Dynamic Nuclear Polarization as an Enabling Technology for Solid State Nuclear Magnetic Resonance Spectroscopy

Resolution in cryogenic solid state NMR: Challenges and solutions

NMR at cryogenic temperatures has the potential to provide rich site-specific details regarding biopolymer structure, function, and mechanistic intermediates. Broad spectral lines compared with room temperature NMR can sometimes present practical challenges. A number of hypotheses regarding the origins of line broadening are explored. One frequently considered explanation is the presence of inhomogeneous conformational distributions. Possibly these arise when the facile characteristic motions that occur near room temperature become dramatically slower or "frozen out" at temperatures below the solvent phase change. Recent studies of low temperature spectra harness the distributions in properties in these low temperature spectra to uncover information regarding the conformational ensembles that drive biological function.

Cryogenic-compatible spherical rotors and stators for magic angle spinning dynamic nuclear polarization

Cryogenic magic angle spinning (MAS) is a standard technique utilized for dynamic nuclear polarization (DNP) in solid-state nuclear magnetic resonance (NMR). Here we describe the optimization and implementation of a stator for cryogenic MAS with 9.5 mm diameter spherical rotors, allowing for DNP experiments on large sample volumes. Designs of the stator and rotor for cryogenic MAS build on recent advancements of MAS spheres and take a step further to incorporate sample insert and eject and a temperature-independent spinning stability of ± 1  Hz. At a field of 7 T and spinning at 2.0 kHz with a sample temperature of 105-107 K, DNP enhancements of 256 and 200 were observed for 124 and 223 µ L sample volumes, respectively, each consisting of 4 M 13 C, 15 N-labeled urea and 20 mM AMUPol in a glycerol-water glassy matrix.

Torsion Angle Analysis of a Thermally Activated Delayed Fluorescence Emitter in an Amorphous State Using Dynamic Nuclear Polarization Enhanced Solid-State NMR

The torsion angle between donor and acceptor segments of a thermally activated delayed fluorescence (TADF) molecule is one of the most critical factors in determining the performance of TADF-based organic light-emitting diodes (OLEDs) because the torsion angle affects not only the energy gap between the singlet and triplet but also the oscillator strength and spin-orbit coupling. However, the torsion angle is difficult to analyze, because organic molecules are in an amorphous state in OLEDs. Here, we determined the torsion angle of a highly efficient TADF emitter, DACT-II, in an amorphous state by dynamic nuclear polarization enhanced solid-state NMR measurements. From the experimentally obtained chemical shift principal values of ¹⁵N on carbazole, we determined the average torsion angle to be 52°. Such quantification of the torsion angles in TADF molecules in amorphous solids will provide deep insight into the TADF mechanism in amorphous OLEDs.

Mechanism underlying the weakening effect of β-glucan on the gluten system

The high β-glucan content in barley disrupts the gluten network in dough. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR), and solid-state nuclear magnetic resonance (NMR) techniques were used to clarify how β-glucan affected the quality of the gluten network structure with β-glucan contents of 0-2%. The results suggest that the physical hindrance of the β-glucan gel destroyed the formation of the gluten network structure. When 1.0-2.0% β-glucan was added, the percentage of α-helical structures increased significantly. When the added amount of β-glucan reached 2.0%, the sulfhydryl group (SH) content increased from 8.06 to 10.27 µmol/g, and the disulfide bond (SS) content decreased from 240.09 to 217.38 µmol/g. The interaction between β-glucan and gluten mainly resulted from the interaction of electron-withdrawing groups, such as carbonyl groups (CO) and double bond carbons (CC), and carbon atoms on the side chains of β-glucan, which play an important role in the central structure of glutenin.

Fast magic angle spinning for the characterization of milligram quantities of organic and biological solids at natural isotopic abundance by 13C-13C correlation DNP-enhanced NMR

We show that multidimensional solid-state NMR ¹³C-¹³C correlation spectra of biomolecular assemblies and microcrystalline organic molecules can be acquired at natural isotopic abundance with only milligram quantities of sample. These experiments combine fast Magic Angle Spinning of the sample, low-power dipolar recoupling, and dynamic nuclear polarization performed with AsymPol biradicals, a recently introduced family of polarizing agents. Such experiments are essential for structural characterization as they provide short- and long-range distance information. This approach is demonstrated on diverse sample types, including polyglutamine fibrils implicated in Huntington's disease and microcrystalline ampicillin, a small antibiotic molecule.

Monitoring the influence of additives on the crystallization processes of glycine with dynamic nuclear polarization solid-state NMR

Crystallization is fundamental in many domains, and the investigation of the sequence of solid phases produced as a function of crystallization time is thus key to understand and control crystallization processes. Here, we used a solid-state nuclear magnetic resonance strategy to monitor the crystallization process of glycine, which is a model compound in polymorphism, under the influence of crystallizing additives, such as methanol or sodium chloride. More specifically, our strategy is based on a combination of low-temperatures and dynamic nuclear polarization (DNP) to trap and detect transient crystallizing forms, which may be present only in low quantities. Interestingly, our results show that these additives yield valuable DNP signal enhancements even in the absence of glycerol within the crystallizing solution.

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.

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.

1H Thermal Mixing Dynamic Nuclear Polarization with BDPA as Polarizing Agents

Dynamic Nuclear Polarization (DNP) is a sensitivity enhancing technique for Nuclear Magnetic Resonance. A recent discovery of Overhauser Effect (OE) DNP in insulating systems under cryogenic conditions using 1,3-bisdiphenylene-2-phenylallyl (BDPA) as the polarizing agent (PA) has caught attention due to its promising DNP performance at a high magnetic field and under fast magic angle spinning conditions. However, the mechanism of OE in insulating-solids/BDPA is unclear. We present an alternative explanation that the dominant underlying DNP mechanism of BDPA is Thermal Mixing (TM). This is ascertained with the discovery that TM effect is enhanced by multi-electron spin coupling, which is corroborated by an asymmetric electron paramagnetic resonance line shape signifying the coexistence of clustered and isolated BDPA species, and by hyperpolarized electron spin populations giving rise to an electron spin polarization gradient which are characteristic signatures of TM DNP. Finally, quantum mechanical simulations using spatially asymmetrically coupled three electron spins and a nuclear spin demonstrate that triple-flip DNP, with hyperfine fluctuations turned off, can yield the ¹H DNP profile as observed with BDPA. Clarifying the DNP mechanism is critical to develop design principles for optimizing the PA for achieving optimal DNP efficiency.

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

The development of magic-angle spinning dynamic nuclear polarization (MAS DNP) has allowed atomic-level characterization of materials for which conventional solid-state NMR is impractical due to the lack of sensitivity. The rapid progress of MAS DNP has been largely enabled through the understanding of rational design concepts for more efficient polarizing agents (PAs). Here, we identify a new design principle which has so far been overlooked. We find that the local geometry around the unpaired electron can change the DNP enhancement by an order of magnitude for two otherwise identical conformers. We present a set of 13 new stable mono- and dinitroxide PAs for MAS DNP NMR where this principle is demonstrated. The radicals are divided into two groups of isomers, named open (O-) and closed (C-), based on the ring conformations in the vicinity of the N-O bond. In all cases, the open conformers exhibit dramatically improved DNP performance as compared to the closed counterparts. In particular, a new urea-based biradical named HydrOPol and a mononitroxide O-MbPyTol yield enhancements of 330 ± 60 and 119 ± 25, respectively, at 9.4 T and 100 K, which are the highest enhancements reported so far in the aqueous solvents used here. We find that while the conformational changes do not significantly affect electron spin-spin distances, they do affect the distribution of the exchange couplings in these biradicals. Electron spin echo envelope modulation (ESEEM) experiments suggest that the improved performance of the open conformers is correlated with higher solvent accessibility.

CCMRD: a solid-state NMR database for complex carbohydrates

Carbohydrates are essential to various life activities in living organisms and serve as the central component in many biomaterials. As an emerging technique with steadily improving resolution, solid-state Nuclear Magnetic Resonance (NMR) spectroscopy has the unique capability in revealing the polymorphic structure and heterogeneous dynamics of insoluble complex carbohydrates. Here, we report the first solid-state NMR database for complex carbohydrates, Complex Carbohydrates Magnetic Resonance Database (CCMRD). This database currently holds the chemical shift information of more than four hundred solid-state NMR compounds and expects rapid expansion. CCMRD provides open portals for data deposition and supports search options based on NMR chemical shifts, carbohydrate names, and compound classes. With the timely implementation, this platform will facilitate spectral analysis and structure determination of carbohydrates and promote software development to benefit the research community. The database is freely accessible at www.ccmrd.org.

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.

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.

Brute-force solvent suppression for DNP studies of powders at natural isotopic abundance

A method based on highly concentrated radical solutions is investigated for the suppression of the NMR signals arising from solvents that are usually used for dynamic nuclear polarization experiments. The presented method is suitable in the case of powders, which are impregnated with a radical-containing solution. It is also demonstrated that the intensity and the resolution of the signals due to the sample of interest is not affected by the high concentration of radicals. The method proposed here is therefore valuable when sensitivity is of the utmost importance, namely samples at natural isotopic abundance.

Stability of nitroxide biradical TOTAPOL in biological samples

We characterize chemical reduction of a nitroxide biradical, TOTAPOL, used in dynamic nuclear polarization (DNP) experiments, specifically probing the stability in whole-cell pellets and lysates, and present a few strategies to stabilize the biradicals for DNP studies. DNP solid-state NMR experiments use paramagnetic species such as nitroxide biradicals to dramatically increase NMR signals. Although there is considerable excitement about using nitroxide-based DNP for detecting the NMR spectra of proteins in whole cells, nitroxide radicals are reduced in minutes in bacterial cell pellets, which we confirm and quantify here. We show that addition of the covalent cysteine blocker N-ethylmaleimide to whole cells significantly slows the rate of reduction, suggesting that cysteine thiol radicals are important to in vivo radical reduction. The use of cell lysates rather than whole cells also slows TOTAPOL reduction, which suggests a possible role for the periplasm and oxidative phosphorylation metabolites in radical degradation. Reduced TOTAPOL in lysates can also be efficiently reoxidized with potassium ferricyanide. These results point to a practical and robust set of strategies for DNP of cellular preparations.

Reducing the computational cost of NMR crystallography of organic powders at natural isotopic abundance with the help of 13 C-13 C dipolar couplings

Structure determination of functional organic compounds remains a formidable challenge when the sample exists as a powder. Nuclear magnetic resonance crystallography approaches based on the comparison of experimental and Density Functional Theory (DFT)-computed ¹ H chemical shifts have already demonstrated great potential for structure determination of organic powders, but limitations still persist. In this study, we discuss the possibility of using ¹³ C-¹³ C dipolar couplings quantified on powdered theophylline at natural isotopic abundance with the help of dynamic nuclear polarization, to realize a DFT-free, rapid screening of a pool of structures predicted by ab initio random structure search. We show that although ¹³ C-¹³ C dipolar couplings can identify structures possessing long range structural motifs and unit cell parameters close to those of the true structure, it must be complemented with other data to recover information about the presence and the chemical nature of the supramolecular interactions.

A Strategy for Probing the Evolution of Crystallization Processes by Low-Temperature Solid-State NMR and Dynamic Nuclear Polarization

Crystallization plays an important role in many areas, and to derive a fundamental understanding of crystallization processes, it is essential to understand the sequence of solid phases produced as a function of time. Here, we introduce a new NMR strategy for studying the time evolution of crystallization processes, in which the crystallizing system is quenched rapidly to low temperature at specific time points during crystallization. The crystallized phase present within the resultant "frozen solution" may be investigated in detail using a range of sophisticated NMR techniques. The low temperatures involved allow dynamic nuclear polarization (DNP) to be exploited to enhance the signal intensity in the solid-state NMR measurements, which is advantageous for detection and structural characterization of transient forms that are present only in small quantities. This work opens up the prospect of studying the very early stages of crystallization, at which the amount of solid phase present is intrinsically low.

Structural Fingerprinting of Protein Aggregates by Dynamic Nuclear Polarization-Enhanced Solid-State NMR at Natural Isotopic Abundance

A pathological hallmark of Huntington’s disease (HD) is the formation of neuronal protein deposits containing mutant huntingtin fragments with expanded polyglutamine (polyQ) domains. Prior studies have shown the strengths of solid-state NMR (ssNMR) to probe the atomic structure of such aggregates, but have required in vitro isotopic labeling. Herein, we present an approach for the structural fingerprinting of fibrils through ssNMR at natural isotopic abundance (NA). These methods will enable the spectroscopic fingerprinting of unlabeled (e.g., ex vivo) protein aggregates and the extraction of valuable new long-range ¹³C–¹³C distance constraints.

Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family

We introduce a new family of highly efficient polarizing agents for dynamic nuclear polarization (DNP)-enhanced nuclear magnetic resonance (NMR) applications, composed of asymmetric bis-nitroxides, in which a piperidine-based radical and a pyrrolinoxyl or a proxyl radical are linked together. The design of the AsymPol family was guided by the use of advanced simulations that allow computation of the impact of the radical structure on DNP efficiency. These simulations suggested the use of a relatively short linker with the intention to generate a sizable intramolecular electron dipolar coupling/ J-exchange interaction, while avoiding parallel nitroxide orientations. The characteristics of AsymPol were further tuned, for instance with the addition of a conjugated carbon-carbon double bond in the 5-membered ring to improve the rigidity and provide a favorable relative orientation, the replacement of methyls by spirocyclohexanolyl groups to slow the electron spin relaxation, and the introduction of phosphate groups to yield highly water-soluble dopants. An in-depth experimental and theoretical study for two members of the family, AsymPol and AsymPolPOK, is presented here. We report substantial sensitivity gains at both 9.4 and 18.8 T. The robust efficiency of this new family is further demonstrated through high-resolution surface characterization of an important industrial catalyst using fast sample spinning at 18.8 T. This work highlights a new direction for polarizing agent design and the critical importance of computations in this process.

Illuminating the dark metabolome to advance the molecular characterisation of biological systems

BACKGROUND

The latest version of the Human Metabolome Database (v4.0) lists 114,100 individual entries. Typically, however, metabolomics studies identify only around 100 compounds and many features identified in mass spectra are listed only as 'unknown compounds'. The lack of ability to detect all metabolites present, and fully identify all metabolites detected (the dark metabolome) means that, despite the great contribution of metabolomics to a range of areas in the last decade, a significant amount of useful information from publically funded studies is being lost or unused each year. This loss of data limits our potential gain in knowledge and understanding of important research areas such as cell biology, environmental pollution, plant science, food chemistry and health and biomedical research. Metabolomics therefore needs to develop new tools and methods for metabolite identification to advance as a field.

AIM OF REVIEW

In this critical review, some potential issues with metabolite identification are identified and discussed. New and novel emerging technologies and tools which may contribute to expanding the number of compounds identified in metabolomics studies (thus illuminating the dark metabolome) are reviewed. The aim is to stimulate debate and research in the molecular characterisation of biological systems to drive forward metabolomic research.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The work specifically discusses dynamic nuclear polarisation nuclear magnetic resonance spectroscopy (DNP-NMR), non-proton NMR active nuclei, two-dimensional liquid chromatography (2DLC) and Raman spectroscopy (RS). It is suggested that developing new methods for metabolomics with these techniques could lead to advances in the field and better characterisation of biological systems.

Amplification of Dynamic Nuclear Polarization at 200 GHz by Arbitrary Pulse Shaping of the Electron Spin Saturation Profile

Dynamic nuclear polarization (DNP) takes center stage in nuclear magnetic resonance (NMR) as a tool to amplify its signal by orders of magnitude through the transfer of polarization from electron to nuclear spins. In contrast to modern NMR and electron paramagnetic resonance (EPR) that extensively rely on pulses for spin manipulation in the time domain, the current mainstream DNP technology exclusively relies on monochromatic continuous wave (CW) irradiation. This study introduces arbitrary phase shaped pulses that constitute a train of coherent chirp pulses in the time domain at 200 GHz (7 T) to dramatically enhance the saturation bandwidth and DNP performance compared to CW DNP, yielding up to 500-fold in NMR signal enhancements. The observed improvement is attributed to the recruitment of additional electron spins contributing to DNP via the cross-effect mechanism, as experimentally confirmed by two-frequency pump-probe electron-electron double resonance (ELDOR).

Exploring Applications of Covalent Organic Frameworks: Homogeneous Reticulation of Radicals for Dynamic Nuclear Polarization

Rapid progress has been witnessed in the past decade in the fields of covalent organic frameworks (COFs) and dynamic nuclear polarization (DNP). In this contribution, we bridge these two fields by constructing radical-embedded COFs as promising DNP agents. Via polarization transfer from unpaired electrons to nuclei, DNP realizes significant enhancement of NMR signal intensities. One of the crucial issues in DNP is to screen for suitable radicals to act as efficient polarizing agents, the basic criteria for which are homogeneous distribution and fixed orientation of unpaired electrons. We therefore envisioned that the crystalline and porous structures of COFs, if evenly embedded with radicals, may work as a new "crystalline sponge" for DNP experiments. As a proof of concept, we constructed a series of proxyl-radical-embedded COFs (denoted as PR( x)-COFs) and successfully applied them to achieve substantial DNP enhancement. Benefiting from the bottom-up and multivariate synthetic strategies, proxyl radicals have been covalently reticulated, homogeneously distributed, and rigidly embedded into the crystalline and mesoporous frameworks with adjustable concentration ( x%). Excellent performance of PR( x)-COFs has been observed for DNP 1H, 13C, and 15N solid-state NMR enhancements. This contribution not only realizes the direct construction of radical COFs from radical monomers, but also explores the new application of COFs as DNP polarizing agents. Given that many radical COFs can therefore be rationally designed and facilely constructed with well-defined composition, distribution, and pore size, we expect that our effort will pave the way for utilizing radical COFs as standard polarizing agents in DNP NMR experiments.

Electron-driven spin diffusion supports crossing the diffusion barrier in MAS DNP

Dynamic nuclear polarization (DNP) can be applied to enhance the sensitivity of solid-state NMR experiments by several orders of magnitude due to microwave-driven transfer of spin polarization from unpaired electrons to nuclei. While the underlying quantum mechanical aspects are sufficiently well understood on a microscopic level, the exact description of the large-scale spin dynamics, usually involving hundreds to thousands of nuclear spins per electron, is still lacking consensus. Generally, it is assumed that nuclear hyperpolarization can only be observed on nuclei which do not experience strong influence of the unpaired electrons and thus being significantly removed from the paramagnetic polarizing agents. At the same time, sufficiently strong hyperfine interaction is required for DNP transfer. Therefore, efficient nuclear spin diffusion from the strongly-interacting nuclei to the NMR-observable bulk is considered to be essential for efficient nuclear hyperpolarization. Based on experimental results obtained on the endohedral fullerene N@C60 as a polarizing agent sparsely diluted in C60, we discuss the effect of the spin-diffusion barrier. We introduce electron-driven spin diffusion (EDSD) as a novel mechanism for nuclear polarization transfer in the proximity of an electron spin which is particularly relevant under magic-angle spinning (MAS) DNP conditions.

Elucidating an Amorphous Form Stabilization Mechanism for Tenapanor Hydrochloride: Crystal Structure Analysis Using X-ray Diffraction, NMR Crystallography, and Molecular Modeling

By the combined use of powder and single-crystal X-ray diffraction, solid-state NMR, and molecular modeling, the crystal structures of two systems containing the unusually large tenapanor drug molecule have been determined: the free form, ANHY, and a dihydrochloride salt form, 2HCl. Dynamic nuclear polarization (DNP) assisted solid-state NMR (SSNMR) crystallography investigations were found essential for the final assignment and were used to validate the crystal structure of ANHY. From a structural informatics analysis of ANHY and 2HCl, conformational ring differences in one part of the molecule were observed which influence the relative orientation of a methyl group on a ring nitrogen and thereby impact the crystallizability of the dihydrochloride salt. From quantum chemistry calculations, the dynamics between different ring conformations in tenapanor is predicted to be fast. Addition of HCl to tenapanor results in general in a mixture of protonated ring conformers and hence a statistical mix of diastereoisomers which builds up the amorphous form, a-2HCl. This was qualitatively verified by ¹³C CP/MAS NMR investigations of the amorphous form. Thus, to form any significant amount of the crystalline material 2HCl, which originates from the minor (i.e., energetically less stable) ring conformations, one needs to involve nitrogen deprotonation to allow exchange between the minor and major conformations of ANHY in solution. Thus, by controlling the solution pH value to well below the p K_a of ANHY, the equilibrium between ANHY and 2HCl can be controlled and by this mechanism the crystallization of 2HCl can be avoided and the amorphous form of the dichloride salt can therefore be stabilized.

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.

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.

NMR Signal Quenching from Bound Biradical Affinity Reagents in DNP Samples

We characterize the effect of specifically bound biradicals on the NMR spectra of dihydrofolate reductase from E. coli. Dynamic nuclear polarization methods enhance the signal-to-noise of solid state NMR experiments by transferring polarization from unpaired electrons of biradicals to nuclei. There has been recent interest in colocalizing the paramagnetic polarizing agents with the analyte of interest through covalent or noncovalent specific interactions. This experimental approach broadens the scope of dynamic nuclear polarization methods by offering the possibility of selective signal enhancements and the potential to work in a broad range of environments. Paramagnetic compounds can have other effects on the NMR spectroscopy of nearby nuclei, including broadening of nuclear resonances due to the proximity of the paramagnetic agent. Understanding the distance dependence of these interactions is important for the success of the technique. Here we explore paramagnetic signal quenching due to a bound biradical, specifically a biradical-derivatized trimethoprim ligand of E. coli dihydrofolate reductase. Biradical-derivatized trimethoprim has nanomolar affinity for its target, and affords strong and selective signal enhancements in dynamic nuclear polarization experiments. In this work, we show that, although the trimethoprim fragment is well ordered, the biradical (TOTAPOL) moiety is disordered when bound to the protein. The distance dependence in bleaching of NMR signal intensity allows us to detect numerous NMR signals in the protein. We present the possibility that static disorder and electron spin diffusion play roles in this observation, among other contributions. The fact that the majority of signals are observed strengthens the case for the use of high affinity or covalent radicals in dynamic nuclear polarization solid state NMR enhancement.

Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy

Molecular orientation in amorphous organic semiconducting thin-film devices is an important issue affecting device performance. However, to date it has not been possible to analyze the "distribution" of the orientations. Although solid-state NMR (ssNMR) spectroscopy can provide information on the "distribution" of molecular orientations, the technique is limited because of the small amount of sample in the device and the low sensitivity of ssNMR. Here, we report the first application of dynamic nuclear polarization enhanced ssNMR (DNP-ssNMR) spectroscopy for the orientational analysis of amorphous phenyldi(pyren-1-yl)phosphine oxide (POPy₂ ). The ³¹ P DNP-ssNMR spectra exhibited a sufficient signal-to-noise ratio to quantify the distribution of molecular orientations in amorphous films: the P=O axis of the vacuum-deposited and drop-cast POPy₂ shows anisotropic and isotropic distribution, respectively. The different molecular orientations reflect the molecular origin of the different charge transport behaviors.

In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR

A principal advantage of magic angle spinning (MAS) NMR spectroscopy lies in its ability to determine molecular structure in a noninvasive and quantitative manner. Accordingly, MAS should be widely applicable to studies of the structure of active pharmaceutical ingredients (API) and formulations. However, the low sensitivity encountered in spectroscopy of natural abundance APIs present at low concentration has limited the success of MAS experiments. Dynamic nuclear polarization (DNP) enhances NMR sensitivity and can be used to circumvent this problem provided that suitable paramagnetic polarizing agent can be incorporated into the system without altering the integrity of solid dosages. Here, we demonstrate that DNP polarizing agents can be added in situ during the preparation of amorphous solid dispersions (ASDs) via spray drying and hot-melt extrusion so that ASDs can be examined during drug development. Specifically, the dependence of DNP enhancement on sample composition, radical concentration, relaxation properties of the API and excipients, types of polarizing agents and proton density, has been thoroughly investigated. Optimal enhancement values are obtained from ASDs containing 1% w/w radical concentration. Both polarizing agents TOTAPOL and AMUPol provided reasonable enhancements. Partial deuteration of the excipient produced 3× higher enhancement values. With these parameters, an ASD containing posaconazole and vinyl acetate yields a 32-fold enhancement which presumably results in a reduction of NMR measurement time by ∼1000. This boost in signal intensity enables the full assignment of the natural abundance pharmaceutical formulation through multidimensional correlation experiments.

New NMR tools for protein structure and function: Spin tags for dynamic nuclear polarization solid state NMR

Magic angle spinning solid state NMR studies of biological macromolecules [1-3] have enabled exciting studies of membrane proteins [4,5], amyloid fibrils [6], viruses, and large macromolecular assemblies [7]. Dynamic nuclear polarization (DNP) provides a means to enhance detection sensitivity for NMR, particularly for solid state NMR, with many recent biological applications and considerable contemporary efforts towards elaboration and optimization of the DNP experiment. This review explores precedents and innovations in biological DNP experiments, especially highlighting novel chemical biology approaches to introduce the radicals that serve as a source of polarization in DNP experiments.

Pharmaceutical cocrystals, salts and polymorphs: Advanced characterization techniques

The main goal of a novel drug development is to obtain it with optimal physiochemical, pharmaceutical and biological properties. Pharmaceutical companies and scientists modify active pharmaceutical ingredients (APIs), which often are cocrystals, salts or carefully selected polymorphs, to improve the properties of a parent drug. To find the best form of a drug, various advanced characterization methods should be used. In this review, we have described such analytical methods, dedicated to solid drug forms. Thus, diffraction, spectroscopic, thermal and also pharmaceutical characterization methods are discussed. They all are necessary to study a solid API in its intrinsic complexity from bulk down to the molecular level, gain information on its structure, properties, purity and possible transformations, and make the characterization efficient, comprehensive and complete. Furthermore, these methods can be used to monitor and investigate physical processes, involved in the drug development, in situ and in real time. The main aim of this paper is to gather information on the current advancements in the analytical methods and highlight their pharmaceutical relevance.

Persistent Radicals of Self-assembled Benzophenone bis-Urea Macrocycles: Characterization and Application as a Polarizing Agent for Solid-state DNP MAS Spectroscopy

UV-irradiation of a self-assembled benzophenone bis-urea macrocycle generates μm amounts of radicals that persist for weeks under ambient conditions. High-field EPR and variable-temperature X-band EPR studies suggest a resonance stabilized radical pair through H-abstraction. These endogenous radicals were applied as a polarizing agent for magic angle spinning (MAS) dynamic nuclear polarization (DNP) NMR enhancement. The field-stepped DNP enhancement profile exhibits a sharp peak with a maximum enhancement of ϵon/off =4 superimposed on a nearly constant DNP enhancement of ϵon/off =2 over a broad field range. This maximum coincides with the high field EPR absorption spectrum, consistent with an Overhauser effect mechanism. DNP enhancement was observed for both the host and guests, suggesting that even low levels of endogenous radicals can facilitate the study of host-guest relationships in the solid-state.

Efficient assignment and NMR analysis of an intact virus using sequential side-chain correlations and DNP sensitization

An experimental strategy has been developed to increase the efficiency of dynamic nuclear polarization (DNP) in solid-state NMR studies. The method makes assignments simpler, faster, and more reliable via sequential correlations of both side-chain and Cα resonances. The approach is particularly suited to complex biomolecules and systems with significant chemical-shift degeneracy. It was designed to overcome the spectral congestion and line broadening that occur due to sample freezing at the cryogenic temperatures required for DNP. Nonuniform sampling (NUS) is incorporated to achieve time-efficient collection of multidimensional data. Additionally, fast (25 kHz) magic-angle spinning (MAS) provides optimal sensitivity and resolution. Data collected in <1 wk produced a virtually complete de novo assignment of the coat protein of Pf1 virus. The peak positions and linewidths for samples near 100 K are perturbed relative to those near 273 K. These temperature-induced perturbations are strongly correlated with hydration surfaces.

Ramped-amplitude NOVEL

We present a pulsed dynamic nuclear polarization (DNP) study using a ramped-amplitude nuclear orientation via electron spin locking (RA-NOVEL) sequence that utilizes a fast arbitrary waveform generator (AWG) to modulate the microwave pulses together with samples doped with narrow-line radicals such as 1,3-bisdiphenylene-2-phenylallyl (BDPA), sulfonated-BDPA (SA-BDPA), and trityl-OX063. Similar to ramped-amplitude cross polarization in solid-state nuclear magnetic resonance, RA-NOVEL improves the DNP efficiency by a factor of up to 1.6 compared to constant-amplitude NOVEL (CA-NOVEL) but requires a longer mixing time. For example, at τmix = 8 μs, the DNP efficiency reaches a plateau at a ramp amplitude of ∼20 MHz for both SA-BDPA and trityl-OX063, regardless of the ramp profile (linear vs. tangent). At shorter mixing times (τmix = 0.8 μs), we found that the tangent ramp is superior to its linear counterpart and in both cases there exists an optimum ramp size and therefore ramp rate. Our results suggest that RA-NOVEL should be used instead of CA-NOVEL as long as the electronic spin lattice relaxation T1e is sufficiently long and/or the duty cycle of the microwave amplifier is not exceeded. To the best of our knowledge, this is the first example of a time domain DNP experiment that utilizes modulated microwave pulses. Our results also suggest that a precise modulation of the microwave pulses can play an important role in optimizing the efficiency of pulsed DNP experiments and an AWG is an elegant instrumental solution for this purpose.

Solvent suppression in DNP enhanced solid state NMR

We show how DNP enhanced solid-state NMR spectra can be dramatically simplified by suppression of solvent signals. This is achieved by (i) exploiting the paramagnetic relaxation enhancement of solvent signals relative to materials substrates, or (ii) by using short cross-polarization contact times to transfer hyperpolarization to only directly bonded carbon-13 nuclei in frozen solutions. The methods are evaluated for organic microcrystals, surfaces and frozen solutions. We show how this allows for the acquisition of high-resolution DNP enhanced proton-proton correlation experiments to measure inter-nuclear proximities in an organic solid.

Donor-acceptor stacking arrangements in bulk and thin-film high-mobility conjugated polymers characterized using molecular modelling and MAS and surface-enhanced solid-state NMR spectroscopy

Conjugated polymers show promising properties as cheap, sustainable and solution-processable semiconductors. A key challenge in the development of these materials is to determine the polymer chain structure, conformation and packing in both the bulk polymer and in thin films typically used in devices. However, many characterisation techniques are unable to provide atomic-level structural information owing to the presence of disorder. Here, we use molecular modelling, magic-angle spinning (MAS) and dynamic nuclear polarisation surface-enhanced NMR spectroscopy (DNP SENS) to characterise the polymer backbone group conformations and packing arrangement in the high-mobility donor-acceptor copolymer diketopyrrolo-pyrrole-dithienylthieno[3,2-b]thiophene (DPP-DTT). Using conventional 1H and 13C solid-state MAS NMR coupled with density functional theory calculations and molecular dynamics simulations, we find that the bulk polymer adopts a highly planar backbone conformation with a laterally-shifted donor-on-acceptor stacking arrangement. DNP SENS enables acquisition of 13C NMR data for polymer films, where sensitivity is limiting owing to small sample volumes. The DNP signal enhancement enables a two-dimensional 1H-13C HETCOR spectrum to be recorded for a drop-cast polymer film, and a 13C CPMAS NMR spectrum to be recorded for a spin-coated thin-film with a thickness of only 400 nm. The results show that the same planar backbone structure and intermolecular stacking arrangement is preserved in the films following solution processing and annealing, thereby rationalizing the favourable device properties of DPP-DTT, and providing a protocol for the study of other thin film materials.

Dynamic Nuclear Polarization Signal Enhancement with High-Affinity Biradical Tags

Dynamic nuclear polarization is an emerging technique for sensitizing solid-state NMR experiments by transferring polarization from electrons to nuclei. Stable biradicals, the polarization source for the cross effect mechanism, are typically codissolved at millimolar concentrations with proteins of interest. Here we describe the high-affinity biradical tag TMP-T, created by covalently linking trimethoprim, a nanomolar affinity ligand of dihydrofolate reductase (DHFR), to the biradical polarizing agent TOTAPOL. With TMP-T bound to DHFR, large enhancements of the protein spectrum are observed, comparable to when TOTAPOL is codissolved with the protein. In contrast to TOTAPOL, the tight binding TMP-T can be added stoichiometrically at radical concentrations orders of magnitude lower than in previously described preparations. Benefits of the reduced radical concentration include reduced spectral bleaching, reduced chemical perturbation of the sample, and the ability to selectively enhance signals for the protein of interest.

Molecular Rationale for Improved Dynamic Nuclear Polarization of Biomembranes

Dynamic nuclear polarization (DNP) enhanced solid-state NMR can provide orders of magnitude in signal enhancement. One of the most important aspects of obtaining efficient DNP enhancements is the optimization of the paramagnetic polarization agents used. To date, the most utilized polarization agents are nitroxide biradicals. However, the efficiency of these polarization agents is diminished when used with samples other than small molecule model compounds. We recently demonstrated the effectiveness of nitroxide labeled lipids as polarization agents for lipids and a membrane embedded peptide. Here, we systematically characterize, via electron paramagnetic (EPR), the dynamics of and the dipolar couplings between nitroxide labeled lipids under conditions relevant to DNP applications. Complemented by DNP enhanced solid-state NMR measurements at 600 MHz/395 GHz, a molecular rationale for the efficiency of nitroxide labeled lipids as DNP polarization agents is developed. Specifically, optimal DNP enhancements are obtained when the nitroxide moiety is attached to the lipid choline headgroup and local nitroxide concentrations yield an average e(-)-e(-) dipolar coupling of 47 MHz. On the basis of these measurements, we propose a framework for development of DNP polarization agents optimal for membrane protein structure determination.