Category Archives: Relaxation

Simultaneous T1 and T2 mapping of hyperpolarized 13C compounds using the bSSFP sequence #DNPNMR

Milshteyn, Eugene, Galen D. Reed, Jeremy W. Gordon, Cornelius von Morze, Peng Cao, Shuyu Tang, Andrew P. Leynes, Peder E.Z. Larson, and Daniel B. Vigneron. “Simultaneous T1 and T2 Mapping of Hyperpolarized 13C Compounds Using the BSSFP Sequence.” Journal of Magnetic Resonance 312 (March 2020): 106691.

https://doi.org/10.1016/j.jmr.2020.106691

As in conventional 1H MRI, T1 and T2 relaxation times of hyperpolarized (HP) 13C nuclei can provide important biomedical information. Two new approaches were developed for simultaneous T1 and T2 mapping of HP 13C probes based on balanced steady state free precession (bSSFP) acquisitions: a method based on sequential T1 and T2 mapping modules, and a model-based joint T1/T2 approach analogous to MR fingerprinting. These new methods were tested in simulations, HP 13C phantoms, and in vivo in normal Sprague-Dawley rats. Non-localized T1 values, low flip angle EPI T1 maps, bSSFP T2 maps, and Bloch-Siegert B1 maps were also acquired for comparison. T1 and T2 maps acquired using both approaches were in good agreement with both literature values and data from comparative acquisitions. Multiple HP 13C compounds were successfully mapped, with their relaxation time parameters measured within heart, liver, kidneys, and vasculature in one acquisition for the first time.

Relaxation Mechanisms #DNPNMR #EPR

This is an excellent review and summary on different relaxation mechanisms observed in EPR spectroscopy. Understanding EPR relaxation is crucial to understand the DNP process.

Eaton, Sandra S., and Gareth R. Eaton. “Relaxation Mechanisms.” In EMagRes, edited by Robin K. Harris and Roderick L. Wasylishen, 1543–56. Chichester, UK: John Wiley & Sons, Ltd, 2016.

https://doi.org/10.1002/9780470034590.emrstm1507

After a paramagnetic species absorbs energy, there are various relaxation processes by which the excitation energy is lost to the surroundings thereby enabling return to the ground state. The focus of this article is on relaxation of species with S= 1∕2 in magnetically dilute samples. The relative importance of various spin–lattice relaxation processes for each paramagnetic species is strongly dependent on temperature, electronic, and molecular structure. The Raman and local-mode processes make significant contributions to T 1 relaxation in rigid and semirigid lattices for a wide range of species at temperature above about 10 K. The Orbach process requires a low-lying excited state. The thermally activated process is significant when a stochastic process averages inequivalent environments on a timescale comparable to the Larmor frequency, as occurs by rotation of methyl groups or hopping of a hydrogen-bonded proton. Spin-echo dephasing at low temperatures is dominated by nuclear spin diffusion. It is enhanced by dynamic processes that average inequivalently coupled nuclei on the time scale of the hyperfine interaction and by motions that average g and A anisotropy. Analysis of the processes that contribute to relaxation as a function of temperature is shown for triarylmethyl radicals, semiquinones, nitroxides, Cu2+ complexes, iron–sulfur complexes, and radicals in irradiated solids. In fluid solution, motion provides additional relaxation mechanisms. Analysis of T2 in solution is a powerful tool to elucidate motion. Experiments as a function of both temperature and resonance frequency are key to distinguishing between relaxation mechanisms.

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.

https://doi.org/10.1016/j.jmr.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.

Understanding Overhauser Dynamic Nuclear Polarisation through NMR relaxometry #DNPNMR

Parigi, Giacomo, Enrico Ravera, Marina Bennati, and Claudio Luchinat. “Understanding Overhauser Dynamic Nuclear Polarisation through NMR Relaxometry.” Molecular Physics 117, no. 7–8 (April 18, 2019): 888–97.

https://doi.org/10.1080/00268976.2018.1527409

Overhauser dynamic nuclear polarisation (DNP) represents a potentially outstanding tool to increase the sensitivity of solution and solid state NMR experiments, as well as of magnetic resonance imaging. DNP signal enhancements are strongly linked to the spin relaxation properties of the system under investigation, which must contain a paramagnetic molecule used as DNP polariser. In turn, nuclear spin relaxation can be monitored through NMR relaxometry, which reports on the field dependence of the nuclear relaxation rates, opening a route to understand the physical processes at the origin of the Overhauser DNP in solution. The contributions of dipole–dipole and Fermi-contact interactions to paramagnetic relaxation are here described and shown to be responsible to both the relaxometry profiles and the DNP enhancements, so that the experimental access to the former can allow for predictions of the latter.

De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization #DNPNMR

Mentink-Vigier, Frédéric, Anne-Laure Barra, Johan van Tol, Sabine Hediger, Daniel Lee, and Gaël De Paëpe. “De Novo Prediction of Cross-Effect Efficiency for Magic Angle Spinning Dynamic Nuclear Polarization.” Physical Chemistry Chemical Physics 21, no. 4 (2019): 2166–76.

https://doi.org/10.1039/C8CP06819D.

Magic angle spinning dynamic nuclear polarization (MAS-DNP) has become a key approach to boost the intrinsic low sensitivity of NMR in solids. This method relies on the use of both stable radicals as polarizing agents (PAs) and suitable high frequency microwave irradiation to hyperpolarize nuclei of interest. Relating PA chemical structure to DNP efficiency has been, and is still, a long-standing problem. The complexity of the polarization transfer mechanism has so far limited the impact of analytical derivation. However, recent numerical approaches have profoundly improved the basic understanding of the phenomenon and have now evolved to a point where they can be used to help design new PAs. In this work, the potential of advanced MAS-DNP simulations combined with DFT calculations and high-field EPR to qualitatively and quantitatively predict hyperpolarization efficiency of particular PAs is analyzed. This approach is demonstrated on AMUPol and TEKPol, two widely-used bis-nitroxide PAs. The results notably highlight how the PA structure and EPR characteristics affect the detailed shape of the DNP field profile. We also show that refined simulations of this profile using the orientation dependency of the electron spin–lattice relaxation times can be used to estimate the microwave B1 field experienced by the sample. Finally, we show how modelling the nuclear spin–lattice relaxation times of close and bulk nuclei while accounting for PA concentration allows for a prediction of DNP enhancement factors and hyperpolarization build-up times.

Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery #DNPNMR

Barnes, Alexander, Nicholas Alaniva, Edward P. Saliba, Erika L. Sesti, and Patrick T. Judge. “Electron Decoupling with Chirped Microwave Pulses for Rapid Signal Acquisition and Electron Saturation Recovery.” Angewandte Chemie, March 28, 2019.

https://doi.org/10.1002/ange.201900139.

Dynamic nuclear polarization (DNP) increases NMR sensitivity by transferring polarization from electron to nuclear spins. Here we demonstrate that electron decoupling enables improved observation of DNP-enhanced 13C spins in direct dipolar contact with electron spins, thereby leading to an optimal delay between transients largely governed by relatively fast electron relaxation. Signal acquisition constitutes 12% of the total experimental time, significantly increasing signal-to-noise per unit time. We report the first measurement of electron longitudinal relaxation (T1e) during magicangle spinning (MAS) NMR through observation of DNP-enhanced NMR (T1e = 40 ± 6 ms, 40 mM trityl 4.0 kHz MAS, 4.3 K). With a 5 ms DNP period, electron decoupling results in a 195% increase in signal intensity. Chirped microwave pulses and MAS at 4.3 K are achieved with a custom spectrometer. MAS at 4.3 K, DNP, electron decoupling, and short recycle delays improves the sensitivity of 13C in the vicinity of the polarizing agent. This is the first demonstration of recovery times between MAS-NMR transients being governed by short electron T1 and fast DNP transfer.

[NMR] Three PhD or Postdoc positions in NMR relaxation and DNP-NMR of polymers and complex fluids at TU Ilmenau, Germany

Employment opportunity

Within the group of Technical Physics II (Technische Physik II), the Technische Universität Ilmenau is offering up to

Three PhD student or postdoctoral positions

in 

Nuclear Magnetic Resonance of Polymers and Complex Fluids

Funded by the German Research Council DFG, we are continuing our research on polymer melts and solutions with an emphasis on new methods for polymer dynamics. Among other approaches, the project involves 1H and 2H relaxation dispersion investigations of isotopically diluted polymers with our two Stelar Fast Field Cycling (FFC) Relaxometers [Lozovoi et al., Macromolecules 51, 10055 (2018)]. Additional experiments will be carried out on a homebuilt Halbach magnet and commercial equipment (Bruker, Magritek). 

The second project involves application of the mentioned techniques to biomacromolecules, in particular to articular cartilage, with the aim of modelling molecular dynamics in biological tissue for which cartilage is a simple model system. Field-dependent relaxation times are well-known from medical MRI, but only empirically described in the literature; we want to establish a theoretical description of the frequency dependence as well as the width of relaxation times distributions in non-exponential signal decays [Petrov et al., Magn. Reson. Med., doi: 10.1002/mrm.27624 (2019)], and develop these findings into biomarkers for diseases such as osteoarthritis.

The third project focusses on the technical improvement or advanced application of DNP-FFC relaxometry studies. By combining DNP hardware with a FFC relaxometer, we have recently developed a novel platform to boost sensitivity and selectivity in complex fluids such as copolymers, porous media and multicomponent systems [Gizatullin et al., ChemPhysChem 18, 2347 (2017)], of which rocks and crude oil represent a naturally occurring example. Stable radicals are introduced and saturated by microwave irradiation, and subsequent magnetization transfer by either Overhauser or Solid Effect enhances the signal of nearby nuclei – including rare, insensitive and quadrupolar X nuclei – in the vicinity of the radical. Enhancement factors of several hundred have been achieved on our equipment. Depending on the skills and expertise of the candidate, the thesis can follow either a technical or an application focus.

We are seeking motivated individuals who are exploring applications of FFC and DNP-FFC within one of these projects. Requirements differ but typically involve sample preparation, modelling and potentially hardware or software development. Regular discussions and research stays with collaboration partners, mostly in Europe and USA, will be part of the project. This position requires skilled and enthusiastic persons, with an MSc degree in physics, chemistry or related disciplines, willing to work and actively participate in an international environment; a proven hands-on experience in ESR or NMR is a requirement, as are a strong background in NMR theory and programming skills.

The projects aim at obtaining a PhD level and are financed for an initial period of 3 years. The salary is according to the TV-L E13 scale of the German public sector (typically ¾ position depending on skills and experience). Under exceptional circumstances, a full salary postdoctoral position may be funded for a holder of a PhD title in one of these projects but with an initial contract period of 2 years. 

The Technische Universität Ilmenau aims to establish gender equality and strongly encourages applications by female candidates. Handicapped applicants with identical qualification will be considered with priority. Special services are available concerning all social matters. 

Please submit your application files (letter of application, complete CV, certificates, possibly references) preferably by February 28, 2019 to:

Prof. Siegfried Stapf, e-mail: siegfried.stapf@tu-ilmenau.de

_______________________________________________

Prof. Dr. Siegfried Stapf Technische Universität Ilmenau Fakultät für Mathematik und Naturwissenschaften FG Technische Physik II/Polymerphysik Postfach 100565 D-98684 Ilmenau Tel: +49 3677 69 3671 Fax: +49 3677 69 3770 

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Time domain measurement of electron spin relaxation at high fields and dynamic nuclear polarization at sub-millimeter wavelengths #DNPNMR

Dubroca, Thierry, Johannes McKay, Xiaoling Wang, and Johan van Tol. “Time Domain Measurement of Electron Spin Relaxation at High Fields and Dynamic Nuclear Polarization at Sub-Millimeter Wavelengths.” In 2017 IEEE MTT-S International Microwave Symposium (IMS), 1400–1403. Honololu, HI, USA: IEEE, 2017.

https://doi.org/10.1109/MWSYM.2017.8058878

We describe a 395 GHz pulsed electron paramagnetic resonance (EPR) setup, and initial results of relaxation measurements and cw EPR at these frequencies in samples used for liquidand solid-state nuclear magnetic resonance enhanced by dynamic nuclear polarization (DNP). Depending on the amount of spin –orbit coupling, the spin lattice relaxation becomes significantly faster at higher fields and frequencies, which has consequences for some DNP applications at high fields and frequencies. We will discuss the requirements for (sub)millimeter-wave sources and components for DNP and pulsed EPR at even higher frequencies and fields, as even higher magnetic fields will become available in the near future.

Temperature-Dependent Nuclear Spin Relaxation Due to Paramagnetic Dopants Below 30 K: Relevance to DNP-Enhanced Magnetic Resonance Imaging #DNPNMR

Chen, Hsueh-Ying, and Robert Tycko. “Temperature-Dependent Nuclear Spin Relaxation Due to Paramagnetic Dopants Below 30 K: Relevance to DNP-Enhanced Magnetic Resonance Imaging.” The Journal of Physical Chemistry B 122, no. 49 (December 13, 2018): 11731–42.

https://doi.org/10.1021/acs.jpcb.8b07958.

Dynamic nuclear polarization (DNP) can increase nuclear magnetic resonance (NMR) signal strengths by factors of 100 or more at low temperatures. In magnetic resonance imaging (MRI), signal enhancements from DNP potentially lead to enhancements in image resolution. However, the paramagnetic dopants required for DNP also reduce nuclear spin relaxation times, producing signal losses that may cancel the signal enhancements from DNP. Here we investigate the dependence of 1H NMR relaxation times, including T1ρ and T2, under conditions of Lee–Goldburg 1H–1H decoupling and pulsed spin locking, on temperature and dopant concentration in frozen solutions that contain the trinitroxide compound DOTOPA. We find that relaxation times become longer at temperatures below 10 K, where DOTOPA electron spins become strongly polarized at equilibrium in a 9.39 T magnetic field. We show that the dependences of relaxation times on temperature and DOTOPA concentration can be reproduced qualitatively (although not quantitatively) by detailed simulations of magnetic field fluctuations due to flip-flop transitions in a system of dipole-coupled electron spin magnetic moments. These results have implications for ongoing attempts to reach submicron resolution in inductively detected MRI at very low temperatures.

Electron-Spin Relaxation of Triarylmethyl Radicals in Glassy Trehalose #DNPNMR

Triarylmethyl radicals are commonly used dissolution-DNP experiments (dDNP). This article is a good reference source for the electronic relaxation times T1e and T2e in different solvents.

Kuzhelev, Andrey A., Olesya A. Krumkacheva, Ivan O. Timofeev, Victor M. Tormyshev, Matvey V. Fedin, and Elena G. Bagryanskaya. “Electron-Spin Relaxation of Triarylmethyl Radicals in Glassy Trehalose.” Applied Magnetic Resonance 49, no. 11 (November 2018): 1171–80. https://doi.org/10.1007/s00723-018-1023-0.

Trehalose was recently proposed as a promising immobilizer of biomolecules for room-temperature electron paramagnetic resonance (EPR) structural studies. The most crucial parameter in these investigations is electron-spin relaxation (namely, phase memory time Tm). Recently, triarylmethyl (TAM) spin labels attached to DNA in trehalose were found to have the longest Tm at room temperature as compared to the existing spin labels and immobilizers. Therefore, in this work, we investigated TAM radicals in trehalose including Finland trityl (H36 form), perdeuterated Finland trityl (D36 form), and a deuterated version of OX063. The temperature dependence of electron-spin relaxation time of these radicals immobilized in trehalose was measured at X-band frequency, and possible mechanisms of relaxation were considered. OX063D in glassy trehalose has longer Tm up to 200 K as compared to Finland trityl, but at higher temperatures, OX063D is inferior in its relaxation properties, and the deuterated form of Finland trityl is preferable for pulse dipolar EPR spectroscopy experiments at 298 K. The influence of various deuterations (TAM or trehalose) on the observed relaxation times was studied, being controlled by the electron-spin-echo envelope modulation at room temperature.

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