Ultra-low temperature MAS-DNP

Rational Design of Dinitroxide Polarizing Agents for Dynamic Nuclear Polarization to Enhance Overall NMR Sensitivity

We evaluate the overall sensitivity gains provided by a series of eighteen nitroxide biradicals for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and 100 K, including eight new biradicals. We find that in the best performing group the factors contributing to the overall sensitivity gains, namely the DNP enhancement, the build-up time, and the contribution factor, often compete with each other leading to very similar overall sensitivity across a range of biradicals. NaphPol and HydroPol are found to provide the best overall sensitivity factors, in organic and aqueous solvents respectively. One of the new biradicals, AMUPolCbm, provides high sensitivity for all three solvent formulations measured here, and can be considered to be a "universal" polarizing agent.

AsymPol-TEKs as efficient polarizing agents for MAS-DNP in glass matrices of non-aqueous solvents

Two polarizing agents from the AsymPol family, AsymPol-TEK and cAsymPol-TEK (methyl-free version) are introduced for MAS-DNP applications in non-aqueous solvents. The performance of these new biradicals is rationalized in detail using a combination of electron paramagnetic resonance spectroscopy, density functional theory, molecular dynamics and quantitative MAS-DNP spin dynamics simulations. By slightly modifying the experimental protocol to keep the sample temperature low at insertion, we are able to obtain reproducable DNP-NMR data with 1,1,2,2-tetrachloroethane (TCE) at 100 K, which facilitates optimization and comparison of different polarizing agents. At intermediate magnetic fields, AsymPol-TEK and cAsymPol-TEK provide 1.5 to 3-fold improvement in sensitivity compared to TEKPol, one of the most widely used polarizing agents for organic solvents, with significantly shorter DNP build-up times of ∼1 s and ∼2 s at 9.4 and 14.1 T respectively. In the course of the work, we also isolated and characterized two diastereoisomers that can form during the synthesis of AsymPol-TEK; their difference in performance is described and discussed. Finally, the advantages of the AsymPol-TEKs are demonstrated by recording 2D 13C-13C correlation experiments at natural 13C-abundance of proton-dense microcrystals and by polarizing the surface of ZnO nanocrystals (NCs) coated with diphenyl phosphate ligands. For those experiments, cAsymPol-TEK yielded a three-fold increase in sensitivity compared to TEKPol, corresponding to a nine-fold time saving.

Sustainable and cost-effective MAS DNP-NMR at 30 K with cryogenic sample exchange

We report here instrumental developments to achieve sustainable, cost-effective cryogenic Helium sample spinning in order to conduct dynamic nuclear polarisation (DNP) and solid-state NMR (ssNMR) at ultra-low temperatures (<30 K). More specifically, we describe an efficient closed-loop helium system composed of a powerful heat exchanger (95% efficient), a single cryocooler, and a single helium compressor to power the sample spinning and cooling. The system is integrated with a newly designed triple-channel NMR probe that minimizes thermal losses without compromising the radio frequency (RF) performance and spinning stability (±0.05%). The probe is equipped with an innovative cryogenic sample exchange system that allows swapping samples in minutes without introducing impurities in the closeloop system. We report that significant gain in sensitivity can be obtained at 30-40 K on large micro-crystalline molecules with unfavorable relaxation timescales, making them difficult or impossible to polarize at 100 K. We also report rotor-synchronized 2D experiments to demonstrate the stability of the system.

Design of a cryogen-free high field dual EPR and DNP probe

We present the design and construction of a cryogen free, dual electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) probe for novel dynamic nuclear polarization (DNP) experiments and concurrent "in situ" analysis of DNP mechanisms. We focus on the probe design that meets the balance between EPR, NMR, and low temperature performance, while maintaining a high degree of versatility: allowing multi-nuclear NMR detection as well as broadband DNP/EPR excitation/detection. To accomplish high NMR/EPR performance, we implement a novel inductively coupled double resonance NMR circuit (¹H-¹³C) in a solid state probe operating at cryogenic temperatures. The components of the circuit were custom built to provide maximum NMR performance, and the physical layout of this circuit was numerically optimized via magnetic field simulations to allow maximum microwave transmission to the sample for optimal EPR performance. Furthermore this probe is based around a cryogen free gas exchange cryostat and has been designed to allow unlimited experiment times down to 8.5 Kelvin with minimal cost. The affordability of EPR/DNP experiment is an extremely important aspect for broader impact with magnetic resonance measurements. The purpose of this article is to provide as complete information as we have available for others with interest in building a dual DNP/EPR instrument based around a cryogen-free cryostat.

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.

Biradical Polarizing Agents at High Fields

The sensitivity enhancements available from dynamic nuclear polarization (DNP) are rapidly reshaping the research landscape and expanding the field of nuclear magnetic resonance (NMR) spectroscopy as a tool for solving complex chemical and structural problems. The past decade has seen considerable advances in this burgeoning method, while efforts to further improve its capabilities continue along many avenues. In this report, we examine the influence of static magnetic field strength and temperature on the reported ¹H DNP enhancements from three conventional organic biradicals: TOTAPOL, AMUPol, and SPIROPOL. In contrast to the conventional wisdom, our findings show that at liquid nitrogen temperatures and 700 MHz/460.5 GHz, these three bisnitroxides all provide similar ¹H DNP enhancements, ε ≈ 60. Furthermore, we investigate the influence of temperature, microwave power, magnetic field strength, and protein sample deuteration on the NMR experimental results.

Magic angle spinning dynamic nuclear polarization solid-state NMR spectroscopy of γ-irradiated molecular organic solids

In the past 15 years, magic angle spinning (MAS) dynamic nuclear polarization (DNP) has emerged as a method to increase the sensitivity of high-resolution solid-state NMR spectroscopy experiments. Recently, γ-irradiation has been used to generate significant concentrations of homogeneously distributed free radicals in a variety of solids, including quartz, glucose, and cellulose. Both γ-irradiated quartz and glucose previously showed significant MAS DNP enhancements. Here, γ-irradiation is applied to twelve small organic molecules to test the applicability of γ-irradiation as a general method of creating stable free radicals for MAS DNP experiments on organic solids and pharmaceuticals. Radical concentrations in the range of 0.25 ​mM-10 ​mM were observed in irradiated glucose, histidine, malic acid, and malonic acid, and significant ¹H DNP enhancements of 32, 130, 19, and 11 were obtained, respectively, as measured by ¹H→¹³C CPMAS experiments. However, concentrations of free radicals below 0.05 ​mM were generally observed in organic molecules containing aromatic rings, preventing sizeable DNP enhancements. DNP sensitivity gains for several of the irradiated compounds exceed that which can be obtained with the relayed DNP approach that uses exogeneous polarizing agent solutions and impregnation procedures. In several cases, significant ¹H DNP enhancements were realized at room temperature. This study demonstrates that in many cases γ-irradiation is a viable alternative to addition of stable exogenous radicals for DNP experiments on organic solids.

Biomolecular and Biological Applications of Solid-State NMR with Dynamic Nuclear Polarization Enhancement

Solid-state NMR spectroscopy (ssNMR) with magic-angle spinning (MAS) enables the investigation of biological systems within their native context, such as lipid membranes, viral capsid assemblies, and cells. However, such ambitious investigations often suffer from low sensitivity due to the presence of significant amounts of other molecular species, which reduces the effective concentration of the biomolecule or interaction of interest. Certain investigations requiring the detection of very low concentration species remain unfeasible even with increasing experimental time for signal averaging. By applying dynamic nuclear polarization (DNP) to overcome the sensitivity challenge, the experimental time required can be reduced by orders of magnitude, broadening the feasible scope of applications for biological solid-state NMR. In this review, we outline strategies commonly adopted for biological applications of DNP, indicate ongoing challenges, and present a comprehensive overview of biological investigations where MAS-DNP has led to unique insights.

Cryogenic signal amplification combined with helium-temperature MAS DNP toward ultimate NMR sensitivity at high field conditions

The low sensitivity of NMR spectroscopy is of historical concern in the field, and various approaches have been developed to mitigate this limitation. On the shoulder of giants, today one can routinely implement, for example, the pulse/Fourier transform NMR with the cross polarization together with the ultra-low temperature MAS DNP under high-field conditions. We show in this work this current opportunity should further be augmented by combining them with the cryogenic signal amplification. Our presented MAS DNP probe operates with the closed-cycle helium MAS system, and cools the internal preamplifier-duplexer module with the "return" helium gas on its way back to the compressor in the loop. The signal-to-noise (S/N) gain relative to the room-temperature measurements of a factor of 4.6 and 2.4 was found for the measurement using the cold- and room-temperature preamplifier, respectively, at the sample temperature of T = 20 K at B₀ = 16.4 T. The ratio of these factors reveals ∼ two-fold sensitivity improvement that results purely from the introduction of the cold signal amplification, i.e., noise reduction. Together with the increase of the thermal Boltzmann polarization at low temperatures, the combined S/N gain of max. ∼70-fold is possible without DNP. The DNP enhancement factor of ∼40 as we found in this work for a microcrystalline MLF sample may be multiplied to this gain. We also demonstrated the sensitivity improvement with a ¹³C-detected 2D NCaCx spectrum, illustrating the generality of the S/N gain from combining DNP with the cold signal amplification.

Dynamic Nuclear Polarization for Sensitivity Enhancement in Biomolecular Solid-State NMR

Solid-state NMR with magic-angle spinning (MAS) is an important method in structural biology. While NMR can provide invaluable information about local geometry on an atomic scale even for large biomolecular assemblies lacking long-range order, it is often limited by low sensitivity due to small nuclear spin polarization in thermal equilibrium. Dynamic nuclear polarization (DNP) has evolved during the last decades to become a powerful method capable of increasing this sensitivity by two to three orders of magnitude, thereby reducing the valuable experimental time from weeks or months to just hours or days; in many cases, this allows experiments that would be otherwise completely unfeasible. In this review, we give an overview of the developments that have opened the field for DNP-enhanced biomolecular solid-state NMR including state-of-the-art applications at fast MAS and high magnetic field. We present DNP mechanisms, polarizing agents, and sample constitution methods suitable for biomolecules. A wide field of biomolecular NMR applications is covered including membrane proteins, amyloid fibrils, large biomolecular assemblies, and biomaterials. Finally, we present perspectives and recent developments that may shape the field of biomolecular DNP in the future.

Solid-State NMR Investigations of Extracellular Matrixes and Cell Walls of Algae, Bacteria, Fungi, and Plants

Extracellular matrixes (ECMs), such as the cell walls and biofilms, are important for supporting cell integrity and function and regulating intercellular communication. These biomaterials are also of significant interest to the production of biofuels and the development of antimicrobial treatment. Solid-state nuclear magnetic resonance (ssNMR) and magic-angle spinning-dynamic nuclear polarization (MAS-DNP) are uniquely powerful for understanding the conformational structure, dynamical characteristics, and supramolecular assemblies of carbohydrates and other biomolecules in ECMs. This review highlights the recent high-resolution investigations of intact ECMs and native cells in many organisms spanning across plants, bacteria, fungi, and algae. We spotlight the structural principles identified in ECMs, discuss the current technical limitation and underexplored biochemical topics, and point out the promising opportunities enabled by the recent advances of the rapidly evolving ssNMR technology.

Solid-state MAS NMR at ultra low temperature of hydrated alanine doped with DNP radicals

Magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments at ultra low temperature (ULT) (≪ 100 K) have demonstrated clear benefits for obtaining large signal sensitivity gain and probing spin dynamics phenomena at ULT. ULT NMR is furthermore a highly promising platform for solid-state dynamic nuclear polarization (DNP). However, ULT NMR is not widely used, given limited availability of such instrumentation from commercial sources. In this paper, we present a comprehensive study of hydrated [U-¹³C]alanine, a standard bio-solid sample, from the first commercial 14.1 Tesla NMR spectrometer equipped with a closed-cycle helium ULT-MAS system. The closed-cycle helium MAS system provides precise temperature control from 25 K to 100 K and stable MAS from 1.5 kHz to 12 kHz. The ¹³C CP-MAS NMR of [U-¹³C]alanine showed 400% signal gain at 28 K compared with at 100 K. The large sensitivity gain results from the Boltzmann factor, radio frequency circuitry quality factor improvement, and the suppression of its methyl group rotation at ULT. We further observed that the addition of organic biradicals widely used for solid-state DNP significantly shortens the ¹H T₁ spin lattice relaxation time at ULT, without further broadening the ¹³C spectral linewidth compared to at 90 K. The mechanism of ¹H T₁ shortening is dominated by the two-electron-one-nucleus triple flip transition underlying the Cross Effect mechanism, widely relied upon to drive solid-state DNP. Our experimental observations suggest that the prospects of MAS NMR and DNP under ULT conditions established with a closed-cycle helium MAS system are bright.

Targeted DNP for biomolecular solid-state NMR

High-field dynamic nuclear polarization is revolutionizing the scope of solid-state NMR with new applications in surface chemistry, materials science and structural biology. In this perspective article, we focus on a specific DNP approach, called targeted DNP, in which the paramagnets introduced to polarize are not uniformly distributed in the sample but site-specifically located on the biomolecular system. After reviewing the various targeting strategies reported to date, including a bio-orthogonal chemistry-based approach, we discuss the potential of targeted DNP to improve the overall NMR sensitivity while avoiding the use of glass-forming DNP matrix. This is especially relevant to the study of diluted biomolecular systems such as, for instance, membrane proteins within their lipidic environment. We also discuss routes towards extracting structural information from paramagnetic relaxation enhancement (PRE) induced by targeted DNP at cryogenic temperature, and the possibility to recover site-specific information in the vicinity of the paramagnetic moieties using high-resolution selective DNP spectra. Finally, we review the potential of targeted DNP for in-cell NMR studies and how it can be used to extract a given protein NMR signal from a complex cellular background.

Efficiency analysis of helium-cooled MAS DNP: case studies of surface-modified nanoparticles and homogeneous small-molecule solutions

Despite the growing number of successful applications of dynamic nuclear polarization (DNP)-enhanced magic-angle spinning (MAS) NMR in structural biology and materials science, the nuclear polarizations achieved by current MAS DNP instrumentation are still considerably lower than the theoretical maximum. The method could be significantly strengthened if experiments were performed at temperatures much lower than those currently widely used (∼100 K). Recently, the prospects of helium (He)-cooled MAS DNP have been increased with the instrumental developments in MAS technology that uses cold helium gas for sample cooling. Despite the additional gains in sensitivity that have been observed with He-cooled MAS DNP, the performance of the technique has not been evaluated in the case of surfaces and interfaces that benefit the most from DNP. Herein, we studied the efficiency of DNP at temperatures between ∼30 K and ∼100 K for organically functionalized silica material and a homogeneous solution of small organic molecules at a magnetic field B0 = 16.4 T. We recorded the changes in signal enhancement, paramagnet-induced quenching and depolarization effects, DNP build-up rate, and Boltzmann polarization. For these samples, the increases in MAS-induced depolarization and DNP build-up times at around 30 K were not as severe as anticipated. In the case of the surface species, we determined that MAS DNP at 30 K provided ∼10 times higher sensitivity than MAS DNP at 90 K, which corresponds to the acceleration of experiments by multiplicative factors of up to 100.

Combining fast magic angle spinning dynamic nuclear polarization with indirect detection to further enhance the sensitivity of solid-state NMR spectroscopy

Dynamic nuclear polarization (DNP) and indirect detection are two commonly applied approaches for enhancing the sensitivity of solid-state NMR spectroscopy. However, their use in tandem has not yet been investigated. With the advent of low-temperature fast magic angle spinning (MAS) probes with 1.3-mm diameter rotors capable of MAS at 40 ​kHz it becomes feasible to combine these two techniques. In this study, we performed DNP-enhanced 2D indirectly detected heteronuclear correlation (idHETCOR) experiments on ¹³C, ¹⁵N, ¹¹³Cd and ⁸⁹Y nuclei in functionalized mesoporous silica, CdS nanoparticles, and Y₂O₃ nanoparticles. The sensitivity of the 2D idHETCOR experiments was compared with those of DNP-enhanced directly-detected 1D cross polarization (CP) and 2D HETCOR experiments performed with a standard 3.2-mm rotor. Due to low CP polarization transfer efficiencies and large proton linewidth, the sensitivity gains achieved by indirect detection alone were lower than in conventional (non-DNP) experiments. Nevertheless, despite the smaller sample volume the 2D idHETCOR experiments showed better absolute sensitivities than 2D HETCOR experiments for nuclei with the lowest gyromagnetic ratios. For ⁸⁹Y, 2D idHETCOR provided 8.2 times better sensitivity than the 1 D⁸⁹Y-detected CP experiment performed with a 3.2-mm rotor.

Dynamic Nuclear Polarization of Biomembrane Assemblies

While atomic scale structural and dynamic information are hallmarks of nuclear magnetic resonance (NMR) methodologies, sensitivity is a fundamental limitation in NMR studies. Fully exploiting NMR capabilities to study membrane proteins is further hampered by their dilution within biological membranes. Recent developments in dynamic nuclear polarization (DNP), which can transfer the relatively high polarization of unpaired electrons to nuclear spins, show promise for overcoming the sensitivity bottleneck and enabling NMR characterization of membrane proteins under native-like conditions. Here we discuss fundamental aspects of DNP-enhanced solid-state NMR spectroscopy, experimental details relevant to the study of lipid assemblies and incorporated proteins, and sensitivity gains which can be realized in biomembrane-based samples. We also present unique insights which can be gained from DNP measurements and prospects for further development of the technique for elucidating structures and orientations of membrane proteins in native lipid environments.

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.

Optimizing nitroxide biradicals for cross-effect MAS-DNP: the role of g-tensors' distance

Nitroxide biradicals are common polarizing agents used to enhance the sensitivity of solid-state NMR experiments via Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP). These biradicals are used to increase the polarization of protons through the cross-effect mechanism, which requires two unpaired electrons with a Larmor frequency difference greater than that of the protons. From their early conception, the relative orientation of the nitroxide rings has been identified as a critical factor determining their MAS-DNP performance. However, the MAS leads to a complex DNP mechanism with time dependent energy level anti-crossings making it difficult to pinpoint the role of relative g-tensor orientation. In this article, a single parameter called "g-tensors' distance" is introduced to characterize the relative orientation's impact on the MAS-DNP field profiles. It is demonstrated for the first time how the g-tensors' distance determines the nuclear hyperpolarization and depolarization properties of a given biradical. This provides a new critical parameter that paves the way for more efficient bis-nitroxides for MAS-DNP.

Modular, triple-resonance, transmission line DNP MAS probe for 500 MHz/330 GHz

We describe the design and construction of a modular, triple-resonance, fully balanced, DNP-MAS probe based on transmission line technology and its integration into a 500 MHz/330 GHz DNP-NMR spectrometer. A novel quantitative probe design and characterization strategy is developed and employed to achieve optimal sensitivity, RF homogeneity and excellent isolation between channels. The resulting three channel HCN probe has a modular design with each individual, swappable module being equipped with connectorized, transmission line ports. This strategy permits attachment of a mating connector that facilitates accurate impedance measurements at these ports and allows characterization and adjustment (e.g. for balancing or tuning/matching) of each component individually. The RF performance of the probe is excellent; for example, the 13C channel attains a Rabi frequency of 280 kHz for a 3.2 mm rotor. In addition, a frequency tunable 330 GHz gyrotron operating at the second harmonic of the electron cyclotron frequency was developed for DNP applications. Careful alignment of the corrugated waveguide led to minimal loss of the microwave power, and an enhancement factor ε = 180 was achieved for U-13C urea in the glassy matrix at 80 K. We demonstrated the operation of the system with acquisition of multidimensional spectra of cross-linked lysozyme crystals which are insoluble in glycerol-water mixtures used for DNP and samples of RNA.

Fast electron paramagnetic resonance magic angle spinning simulations using analytical powder averaging techniques

Simulations describing the spin physics underpinning nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy play an important role in the design of new experiments. When experiments are performed in the solid state, samples are commonly composed of powders or glasses, with molecules oriented at a large number of angles with respect to the laboratory frame. These powder angles must be represented in simulations to account for anisotropic interactions. Numerical techniques are typically used to accurately compute such powder averages. A large number of Euler angles are usually required, leading to lengthy simulation times. This is particularly true in broad spectra, such as those observed in EPR. The combination of the traditionally separate techniques of EPR and magic angle spinning (MAS) NMR could play an important role in future electron detected experiments, combined with dynamic nuclear polarization, which will allow for exceptional detection sensitivity of NMR spin coherences. Here, we present a method of reducing the required number of Euler angles in magnetic resonance simulations by analytically performing the powder average over one of the Euler angles in the static and MAS cases for the TEMPO nitroxide radical in a 7 T field. In the static case, this leads to a 97.5% reduction in simulation time over the fully numerical case and reproduces the expected spinning sideband manifold when simulated with a MAS frequency of 150 kHz. This technique is applicable to more traditional NMR experiments as well, such as those involving quadrupolar nuclei or multiple dimensions.

Recent developments in MAS DNP-NMR of materials

Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS), at static magnetic field B₀ ≥ 5 T, conditions allowing for the acquisition of high-resolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spurred notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B₀ field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes that have been detected by this technique, and the NMR methods that have been combined with DNP. Finally, we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis, optoelectronics etc.

Natural Isotopic Abundance 13C and 15N Multidimensional Solid-State NMR Enabled by Dynamic Nuclear Polarization

Dynamic nuclear polarization (DNP) has made feasible solid-state NMR experiments that were previously thought impractical due to sensitivity limitations. One such class of experiments is the structural characterization of organic and biological samples at natural isotopic abundance (NA). Herein, we describe the many advantages of DNP-enabled ssNMR at NA, including the extraction of long-range distance constraints using dipolar recoupling pulse sequences without the deleterious effects of dipolar truncation. In addition to the theoretical underpinnings in the analysis of these types of experiments, numerous applications of DNP-enabled ssNMR at NA are discussed.

Dynamic Nuclear Polarization / solid-state NMR of membranes. Thermal effects and sample geometry

Whereas specially designed dinitroxide biradicals, reconstitution protocols, oriented sample geometries and NMR probes have helped to much increase the DNP enhancement factors of membrane samples they still lag considerably behind those obtained from glasses made of protein solutions. Here we show that not only the MAS rotor material but also the distribution of the membrane samples within the NMR rotor have a pronounced effect on the DNP enhancement. These observations are rationalized with the cooling efficiency and the internal properties of the sample, monitored by their T₁ relaxation, microwave ON versus OFF signal intensities and DNP effect. The data are suggestive that for membranes the speed of cooling has a pronounced effect on the membrane properties and concomitantly the distribution of biradicals within the sample.

Sensitivity analysis of magic angle spinning dynamic nuclear polarization below 6 K

Dynamic nuclear polarization (DNP) improves signal-to-noise in nuclear magnetic resonance (NMR) spectroscopy. Signal-to-noise in NMR can be further improved with cryogenic sample cooling. Whereas MAS DNP is commonly performed between 25 and 110 K, sample temperatures below 6 K lead to further improvements in sensitivity. Here, we demonstrate that solid effect MAS DNP experiments at 6 K, using trityl, yield 3.2× more sensitivity compared to 90 K. Trityl with solid effect DNP at 6 K yields substantially more signal to noise than biradicals and cross effect DNP. We also characterize cross effect DNP with AMUPol and TEMTriPol-1 biradicals for DNP magic angle spinning at temperatures below 6 K and 7 Tesla. DNP enhancements determined from microwave on/off intensities are 253 from AMUPol and 49 from TEMTriPol-1. The higher thermal Boltzmann polarization at 6 K compared to 298 K, combined with these enhancements, should result in 10,000× signal gain for AMUPol and 2000× gain for TEMTriPol-1. However, we show that AMUPol reduces signal in the absence of microwaves by 90% compared to 41% by TEMTriPol-1 at 7 Tesla as the result of depolarization and other detrimental paramagnetic effects. AMUPol still yields the highest signal-to-noise improvement per unit time between the cross effect radicals due to faster polarization buildup (T_1DNP = 4.3 s and 36 s for AMUPol and TEMTriPol-1, respectively). Overall, AMUPol results in 2.5× better sensitivity compared to TEMTriPol-1 in MAS DNP experiments performed below 6 K at 7 T. Trityl provides 6.0× more sensitivity than TEMTriPol-1 and 1.9× more than AMUPol at 6 K, thus yielding the greatest signal-to-noise per unit time among all three radicals. A DNP enhancement profile of TEMTriPol-1 recorded with a frequency-tunable custom-built gyrotron oscillator operating at 198 GHz is also included. It is determined that at 7 T below 6 K a microwave power level of 0.6 W incident on the sample is sufficient to saturate the cross effect mechanism using TEMTriPol-1, yet increasing the power level up to 5 W results in higher improvements in DNP sensitivity with AMUPol. These results indicate MAS DNP below 6 K will play a prominent role in ultra-sensitive NMR spectroscopy in the future.

Improved waveguide coupling for 1.3 mm MAS DNP probes at 263 GHz

We consider the geometry of a radially irradiated microwave beam in MAS DNP NMR probes and its impact on DNP enhancement. Two related characteristic features are found to be relevant: (i) the focus of the microwave beam on the DNP MAS sample and (ii) the microwave magnetic field magnitude in the sample. We present a waveguide coupler setup that enables us to significantly improve beam focus and field magnitude in 1.3 mm MAS DNP probes at a microwave frequency of 263 GHz, which results in an increase of the DNP enhancement by a factor of 2 compared to previous standard hardware setups. We discuss the implications of improved coupling and its potential to enable cutting-edge applications, such as pulsed high-field DNP and the use of low-power solid-state microwave sources.

Temperature-Dependent Nuclear Spin Relaxation Due to Paramagnetic Dopants Below 30 K: Relevance to DNP-Enhanced Magnetic Resonance Imaging

Dynamic nuclear polarization (DNP) can increase nuclear magnetic resonance (NMR) signal strengths by factors of 100 or more at low temperatures. In magnetic resonance imaging (MRI), signal enhancements from DNP potentially lead to enhancements in image resolution. However, the paramagnetic dopants required for DNP also reduce nuclear spin relaxation times, producing signal losses that may cancel the signal enhancements from DNP. Here we investigate the dependence of 1H NMR relaxation times, including T1ρ and T2, under conditions of Lee-Goldburg 1H-1H decoupling and pulsed spin locking, on temperature and dopant concentration in frozen solutions that contain the trinitroxide compound DOTOPA. We find that relaxation times become longer at temperatures below 10 K, where DOTOPA electron spins become strongly polarized at equilibrium in a 9.39 T magnetic field. We show that the dependences of relaxation times on temperature and DOTOPA concentration can be reproduced qualitatively (although not quantitatively) by detailed simulations of magnetic field fluctuations due to flip-flop transitions in a system of dipole-coupled electron spin magnetic moments. These results have implications for ongoing attempts to reach submicron resolution in inductively detected MRI at very low temperatures.

Structure determination of supra-molecular assemblies by solid-state NMR: Practical considerations

In the cellular environment, biomolecules assemble in large complexes which can act as molecular machines. Determining the structure of intact assemblies can reveal conformations and inter-molecular interactions that are only present in the context of the full assembly. Solid-state NMR (ssNMR) spectroscopy is a technique suitable for the study of samples with high molecular weight that allows the atomic structure determination of such large protein assemblies under nearly physiological conditions. This review provides a practical guide for the first steps of studying biological supra-molecular assemblies using ssNMR. The production of isotope-labeled samples is achievable via several means, which include recombinant expression, cell-free protein synthesis, extraction of assemblies directly from cells, or even the study of assemblies in whole cells in situ. Specialized isotope labeling schemes greatly facilitate the assignment of chemical shifts and the collection of structural data. Advanced strategies such as mixed, diluted, or segmental subunit labeling offer the possibility to study inter-molecular interfaces. Detailed and practical considerations are presented with respect to first setting up magic-angle spinning (MAS) ssNMR experiments, including the selection of the ssNMR rotor, different methods to best transfer the sample and prepare the rotor, as well as common and robust procedures for the calibration of the instrument. Diagnostic spectra to evaluate the resolution and sensitivity of the sample are presented. Possible improvements that can reduce sample heterogeneity and improve the quality of ssNMR spectra are reviewed.

A versatile custom cryostat for dynamic nuclear polarization supports multiple cryogenic magic angle spinning transmission line probes

Dynamic nuclear polarization (DNP) with cryogenic magic angle spinning (MAS) provides significant improvements in NMR sensitivity, yet presents unique technical challenges. Here we describe a custom cryostat and suite of NMR probes capable of manipulating nuclear spins with multi-resonant radiofrequency circuits, cryogenic spinning below 6 K, sample exchange, and microwave coupling for DNP. The corrugated waveguide and six transfer lines needed for DNP and cryogenic spinning functionality are coupled to the probe from the top of the magnet. Transfer lines are vacuum-jacketed and provide bearing and drive gas, variable temperature fluid, two exhaust pathways, and a sample ejection port. The cryostat thermally isolates the magnet bore, thereby protecting the magnet and increasing cryogen efficiency. This novel design supports cryogenic MAS-DNP performance over an array of probes without altering DNP functionality. We present three MAS probes (two supporting 3.2 mm rotors and one supporting 9.5 mm rotors) interfacing with the single cryostat. Mechanical details, transmission line radio frequency design, and performance of the cryostat and three probes are described.

Maximizing NMR signal per unit time by facilitating the e-e-n cross effect DNP rate

The dynamic nuclear polarization (DNP) efficiency is critically dependent on the properties of the radical, solvent, and solute constituting the sample system. In this study, we focused on the three spin e-e-n cross effect (CE)'s influence on the nuclear longitudinal relaxation time constant T1n, the build-up time constants of nuclear magnetic resonance (NMR) signal, TDNP and DNP-enhancement of NMR signal. The dipolar interaction strength between the electron spins driving the e-e-n process was systematically modulated using mono-, di-, tri-, and dendritic-nitroxide radicals, while maintaining a constant global electron spin concentration of 10 mM. Experimental results showed that an increase in electron spin clustering led to an increased electron spin depolarization, as mapped by electron double resonance (ELDOR), and a dramatically shortened T1n and TDNP time constants under static and magic angle spinning (MAS) conditions. A theoretical analysis reveals that strong e-e interactions, caused by electron spin clustering, increase the CE rate. The three spin e-e-n CE is a hitherto little recognized mechanism for shortening T1n and TDNP in solid-state NMR experiments at cryogenic temperatures, and offers a design principle to enhance the effective CE DNP enhancement per unit time. Fast CE rates will benefit DNP at liquid helium temperatures, or at higher magnetic fields and pulsed DNP, where slow e-e-n polarization transfer rate is a key bottleneck to achieving maximal DNP performance.

Electron decoupling with cross polarization and dynamic nuclear polarization below 6 K

Dynamic nuclear polarization (DNP) can improve nuclear magnetic resonance (NMR) sensitivity by orders of magnitude. Polarizing agents containing unpaired electrons required for DNP can broaden nuclear resonances in the presence of appreciable hyperfine couplings. Here we present the first cross polarization experiments implemented with electron decoupling, which attenuates detrimental hyperfine couplings. We also demonstrate magic angle spinning (MAS) DNP experiments below 6 K, producing unprecedented nuclear spin polarization in rotating solids. ¹³C correlation spectra were collected with MAS DNP below 6 K for the first time. Polarization build-up times with MAS DNP (T_{1DNP, 1H}) of urea in a frozen glassy matrix below 6 K were measured for both the solid effect and the cross effect. Trityl radicals exhibit a T_1DNP (¹H) of 18.7 s and the T_1DNP (¹H) of samples doped with 20 mM AMUPol is only 1.3 s. MAS below 6 K with DNP and electron decoupling is an effective strategy to increase NMR signal-to-noise ratios per transient while retaining short polarization periods.

Pulsed Electron Decoupling and Strategies for Time Domain Dynamic Nuclear Polarization with Magic Angle Spinning

Magic angle spinning (MAS) dynamic nuclear polarization (DNP) is widely used to increase nuclear magnetic resonance (NMR) signal intensity. Frequency-chirped microwaves yield superior control of electron spins and are expected to play a central role in the development of DNP MAS experiments. Time domain electron control with MAS has considerable promise to improve DNP performance at higher fields and temperatures. We have recently demonstrated that pulsed electron decoupling using frequency-chirped microwaves improves MAS DNP experiments by partially attenuating detrimental hyperfine interactions. The continued development of pulsed electron decoupling will enable a new suite of MAS DNP experiments that transfer polarization directly to observed spins. Time domain DNP transfers to nuclear spins in conjunction with pulsed electron decoupling is described as a viable avenue toward DNP-enhanced, high-resolution NMR spectroscopy over a range of temperatures from <6 to 320 K.

Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy

High-resolution solid-state NMR spectroscopy is a powerful tool for the study of organic and inorganic materials because it can directly probe the symmetry and structure at nuclear sites, the connectivity/bonding of atoms and precisely measure interatomic distances. However, NMR spectroscopy is hampered by intrinsically poor sensitivity; consequently, the application of NMR spectroscopy to many solid materials is often infeasible. High-field dynamic nuclear polarization (DNP) has emerged as a technique to routinely enhance the sensitivity of solid-state NMR experiments by 1-3 orders of magnitude. This Perspective gives a general overview of how DNP-enhanced solid-state NMR spectroscopy can be applied to a variety of inorganic and organic materials. DNP-enhanced solid-state NMR experiments provide unique insights into the molecular structure, which makes it possible to form structure-activity relationships that ultimately assist in the rational design and improvement of materials.

Magic angle spinning NMR with metallized rotors as cylindrical microwave resonators

We introduce a novel design for millimeter wave electromagnetic structures within magic angle spinning (MAS) rotors. In this demonstration, a copper coating is vacuum deposited onto the outside surface of a sapphire rotor at a thickness of 50 nm. This thickness is sufficient to reflect 197-GHz microwaves, yet not too thick as to interfere with radiofrequency fields at 300 MHz or prevent sample spinning due to eddy currents. Electromagnetic simulations of an idealized rotor geometry show a microwave quality factor of 148. MAS experiments with sample rotation frequencies of ω_r /2π = 5.4 kHz demonstrate that the drag force due to eddy currents within the copper does not prevent sample spinning. Spectra of sodium acetate show resolved ¹³ C J-couplings of 60 Hz and no appreciable broadening between coated and uncoated sapphire rotors, demonstrating that the copper coating does not prevent shimming and high-resolution nuclear magnetic resonance spectroscopy. Additionally, ¹³ C Rabi nutation curves of ω₁ /2π = 103 kHz for both coated and uncoated rotors indicate no detrimental impact of the copper coating on radio frequency coupling of the nuclear spins to the sample coil. We present this metal coated rotor as a first step towards an MAS resonator. MAS resonators are expected to have a significant impact on developments in electron decoupling, pulsed dynamic nuclear polarization (DNP), room temperature DNP, DNP with low-power microwave sources, and electron paramagnetic resonance detection.

Effect of water/glycerol polymorphism on dynamic nuclear polarization

A paramount feature of robust experimental methods is acquiring consistent data. However, in dynamic nuclear polarization (DNP), it has been observed that the DNP-induced NMR signal enhancement of nominally the same sample can vary between different experimental sessions. We investigated the impact of various freezing conditions on the DNP results for a standard sample, a 50/40/10 by volume d8-glycerol/D2O/H2O solution of 40 mM 4-amino TEMPO, and found that annealing the samples 10 K above the glass transition temperature (Tg) causes significant changes to the DNP profiles and enhancements compared to that in rapidly frozen samples. When varying the glycerol composition to yield a solution of 60/30/10 d8-glycerol/D2O/H2O, the DNP performance became markedly more consistent, even for samples prepared under vastly different sample freezing methods, in stark contrast with that of the 50/40/10 solution. The EPR lineshapes, Tm, and glass transition temperature, Tg, were measured under the same sample and experimental conditions as used for the DNP experiments to support the conclusion that different freezing methods change the distribution of 4-amino TEMPO radials in the 50/40/10 solution due to the formation of different polymorphs of the glass, which is mitigated in the 60/30/10 solution and is consistent with the water/glycerol vitrification literature.

Frequency-agile gyrotron for electron decoupling and pulsed dynamic nuclear polarization

We describe a frequency-agile gyrotron which can generate frequency-chirped microwave pulses. An arbitrary waveform generator (AWG) within the NMR spectrometer controls the microwave frequency, enabling synchronized pulsed control of both electron and nuclear spins. We demonstrate that the acceleration of emitted electrons, and thus the microwave frequency, can be quickly changed by varying the anode voltage. This strategy results in much faster frequency response than can be achieved by changing the potential of the electron emitter, and does not require a custom triode electron gun. The gyrotron frequency can be swept with a rate of 20 MHz/μs over a 670 MHz bandwidth in a static magnetic field. We have already implemented time-domain electron decoupling with dynamic nuclear polarization (DNP) magic angle spinning (MAS) with this device. In this contribution, we show frequency-swept DNP enhancement profiles recorded without changing the NMR magnet or probe. The profile of endofullerenes exhibits a DNP profile with a <10 MHz linewidth, indicating that the device also has sufficient frequency stability, and therefore phase stability, to implement pulsed DNP mechanisms such as the frequency-swept solid effect. We describe schematics of the mechanical and vacuum construction of the device which includes a novel flanged sapphire window assembly. Finally, we discuss how commercially available continuous-wave gyrotrons can potentially be converted into similar frequency-agile high-power microwave sources.

Dynamic Nuclear Polarization Opens New Perspectives for NMR Spectroscopy in Analytical Chemistry

Dynamic nuclear polarization (DNP) can boost sensitivity in nuclear magnetic resonance (NMR) experiments by several orders of magnitude. This Feature illustrates how the coupling of DNP with both liquid- and solid-state NMR spectroscopy has the potential to considerably extend the range of applications of NMR in analytical chemistry.

N,N-Diethylmethylamine as lineshape standard for NMR above 130 K

We demonstrate that N,N-Diethylmethylamine (DEMA) is a useful compound for shimming the magnetic field when doing NMR experiments at room temperature and 130 K, near the temperature used in many dynamic nuclear polarization (DNP) experiments. The resonance assigned to the N-methyl carbon in DEMA at 14.7 T and 140 K has a full-width-half-max linewidth of <4 Hz and has a spin-lattice relaxation time of 0.17 ± 0.03 s.

Fast and accurate MAS-DNP simulations of large spin ensembles

A deeper understanding of parameters affecting Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP), an emerging nuclear magnetic resonance hyperpolarization method, is crucial for the development of new polarizing agents and the successful implementation of the technique at higher magnetic fields (>10 T). Such progress is currently impeded by computational limitation which prevents the simulation of large spin ensembles (electron as well as nuclear spins) and to accurately describe the interplay between all the multiple key parameters at play. In this work, we present an alternative approach to existing cross-effect and solid-effect MAS-DNP codes that yields fast and accurate simulations. More specifically we describe the model, the associated Liouville-based formalism (Bloch-type derivation and/or Landau-Zener approximations) and the linear time algorithm that allows computing MAS-DNP mechanisms with unprecedented time savings. As a result, one can easily scan through multiple parameters and disentangle their mutual influences. In addition, the simulation code is able to handle multiple electrons and protons, which allows probing the effect of (hyper)polarizing agents concentration, as well as fully revealing the interplay between the polarizing agent structure and the hyperfine couplings, nuclear dipolar couplings, nuclear relaxation times, both in terms of depolarization effect, but also of polarization gain and buildup times.

Magic angle spinning NMR below 6 K with a computational fluid dynamics analysis of fluid flow and temperature gradients

We report magic angle spinning (MAS) up to 8.5 kHz with a sample temperature below 6 K using liquid helium as a variable temperature fluid. Cross polarization ¹³C NMR spectra exhibit exquisite sensitivity with a single transient. Remarkably, ¹H saturation recovery experiments show a ¹H T₁ of 21 s with MAS below 6 K in the presence of trityl radicals in a glassy matrix. Leveraging the thermal spin polarization available at 4.2 K versus 298 K should result in 71 times higher signal intensity. Taking the ¹H longitudinal relaxation into account, signal averaging times are therefore predicted to be expedited by a factor of >500. Computer assisted design (CAD) and finite element analysis were employed in both the design and diagnostic stages of this cryogenic MAS technology development. Computational fluid dynamics (CFD) models describing temperature gradients and fluid flow are presented. The CFD models bearing and drive gas maintained at 100 K, while a colder helium variable temperature fluid stream cools the center of a zirconia rotor. Results from the CFD were used to optimize the helium exhaust path and determine the sample temperature. This novel cryogenic experimental platform will be integrated with pulsed dynamic nuclear polarization and electron decoupling to interrogate biomolecular structure within intact human cells.

Strategies for Efficient Sample Preparation for Dynamic Nuclear Polarization Solid-State NMR of Biological Macromolecules

Solid-state NMR (SSNMR) is a powerful tool for the elucidation of structure and dynamics in biological macromolecules. Over the years, SSNMR spectroscopists have developed an array of techniques enabling the measurement of internuclear correlations, distances, and torsional angles; these have been applied to the study of a number of biological systems that are difficult to study by X-ray crystallography and solution NMR, including key biological targets such as membrane proteins and amyloid fibrils. Applications of SSNMR to other topic areas, including materials science, pharmaceuticals, and small molecules, have also flourished in recent years. These studies, however, have always been hampered by the low inherent sensitivity of SSNMR, requiring large amounts of both sample and time for data collection. By taking advantage of unpaired electrons doped into a sample as a ready source of additional nuclear polarization, dynamic nuclear polarization (DNP) has brought about large improvements in SSNMR sensitivity. These, in turn, have enabled structural studies of previously inaccessible targets, such as large protein complexes, nucleic acids, viral capsids, and membrane proteins in vivo. Herein, we focus on sample preparation strategies and considerations for scientists interested in applying DNP to challenging systems.

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.

Instrumentation for cryogenic magic angle spinning dynamic nuclear polarization using 90L of liquid nitrogen per day

Cryogenic sample temperatures can enhance NMR sensitivity by extending spin relaxation times to improve dynamic nuclear polarization (DNP) and by increasing Boltzmann spin polarization. We have developed an efficient heat exchanger with a liquid nitrogen consumption rate of only 90L per day to perform magic-angle spinning (MAS) DNP experiments below 85K. In this heat exchanger implementation, cold exhaust gas from the NMR probe is returned to the outer portion of a counterflow coil within an intermediate cooling stage to improve cooling efficiency of the spinning and variable temperature gases. The heat exchange within the counterflow coil is calculated with computational fluid dynamics to optimize the heat transfer. Experimental results using the novel counterflow heat exchanger demonstrate MAS DNP signal enhancements of 328±3 at 81±2K, and 276±4 at 105±2K.

Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization

We demonstrate that NMR/DNP (Dynamic Nuclear Polarization) allows an unprecedented description of carbonate substituted hydroxyapatite (CHAp). Key structural questions related to order/disorder and clustering of carbonates are tackled using distance sensitive DNP experiments using ¹³C-¹³C recoupling. Such experiments are easily implemented due to unprecedented DNP gain (orders of magnitude). DNP is efficiently mediated by quasi one-dimensional spin diffusion through the hydroxyl columns present in the CHAp structure (thought of as "highways" for spin diffusion). For spherical nanoparticles and ϕ < 100 nm, it is numerically shown that spin diffusion allows their study as a whole. Most importantly, we demonstrate also that the DNP study at 100 K leads to data which are comparable to data obtained at room temperature (in terms of spin dynamics and line shape resolution). Finally, all 2D DNP experiments can be interpreted in terms of domains exhibiting well identified types of substitution: local order and carbonate clustering are clearly favored.

Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning

Dynamic nuclear polarization (DNP) has recently emerged as a tool to enhance the sensitivity of solid-state NMR experiments. However, so far high enhancements (>100) are limited to relatively low magnetic fields, and DNP at fields higher than 9.4 T significantly drops in efficiency. Here we report solid-state Overhauser effect DNP enhancements of over 100 at 18.8 T. This is achieved through the unexpected discovery that enhancements increase rapidly with increasing magic angle spinning (MAS) rates. The measurements are made using 1,3-bisdiphenylene-2-phenylallyl dissolved in o-terphenyl at 40 kHz MAS. We introduce a source-sink diffusion model for polarization transfer which is capable of explaining the experimental observations. The advantage of this approach is demonstrated on mesoporous alumina with the acquisition of well-resolved DNP surface-enhanced 27Al cross-polarization spectra.