adsorption/desorption, X-ray diffraction and infrared (IR) spectroscopy. Also, before and after use by utilizing elemental analysis and  1 H - 13 C cross-polarization magic-angle spinning (CPMAS) and dynamic-nuclear polarization (DNP)-enhanced  1 H - 13 C and  1 H - 29 Si CPMAS as well as directly excited  29 Si magic-angle spinning (MAS) NMR methods in solid-state.

Advances in the cellular structural biology of nucleic acids

Conventional biophysical and chemical biology approaches for delineating relationships between the structure and biological function of nucleic acids (NAs) abstract NAs from their native biological context. However, cumulative experimental observations have revealed that the structure, dynamics and interactions of NAs might be strongly influenced by a broad spectrum of specific and nonspecific physical-chemical environmental factors. This consideration has recently sparked interest in the development of novel tools for structural characterization of NAs in the native cellular context. Here, we review the individual methods currently being employed for structural characterization of NA structure in a native cellular environment with a focus on recent advances and developments in the emerging fields of in-cell NMR and electron paramagnetic resonance spectroscopy and in-cell single-molecule FRET of NAs.

High-Field Liquid-State Dynamic Nuclear Polarization in Microliter Samples

Nuclear hyperpolarization in the liquid state by dynamic nuclear polarization (DNP) has been of great interest because of its potential use in NMR spectroscopy of small samples of biological and chemical compounds in aqueous media. Liquid state DNP generally requires microwave resonators in order to generate an alternating magnetic field strong enough to saturate electron spins in the solution. As a consequence, the sample size is limited to dimensions of the order of the wavelength, and this restricts the sample volume to less than 100 nL for DNP at 9 T (∼260 GHz). We show here a new approach that overcomes this sample size limitation. Large saturation of electron spins was obtained with a high-power (∼150 W) gyrotron without microwave resonators. Since high power microwaves can cause serious dielectric heating in polar solutions, we designed a planar probe which effectively alleviates dielectric heating. A thin liquid sample of 100 μm of thickness is placed on a block of high thermal conductivity aluminum nitride, with a gold coating that serves both as a ground plane and as a heat sink. A meander or a coil were used for NMR. We performed ¹H DNP at 9.2 T (∼260 GHz) and at room temperature with 10 μL of water, a volume that is more than 100× larger than reported so far. The ¹H NMR signal is enhanced by a factor of about -10 with 70 W of microwave power. We also demonstrated the liquid state of ³¹P DNP in fluorobenzene containing triphenylphosphine and obtained an enhancement of ∼200.

High-resolution microstrip NMR detectors for subnanoliter samples

We present the numerical optimization and experimental characterization of two microstrip-based nuclear magnetic resonance (NMR) detectors. The first detector, introduced in our previous work, was a flat wire detector with a strip resting on a substrate, and the second detector was created by adding a ground plane on top of the strip conductor, separated by a sample-carrying capillary and a thin layer of insulator. The dimensional parameters of the detectors were optimized using numerical simulations with regards to radio frequency (RF) sensitivity and homogeneity, with particular attention given to the effect of the ground plane. The influence of copper surface finish and substrate surface on the spectral resolution was investigated, and a resolution of 0.8-1.5 Hz was obtained on 1 nL deionized water depending on sample positioning. For 0.13 nmol sucrose (0.2 M in 0.63 nL H₂O) encapsulated between two Fluorinert plugs, high RF homogeneity (A_810°/A_90° = 70-80%) and high sensitivity (expressed in the limit of detection nLOD_m = 0.73-1.21 nmol s^1/2) were achieved, allowing for high-performance 2D NMR spectroscopy of subnanoliter samples.

Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR

The field of dynamic nuclear polarization has undergone tremendous developments and diversification since its inception more than 6 decades ago. In this review we provide an in-depth overview of the relevant topics involved in DNP-enhanced MAS NMR spectroscopy. This includes the theoretical description of DNP mechanisms as well as of the polarization transfer pathways that can lead to a uniform or selective spreading of polarization between nuclear spins. Furthermore, we cover historical and state-of-the art aspects of dedicated instrumentation, polarizing agents, and optimization techniques for efficient MAS DNP. Finally, we present an extensive overview on applications in the fields of structural biology and materials science, which underlines that MAS DNP has moved far beyond the proof-of-concept stage and has become an important tool for research in these fields.

500-fold enhancement of in situ (13)C liquid state NMR using gyrotron-driven temperature-jump DNP

A 550-fold increase in the liquid state (13)C NMR signal of a 50μL sample was obtained by first hyperpolarizing the sample at 20K using a gyrotron (260GHz), then, switching its frequency in order to apply 100W for 1.5s so as to melt the sample, finally, turning off the gyrotron to acquire the (13)C NMR signal. The sample stays in its NMR resonator, so the sequence can be repeated with rapid cooling as the entire cryostat stays cold. DNP and thawing of the sample are performed only by the switchable and tunable gyrotron without external devices. Rapid transition from DNP to thawing in one second time scale was necessary especially in order to enhance liquid (1)H NMR signal.

Optically generated hyperpolarization for sensitivity enhancement in solution-state NMR spectroscopy

Using optical excitation to generate radical triplet pairs the sensitivity of solution-state NMR can be enhanced without microwave pumping.