Dynamic Nuclear Polarization Spectroscopy Research Articles

Shining a Spotlight on Methyl Groups: Photochemically Induced Dynamic Nuclear Polarization Spectroscopy of 5-Deazariboflavin and Its Nor Analogs.

5-Deazaflavins are analogs of naturally occurring flavin cofactors. They serve as substitutes for natural flavin cofactors to investigate and modify the reaction pathways of flavoproteins. Demethylated 5-deazaflavins are potential candidates for artificial cofactors, allowing us to fine-tune the reaction kinetics and absorption characteristics of flavoproteins. In this contribution, demethylated 5-deazariboflavin radicals are investigated (1) to assess the influence of the methyl groups on the electronic structure of the 5-deazaflavin radical and (2) to explore their photophysical properties with regard to their potential as artificial cofactors. We determined the proton hyperfine structure of demethylated 5-deazariboflavins using photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy, as well as density functional theory (DFT). To provide context, we compare our findings to a study of flavin mononucleotide (FMN) derivatives. We found a significant influence of the methylation pattern on the absorption properties, as well as on the proton hyperfine coupling ratios of the xylene moiety, which appears to be solvent-dependent. This effect is enhanced by the replacement of N5 by C5-H in 5-deazaflavin derivatives compared to their respective flavin counterparts.

Design of Soft Material Surfaces with Rationally Tuned Water Diffusivity.

Water structure and dynamics can be key modulators of adsorption, separations, and reactions at soft material interfaces, but systematically tuning water environments in an aqueous, accessible, and functionalizable material platform has been elusive. This work leverages variations in excluded volume to control and measure water diffusivity as a function of position within polymeric micelles using Overhauser dynamic nuclear polarization spectroscopy. Specifically, a versatile materials platform consisting of sequence-defined polypeptoids simultaneously offers a route to controlling the functional group position and a unique opportunity to generate a water diffusivity gradient extending away from the polymer micelle core. These results demonstrate an avenue not only to rationally design the chemical and structural properties of polymer surfaces but also to design and tune the local water dynamics that, in turn, can adjust the local activity for solutes.

Completing the Picture: Determination of 13C Hyperfine Coupling Constants of Flavin Semiquinone Radicals by Photochemically Induced Dynamic Nuclear Polarization Spectroscopy.

We investigate the electronic structure of flavin semiquinone radicals in terms of their 13C hyperfine coupling constants. Photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy was used to study both the neutral and anionic radical species of flavin mononucleotide (FMN) in bulk aqueous solution. Apart from universally 13C-labeled FMN, partially labeled isotopologues are used to increase sensitivity for nuclei exhibiting very small hyperfine couplings and to cope with spectral overlap. In addition, experimental findings are supported by quantum chemical calculations, and implications for the spin density distribution in free flavin radicals are discussed.

Room-temperature dynamic nuclear polarization enhanced NMR spectroscopy of small biological molecules in water

Nuclear magnetic resonance (NMR) spectroscopy is a powerful and popular technique for probing the molecular structures, dynamics and chemical properties. However the conventional NMR spectroscopy is bottlenecked by its low sensitivity. Dynamic nuclear polarization (DNP) boosts NMR sensitivity by orders of magnitude and resolves this limitation. In liquid-state this revolutionizing technique has been restricted to a few specific non-biological model molecules in organic solvents. Here we show that the carbon polarization in small biological molecules, including carbohydrates and amino acids, can be enhanced sizably by in situ Overhauser DNP (ODNP) in water at room temperature and at high magnetic field. An observed connection between ODNP 13C enhancement factor and paramagnetic 13C NMR shift has led to the exploration of biologically relevant heterocyclic compound indole. The QM/MM MD simulation underscores the dynamics of intermolecular hydrogen bonds as the driving force for the scalar ODNP in a long-living radical-substrate complex. Our work reconciles results obtained by DNP spectroscopy, paramagnetic NMR and computational chemistry and provides new mechanistic insights into the high-field scalar ODNP.

Li Plating on Carbon Electrode Surface Probed by Low-Field Dynamic Nuclear Polarization 7Li NMRSupported by the National Key Research and Development Program of China (Grant No. 2018YFC0115000), the National Natural Science Foundation of China (Grant No. 21603267), and the Chinese Academy of Sciences (Grant No. YZ201677 and YZ201551).

Lithium deposition on graphite electrode not only reduces fast-charging capability of lithium ion batteries but also causes safety trouble. Here, a low-field 7Li dynamic nuclear polarization (DNP) is used to probe Li plating on the surfaces of three types of carbon electrodes: hard carbon, soft carbon and graphite. Owing to the strong Fermi contact interaction between 7Li and conduction electrons, the 7Li nuclear-magnetic-resonance (NMR) signal of Li metal deposited on electrode surface could be selectively enhanced by DNP. It is suggested that low-field 7Li DNP spectroscopy is a sensitive tool for investigating Li deposition on electrodes during charging/discharging processes.

Расчет эффективности удвоения частоты излучения субтерагерцового гиротрона за счет решеточной нелинейности в монокристаллической пластине InP

In this work, we calculated the efficiency of second harmonic generation for sub-THz gyrotron radiation in a single-crystal InP plate embedded in a conical metal waveguide. For the values of the losses at the doubled frequency (depending on the degree of purity of the InP crystal), the dimensions of the wafer are revealed that provide the optimal effective frequency conversion. Thus, the conversion factor f –>2f can reach 5 W/kW^2 when pumped at a frequency of 263 GHz and a loss of alpha_2f = 0.1 cm^-1. The possibility of a noticeable increase in the efficiency of frequency doubling with fine tuning of the phase matching due to the selection of the size of the submicron gap between the InP plate and the plates of the metal waveguide is shown. The results obtained can be used to solve applied problems in the field of dynamic nuclear polarization spectroscopy and nuclear magnetic resonance studies.

Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Summary Dynamic nuclear polarization (DNP) harnesses the large polarization of electron spins to dramatically increase nuclear magnetic resonance (NMR) sensitivity. This study expands the scope of DNP beyond its traditional focus on hyperpolarizing the solvent network using exogenous polarizing agents (PAs). We introduce 1H DNP with endogenous V4+ centers positioned in a set of vanadyl complexes with tunable V4+-1H distances. We traced the polarization transfer from V4+ to 1H spins, specifically differentiating between direct V4+-1Hs polarization transfer and the 1H spin-diffusion-mediated bulk solvent 1H polarization buildup and illuminated the effect of the V4+-1H distance on these processes. These results deepen our understanding of polarization pathways and expand the catalog of PAs to broad-line transition metals. This study establishes crucial first steps toward employing strategically positioned endogenous paramagnetic metal centers for DNP and the conceptual framework of hyperfine DNP spectroscopy that merges both spatial and chemical diagnosis of target nuclear spins.

Room temperature CO oxidation catalysed by supported Pt nanoparticles revealed by solid-state NMR and DNP spectroscopy

A series of 1 and 2 nm sized platinum nanoparticles deposited on different support materials are investigated by solid-state NMR combined with dynamic nuclear polarization (DNP).

Wavelength-Dependent Photochemistry of Oxime Ester Photoinitiators

The design of efficient radical photoinitiating systems requires a systematic and detailed evaluation of their photochemical characteristics. Correlating absorbance and the corresponding electronic transitions of a photoinitiator is critical for understanding its photoinduced reaction pathways. In the current contribution, we provide an in-depth investigation into the photochemistry and photophysics of two oxime ester derivatives (O-benzoyl-α-oxooxime, OXE01, and O-acetyloxime, OXE02), known for their excellent performance in pigmented formulations. In particular, we shed light on their wavelength-dependent photopolymerization properties. We utilized a combination of UV–vis spectroscopy, density functional theory (DFT) calculations, photochemically induced dynamic nuclear polarization spectroscopy (photo-CIDNP), and pulsed-laser polymerization with a wavelength-tunable laser with subsequent size exclusion chromatography coupled to high-resolution electrospray ionization mass spectrometry (PLP-SEC-ESI-MS) ...

Higher Order Amyloid Fibril Structure by MAS NMR and DNP Spectroscopy

Protein magic angle spinning (MAS) NMR spectroscopy has generated structural models of several amyloid fibril systems, thus providing valuable information regarding the forces and interactions that confer the extraordinary stability of the amyloid architecture. Despite these advances, however, obtaining atomic resolution information describing the higher levels of structural organization within the fibrils remains a significant challenge. Here, we detail MAS NMR experiments and sample labeling schemes designed specifically to probe such higher order amyloid structure, and we have applied them to the fibrils formed by an eleven-residue segment of the amyloidogenic protein transthyretin (TTR(105-115)). These experiments have allowed us to define unambiguously not only the arrangement of the peptide β-strands into β-sheets but also the β-sheet interfaces within each protofilament, and in addition to identify the nature of the protofilament-to-protofilament contacts that lead to the formation of the complete fibril. Our efforts have resulted in 111 quantitative distance and torsion angle restraints (10 per residue) that describe the various levels of structure organization. The experiments benefited extensively from the use of dynamic nuclear polarization (DNP), which in some cases allowed us to shorten the data acquisition time from days to hours and to improve significantly the signal-to-noise ratios of the spectra. The β-sheet interface and protofilament interactions identified here revealed local variations in the structure that result in multiple peaks for the exposed N- and C-termini of the peptide and in inhomogeneous line-broadening for the residues buried within the interior of the fibrils.

A novel variable field system for field-cycled dynamic nuclear polarization spectroscopy

Dynamic nuclear polarization (DNP) is an NMR-based technique which enables detection and spectral characterization of endogenous and exogenous paramagnetic substances measured via transfer of polarization from the saturated unpaired electron spin system to the NMR active nuclei. A variable field system capable of performing DNP spectroscopy with NMR detection at any magnetic field in the range 0–0.38 T is described. The system is built around a clinical open-MRI system. To obtain EPR spectra via DNP, partial cancellation of the detection field B 0 NMR is required to alter the evolution field B 0 EPR at which the EPR excitation is achieved. The addition of resistive actively shielded field cancellation coils in the gap of the primary magnet provides this field offset in the range of 0–100 mT. A description of the primary magnet, cancellation coils, power supplies, interfacing hardware, RF electronics and console are included. Performance of the instrument has been evaluated by acquiring DNP spectra of phantoms with aqueous nitroxide solutions (TEMPOL) at three NMR detection fields of 97 G, 200 G and 587 G corresponding to 413 kHz, 851.6 kHz and 2.5 MHz respectively and fixed EPR evolution field of 100 G corresponding to an irradiation frequency of 282.3 MHz. This variable-field DNP system offers great flexibility for the performance of DNP spectroscopy with independent optimum choice of EPR excitation and NMR detection fields.

Spin Relaxation Effects in Photochemically Induced Dynamic Nuclear Polarization Spectroscopy of Nuclei with Strongly Anisotropic Hyperfine Couplings

We describe experimental results and theoretical models for nuclear and electron spin relaxation processes occurring during the evolution of 19F-labeled geminate radical pairs on a nanosecond time scale. In magnetic fields of over 10 T, electron-nucleus dipolar cross-relaxation and longitudinal DeltaHFC-Deltag (hyperfine coupling anisotropy--g-tensor anisotropy) cross-correlation are shown to be negligibly slow. The dominant relaxation process is transverse DeltaHFC-Deltag cross-correlation, which is shown to lead to an inversion in the geminate 19F chemically induced dynamic nuclear polarization (CIDNP) phase for sufficiently large rotational correlation times. This inversion has recently been observed experimentally and used as a probe of local mobility in partially denatured proteins (Khan, F.; et al. J. Am. Chem. Soc. 2006, 128, 10729-10737). The essential feature of the spin dynamics model employed here is the use of the complete spin state space and the complete relaxation superoperator. On the basis of the results reported, we recommend this approach for reliable treatment of magnetokinetic systems in which relaxation effects are important.

Real-time calculation and visualization of spectra in field-cycled dynamic nuclear polarization spectroscopy

Field-cycled dynamic nuclear polarization (FC-DNP), which is based on the Overhauser effect, provides a new way to perform in vivo measurements of free radicals in biological systems. Since it measures the alterations of the nuclear magnetic resonance (NMR) signal in the presence of paramagnetic molecules, a customized program is usually needed in FC-DNP experiments to extract spectral information from the acquired NMR signals. While this program can be designed to calculate the spectrum after all the NMR signals are collected, the batch-processing mode inevitably causes delay and is not convenient for in vivo applications. In this paper, we report the development of a real-time DNP spectrum calculation and visualization program, called RT_DNP, for FC-DNP experiments. A dynamic data exchange (DDE) client was implemented to enable real-time receipt of the system information and the NMR signals from a commercial NMR console. The received NMR signals and experimental parameters were then used to calculate the DNP spectrum during the data acquisition. The real-time DNP spectrum calculation and visualization program was tested in experiments. A seamless integration of the program into a commercial NMR console has been achieved.

A violation of the magnetic equivalence of nuclei in chemically induced dynamic nuclear polarization spectroscopy

It has been shown that a correlation between the hyperfine coupling constant and the ability of a given nucleus to be abstracted in the radical reaction could cause a violation of the equality in CIDNP signals from protons constituting a group of magnetically equivalent nuclei in the radical undergoing the abstraction. The scale of the effect is analyzed and optimal conditions for its detection are found.

Kringle 4 from human plasminogen: a proton magnetic resonance study via two-dimensional photochemically induced dynamic nuclear polarization spectroscopy

Two-dimensional (2D) proton magnetic resonance techniques used in conjunction with laser photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy have been applied to studying the kringle 4 domain from human plasminogen at 360 MHz. Out of 11 potential CIDNP-sensitive aromatic side chains, only 5 (His3, Tyr41, Tyr50, Trp72, and Tyr74) appear to be accessible to 3-(carboxymethyl)lumiflavin, the dye used to photogenerate spin polarization. Of these, Trp72 and Tyr74 are known to be at, or near, the lysine-binding site. The spin-spin scalar (J) and phase-sensitive dipolar (Overhauser) connectivities in the 2D experiments yield absolute assignments for the aromatic signals stemming from the exposed tyrosyl and tryptophanyl rings. Moreover, a number of side-chain H beta resonances can be identified and assigned to specific types of aromatic amino acid residues.

Applications of two-dimensional NMR methods in photochemically induced dynamic nuclear polarization spectroscopy

On montre que les spectroscopies CIDNP-COSY et CIDNP-NOESY sont utiles pour etudier des residus de surface et leur environnement immediat dans des macromolecules biologiques

Proton nuclear magnetic resonance assignments and surface accessibility of Tryptophan residues in lysozyme using photochemically induced dynamic nuclear polarization spectroscopy.

Tryptophan resonances in the 360-MHz 1H photochemically induced dynamic nuclear polarization spectrum of hen egg white lysozyme are investigated in detail. All resonances of one tryptophan and six of another are identified and assigned to their respective protons. The methods employed, all involving nuclear spin polarization, include the study of cross-relaxation effects and the use of selective radio-frequency irradiation, Gd3+ as a paramagnetic probe, and riboflavin as the chemically induced dynamic nuclear polarization generating dye. From a comparison of the experimental results with the known X-ray structure of lysozyme, second-stage assignments of the two tryptophan residues (Trp-62 and Trp-123) are proposed. A number of other resonances are characterized, among them Trp-63 C(2)H and four indirectly polarized methyl groups.