Category Archives: Polarizing Agents

Platforms for Stable Carbon‐Centered Radicals #DNPNMR

Kato, Kenichi, and Atsuhiro Osuka. “Platforms for Stable Carbon‐Centered Radicals.” Angewandte Chemie International Edition 58, no. 27 (July 2019): 8978–86.

https://doi.org/10.1002/anie.201900307

Organic radicals can play important roles potentially in diverse functional materials owing to an unpaired electron but are usually highly reactive and difficult to use. Therefore, stabilization of organic radicals is crucially important. Among organic radicals, carbon-centered radicals are promising because of their trivalent nature that enables structural diversity and elaborate designs but they show less stabilities because of reactivities toward carboncarbon bond formation and atmospheric oxygen. Recently, stable carbon-centered radicals have been increasingly explored on the basis of diverse molecular platforms. This minireview highlights these newly explored stable carbon-centered radicals with a particular focus on porphyrinoid-stabilized radicals owing to their remarkable spin delocalization abilities.

Molecular Dynamics and Hyperpolarization Performance of Deuterated β-Cyclodextrins #DNP

Caracciolo, Filippo, Efstathios Charlaftis, Lucio Melone, and Pietro Carretta. “Molecular Dynamics and Hyperpolarization Performance of Deuterated β-Cyclodextrins.” The Journal of Physical Chemistry B 123, no. 17 (May 2, 2019): 3731–37.

https://doi.org/10.1021/acs.jpcb.9b01857.

We discuss the temperature dependence of the 1H and 13C nuclear spin−lattice relaxation rate 1/T1 and dynamic nuclear polarization (DNP) performance in β-cyclodextrins with deuterated methyl groups. It is shown that 13C DNP-enhanced polarization is raised up to 10%. The temperature dependence of the buildup rate for nuclear spin polarization and of 1/T1, below 4.2 K, is analyzed in the framework of the thermal mixing regime and the origin of the deviations from the theoretical behavior discussed. 13C 1/T1 is determined at low temperature by the glassy dynamics and at high temperature by the rotational molecular motions of the deuterated methyl groups. Thanks to deuteration, relaxation times approaching 30 s are achieved at room temperature, making this material interesting for molecular imaging applications. The effect of molecular dynamics on the line width of the NMR spectra is also discussed.

Water-soluble BDPA radicals with improved persistence #DNPNMR

Mandal, Sucharita, and Snorri Th. Sigurdsson. “Water-Soluble BDPA Radicals with Improved Persistence.” Chemical Communications, 2020, 10.1039.D0CC04920D. 

https://doi.org/10.1039/D0CC04920D.

1,3-Bis(diphenylene)-2-phenylallyl (BDPA) radicals are promising polarizing agents for increasing the sensitivity of NMR spectroscopy through dynamic nuclear polarization (DNP), but have low persistence and solubility in aqueous media. New tetraalkyl/aryl-ammonium derivatives of BDPA are soluble in polar solvents and are highly persistent, with 5–20-fold lower initial rates of degradation than BDPA.

Balancing dipolar and exchange coupling in biradicals to maximize cross effect dynamic nuclear polarization #DNPNMR

Equbal, Asif, Kan Tagami, and Songi Han. “Balancing Dipolar and Exchange Coupling in Biradicals to Maximize Cross Effect Dynamic Nuclear Polarization.” Physical Chemistry Chemical Physics 22, no. 24 (2020): 13569–79.

https://doi.org/10.1039/D0CP02051F

Dynamic nuclear polarization (DNP) by the Cross Eect (CE) has become a game changer for solid-state nuclear magnetic resonance (NMR) spectroscopy. The eciency of CE-DNP depends on the strength of the electron-electron coupling in biradical polarizing agents. Hence, the focus lately has been on designing biradicals with a large net exchange (J) and dipolar (D) coupling. In this study, we reveal that the crucial factor for CE-DNP is not the sum, J+D, but rather the relative magnitude of J and D, expressed as the J/D ratio. We show that the mechanistic basis of this interference lies in the isotropic v.s. the anisotropic nature of the J and D couplings, respectively. This interference can lead to a small (eective) electron-electron coupling for many orientations even when J+D is large, resulting in non-adiabatic rotor-events. We find that when 0< jJ/Dj < 1 the CE-DNP eciency is attenuated for the majority of orientations, with greater attenuation observed at higher magnetic elds and faster Magic-Angle Spinning (MAS) frequency. The interference eect of J and D coupling introduced in this study can explain why many biradicals with high or comparable J + D still show signicantly divergent DNP performances. We debut J/D as a consequential criteria for designing ecient biradicals to robustly perform across a large range of B0 elds and MAS frequencies.

[NMR] Paramagnetic metal ion DNP | Tutorial by Björn Corzilius| Tuesday, September 29, 8:00am California #DNPNMR

Dear NMR Enthusiast,

The 15th Educational Tutorial will be given by Prof. Björn Corzilius, University of Rostock, Germany, on the topic:

“Paramagnetic metal ions polarizing agents for DNP”.

Abstract:

Paramagnetic metal ions are intriguing alternatives to radical polarizing agents for DNP. In the first part of my presentation I will introduce their magnetic resonance properties and the peculiarities in DNP theory of high-spin metal ion polarizing agents. In the second part I will give an overview of recent DNP applications in biomolecular and materials science using high-spin polarizing agents Mn(II), Fe(III), Cr(III), Gd(III).

Speaker’s biography:

2005-2008: PhD, Physical Chemistry, Technical University Darmstadt
2009-2013: Postdoctoral Fellow and Associate, Massachusetts Institute of Technology
2013-2019: Emmy Noether Research Group Leader, Goethe University Frankfurt2019-present: Professor of Physical Chemistry, University of Rostock

Webinar details:
Time: Tuesday, September 29, 2020, 08:00 AM California or 11:00 am Boston or 5:00 PM Paris or 8:30 PM Delhi

Join Meeting: https://ucsb.zoom.us/j/92480496788
Meeting ID: 924 8049 6788

Best regards,

Global NMR Discussion Meetings

[Organizers:
Adrian Draney (Guido Pintacuda Lab, CRMN lyon)
Amrit Venkatesh (Aaron Rossini Lab, Iowa)
Asif Equbal (Songi Han Lab, UCSB)
Blake Wilson (Robert Tycko Lab, NIH)
Michael Hope (Lyndon Emsley Lab, EPFL)
Mona Mohammadi (Alexej Jerschow, NYU)
PinelopiMoutzouri (Lyndon Emsley Lab, EPFL) ]
====================================
This is the AMPERE MAGNETIC RESONANCE mailing list:
http://www.drorlist.com/nmrlist.html

NMR web database:
http://www.drorlist.com/nmr.html

EPR Spectroscopy: A Powerful Tool to Analyze Supramolecular Host•Guest Complexes of Stable Radicals with Cucurbiturils

Liu, Fengbo, Hakim Karoui, Antal Rockenbauer, Simin Liu, Olivier Ouari, and David Bardelang. “EPR Spectroscopy: A Powerful Tool to Analyze Supramolecular Host•Guest Complexes of Stable Radicals with Cucurbiturils.” Molecules 25, no. 4 (February 11, 2020): 776.

https://doi.org/10.3390/molecules25040776

Stable organic free radicals are increasingly studied compounds due to the multiple and unusual properties imparted by the single electron(s). However, being paramagnetic, classical methods such as NMR spectroscopy can hardly be used due to relaxation and line broadening effects. EPR spectroscopy is thus better suited to get information about the immediate surroundings of the single electrons. EPR has enabled obtaining useful data in the context of host•guest chemistry, and a classical example is reported here for the stable (2,2,6,6-tetramethyl-4-oxo-piperidin-1-yl)oxyl or 4-oxo-TEMPO nitroxide (TEMPONE) inside the macrocycle host cucurbit[7]uril (CB[7]). Generally and also observed here, a contraction of the spectrum is observed as a result of the reduced nitrogen coupling constant due to inclusion complexation in the hydrophobic cavity of the host. Simulations of EPR spectra allowed determining the corresponding binding constant pointing to a weaker affinity for CB[7], compared to TEMPO with CB[7]. We complement this work by the results of EPR spectroscopy of a biradical: bis-TEMPO-bis-ketal (bTbk) with cucurbit[8]uril (CB[8]). Initial investigations pointed to very weak effects on the spectrum of the guest and incorrectly led us to conclude an absence of binding. However, simulations of EPR spectra combined with NMR data of reduced bTbk allowed showing inclusion complexation. EPR titrations were performed, and the corresponding binding constant was determined. 1H NMR spectra with reduced bTbk suggested a shuttle mechanism, at nearly one equivalent of CB[8], for which the host moves rapidly between two stations.

Hyperpolarized water through dissolution dynamic nuclear polarization with UV-generated radicals #DNPNMR

Pinon, Arthur C., Andrea Capozzi, and Jan Henrik Ardenkjær-Larsen. “Hyperpolarized Water through Dissolution Dynamic Nuclear Polarization with UV-Generated Radicals.” Communications Chemistry 3, no. 1 (December 2020): 57.

https://doi.org/10.1038/s42004-020-0301-6

In recent years, hyperpolarization of water protons via dissolution Dynamic Nuclear Polarization (dDNP) has attracted increasing interest in the magnetic resonance community. Hyperpolarized water may provide an alternative to Gd-based contrast agents for angiographic and perfusion Magnetic Resonance Imaging (MRI) examinations, and it may report on chemical and biochemical reactions and proton exchange while perfoming Nuclear Magnetic Resonance (NMR) investigations. However, hyperpolarizing water protons is challenging. The main reason is the presence of radicals, required to create the hyperpolarized nuclear spin state. Indeed, the radicals will also be the main source of relaxation during the dissolution and transfer to the NMR or MRI system. In this work, we report water magnetizations otherwise requiring a field of 10,000 T at room temperature on a sample of pure water, by employing dDNP via UV-generated, labile radicals. We demonstrate the potential of our methodology by acquiring a 15N spectrum from natural abundance urea with a single scan, after spontaneous magnetization transfer from water protons to nitrogen nuclei.

TinyPols: a family of water-soluble binitroxides tailored for dynamic nuclear polarization enhanced NMR spectroscopy at 18.8 and 21.1 T #DNPNMR

Lund, Alicia, Gilles Casano, Georges Menzildjian, Monu Kaushik, Gabriele Stevanato, Maxim Yulikov, Ribal Jabbour, et al. “TinyPols: A Family of Water-Soluble Binitroxides Tailored for Dynamic Nuclear Polarization Enhanced NMR Spectroscopy at 18.8 and 21.1 T.” Chemical Science 11, no. 10 (2020): 2810–18.

https://doi.org/10.1039/C9SC05384K.

Dynamic Nuclear Polarization (DNP) has recently emerged as a key method to increase the sensitivity of solid-state NMR spectroscopy under Magic Angle Spinning (MAS). While efficient binitroxide polarizing agents such as AMUPol have been developed for MAS DNP NMR at magnetic fields up to 9.4 T, their performance drops rapidly at higher fields due to the unfavorable field dependence of the cross-effect (CE) mechanism and AMUPol-like radicals were so far disregarded in the context of the development of polarizing agents for very high-field DNP. Here, we introduce a new family of water-soluble binitroxides, dubbed TinyPols, which have a three-bond non-conjugated flexible amine linker allowing sizable couplings between the two unpaired electrons. We show that this adjustment of the linker is crucial and leads to unexpectedly high DNP enhancement factors at 18.8 T and 21.1 T: an improvement of about a factor 2 compared to AMUPol is reported for spinning frequencies ranging from 5 to 40 kHz, with εH of up to 90 at 18.8 T and 38 at 21.1 T for the best radical in this series, which are the highest MAS DNP enhancements measured so far in aqueous solutions at these magnetic fields. This work not only breathes a new momentum into the design of binitroxides tailored towards high magnetic fields, but also is expected to push the application frontiers of high-resolution DNP MAS NMR, as demonstrated here on a hybrid mesostructured silica material.

Paramagnetic Metal Ions for Dynamic Nuclear Polarization #DNPNMR

Corzilius, Björn. “Paramagnetic Metal Ions for Dynamic Nuclear Polarization,” 7:16, 2018.

https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470034590.emrstm1593

Paramagnetic metal ions have been utilized as polarizing agents already in the early days of dynamic nuclear polarization (DNP) and have been more recently introduced for magic-angle spinning (MAS) DNP at high magnetic field. In this article, a comprehensive overview is given about the concepts relevant to DNP with high-spin metal ions. The theoretical basis covering the peculiar electron spin dynamics including spin-orbit coupling and zero-field splitting is reviewed, and prerequisites for efficient DNP are introduced. Subsequently, special considerations about the relevant DNP mechanisms (i.e., solid effect and cross effect) are derived. The practical aspects particular to high-spin metal ion DNP are discussed, focusing on differences with respect to conventional (i.e., radical) polarizing agents. In the final section, several demonstrations of MAS DNP on model systems as well as samples relevant to structural biology and materials research are presented. At last, an outlook is given about the prospects of metal ion DNP in light of recent and future advances in modern DNP.

Polarizing Agents: Evolution and Outlook in Free Radical Development for DNP #DNPNMR

Casano, Gilles, Hakim Karoui, and Olivier Ouari. “Polarizing Agents: Evolution and Outlook in Free Radical Development for DNP” 7 (2018): 14.

https://doi.org/10.1002/9780470034590.emrstm1547.

In this article, we describe an in-depth overview of the development of paramagnetic polarizing agents for dynamic nuclear polarization (DNP)-enhanced solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR). In DNP experiments, the large polarization of unpaired electrons is transferred to surrounding nuclei, which provides a maximum theoretical DNP enhancement of 660 for 1H NMR. The article includes a description of the different polarizing mechanisms and outlines key structural and magnetic parameters that contributed to the rational design of improved polarizing sources. The application of (di)nitroxides, heterobiradicals, narrow-line radicals, paramagnetic metal ions as well as site-specific polarizing agents is discussed. With the best polarizing agents, ssNMR MAS NMR/DNP enhances sensitivity by a factor of up to 200, providing decreased experiment time by five orders of magnitude and opening new avenues for NMR.

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