Monday, December 10, 2018

DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers #DNPNMR

Verde-Sesto, Ester, Nicolas Goujon, Haritz Sardon, Pauline Ruiz, Tan Vu Huynh, Fermin Elizalde, David Mecerreyes, Maria Forsyth, and Luke A. O’Dell. “DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers.” Macromolecules 51, no. 20 (October 23, 2018): 8046–53.


Dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy is increasingly recognized as a powerful and versatile tool for the characterization of polymers and polymer-based materials. DNP requires the presence of unpaired electrons, usually mono- or biradicals, and the method of incorporation of these groups and their distribution within the structure is crucial. Methods for covalently binding the radicals to the polymer and controlling their location (e.g., exclusively within a specific phase or at an interface) can allow the selective enhancement of a particular region or the measurement of domain sizes. We have prepared a series of polyurethanes by copolymerization of a nitroxide radical monomer with poly(ethylene glycol) (PEO) and diisocyanate linkers. The PEO is shown to form crystalline domains with the radical monomers in a separate phase, providing DNP enhancements of around 10 and allowing the domain size and morphology to be probed with the aid of X-ray scattering data. Additionally, electron paramagnetic resonance is used to estimate the inter-radical distances and density functional theory calculations are used to refine the PEO crystal structure.

Saturday, December 8, 2018

[NMR] Post-doctoral position available #DNPNMR

A postdoctoral position is available to join the Emsley group at EPFL, Lausanne

We are looking for highly motivated candidates to take up a Postdoctoral position developing new methods in NMR spectroscopy to address challenging problems in chemistry and materials science. In particular we will be working on dynamic nuclear polarization enhanced NMR methods for materials. Examples of our recent work and the application areas that we work on can be found on our website: http://lrm.epfl.ch/publications/


We are looking for highly motivated candidates with strong scientific background, independence, and who enjoy teamwork. You should hold a relevant qualification in chemistry, physics or related disciplines. Skills in developing experimental multi-dimensional nuclear magnetic resonance are a plus. 

Our laboratory at EPFL is part of one the world’s leading chemistry departments, and is located Lausanne on the north shore of Lake Geneva. The laboratory is equipped with unique state of the art NMR spectrometers (including gyrotron DNP accessories at 400 and 900 MHz, a dissolution-DNP machine, and 100 kHz magic angle spinning probes). 
Motivated candidates should contact Lyndon Emsley by email at lyndon.emsley@epfl.ch



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Friday, December 7, 2018

Long-range heteronuclear J-coupling constants in esters: Implications for 13C metabolic MRI by side-arm parahydrogen-induced polarization

Stewart, Neil J., Hiroyuki Kumeta, Mitsushi Tomohiro, Takuya Hashimoto, Noriyuki Hatae, and Shingo Matsumoto. “Long-Range Heteronuclear J-Coupling Constants in Esters: Implications for 13C Metabolic MRI by Side-Arm Parahydrogen-Induced Polarization.” Journal of Magnetic Resonance 296 (November 2018): 85–92.


Side-arm parahydrogen induced polarization (PHIP-SAH) presents a cost-effective method for hyperpolarization of 13C metabolites (e.g. acetate, pyruvate) for metabolic MRI. The timing and efficiency of typical spin order transfer methods including magnetic field cycling and tailored RF pulse sequences crucially depends on the heteronuclear J coupling network between nascent parahydrogen protons and 13C, post-parahydrogenation of the target compound. In this work, heteronuclear nJHC (1<n≤5) couplings of acetate and pyruvate esters pertinent for PHIP-SAH were investigated experimentally using selective HSQMBC-based pulse sequences and numerically using DFT simulations. The CLIP-HSQMBC technique was used to quantify 2/3-bond JHC couplings, and 4/5-bond JHC ≲ 0.5 Hz were estimated by the sel-HSQMBC-TOCSY approach. Experimental and numerical (DFT-simulated) nJHC couplings were strongly correlated (P < 0.001). Implications for 13C hyperpolarization by magnetic field cycling, and PH-INEPT and ESOTHERIC type spin order transfer methods for PHIP-SAH were assessed, and the influence of direct nascent parahydrogen proton to 13C coupling when compared with indirect TOCSY-type transfer through intermediate (non-nascent parahydrogen) protons was studied by the density matrix approach.

Wednesday, December 5, 2018

Electron decoupling with cross polarization and dynamic nuclear polarization below 6 K #DNPNMR

Sesti, Erika L., Edward P. Saliba, Nicholas Alaniva, and Alexander B. Barnes. “Electron Decoupling with Cross Polarization and Dynamic Nuclear Polarization below 6 K.” Journal of Magnetic Resonance 295 (October 2018): 1–5.


Dynamic nuclear polarization (DNP) can improve nuclear magnetic resonance (NMR) sensitivity by orders of magnitude. Polarizing agents containing unpaired electrons required for DNP can broaden nuclear resonances in the presence of appreciable hyperfine couplings. Here we present the first cross polarization experiments implemented with electron decoupling, which attenuates detrimental hyperfine couplings. We also demonstrate magic angle spinning (MAS) DNP experiments below 6 K, producing unprecedented nuclear spin polarization in rotating solids. 13C correlation spectra were collected with MAS DNP below 6 K for the first time. Longitudinal magnetization recovery times with MAS DNP (T1DNP, 1H) of urea in a frozen glassy matrix below 6 K were measured for both the solid effect and the cross effect. Trityl radicals exhibit a T1DNP (1H) of 18.7 s and the T1DNP (1H) of samples doped with 20 mM AMUPol is only 1.3 s. MAS below 6 K with DNP and electron decoupling is an effective strategy to increase NMR signal-to-noise ratios per transient while retaining short recovery periods.

Monday, December 3, 2018

Multi-resonant photonic band-gap/saddle coil DNP probehead for static solid state NMR of microliter volume samples #DNPNMR

Nevzorov, Alexander A., Sergey Milikisiyants, Antonin N. Marek, and Alex I. Smirnov. “Multi-Resonant Photonic Band-Gap/Saddle Coil DNP Probehead for Static Solid State NMR of Microliter Volume Samples.” Journal of Magnetic Resonance 297 (December 2018): 113–23. 


The most critical condition for performing Dynamic Nuclear Polarization (DNP) is achieving sufficiently high electronic B1e fields over the sample at the matched EPR frequencies, which for modern high-resolution NMR instruments fall into the millimeter wave (mmW) range. Typically, mmWs are generated by powerful gyrotrons and/or extended interaction klystrons (EIKs) sources and then focused onto the sample by dielectric lenses. However, further development of DNP methods including new DNP pulse sequences may require B1e fields higher than one could achieve with the current mmW technology. In order to address the challenge of significantly enhancing the mmW field at the sample, we have constructed and tested one-dimensional photonic band-gap (PBG) mmW resonator that was incorporated inside a double-tuned radiofrequency (rf) NMR saddle coil. The photonic crystal is formed by stacking ceramic discs with alternating high and low dielectric constants. The thicknesses of the discs are chosen to be λ/4 or 3λ/4, where λ is the wavelength of the incident mmW field in the corresponding dielectric material. When the mmW frequency is within the band gap of the photonic crystal, a defect created in the middle of the crystal confines the mmW energy, thus forming a resonant structure. An aluminum mirror in the middle of the defect has been used to split the structure in order to reduce its size and simplify the resonator tuning. The latter is achieved by adjusting the width of the defect by moving the aluminum mirror with respect to the dielectric stack using a gear mechanism. The 1D PBG resonator was the key element for constructing a multi-resonant integrated DNP/NMR probehead operating at 198 GHz EPR / 300 MHz 1H / 75.5 MHz 13C NMR frequencies. Initial tests of the multi-resonant DNP/NMR probehead were carried out using a quasioptical 200 GHz bridge and a Bruker Biospin Avance II spectrometer equipped with a standard Bruker 7 T wide-bore 89 mm magnet parked at 300.13 MHz 1H NMR frequency. The mmW bridge built with all solid-state active components allows for the frequency tuning between ca. 190 to ca. 198 GHz with the output power up to 27 dBm (0.5 W) at 192 GHz and up to 23 dBm (0.2 W) at 197.5 GHz. Room temperature DNP experiments with a synthetic single crystal high-pressure high-temperature (HPHT) diamond (0.3Å~0.3Å~3.0 mm3) demonstrated dramatic 1,500–fold enhancement of 13C natural abundance NMR signal at full incident mmW power. Significant 13C DNP enhancement (of about 90) have been obtained at incident mmW powers of as low as <100 μW. Further tests of the resonator performance have been carried out with a thin (ca. 100 μm thickness) composite polystyrene-microdiamond film by controlling the average mmW power at the optimal DNP conditions via a gated mode of operation. From these experiments, the PBG resonator with loaded Q≃250 and finesse provides up to 12-fold/11 db gain in the average mmW power vs. the non-resonant probehead configuration employing only a reflective mirror.