Friday, November 29, 2013

Over 35% liquid-state (13)C polarization obtained via dissolution dynamic nuclear polarization at 7 T and 1 K using ubiquitous nitroxyl radicals


Cheng, T., et al., Over 35% liquid-state (13)C polarization obtained via dissolution dynamic nuclear polarization at 7 T and 1 K using ubiquitous nitroxyl radicals. Phys Chem Chem Phys, 2013. 15(48): p. 20819-22.


The most versatile method to increase liquid-state (13)C NMR sensitivity is dissolution dynamic nuclear polarization. The use of trityl radicals is usually required to obtain very large (13)C polarization via this technique. We herein demonstrate that up to 35% liquid-state (13)C polarization can be obtained in about 1.5 h using ubiquitous nitroxyl radicals in (13)C-labeled sodium salts by partially deuterating the solvents and using a polarizer operating at 1 K and 7 T.

Wednesday, November 27, 2013

A comparative study of (1)H and (19)F Overhauser DNP in fluorinated benzenes


Neudert, O., et al., A comparative study of (1)H and (19)F Overhauser DNP in fluorinated benzenes. Phys Chem Chem Phys, 2013. 15(47): p. 20717-26.


Hyperpolarization techniques, such as Overhauser dynamic nuclear polarization (DNP), can provide a dramatic increase in the signal obtained from nuclear magnetic resonance experiments and may therefore enable new applications where sensitivity is a limiting factor. In this contribution, studies of the (1)H and (19)F Overhauser dynamic nuclear polarization enhancements at 345 mT are presented for three different aromatic solvents with the TEMPO radical for a range of radical concentrations. Furthermore, nuclear magnetic relaxation dispersion measurements of the same solutions are analyzed, showing contributions from dipolar and scalar coupling modulated by translational diffusion and different coupling efficiency for different solvents and nuclei. Measurements of the electron paramagnetic resonance linewidth are included to support the analysis of the DNP saturation factor for varying radical concentration. The results of our study give an insight into the characteristics of nitroxide radicals as polarizing agents for (19)F Overhauser DNP of aromatic fluorinated solvents. Furthermore, we compare our results with the findings of the extensive research on Overhauser DNP that was conducted in the past for a large variety of other radicals.

Monday, November 25, 2013

Sensitivity-enhanced solid-state NMR detection of expansin's target in plant cell walls


Wang, T., et al., Sensitivity-enhanced solid-state NMR detection of expansin's target in plant cell walls. Proc Natl Acad Sci U S A, 2013. 110(41): p. 16444-9.


Structure determination of protein binding to noncrystalline macromolecular assemblies such as plant cell walls (CWs) poses a significant structural biology challenge. CWs are loosened during growth by expansin proteins, which weaken the noncovalent network formed by cellulose, hemicellulose, and pectins, but the CW target of expansins has remained elusive because of the minute amount of the protein required for activity and the complex nature of the CW. Using solid-state NMR spectroscopy, combined with sensitivity-enhancing dynamic nuclear polarization (DNP) and differential isotopic labeling of expansin and polysaccharides, we have now determined the functional binding target of expansin in the Arabidopsis thaliana CW. By transferring the electron polarization of a biradical dopant to the nuclei, DNP allowed selective detection of (13)C spin diffusion from trace concentrations of (13)C, (15)N-labeled expansin in the CW to nearby polysaccharides. From the spin diffusion data of wild-type and mutant expansins, we conclude that to loosen the CW, expansin binds highly specific cellulose domains enriched in xyloglucan, whereas more abundant binding to pectins is unrelated to activity. Molecular dynamics simulations indicate short (13)C-(13)C distances of 4-6 A between a hydrophobic surface of the cellulose microfibril and an aromatic motif on the expansin surface, consistent with the observed NMR signals. DNP-enhanced 2D (13)C correlation spectra further reveal that the expansin-bound cellulose has altered conformation and is enriched in xyloglucan, thus providing unique insight into the mechanism of CW loosening. DNP-enhanced NMR provides a powerful, generalizable approach for investigating protein binding to complex macromolecular targets.

Sunday, November 24, 2013

[NMR] NMR Post-doc position at CEA Saclay, France

From the Ampere Magnetic Resonance List



Post-doc in hyperpolarized 129Xe MRI

A project untitled: « Polarized Xenon-MRI for diagnosis and follow up of Chest Tumors » is granted by the French programme « Projets de recherche dans le domaine de la physique, des mathématiques ou des sciences de l'ingénieur appliqués au cancer ». We are looking for a post-doctoral fellow for one year starting January 2014. The place is CEA-Saclay (25 km in the S-W of Paris).

Abstract
Diagnosis and treatment of early stages of non-small-cell lung cancer (NSCLC) could drastically modify the outcome in terms of disease-free and overall survival.
MRI offers several advantages owing to its low invasiveness, its harmlessness and its spatial in-depth resolution but suffers from poor sensitivity. Very few MRI methods are able to detect events at the air-liquid interface (i.e. lung epithelium), due to the low number of detectable atoms present and the strong local magnetic field inhomogeneity. Different strategies can be used to improve MRI detection threshold in lungs including the use of hyperpolarized gases such as 3He and 129Xe. These species can be detected at very low concentration thanks to the gain by several orders of magnitude in nuclear polarization. Hyperpolarized xenon presents interesting properties for medical applications:

  • it is a non-toxic, freely diffusive tracer that crosses the cell membrane in some tens of milliseconds without polarization loss,
  • its spectral signature is strongly dependent on its environment,
  • it can be encapsulated in host molecules functionalized to target specific receptors (hereafter denoted biosensors).

In the project with Institut Gustave Roussy (IGR) and CEA/DSV, we propose a new approach based on 129Xe MRI biosensors for the early diagnosis of NSCLC and follow up of treatment efficacy.
The candidate will belong to the team having in charge the NMR/MRI part. He/she must have a strong background in MRI, an experience of in vivo experiments, and some notions of spin hyperpolarization. The laboratory owns several high-field spectrometers and two home-built setups for optical pumping of noble gases, and has access to the high-field small animal MR imagers at NeuroSpin (40 meters apart).
Interested candidate must send a detailed curriculum vitae and a letter of motivation to : 
Patrick Berthault 
CEA Saclay
IRAMIS/SIS2M/UMR3299
PC #9
91191 Gif sur Yvette, France
 Tel: (+33) 169 08 42 45


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[NMR] Postdoctoral position in NMR Hyperpolarisation

From the Ampere Magnetic Resonance List



The Korvink group at the Department of Microsystems Engineering, University of Freiburg, is inviting applications for a postdoctoral position in NMR hyperpolarisation. The research work is funded by an ERC project that targets novel NMR detector concepts in the broad area of miniaturised NMR. The successful candidate brings along research experience in para Hydrogen induced polarisation (PHiP). Any additional experience from the broad areas of NMR or MEMS will be of benefit.

The ideal candidate will be a recent PhD graduate with practical experience in PHiP. This may include as examples: catalyst design, substrate design, and/or pulse sequence development for efficient polarization transfer.  Responsibilities will include supervision of graduate students and leading PHiP activities within the context of the broader goals of the NMCEL project. As a highly interdisciplinary project, strong team and communication skills are imperative. 

The environment at IMTEK offers an outstanding scientific setting, including state-of-the-art microsystem fabrication facilities, an RF lab, and a dedicated 500 MHz wide-bore Bruker NMR spectrometer. The NMCEL team will be composed of 1-2 postdocs, 3-4 PhD students, and 3-4 masters students. The University of Freiburg offers several facilities suitable for supporting this project and is within one hour from Bruker headquarters in Karlsruhe for potential collaborative efforts.

The position is immediately available, at the E13 level for one year, and is extendable to a total duration of 2 years. Applications for the position should contain a motivational cover letter, a curriculum vitae, a publications list, and a copy of the applicant's most significant journal publication, preferably submitted via email as a single PDF file, to korvink@imtek.uni-freiburg.de. Applicants should be willing to travel to Freiburg for an interview. The university is currently seeking to increase the amount of female employees and is thus especially pleased to receive applications from qualified female candidates.

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Friday, November 22, 2013

Highly Efficient, Water-Soluble Polarizing Agents for Dynamic Nuclear Polarization at High Frequency


Sauvee, C., et al., Highly Efficient, Water-Soluble Polarizing Agents for Dynamic Nuclear Polarization at High Frequency. Angew Chem Int Ed Engl, 2013. 52(41): p. 10858-10861.

Well polarized: Two new polarizing agents PyPol and AMUPol soluble in glycerol/water mixtures are used for dynamic nuclear polarization (DNP) NMR spectroscopy. The enhancement factors (epsilon) are about 3.5 to 4 times larger than for the established agent TOTAPOL at 263 and 395 GHz. For AMUPol, the temperature dependence of epsilon allows DNP experiments to be performed at temperatures significantly higher than for typical high-field DNP NMR experiments.

Wednesday, November 20, 2013

Quantitative cw Overhauser effect dynamic nuclear polarization for the analysis of local water dynamics


Franck, J.M., et al., Quantitative cw Overhauser effect dynamic nuclear polarization for the analysis of local water dynamics. Prog Nucl Magn Reson Spectrosc, 2013. 74(0): p. 33-56.

Liquid state Overhauser effect Dynamic Nuclear Polarization (ODNP) has experienced a recent resurgence of interest. The ODNP technique described here relies on the double resonance of electron spin resonance (ESR) at the most common, i.e. X-band ( approximately 10GHz), frequency and (1)H nuclear magnetic resonance (NMR) at approximately 15MHz. It requires only a standard continuous wave (cw) ESR spectrometer with an NMR probe inserted or built into an X-band cavity. We focus on reviewing a new and powerful manifestation of ODNP as a high frequency NMR relaxometry tool that probes dipolar cross relaxation between the electron spins and the (1)H nuclear spins at X-band frequencies. This technique selectively measures the translational mobility of water within a volume extending 0.5-1.5nm outward from a nitroxide radical spin probe that is attached to a targeted site of a macromolecule. It allows one to study the dynamics of water that hydrates or permeates the surface or interior of proteins, polymers, and lipid membrane vesicles. We begin by reviewing the recent advances that have helped develop ODNP into a tool for mapping the dynamic landscape of hydration water with sub-nanometer locality. In order to bind this work coherently together and to place it in the context of the extensive body of research in the field of NMR relaxometry, we then rephrase the analytical model and extend the description of the ODNP-derived NMR signal enhancements. This extended model highlights several aspects of ODNP data analysis, including the importance of considering all possible effects of microwave sample heating, the need to consider the error associated with various relaxation rates, and the unique ability of ODNP to probe the electron-(1)H cross-relaxation process, which is uniquely sensitive to fast (tens of ps) dynamical processes. By implementing the relevant corrections in a stepwise fashion, this paper draws a consensus result from previous ODNP procedures and then shows how such data can be further corrected to yield clear and reproducible saturation of the NMR hyperpolarization process. Finally, drawing on these results, we broadly survey the previous ODNP dynamics literature. We find that the vast number of published, empirical hydration dynamics data can be reproducibly classified into regimes of surface, interfacial, vs. buried water dynamics.

Monday, November 18, 2013

Cross Polarization for Dissolution Dynamic Nuclear Polarization Experiments at Readily Accessible Temperatures 1.2 < T < 4.2 K


Bornet, A., et al., Cross Polarization for Dissolution Dynamic Nuclear Polarization Experiments at Readily Accessible Temperatures 1.2 < T < 4.2 K. Appl. Magn. Reson., 2012. 43(1-2): p. 107-117.


Cross polarization can provide significant enhancements with respect to direct polarization of low-γ nuclei such as 13 C. Substantial gains in sample throughput (shorter polarization times) can be achieved by exploiting shorter build-up times τ DNP ( 1 H) < τ DNP ( 13 C). To polarize protons rather than low-γ nuclei, nitroxide radicals with broad ESR resonances such as TEMPO are more appropriate than Trityl and similar carbon-based radicals that have narrow lines. With TEMPO as polarizing agent, the main Dynamic Nuclear Polarization (DNP) mechanism is thermal mixing (TM). Cross polarization makes it possible to attain higher polarization levels at 2.2 K than one can obtain with direct DNP of low-γ nuclei with TEMPO at 1.2 K, thus avoiding complex cryogenic technology.

Friday, November 15, 2013

Diradicals


For solid-state DNP-NMR spectroscopy very large enhancement factors can be achieved when employing the cross-effect, which is especially sufficient when using bi-radicals (e.g. TOTAPOL,bTbk ...). Bi-radicals (or Di-radicals) were not invented for DNP-NMR spectroscopy and a huge amount of EPR literature is available describing their magnetic resonance related properties.
The article cited below gives a comprehensive overview and can a source of new inspiration in the hunt for more efficient polarizing agents for DNP-NMR spectroscopy.

Abe, M., Diradicals. Chem. Rev., 2013. 113(9): p. 7011-7088.

Unfortunately no abstract available.


Wednesday, November 13, 2013

Level anti-crossings in ParaHydrogen Induced Polarization experiments with Cs-symmetric molecules


Buljubasich, L., et al., Level anti-crossings in ParaHydrogen Induced Polarization experiments with Cs-symmetric molecules. J Magn Reson, 2012. 219(0): p. 33-40.


Hyperpolarization by means of ParaHydrogen Induced Polarization (PHIP) has found increasing applications since its discovery. However, in the last decade only a few experiments have been reported describing the hydrogenation of symmetric molecules. A general AA'BB' system is studied here. Calculations of the spin dynamics with the density matrix formalism support the experimental findings, providing profound understanding of the experiments in Cs-symmetric molecules. Level anti-crossings between states related to the triplet and the singlet state of one pair of the protons are identified as being responsible for hyperpolarization transfer in a PHIP experiment, when the former p-H(2) protons occupy the sites AA'. The hydrogenation of acetylene dicarboxylic acid dimethylester with parahydrogen is used to illustrate the case. The theoretical treatment applied to this particular reaction explains the signal enhancements in both groups of protons in the spectrum when the sample is placed in the proper magnetic field strength, including the phase inversion of the signal of the methyl group. The treatment described here can be extended to every molecule which can be approximated as an AA'BB' system.

Friday, November 8, 2013

Factors Affecting DNP NMR in Polycrystalline Diamond Samples


Casabianca, L.B., et al., Factors Affecting DNP NMR in Polycrystalline Diamond Samples. The Journal of Physical Chemistry C, 2011. 115(39): p. 19041-19048.


This work examines several polycrystalline diamond samples for their potential as polarizing agents for dynamic nuclear polarization (DNP) in NMR. Diamond samples of various origin and particle sizes ranging from a few nanometers to micrometers were examined by EPR, solid-state NMR and DNP techniques. A correlation was found between the size of the diamond particles and the electron spin–lattice relaxation time, the 13C nuclear spin–lattice relaxation times in room temperature magic-angle-spinning experiments, and the ability of the diamond carbons to be hyperpolarized by irradiating unpaired electrons of inherent defects by microwaves at cryogenic temperatures. As the size of the diamond particles approaches that of bulk diamond, both electron and nuclear relaxation times become longer. NMR signal enhancement through DNP was found to be very efficient only for these larger size diamond samples. The reasons and implications of these results are briefly discussed, in the light of these EPR, DNP, and NMR observations.

Wednesday, November 6, 2013

Transmit-Only/Receive-Only Radiofrequency System for Hyperpolarized 13C MRS Cardiac Metabolism Studies in Pigs


Giovannetti, G., et al., Transmit-Only/Receive-Only Radiofrequency System for Hyperpolarized 13C MRS Cardiac Metabolism Studies in Pigs. Appl. Magn. Reson., 2013. 44(10): p. 1125-1138.


Hyperpolarized 13C magnetic resonance spectroscopy in pig models enables metabolic activity mapping, providing a powerful tool for the study of the heart physiology, but requires the development of dedicated radiofrequency coils, capable of providing large field of view with high signal-to-noise ratio (SNR) data. This work describes the simulations and the tests of a transmit-only (TX) volume coil/receive-only (RX) surface coil both designed for hyperpolarized studies of pig heart with a clinical 3T scanner. The coil characterization is performed by developing an SNR model for coil performance in terms of coil resistance, sample induced resistance and magnetic field pattern. In particular, coil resistances were calculated from Ohm’s law, while magnetic field patterns and sample-induced resistances were calculated using a numerical finite-difference time-domain algorithm. Experimental phantom chemical shift image, showed good agreement with the theoretical SNR-vs-depth profiles and highlighted the advantage of the novel configuration over the single transmit–receive coils throughout the volume of interest for cardiac imaging in pig. Finally, the TX-birdcage/RX-circular configuration was tested by acquiring metabolic maps with hyperpolarized [1-13C] pyruvate injected i.v. in a pig. The results of the phantom and pig experiments show the ability of the coil configuration to image well the metabolites distribution.

Monday, November 4, 2013

Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms


Hovav, Y., et al., Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms. Appl. Magn. Reson., 2012. 43(1-2): p. 21-41.


Dynamic nuclear polarization (DNP) is used to enhance signals in NMR and MRI experiments. During these experiments microwave (MW) irradiation mediates transfer of spin polarization from unpaired electrons to their neighboring nuclei. Solid state DNP is typically applied to samples containing high concentrations (i.e. 10–40 mM) of stable radicals that are dissolved in glass forming solvents together with molecules of interest. Three DNP mechanisms can be responsible for enhancing the NMR signals: the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Recently, numerical simulations were performed to describe the SE and CE mechanisms in model systems composed of several nuclei and one or two electrons. It was shown that the presence of core nuclei, close to DNP active electrons, can result in a decrease of the nuclear polarization, due to broadening of the double quantum (DQ) and zero quantum (ZQ) spectra. In this publication we consider samples with high radical concentrations, exhibiting broad inhomogeneous EPR line-shapes and slow electron cross-relaxation rates, where the TM mechanism is not the main source for the signal enhancements. In this case most of the electrons in the sample are not affected by the MW field applied at a discrete frequency. Numerical simulations are performed on spin systems composed of several electrons and nuclei in an effort to examine the role of the DNP inactive electrons. Here we show that these electrons also broaden the DQ and ZQ spectra, but that they hardly cause any loss to the DNP enhanced nuclear polarization due to their spin-lattice relaxation mechanism. Their presence can also prevent some of the polarization losses due to the core nuclei.

Friday, November 1, 2013

DAC-board based X-band EPR spectrometer with arbitrary waveform control

Some instrumentation that is very interesting.



Kaufmann, T., et al., DAC-board based X-band EPR spectrometer with arbitrary waveform control. J Magn Reson, 2013. 235(0): p. 95-108.


We present arbitrary control over a homogenous spin system, demonstrated on a simple, home-built, electron paramagnetic resonance (EPR) spectrometer operating at 8-10 GHz (X-band) and controlled by a 1 GHz arbitrary waveform generator (AWG) with 42 dB (i.e. 14-bit) of dynamic range. Such a spectrometer can be relatively easily built from a single DAC (digital to analog converter) board with a modest number of stock components and offers powerful capabilities for automated digital calibration and correction routines that allow it to generate shaped X-band pulses with precise amplitude and phase control. It can precisely tailor the excitation profiles "seen" by the spins in the microwave resonator, based on feedback calibration with experimental input. We demonstrate the capability to generate a variety of pulse shapes, including rectangular, triangular, Gaussian, sinc, and adiabatic rapid passage waveforms. We then show how one can precisely compensate for the distortion and broadening caused by transmission into the microwave cavity in order to optimize corrected waveforms that are distinctly different from the initial, uncorrected waveforms. Specifically, we exploit a narrow EPR signal whose width is finer than the features of any distortions in order to map out the response to a short pulse, which, in turn, yields the precise transfer function of the spectrometer system. This transfer function is found to be consistent for all pulse shapes in the linear response regime. In addition to allowing precise waveform shaping capabilities, the spectrometer presented here offers complete digital control and calibration of the spectrometer that allows one to phase cycle the pulse phase with 0.007 degrees resolution and to specify the inter-pulse delays and pulse durations to </= 250 ps resolution. The implications and potential applications of these capabilities will be discussed.