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.

Breaking Strong Alkynyl-Phenyl Bonds: Poly(para-phenylene ethynylene)s under Mechanical Stress.

Stronger chemical bonds withstand higher mechanical forces; thus, the rupture of single bonds is preferred over the rupture of double or triple bonds or aromatic rings. We investigated bond scission in poly(dialkyl-p-phenylene ethynylene)s (PPEs), a fully conjugated polymer. In a scale-bridging approach using electron-paramagnetic resonance spectroscopy and gel permeation chromatography of cryomilled samples, in combination with density functional theory calculations and coarse-grained simulations, we conclude that mechanical force cleaves the sp-sp2 bond of PPEs (bond dissociation energy as high as 600 kJ mol-1). Bond scission primarily occurs in shear bands with locally increased shear stresses. The scission occurs in the middle of the PPE chains. Breaking sp-sp2 bonds into free radicals thus is feasible but requires significant mechanical force and an efficient stress concentration.

Structure and Internal Dynamics of Short RNA Duplexes Determined by a Combination of Pulsed EPR Methods and MD Simulations.

We used EPR spectroscopy to characterize the structure of RNA duplexes and their internal twist, stretch and bending motions. We prepared eight 20-base-pair-long RNA duplexes containing the rigid spin-label Çm, a cytidine analogue, at two positions and acquired orientation-selective PELDOR/DEER data. By using different frequency bands (X-, Q-, G-band), detailed information about the distance and orientation of the labels was obtained and provided insights into the global conformational dynamics of the RNA duplex. We used 19F Mims ENDOR experiments on three singly Çm- and singly fluorine-labeled RNA duplexes to determine the exact position of the Çm spin label in the helix. In a quantitative comparison to MD simulations of RNA with and without Çm spin labels, we found that state-of-the-art force fields with explicit parameterization of the spin label were able to describe the conformational ensemble present in our experiments. The MD simulations further confirmed that the Çm spin labels are excellent mimics of cytidine inducing only small local changes in the RNA structure. Çm spin labels are thus ideally suited for high-precision EPR experiments to probe the structure and, in conjunction with MD simulations, motions of RNA.

Solid-Effect Dynamic Nuclear Polarization in Viscous Liquids at 9.4 T Using Narrow-Line Polarizing Agents.

Dynamic nuclear polarization (DNP) is a hyperpolarization method that is widely used for increasing the sensitivity of nuclear magnetic resonance (NMR) experiments. DNP is efficient in solid-state and liquid-state NMR, but its implementation in the intermediate state, namely, viscous media, is still less explored. Here, we show that a 1H DNP enhancement of over 50 can be obtained in viscous liquids at a magnetic field of 9.4 T and a temperature of 315 K. This was accomplished by using narrow-line polarizing agents in glycerol, both the water-soluble α,γ-bisdiphenylen-ß-phenylallyl (BDPA) and triarylmethyl radicals, and a microwave/RF double-resonance probehead. We observed DNP enhancements with a field profile indicative of the solid effect and investigated the influence of microwave power, temperature, and concentration on the 1H NMR results. To demonstrate potential applications of this new DNP approach for chemistry and biology, we show hyperpolarized 1H NMR spectra of tripeptides, triglycine, and glypromate, in glycerol-d8.

Efficient determination of the accessible conformation space of multi-domain complexes based on EPR PELDOR data.

Many proteins can adopt multiple conformations which are important for their function. This is also true for proteins and domains that are covalently linked to each other. One important example is ubiquitin, which can form chains of different conformations depending on which of its lysine side chains is used to form an isopeptide bond with the C-terminus of another ubiquitin molecule. Similarly, ubiquitin gets covalently attached to active-site residues of E2 ubiquitin-conjugating enzymes. Due to weak interactions between ubiquitin and its interaction partners, these covalent complexes adopt multiple conformations. Understanding the function of these complexes requires the characterization of the entire accessible conformation space and its modulation by interaction partners. Long-range (1.8-10 nm) distance restraints obtained by EPR spectroscopy in the form of probability distributions are ideally suited for this task as not only the mean distance but also information about the conformation dynamics is encoded in the experimental data. Here we describe a computational method that we have developed based on well-established structure determination software using NMR restraints to calculate the accessible conformation space using PELDOR/DEER data.

DOPA Residues Endow Collagen with Radical Scavenging Capacity.

Here we uncover collagen, the main structural protein of all connective tissues, as a redox-active material. We identify dihydroxyphenylalanine (DOPA) residues, post-translational oxidation products of tyrosine residues, to be common in collagen derived from different connective tissues. We observe that these DOPA residues endow collagen with substantial radical scavenging capacity. When reducing radicals, DOPA residues work as redox relay: they convert to the quinone and generate hydrogen peroxide. In this dual function, DOPA outcompetes its amino acid precursors and ascorbic acid. Our results establish DOPA residues as redox-active side chains of collagens, probably protecting connective tissues against radicals formed under mechanical stress and/or inflammation.

Long-range distance determination in fully deuterated RNA with pulsed EPR spectroscopy.

Pulsed electron-electron double resonance (PELDOR or DEER) spectroscopy is powerful in structure and dynamics study of biological macromolecules by providing distance distribution information ranging from 1.8 to 6 nm, providing that the biomolecules are site-specifically labeled with paramagnetic tags. However, long distances up to 16 nm have been measured on perdeuterated and spin-labeled proteins in deuterated solvent by PELDOR. Here we demonstrate long-range distance measurement on a large RNA, the 97-nucleotide 3'SL RNA element of the Dengue virus 2 genome, by combining a posttranscriptional site-directed spin labeling method using an unnatural basepair system with RNA perdeuteration by enzymatic synthesis using deuterated nucleotides. The perdeuteration removes the coupling of the electron spins of the nitroxide spin labels from the proton nuclear spin system of the RNA and does extend the observation time windows of PELDOR up to 50 µs. This enables one to determine long distances up to 14 nm for large RNAs and their conformational flexibility.

Solid-like Dynamic Nuclear Polarization Observed in the Fluid Phase of Lipid Bilayers at 9.4 T.

Dynamic nuclear polarization (DNP) is a powerful method to enhance NMR sensitivity. Much progress has been achieved recently to optimize DNP performance at high magnetic fields in solid-state samples, mostly by utilizing the solid or the cross effect. In liquids, only the Overhauser mechanism is active, which exhibits a DNP field profile matching the EPR line shape of the radical, distinguishable from other DNP mechanisms. Here, we observe DNP enhancements with a field profile indicative of the solid effect and thermal mixing at ∼320 K and a magnetic field of 9.4 T in the fluid phase of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers doped with the radical BDPA (1,3-bis(diphenylene)-2-phenylallyl). This interesting observation might open up new perspectives for DNP applications in macromolecular systems at ambient temperatures.

Spin-Labeled Riboswitch Synthesized from a Protected TPA Phosphoramidite Building Block.

The nitroxide TPA (2,2,5,5-tetramethyl-pyrrolin-1-oxyl-3-acetylene) is an excellent spin label for EPR studies of RNA. Previous synthetic methods, however, are complicated and require special equipment. Herein, we describe a uridine derived phosphoramidite with a photocaged TPA unit attached. The light sensitive 2-nitrobenzyloxymethyl group can be removed in high yield by short irradiation at 365 nm. Based on this approach, a doubly spin-labeled 27mer neomycin sensing riboswitch was synthesized and studied by PELDOR. The overall thermal stability of the fold is not much reduced by TPA. In-line probing nevertheless detected changes in local mobility.

High-resolution EPR distance measurements on RNA and DNA with the non-covalent Ç´ spin label.

Pulsed electron paramagnetic resonance (EPR) experiments, among them most prominently pulsed electron-electron double resonance experiments (PELDOR/DEER), resolve the conformational dynamics of nucleic acids with high resolution. The wide application of these powerful experiments is limited by the synthetic complexity of some of the best-performing spin labels. The recently developed $\bf\acute{G}$ (G-spin) label, an isoindoline-nitroxide derivative of guanine, can be incorporated non-covalently into DNA and RNA duplexes via Watson-Crick base pairing in an abasic site. We used PELDOR and molecular dynamics (MD) simulations to characterize $\bf\acute{G}$, obtaining excellent agreement between experiments and time traces calculated from MD simulations of RNA and DNA double helices with explicitly modeled $\bf\acute{G}$ bound in two abasic sites. The MD simulations reveal stable hydrogen bonds between the spin labels and the paired cytosines. The abasic sites do not significantly perturb the helical structure. $\bf\acute{G}$ remains rigidly bound to helical RNA and DNA. The distance distributions between the two bound $\bf\acute{G}$ labels are not substantially broadened by spin-label motions in the abasic site and agree well between experiment and MD. $\bf\acute{G}$ and similar non-covalently attached spin labels promise high-quality distance and orientation information, also of complexes of nucleic acids and proteins.

Benchmark Test and Guidelines for DEER/PELDOR Experiments on Nitroxide-Labeled Biomolecules.

Distance distribution information obtained by pulsed dipolar EPR spectroscopy provides an important contribution to many studies in structural biology. Increasingly, such information is used in integrative structural modeling, where it delivers unique restraints on the width of conformational ensembles. In order to ensure reliability of the structural models and of biological conclusions, we herein define quality standards for sample preparation and characterization, for measurements of distributed dipole-dipole couplings between paramagnetic labels, for conversion of the primary time-domain data into distance distributions, for interpreting these distributions, and for reporting results. These guidelines are substantiated by a multi-laboratory benchmark study and by analysis of data sets with known distance distribution ground truth. The study and the guidelines focus on proteins labeled with nitroxides and on double electron-electron resonance (DEER aka PELDOR) measurements and provide suggestions on how to proceed analogously in other cases.

"End-to-end" stacking of small dsRNA.

PELDOR (pulsed electron-electron double resonance) is an established method to study intramolecular distances and can give evidence for conformational changes and flexibilities. However, it can also be used to study intermolecular interactions as for example oligerimization. Here, we used PELDOR to study the "end-to-end" stacking of small double-stranded (ds) RNAs. For this study, the dsRNA molecules were only singly labeled with the spin label TPA to avoid multispin effects and to measure only the intermolecular stacking interactions. It can be shown that small dsRNAs tend to assemble to rod-like structures due to π-π interactions between the base pairs at the end of the strands. On the one hand, these interactions can influence or complicate measurements aimed at the determining of the structure and dynamics of the dsRNA molecule itself. On the other hand, it can be interesting to study such intermolecular stacking interactions in more detail, as for example their dependence on ion concentration. We quantitatively determined the stacking probability as a function of the monovalent NaCl salt and the dsRNA concentration. From these data, the dissociation constant Kd was deduced and found to depend on the ratio between the NaCl salt and dsRNA concentrations. Additionally, the distances and distance distributions obtained predict a model for the stacking geometry of dsRNAs. Introducing a nucleotide overhangs at one end of the dsRNA molecule restricts the stacking to the other end, leading only to dimer formations. Introducing such an overhang at both ends of the dsRNA molecule fully suppresses stacking, as we demonstrate by PELDOR experiments quantitatively.

Influence of Mg2+ on the conformational flexibility of a tetracycline aptamer.

The tetracycline-binding RNA aptamer (TC-aptamer) is a synthetic riboswitch that binds the antibiotic tetracycline (TC) with exceptionally high affinity. Although a crystal structure exists of the TC-bound state, little is known about the conformational dynamics and changes upon ligand binding. In this study, pulsed electron paramagnetic resonance techniques for measuring distances (PELDOR) in combination with rigid nitroxide spin labels (Çm spin label) were used to investigate the conformational flexibility of the TC-aptamer in the presence and absence of TC at different Mg2+ concentrations. TC was found to be the essential factor for stabilizing the tertiary structure at intermediate Mg2+ concentrations. At higher Mg2+ concentrations, Mg2+ alone is sufficient to stabilize the tertiary structure. In addition, the orientation of the two spin-labeled RNA helices with respect to each other was analyzed with orientation-selective PELDOR and compared to the crystal structure. These results demonstrate for the first time the unique value of the Çm spin label in combination with PELDOR to provide information about conformational flexibilities and orientations of secondary structure elements of biologically relevant RNAs.

Mixed-Valence Compounds as Polarizing Agents for Overhauser Dynamic Nuclear Polarization in Solids*.

Herein, we investigate a novel set of polarizing agents-mixed-valence compounds-by theoretical and experimental methods and demonstrate their performance in high-field dynamic nuclear polarization (DNP) NMR experiments in the solid state. Mixed-valence compounds constitute a group of molecules in which molecular mobility persists even in solids. Consequently, such polarizing agents can be used to perform Overhauser-DNP experiments in the solid state, with favorable conditions for dynamic nuclear polarization formation at ultra-high magnetic fields.

Compaction of RNA Duplexes in the Cell*.

The structure and flexibility of RNA depends sensitively on the microenvironment. Using pulsed electron-electron double-resonance (PELDOR)/double electron-electron resonance (DEER) spectroscopy combined with advanced labeling techniques, we show that the structure of double-stranded RNA (dsRNA) changes upon internalization into Xenopus laevis oocytes. Compared to dilute solution, the dsRNA A-helix is more compact in cells. We recapitulate this compaction in a densely crowded protein solution. Atomic-resolution molecular dynamics simulations of dsRNA semi-quantitatively capture the compaction, and identify non-specific electrostatic interactions between proteins and dsRNA as a possible driver of this effect.

Dynamic Nuclear Polarization of 13 C Nuclei in the Liquid State over a 10†Tesla Field Range.

Nuclear magnetic resonance (NMR) techniques play an essential role in natural science and medicine. In spite of the tremendous utility associated with the small energies detected, the most severe limitation is the low signal-to-noise ratio. Dynamic nuclear polarization (DNP), a technique based on transfer of polarization from electron to nuclear spins, has emerged as a tool to enhance sensitivity of NMR. However, the approach in liquids still faces several challenges. Herein we report the observation of room-temperature, liquid DNP 13 C signal enhancements in organic small molecules as high as 600 at 9.4 Tesla and 800 at 1.2 Tesla. A mechanistic investigation of the 13 C-DNP field dependence shows that DNP efficiency is raised by proper choice of the polarizing agent (paramagnetic center) and by halogen atoms as mediators of scalar hyperfine interaction. Observation of sizable DNP of 13 CH2 and 13 CH3 groups in organic molecules at 9.4 T opens perspective for a broader application of this method.

Conformational Coupling and trans-Inhibition in the Human Antigen Transporter Ortholog TmrAB Resolved with Dipolar EPR Spectroscopy.

ATP-binding cassette (ABC) exporters actively move chemically diverse substrates across biological membranes. Their malfunction leads to human diseases. Many ABC exporters encompass asymmetric nucleotide-binding sites (NBSs), and some of them are inhibited by the transported substrate. The functional relevance of the catalytic asymmetry or the mechanism for trans-inhibition remains elusive. Here, we investigated TmrAB, a functional homologue of the human antigen translocation complex TAP using advanced electron-electron double resonance spectroscopy. In the presence of ATP, the heterodimeric ABC exporter exists in a tunable equilibrium between inward- and outward-facing conformations. The two NBSs exhibit pronounced asymmetry in the open-to-close equilibrium. The closed conformation is more favored at the degenerate NBS, and closure of either of the NBS is sufficient to open the extracellular gate. We define the mechanistic basis for trans-inhibition, which operates by a reverse transition from the outward-facing state through an occluded conformation. These novel findings uncover the central role of reversible conformational equilibrium in the function and regulation of an ABC exporter and establish a mechanistic framework for future investigations on other medically important transporters with imprinted asymmetry. Also, this study demonstrates for the first-time the feasibility to resolve equilibrium populations at multiple domains and their interdependence for global conformational changes in a large membrane protein complex.

Unexplored Nucleotide Binding Modes for the ABC Exporter MsbA.

The ATP-binding cassette (ABC) transporter MsbA is an ATP-driven lipid-A flippase. It belongs to the ABC protein superfamily whose members are characterized by conserved motifs in their nucleotide binding domains (NBDs), which are responsible for ATP hydrolysis. Recently, it was found that MsbA could catalyze a reverse adenylate kinase (rAK)-like reaction in addition to ATP hydrolysis. Both reactions are connected and mediated by the same conserved NBD domains. Here, the structural foundations underlying the nucleotide binding to MsbA were therefore explored using a concerted approach based on conventional- and DNP-enhanced solid-state NMR, pulsed-EPR, and MD simulations. MsbA reconstituted into lipid bilayers was trapped in various catalytic states corresponding to intermediates of the coupled ATPase-rAK mechanism. The analysis of nucleotide-binding dependent chemical shift changes, and the detection of through-space contacts between bound nucleotides and MsbA within these states provides evidence for an additional nucleotide-binding site in close proximity to the Q-loop and the His-Switch. By replacing Mg2+ with Mn2+ and employing pulsed EPR spectroscopy, evidence is provided that this newly found nucleotide binding site does not interfere with the coordination of the required metal ion. Molecular dynamic (MD) simulations of nucleotide and metal binding required for the coupled ATPase-rAK mechanism have been used to corroborate these experimental findings and provide additional insight into nucleotide location, orientation, and possible binding modes.

A Cytidine Phosphoramidite with Protected Nitroxide Spin Label: Synthesis of a Full-Length TAR RNA and Investigation by In-Line Probing and EPR Spectroscopy.

EPR studies on RNA are complicated by three major obstacles related to the chemical nature of nitroxide spin labels: Decomposition while oligonucleotides are chemically synthesized, further decay during enzymatic strand ligation, and undetected changes in conformational equilibria due to the steric demand of the label. Herein possible solutions for all three problems are presented: A 2-nitrobenzyloxymethyl protective group for nitroxides that is stable under all conditions of chemical RNA synthesis and can be removed photochemically. By careful selection of ligation sites and splint oligonucleotides, high yields were achieved in the assembly of a full-length HIV-1 TAR RNA labeled with two protected nitroxide groups. PELDOR measurements on spin-labeled TAR in the absence and presence of arginine amide indicated arrest of interhelical motions on ligand binding. Finally, even minor changes in conformation due to the presence of spin labels are detected with high sensitivity by in-line probing.

A semi-rigid isoindoline-derived nitroxide spin label for RNA.

A new isoindoline-derived benzimidazole nitroxide spin label, ImUm, was synthesized and incorporated into RNA oligoribonucleotides. ImUm is the first example of a conformationally unambiguous spin label for RNA, in which the nitroxide N-O bond lies on the same axis as the single bond used to attach the rigid isoindoline-based spin label to a uridine base. This results in minimal displacement of the nitroxide upon rotation of this single bond, which is a useful property for a label to be used for distance measurements. Continuous-wave (CW) EPR measurements of RNA duplexes containing ImUm indicate a restricted rotation around this single bond, presumably due to an intramolecular hydrogen bond between the benzimidazole N-H and O4 of the uracil. Orientation-selective pulsed electron-electron double resonance (PELDOR, also called double electron-electron resonance, or DEER) distance measurements between two spin labels in two RNA duplexes showed in one case a strong orientation dependence, further confirming the restricted motion of the spin labels in RNA duplexes.