Static (1)H dynamic nuclear polarization with the biradical TOTAPOL: a transition between the solid effect and the cross effect

Missing Pieces in Structure Puzzles: How Hyperpolarized NMR Spectroscopy Can Complement Structural Biology and Biochemistry

Structure determination lies at the heart of many biochemical research programs. However, the "giants": X-ray diffraction, electron microscopy, molecular dynamics simulations, and nuclear magnetic resonance, among others, leave quite a few dark spots on the structural pictures drawn of proteins, nucleic acids, membranes, and other biomacromolecules. For example, structural models under physiological conditions or of short-lived intermediates often remain out of reach of the established experimental methods. This account frames the possibility of including hyperpolarized, that is, dramatically signal-enhanced NMR in existing workflows to fill these spots with detailed depictions. We highlight how integrating methods based on dissolution dynamic nuclear polarization can provide valuable complementary information about formerly inaccessible conformational spaces for many systems. A particular focus will be on hyperpolarized buffers to facilitate the NMR structure determination of challenging systems.

Site-specific dynamic nuclear polarization in a Gd(III)-labeled protein

Dynamic nuclear polarization (DNP) of a biomolecule tagged with a polarizing agent has the potential to not only increase NMR sensitivity but also to provide specificity towards the tagging site. Although the general concept has been often discussed, the observation of true site-specific DNP and its dependence on the electron-nuclear distance has been elusive. Here, we demonstrate site-specific DNP in a uniformly isotope-labeled ubiquitin. By recombinant expression of three different ubiquitin point mutants (F4C, A28C, and G75C) post-translationally modified with a Gd3+-chelator tag, localized metal-ion DNP of 13C and 15N is investigated. Effects counteracting the site-specificity of DNP such as nuclear spin-lattice relaxation and proton-driven spin diffusion have been attenuated by perdeuteration of the protein. Particularly for 15N, large DNP enhancement factors on the order of 100 and above as well as localized effects within side-chain resonances differently distributed over the protein are observed. By analyzing the experimental DNP built-up dynamics combined with structural modeling of Gd3+-tags in ubiquitin supported by paramagnetic relaxation enhancement (PRE) in solution, we provide, for the first time, quantitative information on the distance dependence of the initial DNP transfer. We show that the direct 15N DNP transfer rate indeed linearly depends on the square of the hyperfine interaction between the electron and the nucleus following Fermi's golden rule, however, below a certain distance cutoff paramagnetic signal bleaching may dramatically skew the correlation.

Study of electron spectral diffusion process under DNP conditions by ELDOR spectroscopy focusing on the 14N solid effect

Electron spectral diffusion (eSD) plays an important role in solid-state, static dynamic nuclear polarization (DNP) with polarizers that have inhomogeneously broadened EPR spectra, such as nitroxide radicals. It affects the electron spin polarization gradient within the EPR spectrum during microwave irradiation and thereby determines the effectiveness of the DNP process via the so-called indirect cross-effect (iCE) mechanism. The electron depolarization profile can be measured by electron-electron double resonance (ELDOR) experiments, and a theoretical framework for deriving eSD parameters from ELDOR spectra and employing them to calculate DNP profiles has been developed. The inclusion of electron depolarization arising from the 14 N solid effect (SE) has not yet been taken into account in this theoretical framework and is the subject of the present work. The 14 N SE depolarization was studied using W-band ELDOR of a 0.5 mM TEMPOL solution, where eSD is negligible, taking into account the hyperfine interaction of both 14 N and 1 H nuclei, the long microwave irradiation applied under DNP conditions, and electron and nuclear relaxation. The results of this analysis were then used in simulations of ELDOR spectra of 10 and 20 mM TEMPOL solutions, where eSD is significant using the eSD model and the SE contributions were added ad hoc employing the 1 H and 14 N frequencies and their combinations, as found from the analysis of the 0.5 mM sample. This approach worked well for the 20 mM solution, where a good fit for all ELDOR spectra recorded along the EPR spectrum was obtained and the inclusion of the 14 N SE mechanism improved the agreement with the experimental spectra. For the 10 mM solution, simulations of the ELDOR spectra recorded along the g z position gave a lower-quality fit than for spectra recorded in the center of the EPR spectrum. This indicates that the simple approach we used to describe the 14 N SE is limited when its contribution is relatively high as the anisotropy of its magnetic interactions was not considered explicitly.

Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials

Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid-state NMR spectroscopy is a powerful approach for gaining chemical and structural insights at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here, we employ for the first time metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li₄Ti₅O₁₂. In addition for being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect that the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.

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.

Application and methodology of dissolution dynamic nuclear polarization in physical, chemical and biological contexts

Dissolution dynamic nuclear polarization (d-DNP) is a versatile method to enhance nuclear magnetic resonance (NMR) spectroscopy. It boosts signal intensities by four to five orders of magnitude thereby providing the potential to improve and enable a plethora of applications ranging from the real-time monitoring of chemical or biological processes to metabolomics and in-cell investigations. This perspectives article highlights possible avenues for developments and applications of d-DNP in biochemical and physicochemical studies. It outlines how chemists, biologists and physicists with various fields of interest can transform and employ d-DNP as a powerful characterization method for their research.

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.

Anisotropic longitudinal electronic relaxation affects DNP at cryogenic temperatures

We report the observation of anisotropic longitudinal electronic relaxation in nitroxide radicals under typical dynamic nuclear polarization conditions. This anisotropy affects the efficiency of dynamic nuclear polarization at cryogenic temperatures of 4 K and high magnetic fields of 6.7 T. Under our experimental conditions, the electron paramagnetic resonance spectrum of nitroxides such as TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) is only partly averaged by electronic spectral diffusion, so that the relaxation times T_1e(ω) vary across the spectrum. We demonstrate how the anisotropy of T_1e(ω) can be taken into account in simple DNP models.

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.

Dynamic Nuclear Polarization/Solid-State NMR Spectroscopy of Membrane Polypeptides: Free-Radical Optimization for Matrix-Free Lipid Bilayer Samples

Dynamic nuclear polarization (DNP) boosts the sensitivity of NMR spectroscopy by orders of magnitude and makes investigations previously out of scope possible. For magic-angle-spinning (MAS) solid-state NMR spectroscopy studies, the samples are typically mixed with biradicals dissolved in a glass-forming solvent and are investigated at cryotemperatures. Herein, we present new biradical polarizing agents developed for matrix-free samples such as supported lipid bilayers, which are systems widely used for the investigation of membrane polypeptides of high biomedical importance. A series of 11 biradicals with different structures, geometries, and physicochemical properties were comprehensively tested for DNP performance in lipid bilayers, some of them developed specifically for DNP investigations of membranes. The membrane-anchored biradicals PyPol-C16, AMUPOL-cholesterol, and bTurea-C16 were found to exhibit improved g-tensor alignment, inter-radical distance, and dispersion. Consequently, these biradicals show the highest signal enhancement factors so far obtained for matrix-free membranes or other matrix-free samples and may potentially shorten NMR acquisition times by three orders of magnitude. Furthermore, the optimal biradical-to-lipid ratio, sample deuteration, and membrane lipid composition were determined under static and MAS conditions. To rationalize biradical performance better, DNP enhancement was measured by using the ¹³ C and ¹⁵ N signals of lipids and a peptide as a function of the biradical concentration, DNP build-up time, resonance line width, quenching effect, microwave power, and MAS frequency.

On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid-State and Dissolution (13) C NMR Spectroscopy

Dynamic nuclear polarization (DNP) is a versatile option to improve the sensitivity of NMR and MRI. This versatility has elicited interest for overcoming potential limitations of these techniques, including the achievement of solid-state polarization enhancement at ambient conditions, and the maximization of (13) C signal lifetimes for performing in vivo MRI scans. This study explores whether diamond's (13) C behavior in nano- and micro-particles could be used to achieve these ends. The characteristics of diamond's DNP enhancement were analyzed for different magnetic fields, grain sizes, and sample environments ranging from cryogenic to ambient temperatures, in both solution and solid-state experiments. It was found that (13) C NMR signals could be boosted by orders of magnitude in either low- or room-temperature solid-state DNP experiments by utilizing naturally occurring paramagnetic P1 substitutional nitrogen defects. We attribute this behavior to the unusually long electronic/nuclear spin-lattice relaxation times characteristic of diamond, coupled with a time-independent cross-effect-like polarization transfer mechanism facilitated by a matching of the nitrogen-related hyperfine coupling and the (13) C Zeeman splitting. The efficiency of this solid-state polarization process, however, is harder to exploit in dissolution DNP-enhanced MRI contexts. The prospects for utilizing polarized diamond approaching nanoscale dimensions for both solid and solution applications are briefly discussed.

Tailoring of Polarizing Agents in the bTurea Series for Cross-Effect Dynamic Nuclear Polarization in Aqueous Media

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors ɛ ((1)H) in cross-effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP-optimized glycerol/water matrix ("DNP juice") have been studied. We observe that ɛ ((1)H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e-e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e-e distance or too long a T1e can dramatically limit ɛ ((1)H). Our study also shows that the molecular structure of AMUPol is not optimal and its ɛ ((1)H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better ɛ ((1)H) than AMUPol itself (by a factor of ca. 1.2).

A versatile and modular quasi optics-based 200GHz dual dynamic nuclear polarization and electron paramagnetic resonance instrument

Solid-state dynamic nuclear polarization (DNP) at higher magnetic fields (>3T) and cryogenic temperatures (∼ 2-90K) has gained enormous interest and seen major technological advances as an NMR signal enhancing technique. Still, the current state of the art DNP operation is not at a state at which sample and freezing conditions can be rationally chosen and the DNP performance predicted a priori, but relies on purely empirical approaches. An important step towards rational optimization of DNP conditions is to have access to DNP instrumental capabilities to diagnose DNP performance and elucidate DNP mechanisms. The desired diagnoses include the measurement of the "DNP power curve", i.e. the microwave (MW) power dependence of DNP enhancement, the "DNP spectrum", i.e. the MW frequency dependence of DNP enhancement, the electron paramagnetic resonance (EPR) spectrum, and the saturation and spectral diffusion properties of the EPR spectrum upon prolonged MW irradiation typical of continuous wave (CW) DNP, as well as various electron and nuclear spin relaxation parameters. Even basic measurements of these DNP parameters require versatile instrumentation at high magnetic fields not commercially available to date. In this article, we describe the detailed design of such a DNP instrument, powered by a solid-state MW source that is tunable between 193 and 201 GHz and outputs up to 140 mW of MW power. The quality and pathway of the transmitted and reflected MWs is controlled by a quasi-optics (QO) bridge and a corrugated waveguide, where the latter couples the MW from an open-space QO bridge to the sample located inside the superconducting magnet and vice versa. Crucially, the versatility of the solid-state MW source enables the automated acquisition of frequency swept DNP spectra, DNP power curves, the diagnosis of MW power and transmission, and frequency swept continuous wave (CW) and pulsed EPR experiments. The flexibility of the DNP instrument centered around the QO MW bridge will provide an efficient means to collect DNP data that is crucial for understanding the relationship between experimental and sample conditions, and the DNP performance. The modularity of this instrumental platform is suitable for future upgrades and extensions to include new experimental capabilities to meet contemporary DNP needs, including the simultaneous operation of two or more MW sources, time domain DNP, electron double resonance measurements, pulsed EPR operation, or simply the implementation of higher power MW amplifiers.

Pulsed Dynamic Nuclear Polarization with Trityl Radicals

Continuous-wave (CW) dynamic nuclear polarization (DNP) is now established as a method of choice to enhance the sensitivity in a variety of NMR experiments. Nevertheless, there remains a need for the development of more efficient methods to transfer polarization from electrons to nuclei. Of particular interest are pulsed DNP methods because they enable a rapid and efficient polarization transfer that, in contrast with CW DNP methods, is not attenuated at high magnetic fields. Here we report nuclear spin orientation via electron spin-locking (NOVEL) experiments using the polarizing agent trityl OX063 in glycerol/water at a temperature of 80 K and a magnetic field of 0.34 T. (1)H NMR signal enhancements up to 430 are observed, and the buildup of the local polarization occurs in a few hundred nanoseconds. Thus, NOVEL can efficiently dynamically polarize (1)H atoms in a system that is of general interest to the solid-state DNP NMR community. This is a first, important step toward the general application of pulsed DNP at higher fields.

Heteronuclear DNP of protons and deuterons with TEMPOL

Dynamic nuclear polarization (DNP) experiments on samples with several types of magnetic nuclei sometimes exhibit “cross-talk” between the nuclei, such as different nuclei having DNP spectra with similar shapes and enhancements.

Design and characterization of a W-band system for modulated DNP experiments

Magnetic-field and microwave-frequency modulated DNP experiments have been shown to yield improved enhancements over conventional DNP techniques, and even to shorten polarization build-up times. The resulting increase in signal-to-noise ratios can lead to significantly shorter acquisition times in signal-limited multi-dimensional NMR experiments and pave the way to the study of even smaller sample volumes. In this paper we describe the design and performance of a broadband system for microwave frequency- and amplitude-modulated DNP that has been engineered to minimize both microwave and thermal losses during operation at liquid helium temperatures. The system incorporates a flexible source that can generate arbitrary waveforms at 94GHz with a bandwidth greater than 1GHz, as well as a probe that efficiently transmits the millimeter waves from room temperature outside the magnet to a cryogenic environment inside the magnet. Using a thin-walled brass tube as an overmoded waveguide to transmit a hybrid HE11 mode, it is possible to limit the losses to 1dB across a 2GHz bandwidth. The loss is dominated by the presence of a quartz window used to isolate the waveguide pipe. This performance is comparable to systems with corrugated waveguide or quasi-optical components. The overall excitation bandwidth of the probe is seen to be primarily determined by the final antenna or resonator used to excite the sample and its coupling to the NMR RF coil. Understanding the instrumental limitations imposed on any modulation scheme is key to understanding the observed DNP results and potentially identifying the underlying mechanisms. We demonstrate the utility of our design with a set of triangular frequency-modulated DNP experiments.

Theoretical aspects of Magic Angle Spinning - Dynamic Nuclear Polarization

Magic Angle Spinning (MAS) combined with Dynamic Nuclear Polarization (DNP) has been proven in recent years to be a very powerful method for increasing solid-state NMR signals. Since the advent of biradicals such as TOTAPOL to increase the nuclear polarization new classes of radicals, with larger molecular weight and/or different spin properties have been developed. These have led to unprecedented signal gain, with varying results for different experimental parameters, in particular the microwave irradiation strength, the static field, and the spinning frequency. Recently it has been demonstrated that sample spinning imposes DNP enhancement processes that differ from the active DNP mechanism in static samples as upon sample spinning the DNP enhancements are the results of energy level anticrossings occurring periodically during each rotor cycle. In this work we present experimental results with regards to the MAS frequency dependence of the DNP enhancement profiles of four nitroxide-based radicals at two different sets of temperature, 110 and 160K. In fact, different magnitudes of reduction in enhancement are observed with increasing spinning frequency. Our simulation code for calculating MAS-DNP powder enhancements of small model spin systems has been improved to extend our studies of the influence of the interaction and relaxation parameters on powder enhancements. To achieve a better understanding we simulated the spin dynamics of a single three-spin system {ea-eb-n} during its steady state rotor periods and used the Landau-Zener formula to characterize the influence of the different anti-crossings on the polarizations of the system and their necessary action for reaching steady state conditions together with spin relaxation processes. Based on these model calculations we demonstrate that the maximum steady state nuclear polarization cannot become larger than the maximum polarization difference between the two electrons during the steady state rotor cycle. This study also shows the complexity of the MAS-DNP process and therefore the necessity to rely on numerical simulations for understanding parametric dependencies of the enhancements. Finally an extension of the spin system up to five spins allowed us to probe the first steps of the transfer of polarization from the nuclei coupled to the electrons to further away nuclei, demonstrating a decrease in the spin-diffusion barrier under MAS conditions.

Simultaneous DNP enhancements of (1)H and (13)C nuclei: theory and experiments

DNP on heteronuclear spin systems often results in interesting phenomena such as the polarization enhancement of one nucleus during MW irradiation at the "forbidden" transition frequencies of another nucleus or the polarization transfer between the nuclei without MW irradiation. In this work we discuss the spin dynamics in a four-spin model system of the form {ea-eb-((1)H,(13)C)}, with the Larmor frequencies ωa, ωb, ωH and ωC, by performing Liouville space simulations. This spin system exhibits the common (1)H solid effect (SE), (13)C cross effect (CE) and in addition high order CE-DNP enhancements. Here we show, in particular, the "proton shifted (13)C-CE" mechanism that results in (13)C polarization when the model system, at one of its (13)C-CE conditions, is excited by a MW field at the zero quantum or double quantum electron-proton transitions ωMW = ωa ± ωH and ωMW = ωb ± ωH. Furthermore, we introduce the "heteronuclear" CE mechanism that becomes efficient when the system is at one of its combined CE conditions |ωa - ωb| = |ωH ± ωC|. At these conditions, simulations of the four-spin system show polarization transfer processes between the nuclei, during and without MW irradiation, resembling the polarization exchange effects often discussed in the literature. To link the "microscopic" four-spin simulations to the experimental results we use DNP lineshape simulations based on "macroscopic" rate equations describing the electron and nuclear polarization dynamics in large spin systems. This approach is applied based on electron-electron double resonance (ELDOR) measurements that show strong (1)H-SE features outside the EPR frequency range. Simulated ELDOR spectra combined with the indirect (13)C-CE (iCE) mechanism, result in additional "proton shifted (13)C-CE" features that are similar to the experimental ones. These features are also observed experimentally in (13)C-DNP spectra of a sample containing 15 mM of trityl in a glass forming solution of (13)C-glycerol/H2O and are analyzed by calculating the basic (13)C-SE and (13)C-iCE shapes using simulated ELDOR spectra that were fitted to the experimental ones.

Mechanisms of dynamic nuclear polarization in insulating solids

Dynamic nuclear polarization (DNP) is a technique used to enhance signal intensities in NMR experiments by transferring the high polarization of electrons to their surrounding nuclei. The past decade has witnessed a renaissance in the development of DNP, especially at high magnetic fields, and its application in several areas including biophysics, chemistry, structural biology and materials science. Recent technical and theoretical advances have expanded our understanding of established experiments: for example, the cross effect DNP in samples spinning at the magic angle. Furthermore, new experiments suggest that our understanding of the Overhauser effect and its applicability to insulating solids needs to be re-examined. In this article, we summarize important results of the past few years and provide quantum mechanical explanations underlying these results. We also discuss future directions of DNP and current limitations, including the problem of resolution in protein spectra recorded at 80-100 K.

Effects of the electron polarization on dynamic nuclear polarization in solids

The effect of the electron polarization distribution on the DNP line-shapes: theory and a demonstration on a 40 mM TEMPOL sample.

The effect of Gd on trityl-based dynamic nuclear polarisation in solids

The increase in ¹³C polarisation of ¹³C-urea dissolved in samples containing water/DMSO mixtures and trityl radical (OX063) in the presence of Gd³⁺ is explained by changes in electron relaxation, electron spectral diffusion and effective electron–proton hyperfine interaction.

The electron depolarization during dynamic nuclear polarization: measurements and simulations

Measurements and simulations of the electron spin polarization along the EPR spectrum of TEMPOL and trityl radicals, under DNP conditions.

The 13C solid DNP mechanisms with perchlorotriphenylmethyl radicals – the role of 35,37Cl

Chlorinated trityl radicals generate ¹³C DNP frequency sweeps that indicate contributions of e → ^36,37Cl → ¹³C polarization transfer mechanism at low temperatures and that the relative contributions of the solid effect and cross effects mechanism can be controlled by the microwave irradiation power.