Category Archives: Overhauser Effect

Overhauser Dynamic Nuclear Polarization: A Tool for Building Maps of Hydration Water #DNPNMR #ODNP #Review

Franck, John M. “Overhauser Dynamic Nuclear Polarization: A Tool for Building Maps of Hydration Water.” Biophysical Journal 118, no. 3, Supplement 1 (February 7, 2020): 487a.

Coating the surface of every macromolecule or macromolecular assembly, one finds a hydration layer composed of water molecules that move typically between 3× and 10× slower than water molecules in bulk water. The interaction between the water molecules in the hydration layer and the macromolecules contributes to the structural stability and sometimes the function of, e.g., proteins and lipid bilayers. Overhauser Dynamic Nuclear Polarization (ODNP) is an emerging electron-spin nuclear-spin (EPR-NMR) double-resonance tool that has demonstrated a capability of measuring the translational dynamics of water in the hydration layer. Here we discuss our efforts on two fronts: First, we design a scheme for measuring the thickness of the hydration layer and the effect of confinement on translational dynamics, as measured by ODNP, with controlled, appropriately labeled reverse micelle systems. Second, we describe the development of an a priori technique for converting ODNP measurements into a 3D “map” of hydration layer properties in dynamic room temperature samples that explore an ensemble of structures. This latter effort focuses on transmembrane model systems and utilizes the modern structure-prediction tool Rosetta in a fashion analogous to successful efforts to predict NMR order parameters. Particular focus is given to improving the quality and automation of the ODNP measurement, as well as validating predicted ensemble structures against both continuous wave EPR and NMR Paramagnetic Relaxation Enhancement (PRE) data.

Motional Dynamics of Halogen‐Bonded Complexes Probed by Low‐Field NMR Relaxometry and Overhauser Dynamic Nuclear Polarization #DNPNMR #ODNP

Banerjee, Abhishek, Arnab Dey, and N. Chandrakumar. “Motional Dynamics of Halogen‐Bonded Complexes Probed by Low‐Field NMR Relaxometry and Overhauser Dynamic Nuclear Polarization.” Chemistry – An Asian Journal, July 9, 2019, asia.201900754.

Halogen bonding is a subject of considerable interest owing to wide-ranging chemical, materials and biological applications. The motional dynamics of halogenbonded complexes play a pivotal role in comprehending the nature of the halogen-bonding interaction. However, not many attempts appear to have been made to shed light on the dynamical characteristics of halogen-bonded species. For the first time, we demonstrate here that the combination of low-field NMR relaxometry and Overhauser dynamic nuclear polarization (ODNP) makes it possible to obtain a cogent picture of the motional dynamics of halogen-bonded species. We discuss here the advantages of this combined approach. Low-field relaxometry allows us to infer the hydrodynamic radius and rotational correlation time, whereas ODNP probes the molecular translational correlation times (involving the substrate as well as the organic radical) with high sensitivity at low field.

Liquid-state Overhauser DNP at High Magnetic Fields #DNPNMR

Denysenkov, Vasyl P, and Thomas F Prisner. “Liquid-State Overhauser DNP at High Magnetic Fields,” 8:14, 2019.

Over the past few years, dynamic nuclear polarization at high magnetic fields has become one of the most popular technical and methodical research areas in the field of magnetic resonance spectroscopy and imaging. Much work has been devoted to experiments with the polarization transfer from the electron spin to the nuclear spin performed in the solid state, whereas only few examples exist for polarization transfer in the liquid‐state, which is driven by the Overhauser effect. The technical requirements to perform such experiments at high magnetic fields will be discussed in this review article, with special emphasis on double‐resonance structures, which allow performing such experiments in microwave‐absorbing solutions. Experimental procedures to individually evaluate the factors determining the proton Overhauser dynamic nuclear polarization (DNP) enhancements of several organic solvents using nitroxide radicals are described and compared to theoretical predictions. We show that at high magnetic fields the models based on Stokes–Einstein motion underestimate the coupling factor and therefore the DNP efficiency. This failure of the classical model is most obvious for DNP results obtained with lipid bilayer samples.

Characterizing oils in oil-water mixtures inside porous media by Overhauser dynamic nuclear polarization #DNPNMR #ODNP

Chen, Junfei, Jiwen Feng, Fang Chen, Zhen Zhang, Li Chen, Zhekai Zhang, Rugang Liao, Maili Liu, and Chaoyang Liu. “Characterizing Oils in Oil-Water Mixtures inside Porous Media by Overhauser Dynamic Nuclear Polarization.” Fuel 257 (December 2019): 116107.

We present a method to identify and sort the oils in oil-water mixtures based on the Overhauser dynamic nuclear polarization (ODNP) enhancement at low field. Through combining two types of radicals, e.g. ODNP enhancer TEMPO and relaxation reagent Mn2+, we can selectively enhance the 1H NMR signal of oil in oil-water mixture infiltrated in porous media. More importantly, we have found that the enhancements of light oils in porous materials are inversely dependent of their viscosities but independent of pore size approximately above 10 μm. This allows us to roughly sort oils according to their ODNP enhancement values. The verification experiments in sandstones saturated with several oil and water mixtures show that the method is useful for oils identification and classification in porous media, especially for reservoir assessment or development.

Optically-generated Overhauser dynamic nuclear polarization: A numerical analysis #ODNP #DNPNMR

Cheney, Daniel J., and Christopher J. Wedge. “Optically-Generated Overhauser Dynamic Nuclear Polarization: A Numerical Analysis.” The Journal of Chemical Physics 152, no. 3 (January 21, 2020): 034202.

Recently, an alternative approach to dynamic nuclear polarization (DNP) in the liquid state was introduced using optical illumination instead of microwave pumping. By exciting a suitable dye to the triplet state which undergoes a diffusive encounter with a persistent radical forming a quartet-doublet pair in the encounter complex, dynamic electron polarization (DEP) is generated via the radical-triplet pair mechanism. Subsequent cross-relaxation generates nuclear polarization without the need for microwave saturation of the electronic transitions. Here, we present a theoretical justification for the initial experimental results by means of numerical simulations. These allow investigation of the effects of various experimental parameters, such as radical and dye concentrations, sample geometry, and laser power, on the DNP enhancement factors, providing targets for experimental optimization. It is predicted that reducing the sample volume will result in larger enhancements by permitting a higher concentration of triplets in a sample of increased optical density. We also explore the effects of the pulsed laser rather than continuous-wave illumination, rationalizing the failure to observe the optical DNP effect under illumination conditions common to DEP experiments. Examining the influence of the illumination duty cycle, the conditions necessary to permit the use of pulsed illumination without compromising signal enhancement are determined, which may reduce undesirable laser heating effects. This first simulation of the optical DNP method therefore underpins the further development of the technology.

Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions #DNPNMR

Liu, Xiaoyang, Juan Gu, James Wightman, and Harry C. Dorn. “Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions.” ACS Applied Nano Materials, November 25, 2019, acsanm.9b01308.

Activated carbons (ACs) are widely used in many industrial and medical adsorbent applications because of their distinct ability to adsorb numerous gaseous and/or liquid analytes. More recently, ACs have been actively explored as an inexpensive alternative to metal catalysts (Pt) for numerous oxygen reduction reactions including microbial fuel cells (MFCs) for wastewater treatment. Although it is well established that O2 is chemisorbed on ACs, the actual chemical site has not been elucidated. In this study, we characterize adsorption of benzene on the surface of ACs in the presence and absence of O2. The AC samples have been heat treated and cover the range of 350−600 °C. The flowing benzene is monitored by solid/liquid intermolecular transfer (SLIT) 1H dynamic nuclear polarization (DNP). We find that the introduction of benzene (N2 atmosphere) flowing over an AC interface leads to a scalar (positive) 1H Overhauser effect in high temperature activated carbons (550−600 °C), whereas this nanoscale close-in Fermi interaction is completely suppressed upon introduction of oxygen (air) to the flowing benzene/ activated carbon interface. We propose these results are consistent with a benzene/delocalized singlet−triplet radical carbene or diradical interaction at the zigzag sites edges of disordered graphene motifs. These unique radical sites chemically react with O2 to form quenched diamagnetic sites. In contrast, a solid-state 1H DNP effect is observed at lower heat treatment temperatures representing different radical sites (e.g., aromatic heteroatom radical sites) in ACs.

Native Vanadyl Complexes in Crude Oil as Polarizing Agents for In Situ Proton Dynamic Nuclear Polarization #DNPNMR

This is really exciting work and the first time a vanadyl complex is used as the polarizing agent for DNP. Typically, nitroxides are used as polarizing agents, and this work clearly demonstrates the potential of metal complexes for DNP.

Gizatullin, Bulat, Marat Gafurov, Alexey Vakhin, Alexander Rodionov, Georgy Mamin, Sergei Orlinskii, Carlos Mattea, and Siegfried Stapf. “Native Vanadyl Complexes in Crude Oil as Polarizing Agents for In Situ Proton Dynamic Nuclear Polarization.” Energy & Fuels 33, no. 11 (November 21, 2019): 10923–32.

The presence of paramagnetic species such as vanadyl complexes (VO2+) and free carbon radicals in petroleum disperse systems (PDSs) such as crude oil, bitumen, or kerogen causes significant interest of studying the structure of PDS, high-molecular weight components, and their effects on the physical and chemical properties of PDS products by magnetic resonance techniques. However, the lack of detailed studies keeps the exact structure, aggregation mechanism, and interaction with complex composites of the PDS still disputable. In this contribution, detailed electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) investigations, including advanced fast field cycling dynamic nuclear polarization, of heavy crude oil focused on vanadyl complexes are presented. A perceptible room-temperature 1H dynamic nuclear polarization (DNP) solid effect at the X-band (magnetic field of 300−400 mT corresponding to the EPR frequency of 9.5 GHz and NMR frequency of 14.6 MHz), with enhancement ±5, is observed at moderate microwave irradiation power in crude oil with a high concentration of VO2+, while no Overhauser DNP contribution is found. Using NMR T2-encoding, DNP spectra and molecular dynamics, two components are distinguished, from which the one with slower dynamics exhibits higher DNP enhancement via VO2+ complexes. The observed difference is discussed in terms of electron−nuclear interaction and relative parts of hyperpolarized nuclear spins using an advanced model for DNP data simulation.

X-nuclei hyperpolarization for studying molecular dynamics by DNP-FFC #DNPNMR

Gizatullin, Bulat, Carlos Mattea, and Siegfried Stapf. “X-Nuclei Hyperpolarization for Studying Molecular Dynamics by DNP-FFC.” Journal of Magnetic Resonance 307 (October 2019): 106583.

Dynamic Nuclear Polarization methods are used for improving the quality of the NMR data, opening new possibilities by increasing both the sensitivity and the selectivity in NMR relaxation experiments. Recently, Fast Field Cycling relaxometry combined with DNP was introduced, demonstrating that molecular dynamics studies in the presence of natural or artificial radicals are indeed feasible under conditions where the signal-to-noise ratio is frequently critical. In this work, the extension of NMR relaxation dispersion beyond 1H NMR, by hyperpolarization of X-nuclei, is demonstrated. Overhauser effect via nitroxide radicals in simple (low viscous) liquids and saline solutions was observed for 2H, 7Li and 13C nuclei at ambient temperature. Substantial NMR signal enhancement up to several hundred was achieved for the studied samples. An advanced approach for reconstructing of the original relaxation dispersion of pure substances is used to eliminate the effect of the additional radical relaxivity of the X-nuclei.

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

Orlando, Tomas, Rıza Dervişoğlu, Marcel Levien, Igor Tkach, Thomas F. Prisner, Loren B. Andreas, Vasyl P. Denysenkov, and Marina Bennati. “Dynamic Nuclear Polarization of 13C Nuclei in the Liquid State over a 10 Tesla Field Range.” Angewandte Chemie International Edition 58, no. 5 (January 28, 2019): 1402–6.

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 is still facing several challenges. Here we report the observation of room temperature, liquid DNP 13C 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 13CDNP field dependence shines light on parameters governing the underlying scalar DNP, indicating 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 13CH2 and 13CH3 groups in organic molecules at 9.4 T opens perspective for a broader application of this method.

Large volume liquid state scalar Overhauser dynamic nuclear polarization at high magnetic field #DNPNMR

Dubroca, Thierry, Sungsool Wi, Johan van Tol, Lucio Frydman, and Stephen Hill. “Large Volume Liquid State Scalar Overhauser Dynamic Nuclear Polarization at High Magnetic Field.” Physical Chemistry Chemical Physics 21, no. 38 (2019): 21200–204.

Dynamic Nuclear Polarization (DNP) can increase the sensitivity of Nuclear Magnetic Resonance (NMR), but it is challenging in the liquid state at high magnetic fields. In this study we demonstrate significant enhancements of NMR signals (up to 70 on 13C) in the liquid state by scalar Overhauser DNP at 14.1 T, with high resolution (∼0.1 ppm) and relatively large sample volume (∼100 μL).

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