Category Archives: 9 GHz ODNP

Electrochemical Overhauser dynamic nuclear polarization #DNPNMR #ODNP

Tamski, Mika, Jonas Milani, Christophe Roussel, and Jean-Philippe Ansermet. “Electrochemical Overhauser Dynamic Nuclear Polarization.” Physical Chemistry Chemical Physics 22, no. 32 (2020): 17769–76.

Nuclear Magnetic Resonance (NMR) spectroscopy suffers from low sensitivity due to the low nuclear spin polarization obtained within practically achievable external magnetic fields. Dynamic Nuclear Polarization (DNP) refers to the techniques that increases NMR signal intensity by transferring spin polarization from electrons to the nuclei.

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.

A compact X-Band ODNP spectrometer towards hyperpolarized 1H spectroscopy #DNPNMR #ODNP

Überrück, Till, Michael Adams, Josef Granwehr, and Bernhard Blümich. “A Compact X-Band ODNP Spectrometer towards Hyperpolarized 1H Spectroscopy.” Journal of Magnetic Resonance, April 2020, 106724.

The demand for compact benchtop NMR systems that can resolve chemical shift differences in the ppm to sub-ppm range is growing. However due to material and size restrictions these magnets are limited in field strength and thus in signal intensity and quality. The implementation of standard hyperpolarization techniques is a next step in an effort to boost the signal. Here we present a compact Overhauser Dynamic Nuclear Polarization (ODNP) setup with a permanent magnet that can resolve 1H chemical shift differences in the ppm range. The assembly of the setup and its components are described in detail, and the functionality of the setup is demonstrated experimentally with ODNP enhanced relaxation measurements yielding a maximal enhancement of -140 for an aqueous 4Hydroxy-TEMPO solution. Additionally, 1H spectroscopic resolution and significant enhancements are demonstrated on acetic acid as a solvent.

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.

Nitroxide Derivatives for Dynamic Nuclear Polarization in Liquids: The Role of Rotational Diffusion #DNPNMR

Levien, M., M. Hiller, I. Tkach, M. Bennati, and T. Orlando. “Nitroxide Derivatives for Dynamic Nuclear Polarization in Liquids: The Role of Rotational Diffusion.” The Journal of Physical Chemistry Letters 11, no. 5 (March 5, 2020): 1629–35.

Polarization transfer efficiency in liquid-state dynamic nuclear polarization (DNP) depends on the interaction between polarizing agents (PAs) and target nuclei modulated by molecular motions. We show how translational and rotational diffusion differently affect the DNP efficiency. These contributions were disentangled by measuring 1HDNP enhancements of toluene and chloroform doped with nitroxide derivatives at 0.34 T as a function of either the temperature or the size of the PA. The results were employed to analyze 13C-DNP data at higher fields, where the polarization transfer is also driven by the Fermi contact interaction. In this case, bulky nitroxide PAs perform better than the small TEMPONE radical due to structural fluctuations of the ring conformation. These findings will help in designing PAs with features specifically optimized for liquid-state DNP at various magnetic fields.

High-Resolution Overhauser Dynamic Nuclear Polarization Enhanced Proton NMR Spectroscopy at Low Magnetic Fields #DNPNMR #Bridge12

Overhauser DNP spectroscopy at X-Band is mostly used to study hydration dynamics. However, using a hybrid magnet (permanent magnet in combination with sweep coils) with active shimming it is possible to record high-resolution NMR spectra with chemical shift resolution. This is an example of the research and development activities performed at Bridge12.

Keller, Timothy J., Alexander J. Laut, Jagadishwar Sirigiri, and Thorsten Maly. “High-Resolution Overhauser Dynamic Nuclear Polarization Enhanced Proton NMR Spectroscopy at Low Magnetic Fields.” Journal of Magnetic Resonance, March 2020, 106719.

Dynamic nuclear polarization (DNP) has gained large interest due to its ability to increase signal intensities in nuclear magnetic resonance (NMR) experiments by several orders of magnitude. Currently, DNP is typically used to enhance high-field, solid-state NMR experiments. However, the method is also capable of dramatically increasing the observed signal intensities in solution-state NMR spectroscopy. In this work, we demonstrate the application of Overhauser dynamic nuclear polarization (ODNP) spectroscopy at an NMR frequency of 14.5 MHz (0.35 T) to observe DNP-enhanced highresolution NMR spectra of small molecules in solutions. Using a compact hybrid magnet with integrated shim coils to improve the magnetic field homogeneity we are able to routinely obtain proton linewidths of less than 4 Hz and enhancement factors > 30. The excellent field resolution allows us to perform chemical-shift resolved ODNP experiments on ethyl crotonate to observe proton J-coupling. Furthermore, recording high-resolution ODNP-enhanced NMR spectra of ethylene glycol allows us to characterize the microwave induced sample heating in-situ, by measuring the separation of the OH and CH2 proton peaks.

Slow Molecular Motions in Ionic Liquids Probed by Cross-Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization #DNPNMR

Banerjee, Abhishek, Arnab Dey, and Narayanan Chandrakumar. “Slow Molecular Motions in Ionic Liquids Probed by Cross-Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization.” Angewandte Chemie International Edition 55, no. 47 (November 14, 2016): 14756–61.

Solution-state Overhauser dynamic nuclear polarization (ODNP) at moderate fields, performed by saturating the electron spin resonance (ESR) of a free radical added to the sample of interest, is well known to lead to significant NMR signal enhancements in the steady state, owing to electron–nuclear cross-relaxation. Here it is shown that under conditions which limit radical access to the molecules of interest, the time course of establishment of ODNP can provide a unique window into internuclear cross-relaxation, and reflects relatively slow molecular motions. This behavior, modeled mathematically by a three-spin version of the Solomon equations (one unpaired electron and two nuclear spins), is demonstrated experimentally on the 19F/1H system in ionic liquids. Bulky radicals in these viscous environments turn out to be just the right setting to exploit these effects. Compared to standard nuclear Overhauser effect (NOE) work, the present experiment offers significant improvement in dynamic range and sensitivity, retains usable chemical shift information, and reports on molecular motions in the sub-megahertz (MHz) to tens of MHz range—motions which are not accessed at high fields.

Effect of nitroxide spin probes on the transport properties of Nafion membranes #DNPNMR #ODNP

Überrück, Till, Oliver Neudert, Klaus-Dieter Kreuer, Bernhard Blümich, Josef Granwehr, Siegfried Stapf, and Songi Han. “Effect of Nitroxide Spin Probes on the Transport Properties of Nafion Membranes.” Physical Chemistry Chemical Physics 20, no. 41 (2018): 26660–74.

Nafion is the most common material used as proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still debated. Overhauser Dynamic Nuclear Polarization (ODNP) has proven a useful tool for probing hydration dynamics and interactions within 5–8 Å of protein and soft material surfaces. Recently it was suggested that ODNP can also be applied to analyze surface water dynamics along Nafion’s inner membrane. Here we interrogate the viability of this method for Nafion by carrying out a series of measurements relying on 1H nuclear magnetic resonance (NMR) relaxometry and diffusometry experiments with and without ODNP hyperpolarization, accompanied by other complementary characterization methods including small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA) of hydration, and proton conductivity by AC impedance spectroscopy. Our comprehensive study shows that the commonly used paramagnetic spin probes—here, stable nitroxide radicals—for ODNP, as well as their diamagnetic analogues, reduce the inner membrane surface hydrophilicity, depending on the location and concentration of the spin probe. This heavily reduces the hydration of Nafion, hence increases the tortuosity of the inner membrane morphology and/or increases the activiation barrier for water transport, and consequently impedes water diffusion, transport, and proton conductivity.

Contrasting effects of glycerol and DMSO on lipid membrane surface hydration dynamics and forces #DNPNMR #ODNP

Schrader, Alex M., Chi-Yuan Cheng, Jacob N. Israelachvili, and Songi Han. “Communication: Contrasting Effects of Glycerol and DMSO on Lipid Membrane Surface Hydration Dynamics and Forces.” The Journal of Chemical Physics 145, no. 4 (July 28, 2016): 041101.

Glycerol and dimethyl sulfoxide (DMSO) are commonly used cryoprotectants in cellular systems, but due to the challenges of measuring the properties of surface-bound solvent, fundamental questions remain regarding the concentration, interactions, and conformation of these solutes at lipid membrane surfaces. We measured the surface water diffusivity at gel-phase dipalmitoylphosphatidylcholine (DPPC) bilayer surfaces in aqueous solutions containing ≤7.5 mol. % of DMSO or glycerol using Overhauser dynamic nuclear polarization. We found that glycerol similarly affects the diffusivity of water near the bilayer surface and that in the bulk solution (within 20%), while DMSO substantially increases the diffusivity of surface water relative to bulk water. We compare these measurements of water dynamics with those of equilibrium forces between DPPC bilayers in the same solvent mixtures. DMSO greatly decreases the range and magnitude of the repulsive forces between the bilayers, whereas glycerol increases it. We propose that the differences in hydrogen bonding capability of the two solutes leads DMSO to dehydrate the lipid head groups, while glycerol affects surface hydration only as much as it affects the bulk water properties. The results suggest that the mechanism of the two most common cryoprotectants must be fundamentally different: in the case of DMSO by decoupling the solvent from the lipid surface, and in the case of glycerol by altering the hydrogen bond structure and intermolecular cohesion of the global solvent, as manifested by increased solvent viscosity.

Surface chemical heterogeneity modulates silica surface hydration #DNPNMR #ODNP

Schrader, Alex M., Jacob I. Monroe, Ryan Sheil, Howard A. Dobbs, Timothy J. Keller, Yuanxin Li, Sheetal Jain, M. Scott Shell, Jacob N. Israelachvili, and Songi Han. “Surface Chemical Heterogeneity Modulates Silica Surface Hydration.” Proceedings of the National Academy of Sciences 115, no. 12 (March 20, 2018): 2890–95.

An in-depth knowledge of the interaction of water with amorphous silica is critical to fundamental studies of interfacial hydration water, as well as to industrial processes such as catalysis, nanofabrication, and chromatography. Silica has a tunable surface comprising hydrophilic silanol groups and moderately hydrophobic siloxane groups that can be interchanged through thermal and chemical treatments. Despite extensive studies of silica surfaces, the influence of surface hydrophilicity and chemical topology on the molecular properties of interfacial water is not well understood. In this work, we controllably altered the surface silanol density, and measured surface water diffusivity using Overhauser dynamic nuclear polarization (ODNP) and complementary silica–silica interaction forces acrosswater using a surface forces apparatus (SFA). The results show that increased silanol density generally leads to slower water diffusivity and stronger silica– silica repulsion at short aqueous separations (less than ∼4 nm). Both techniques show sharp changes in hydration properties at intermediate silanol densities (2.0–2.9 nm−2). Molecular dynamics simulations of model silica–water interfaces corroborate the increase in water diffusivity with silanol density, and furthermore show that even on a smooth and crystalline surface at a fixed silanol density, adjusting the spatial distribution of silanols results in a range of surface water diffusivities spanning ∼10%. We speculate that a critical silanol cluster size or connectivity parameter could explain the sharp transition in our results, and can modulate wettability, colloidal interactions, and surface reactions, and thus is a phenomenon worth further investigation on silica and chemically heterogeneous surfaces.

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