Nuclear Polarization Research Articles

Core-polarization effects on electron scattering in 10B, 19F and 39K nuclei

In the study of NN interactions, calculations are performed from the longitudinal shape factor through inelastic scattering of electrons. The determination of the longitudinal shape factor in the inelastic scattering of electrons is based on the transfer momentum and angular momentum. We delve into the analysis of electron scattering shape factors as well as CP — standing for nuclear polarization — in longitudinal inelastic shell models. This is done using NuShellX, where we employ a sum potential HO to look at 10B, 39K, and 19F nuclei. Among the computations carried out include those of Single Particle Matrix Elements and Inelastic Electron Scattering Shape Factors which are then pitted against experimental data. The outcomes depict a scenario where the model space provides a full explanation on NushellX shape factor compared to nuclear polarization; this thereby implies less contribution from certain quarters towards shaping this factor. On another front, the comparison shows that results from longitudinal form factor polarization for NuShellX core find resonance with what is already available experimentally— a pointer towards reliability if not outright correctness.

Crystal structure validation of verinurad via proton-detected ultra-fast MAS NMR and machine learning.

The recent development of ultra-fast magic-angle spinning (MAS) (>100 kHz) provides new opportunities for structural characterization in solids. Here, we use NMR crystallography to validate the structure of verinurad, a microcrystalline active pharmaceutical ingredient. To do this, we take advantage of 1H resolution improvement at ultra-fast MAS and use solely 1H-detected experiments and machine learning methods to assign all the experimental proton and carbon chemical shifts. This framework provides a new tool for elucidating chemical information from crystalline samples with limited sample volume and yields remarkably faster acquisition times compared to 13C-detected experiments, without the need to employ dynamic nuclear polarization.

On the peculiar EPR spectra of P1 centers at high (12-20 T) magnetic fields.

The most common lattice defect in high-pressure high-temperature (HPHT) diamonds is the nitrogen substitution (P1) center. This is a paramagnetic defect with a single unpaired electron spin coupled to a 14N nuclear spin forming an S = 1/2, I = 1 spin system. While P1 centers have been studied by electron paramagnetic resonance (EPR) spectroscopy for decades, only recently did their behavior at ultra-high (>12 T) magnetic fields become of interest. This is because P1 centers were recently found to be very efficient polarizing agents in dynamic nuclear polarization (DNP) experiments, which are typically carried out at high magnetic fields. The P1 ultra-high field EPR spectra show multiple peaks which the lower fields spectra do not. In this paper, we present an account of the EPR spectra of P1 centers at ultra-high fields and show that the more complex spectra at 12-20 T are the result of significant state mixing in the mS = +1/2 electron spin manifold. The state mixing is a result of fulfilling the cancellation condition, meaning the e-14N hyperfine interaction equals twice the Larmor frequency of the 14N nuclear spin. We illustrate the influence of the cancellation condition on the EPR spectra by comparing EPR spectra acquired at 6.9 and 13.8 T. While the former are similar to the consensus spectra observed at lower fields, the latter are very different. We present numerical simulations that quantitatively account for the experimental spectra at both 6.9 and 13.8 T. Finally, we use electron electron double resonance (ELDOR) measurements to show that the cancellation condition results in increased spectral diffusion in the 13.8 T spectrum. This work sheds light on the spin properties of P1 centers under DNP-characteristic conditions, which will be conducive to their efficient utilization in DNP.

Endogenous metal-ion dynamic nuclear polarization for NMR signal enhancement in metal organic frameworks.

Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy is an established tool for obtaining such structural information, however its low sensitivity limits its application. This limitation can be overcome with dynamic nuclear polarization (DNP) which is based on polarization transfer from unpaired electrons to the nuclei of interest and, as a result, enhancement of the NMR signal. Typically, DNP is achieved by impregnating or wetting the MOF material with a solution of nitroxide biradicals, which prevents or interferes with the study of host-guest interactions. Here we demonstrate how Gd(iii) ions doped into the MOF structure, LaBTB (BTB = 4,4',4''-benzene-1,3,5-triyl-trisbenzoate), can be employed as an efficient polarization agent, yielding up to 30-fold 13C signal enhancement for the MOF linkers, while leaving the pores empty for potential guests. Furthermore, we demonstrate that ethylene glycol, loaded into the MOF as a guest, can also be polarized using our approach. We identify specific challenges in DNP studies of MOFs, associated with residual oxygen trapped within the MOF pores and the dynamics of the framework and its guests, even at cryogenic temperatures. To address these, we describe optimal conditions for carrying out and maximizing the enhancement achieved in DNP-NMR experiments. The approach presented here can be expanded to other porous materials which are currently the state-of-the-art in energy and sustainability research.

Computation of Overhauser dynamic nuclear polarization processes reveals fundamental correlation between water dynamics, structure, and solvent restructuring entropy.

Hydration water dynamics, structure, and thermodynamics are crucially important to understand and predict water-mediated properties at molecular interfaces. Yet experimentally and directly quantifying water behavior locally near interfaces at the sub-nanometer scale is challenging, especially at interfaces submerged in biological solutions. Overhauser dynamic nuclear polarization (ODNP) experiments measure equilibrium hydration water dynamics within 8-15 angstroms of a nitroxide spin probe on instantaneous timescales (10 picoseconds to nanoseconds), making ODNP a powerful tool for probing local water dynamics in the vicinity of the spin probe. As with other spectroscopic techniques, concurrent computational analysis is necessary to gain access to detailed molecular level information about the dynamic, structural, and thermodynamic properties of water from experimental ODNP data. We chose a model system that can systematically tune the dynamics of water, a water-glycerol mixture with compositions ranging from 0 to 0.3 mole fraction glycerol. We demonstrate the ability of molecular dynamics (MD) simulations to compute ODNP spectroscopic quantities, and show that translational, rotational, and hydrogen bonding dynamics of hydration water align strongly with spectroscopic ODNP parameters. Moreover, MD simulations show tight correlations between the dynamic properties of water that ODNP captures and the structural and thermodynamic behavior of water. Hence, experimental ODNP readouts of varying water dynamics suggest changes in local structural and thermodynamic hydration water properties.

Exploring the crystallisation of aspirin in a confined porous material using solid-state nuclear magnetic resonance.

In this study, nuclear magnetic resonance (NMR) is used to investigate the crystallisation behaviour of aspirin within a mesoporous SBA-15 silica material. The potential of dynamic nuclear polarisation (DNP) experiments is also investigated using specifically designed porous materials that incorporate polarising agents within their walls. The formation of the metastable crystalline form II is observed when crystallisation occurs within the pores of the mesoporous structure. Conversely, bulk crystallisation yields the most stable form, namely form I, of aspirin. Remarkably, the metastable form II remains trapped within the pores of mesoporous SBA-15 silica material even 30 days after impregnation, underscoring its persistent stability within this confined environment.

LC-Photo-CIDNP hyperpolarization of biomolecules bearing a quasi-isolated spin pair: Magnetic-Field dependence via a rapid-shuttling device

Liquid-state low-concentration photochemically induced dynamic nuclear polarization (LC-photo-CIDNP) is an emerging technology tailored to enhance the sensitivity of NMR spectroscopy via LED- or laser-mediated optical irradiation. LC-photo-CIDNP is particularly useful to detect solvent-exposed aromatic residues (Trp, Tyr), either in isolation or within polypeptides and proteins. This study investigates the magnetic-field dependence of the LC-photo-CIDNP of Trp-α-13C-β,β,2,4,5,6,7-d7, a Trp isotopolog bearing a quasi-isolated 1Hα-13Cαspin pair (QISP). We employed a new rapid-shuttling side-illumination field-cycling device that enables ultra-fast (90–120 ms) vertical movements of NMR samples within the bore of a superconducting magnet. Thus, LC-photo-CIDNP hyperpolarization occurs at low field, while hyperpolarized signals are detected at high field (700 MHz). Resonance lineshapes were excellent, and the effect of several fields (1.18–7.08 T range) on hyperpolarization efficiency could be readily explored. Remarkably, unprecedented LC-photo-CIDNP enhancements ε ≅ 1,200 were obtained at 50 MHz (1.18 T), suggesting exciting avenues to hypersensitive LED-enhanced NMR in liquids at low field.

Perfluorinated Iridium Catalyst for Signal Amplification by Reversible Exchange Provides Metal-Free Aqueous Hyperpolarized [1-13C]-Pyruvate.

Hyperpolarized (HP) carbon-13 [13C] enables the specific investigation of dynamic metabolic and physiologic processes via in vivo MRI-based molecular imaging. As the leading HP metabolic agent, [1-13C]pyruvate plays a pivotal role due to its rapid tissue uptake and central role in cellular energetics. Dissolution dynamic nuclear polarization (d-DNP) is considered the gold standard method for the production of HP metabolic probes; however, development of a faster, less expensive technique could accelerate the translation of metabolic imaging via HP MRI to routine clinical use. Signal Amplification by Reversible Exchange in SHield Enabled Alignment Transfer (SABRE-SHEATH) achieves rapid hyperpolarization by using parahydrogen (p-H2) as the source of nuclear spin order. Currently, SABRE is clinically limited due to the toxicity of the iridium catalyst, which is crucial to the SABRE process. To mitigate Ir contamination, we introduce a novel iteration of the SABRE catalyst, incorporating bis(polyfluoroalkylated) imidazolium salts. This novel perfluorinated SABRE catalyst retained polarization properties while exhibiting an enhanced hydrophobicity. This modification allows the easy removal of the perfluorinated SABRE catalyst from HP [1-13C]-pyruvate after polarization in an aqueous solution, using the ReD-SABRE protocol. The residual Ir content after removal was measured via ICP-MS at 177 ppb, which is the lowest reported to date for pyruvate and is sufficiently safe for use in clinical investigations. Further improvement is anticipated once automated processes for delivery and recovery are initiated. SABRE-SHEATH using the perfluorinated SABRE catalyst can become an attractive low-cost alternative to d-DNP to prepare biocompatible HP [1-13C]-pyruvate formulations for in vivo applications in next-generation molecular imaging modalities.

Dynamic Nuclear Polarization from Lithium Metal, a New Method to Study Lithium Batteries

The massive development of portable electronics, electric vehicles, and the increased need for energy storage require higher battery energy density, improved efficiency, and safety. In this context, lithium metal is the ultimate high-energy-density battery anode material. However, its use as an anode material is limited due to the formation of lithium dendrites that could lead to low efficiency and potential safety issues. Therefore, understanding and controlling the formation of lithium dendrites is key to realise lithium metal batteries.Here we show a new approach combining Scanning Electron Microscopy (SEM) and Magic-Angle Spinning Dynamic Nuclear Polarization (MAS DNP) to study the interplay between lithium morphology and interface reactivity. We aim to understand how the Solid-Electrolyte-Interphase (SEI), formed at the surface of lithium metal, affects lithium morphology. 1,2. Whereas the morphology of lithium dendrites can easily be identified using SEM, information about the SEI is more challenging to access. Among the different techniques used to characterize battery material, Nuclear Magnetic Resonance (NMR), has emerged as a powerful method providing information about the chemical composition as well as dynamic properties of a given material, at the atomic scale. Yet, the main limitations of NMR are its low intrinsic sensitivity and lack of selectivity which highly complicate the study of thin interphase as the SEI. In the present work, we use MAS DNP to overcome the low sensitivity of NMR. MAS DNP is based on the transfer of the large electron spin polarization of unpaired electrons to the nuclear spins, leading to the enhancement of the NMR signal intensity. However, the addition of free organic radicals solution to the typical exogenous DNP may alter the nature of the SEI. Furthermore, exogenous DNP is not selective to the interfaces between the lithium metal and the SEI. In this respect, we use the recently developed Lithium metal DNP method which exploits lithium metal's conduction electrons to perform endogenous MAS DNP and achieve one order of magnitude hyperpolarization at room temperature3. In addition to the gain of sensitivity, this technique allows to selectively enhance the chemical species close to the lithium metal providing critical information about the chemical structure of the SEI. This approach, currently under development, provides a powerful tool in research to control and prevent the formation of lithium dendrites which typically result in rapid degradation and potential safety issues. Figure: A) Schematic representation of Lithium dendrites with SEM of lithium dendrites formed on a copper current collector. B) Schematic representation of SEI formed on lithium metal with 7Li MAS DNP spectra recorded with and without microwave irradiation on lithium dendrites

Nitrogen Substitutions Aggregation and Clustering in Diamonds as Revealed by High-Field Electron Paramagnetic Resonance.

Diamonds have been shown to be an excellent platform for quantum computing and quantum sensing applications. These applications are enabled by the presence of defects in the lattice, which are also known as color centers. The most common nitrogen-based defect in synthetic diamonds is the paramagnetic nitrogen substitution (P1) center. While the majority of quantum applications rely on nitrogen-vacancy (NV) centers, the properties of the latter are heavily influenced by the presence and the spatial distribution of the P1 centers. Hence, understanding the spatial distribution and mutual interactions of P1 centers is crucial for the successful development of diamond-based quantum devices. Unlike NV centers, P1 centers do not have a spin-dependent optical signature, and their spin-related properties, therefore, have to be detected and characterized using magnetic resonance methods. We show that using high-field (6.9 and 13.8 T) pulsed electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) experiments, we can distinguish and quantify three distinct populations of P1 centers: isolated P1 centers, weakly interacting ones, and exchange-coupled ones that are clustered together. While such clustering was suggested before, these clusters were never detected directly and unambiguously. Moreover, by using electron-electron double resonance (ELDOR) pump-probe experiments, we demonstrate that the latter clustered population does not exist in isolation but coexists with the more weakly interacting P1 centers throughout the diamond lattice. Its presence thus strongly affects the quantum properties of the diamond. We also show that the existence of this population can explain recent hyperpolarization results in type Ib high-pressure, high-temperature (HPHT) diamonds. We propose a combination of high-field pulsed EPR, ELDOR, and DNP as a tool for probing the aggregation state and interactions among different populations of nitrogen substitution centers.

P1 Center Electron Spin Clusters Are Prevalent in Type Ib Diamonds.

Understanding the spatial distribution of the P1 centers is crucial for diamond-based sensors and quantum devices. P1 centers serve as polarization sources for dynamic nuclear polarization (DNP) quantum sensing and play a significant role in the relaxation of nitrogen vacancy (NV) centers. Additionally, the distribution of NV centers correlates with the distribution of P1 centers, as NV centers are formed through the conversion of P1 centers. We utilized DNP and pulsed electron paramagnetic resonance (EPR) techniques that revealed strong clustering of a significant population of P1 centers that exhibit exchange coupling and produce asymmetric line shapes. The 13C DNP frequency profile at a high magnetic field revealed a pattern that requires an asymmetric EPR line shape of the P1 clusters with electron-electron (e-e) coupling strengths exceeding the 13C nuclear Larmor frequency. EPR and DNP characterization at high magnetic fields was necessary to resolve energy contributions from different e-e couplings. We employed a two-frequency pump-probe pulsed electron double resonance technique to show cross-talk between the isolated and clustered P1 centers. This finding implies that the clustered P1 centers affect all of the P1 populations. Direct observation of clustered P1 centers and their asymmetric line shape offers a novel and crucial insight into understanding magnetic noise sources for quantum information applications of diamonds and for designing diamond-based polarizing agents with optimized DNP efficiency for 13C and other nuclear spins of analytes. We propose that room temperature 13C DNP at a high field, achievable through straightforward modifications to existing solution-state NMR systems, is a potent tool for evaluating and controlling diamond defects.

Hyperpolarization-Enhanced NMR Spectroscopy of Unaltered Biofluids Using Photo-CIDNP.

The direct and unambiguous detection and identification of individual metabolite molecules present in complex biological mixtures constitute a major challenge in (bio)analytical research. In this context, nuclear magnetic resonance (NMR) spectroscopy has proven to be particularly powerful owing to its ability to provide both qualitative and quantitative atomic-level information on multiple analytes simultaneously in a noninvasive manner. Nevertheless, NMR suffers from a low inherent sensitivity and, moreover, lacks selectivity regarding the number of individual analytes to be studied in a mixture of a myriad of structurally and chemically very different molecules, e.g., metabolites in a biofluid. Here, we describe a method that circumvents these shortcomings via performing selective, photochemically induced dynamic nuclear polarization (photo-CIDNP) enhanced NMR spectroscopy on unmodified complex biological mixtures, i.e., human urine and serum, which yields a single, background-free one-dimensional NMR spectrum. In doing this, we demonstrate that photo-CIDNP experiments on unmodified complex mixtures of biological origin are feasible, can be performed straightforwardly in the native aqueous medium at physiological metabolite concentrations, and act as a spectral filter, facilitating the analysis of NMR spectra of complex biofluids. Due to its noninvasive nature, the method is fully compatible with state-of-the-art metabolomic protocols providing direct spectroscopic information on a small, carefully selected subset of clinically relevant metabolites. We anticipate that this approach, which, in addition, can be combined with existing high-throughput/high-sensitivity NMR methodology, holds great promise for further in-depth studies and development for use in metabolomics and many other areas of analytical research.

Multi Electron Spin Cluster Enabled Dynamic Nuclear Polarization with Sulfonated BDPA.

Dynamic nuclear polarization (DNP) can amplify the solid-state nuclear magnetic resonance (NMR) signal by several orders of magnitude. The mechanism of DNP utilizing α,γ-bisdiphenylene-β-phenylallyl (BDPA) variants as Polarizing Agents (PA) has been the subject of lively discussions on account of their remarkable DNP efficiency with low demand for microwave power. We propose that electron spin clustering of sulfonated BDPA is responsible for its DNP performance, as revealed by the temperature-dependent shape of the central DNP profile and strong electron-electron (e-e) crosstalk seen by Electron Double Resonance. We demonstrate that a multielectron spin cluster can be modeled with three coupled spins, where electron J (exchange) coupling between one of the e-e pairs matching the NMR Larmor frequency induces the experimentally observed absorptive central DNP profile, and the electron T1e modulated by temperature and magic-angle spinning alters the shape between an absorptive and dispersive feature. Understanding the microscopic origin is key to designing new PAs to harness the microwave-power-efficient DNP effect observed with BDPA variants.

Vosstanovlenie po trebovaniyu volnovoy formy messbauerovskogo gamma-fotona posredstvom zaderzhannoy akusticheski indutsirovannoy prozrachnosti

A method has been proposed to reconstruct at arbitrary time the spectral–temporal characteristics of a14.4-keV single-photon wave packet that is emitted by a 57Co source and is resonantly absorbed in themedium of 57Fe nuclei. The method is based on the frequency separation of the field emitted by the sourceand resonance nuclear polarization induced by this field by means of delayed acoustically induced transparencyof the absorber, which appears after the activation of oscillations of the absorber at the correspondingfrequency and amplitude. The proposed method has been compared to the known quantum-optical memorymethods and methods of nuclear polarization control in the gamma range. Experimental conditions havebeen proposed to implement the method. It has been shown that this method allows the implementation ofthe time-resolved Mössbauer spectroscopy of various media.

Spectroscopic Techniques to Unravel Mechanistic Details in Light‐Induced Transformations and Photoredox Catalysis

AbstractThe rapid development of photo(redox) catalysis within the last decades is remarkable to the extent that the utilization of light‐driven processes in organic chemistry has become a credible alternative to current thermal processes. Such advances offer tremendous opportunities of collaborations between scientific realms that can have a drastic impact on the development of the field. In this concept article, a special emphasis is placed on spectroscopic techniques that are used, or could be used, for light‐induced transformations and photoredox catalysis applications. These include spectroelectrochemistry, UV‐VIS, IR and X‐Ray transient absorption spectroscopy, laser pulsed radiolysis (PR), photo‐induced chemically induced dynamic nuclear polarization (Photo‐CIDNP), photoacoustic spectroscopy, time‐resolved Raman spectroscopy (TRRS), time‐resolved Electron Paramagnetic Resonance (TREPR) and time‐resolved dielectric loss spectroscopy (TRDL). The theoretical background behind each technique is briefly introduced followed by selected relevant examples from the literature.

Spin-Polarized Radicals with Extremely Long Spin-Lattice Relaxation Time at Room Temperature in a Metal-Organic Framework.

The generation of spin polarization is key in quantum information science and dynamic nuclear polarization. Polarized electron spins with long spin-lattice relaxation times (T1) at room temperature are important for these applications but have been difficult to achieve. We report the realization of spin-polarized radicals with extremely long T1 at room temperature in a metal-organic framework (MOF) in which azaacene chromophores are densely integrated. Persistent radicals are generated in the MOF by charge separation after photoexcitation. Spin polarization of a triplet generated by photoexcitation is successfully transferred to the persistent radicals. Pulse electron spin resonance measurements reveal that the T1 of the polarized radical in the MOF is as long as 214 μs with a relatively long spin-spin relaxation time T2 of the radicals of up to 0.98 μs at room temperature. The achievement of extremely long spin polarization in MOFs with nanopores accessible to guest molecules will be an important cornerstone for future highly sensitive quantum sensing and efficient dynamic nuclear polarization.

Rapid and Simple 13C-Hyperpolarization by 1H Dissolution Dynamic Nuclear Polarization Followed by an Inline Magnetic Field Inversion.

Dissolution dynamic nuclear polarization (dDNP) is a method of choice for preparing hyperpolarized 13C metabolites such as 1-13C-pyruvate used for in vivo applications, including the real-time monitoring of cancer cell metabolism in human patients. The approach consists of transferring the high polarization of electron spins to nuclear spins via microwave irradiation at low temperatures (1.0-1.5 K) and moderate magnetic fields (3.3-7 T). The solid sample is then dissolved and transferred to an NMR spectrometer or MRI scanner for detection in the liquid state. Common dDNP protocols use direct hyperpolarization of 13C spins reaching polarizations of >50% in ∼1-2 h. Alternatively, 1H spins are polarized before transferring their polarization to 13C spins using cross-polarization, reaching polarization levels similar to those of direct DNP in only ∼20 min. However, it relies on more complex instrumentation, requiring highly skilled personnel. Here, we explore an alternative route using 1H dDNP followed by inline adiabatic magnetic field inversion in the liquid state during the transfer. 1H polarizations of >70% in the solid state are obtained in ∼5-10 min. As the hyperpolarized sample travels from the dDNP polarizer to the NMR spectrometer, it goes through a field inversion chamber, which causes the 1H → 13C polarization transfer. This transfer is made possible by the J-coupling between the heteronuclei, which mixes the Zeeman states at zero-field and causes an antilevel crossing. We report liquid-state 13C polarization up to ∼17% for 3-13C-pyruvate and 13C-formate. The instrumentation needed to perform this experiment in addition to a conventional dDNP polarizer is simple and readily assembled.

Elementary processes triggered in curcumin molecule by gas-phase resonance electron attachment and by photoexcitation in solution.

Electron-driven processes in isolated curcumin (CUR) molecules are studied by means of dissociative electron attachment (DEA) spectroscopy under gas-phase conditions. Elementary photostimulated reactions initiated in CUR molecules under UV irradiation are studied using the chemically induced dynamic nuclear polarization method in an acetonitrile solvent. Density functional theory is applied to elucidate the energetics of fragmentation of CUR by low-energy (0-15eV) resonance electron attachment and to characterize various CUR radical forms. The adiabatic electron affinity of CUR molecule is experimentally estimated to be about 1eV. An extra electron attachment to the π1* LUMO and π2* molecular orbitals is responsible for the most intense DEA signals observed at thermal electron energy. The most abundant long-lived (hundreds of micro- to milliseconds) molecular negative ions CUR- are detected not only at the thermal energy of incident electrons but also at 0.6eV, which is due to the formation of the π3* and π4* temporary negative ion states predicted to lie around 1eV. Proton-assisted electron transfer between CUR molecules is registered under UV irradiation. The formation of both radical-anions and radical-cations of CUR is found to be more favorable in its enol form. The present findings shed some light on the elementary processes triggered in CUR by electrons and photons and, therefore, can be useful to understand the molecular mechanisms responsible for a variety of biological effects produced by CUR.

Hydrogen bonding in duplex DNA probed by DNP enhanced solid-state NMR N-H bond length measurements.

Numerous biological processes and mechanisms depend on details of base pairing and hydrogen bonding in DNA. Hydrogen bonds are challenging to quantify by X-ray crystallography and cryo-EM due to difficulty of visualizing hydrogen atom locations but can be probed with site specificity by NMR spectroscopy in solution and the solid state with the latter particularly suited to large, slowly tumbling DNA complexes. Recently, we showed that low-temperature dynamic nuclear polarization (DNP) enhanced solid-state NMR is a valuable tool for distinguishing Hoogsteen base pairs (bps) from canonical Watson-Crick bps in various DNA systems under native-like conditions. Here, using a model 12-mer DNA duplex containing two central adenine-thymine (A-T) bps in either Watson-Crick or Hoogsteen confirmation, we demonstrate DNP solid-state NMR measurements of thymine N3-H3 bond lengths, which are sensitive to details of N-H···N hydrogen bonding and permit hydrogen bonds for the two bp conformers to be systematically compared within the same DNA sequence context. For this DNA duplex, effectively identical TN3-H3 bond lengths of 1.055 ± 0.011Å and 1.060 ± 0.011Å were found for Watson-Crick A-T and Hoogsteen A (syn)-T base pairs, respectively, relative to a reference amide bond length of 1.015 ± 0.010Å determined for N-acetyl-valine under comparable experimental conditions. Considering that prior quantum chemical calculations which account for zero-point motions predict a somewhat longer effective peptide N-H bond length of 1.041Å, in agreement with solution and solid-state NMR studies of peptides and proteins at ambient temperature, to facilitate direct comparisons with these earlier studies TN3-H3 bond lengths for the DNA samples can be readily scaled appropriately to yield 1.083Å and 1.087Å for Watson-Crick A-T and Hoogsteen A (syn)-T bps, respectively, relative to the 1.041Å reference peptide N-H bond length. Remarkably, in the context of the model DNA duplex, these results indicate that there are no significant differences in N-H···N A-T hydrogen bonds between Watson-Crick and Hoogsteen bp conformers. More generally, high precision measurements of N-H bond lengths by low-temperature DNP solid-state NMR based methods are expected to facilitate detailed comparative analysis of hydrogen bonding for a range of DNA complexes and base pairing environments.

On-Demand Reconstruction of the Waveform of a Mössbauer Gamma-Ray Photon by Means of Delayed Acoustically Induced Transparency

A method has been proposed to reconstruct at arbitrary time the spectral–temporal characteristics of a 14.4‑keV single-photon wave packet that is emitted by a 57Co source and is resonantly absorbed in the medium of 57Fe nuclei. The method is based on the frequency separation of the field emitted by the source and resonance nuclear polarization induced by this field by means of delayed acoustically induced transparency of the absorber, which appears after the activation of oscillations of the absorber at the corresponding frequency and amplitude. The proposed method has been compared to the known quantum-optical memory methods and methods of nuclear polarization control in the gamma-ray range. Experimental conditions have been proposed to implement the method. It has been shown that this method allows the implementation of the time-resolved Mössbauer spectroscopy of various media.