Spin density localization and accessibility of organic radicals affect liquid-state DNP efficiency

Levien, Marcel; Reinhard, Maik; Hiller, Markus; Tkach, Igor; Bennati, Marina; Orlando, Tomas

doi:10.1039/d0cp05796g

Cited by 15 publications

(14 citation statements)

References 48 publications

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“…Therefore, we can predict that at low magnetic fields, the liquid-state Overhauser DNP efficiency will practically be the same for all of the nitroxides studied in the present work. This agrees well with previous studies, where relatively small polarizing agents (except BDPA and nitroxides attached to fullerene) showed very similar coupling factors at the X-band frequency. , Thus, experiments at 9.4 T are perfectly suited to observe the dependence of the Overhauser DNP effect on the inner-sphere rotational dynamics for the nitroxide radicals studied here.…”

Section: Discussionsupporting

confidence: 92%

“…This agrees well with previous studies, where relatively small polarizing agents (except BDPA and nitroxides attached to fullerene) showed very similar coupling factors at the X-band frequency. 23,42 Thus, experiments at 9.4 T are perfectly suited to observe the dependence of the Overhauser DNP effect on the inner-sphere rotational dynamics for the nitroxide radicals studied here.…”

Section: ■ Discussionmentioning

confidence: 93%

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Influence of Rotational Motion of Nitroxides on Overhauser Dynamic Nuclear Polarization: A Systematic Study at High Magnetic Fields

et al. 2021

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TEMPOL nitroxide radicals are frequently used as a polarizing agent for liquid-state Overhauser dynamic nuclear polarization. To achieve large signal enhancements at high magnetic fields (9.4 T), fast picosecond to sub-picosecond dynamics between nitroxide and the target solvent molecule are required. Such dynamics have been predicted by molecular dynamic studies and attributed to fast inner-sphere motions of a transient radical–solvent complex. Here, we systematically study a series of nitroxide radicals with different substituents around the electron spin-bearing NO moiety and different overall sizes to quantify the contribution of the rotational dynamics of the radical to these inner-sphere dynamics. The experiments are performed at a 9.4 T magnetic field, which exhibits high sensitivity to rotational motion contributions in a low picosecond time range. We can show that the observed enhancements can be quantitatively predicted taking the rotational motion of the radical into account.

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Section: Discussionsupporting

confidence: 92%

Section: ■ Discussionmentioning

confidence: 93%

Influence of Rotational Motion of Nitroxides on Overhauser Dynamic Nuclear Polarization: A Systematic Study at High Magnetic Fields

et al. 2021

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“…We conclude that high-field ENDOR in conjunction with DFT calculation is the method of choice for the investigation of hydrogen bonding of nitroxide radicals. We believe that the new information about hydrogen bonding of nitroxide radicals will allow to improve the prediction methods for investigation of nitroxide-labeled (bio)-macromolecules (DEER rotamer library [89]) and also lead to a better understanding of the dependencies of Overhauser DNP [90] enhancement factors on the structure of nitroxides [91].…”

Section: Discussionmentioning

confidence: 99%

Hydrogen-Bonded Complexes of Neutral Nitroxide Radicals with 2-Propanol Studied by Multifrequency EPR/ENDOR

et al. 2021

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The hydrogen bond plays a key role in weak directional intermolecular interactions. It is operative in determining molecular conformation and aggregation, and controls the function of many chemical systems, ranging from inorganic, organic to biological molecules. Although an enormous amount of spectroscopic information has been collected about hydrogen-bond formation between molecules with closed-shell electronic configuration, the details of such interactions between open-shell radicals and closed-shell molecules are still rare. Here we report on an investigation of hydrogen-bonded complexes between pyrroline-type as well as piperidine-type neutral nitroxide radicals and an alcohol, i.e., 2-propanol. These nitroxide radicals are commonly used as EPR spin labels and probes. To obtain information on the geometry of the complexes and their electronic structure, multi-resonance EPR techniques at various microwave frequencies (X-, Q-, W-band, 244 GHz) have been employed in conjunction with DFT calculations. The planar five-membered ring system of the pyrroline-type nitroxide radical was found to form exclusively well-defined in-plane σ-type hydrogen-bonded complexes with one 2-propanol molecule in the first solvation shell in frozen solution. The measured hyperfine parameters of the hydrogen-bridge proton and the internal magnetic parameters describing the electron Zeeman and the electron-nuclear hyperfine and nuclear quadrupole interactions are in good agreement with values predicted by state-of-the-art DFT calculations. In contrast, multi-resonance EPR on the non-planar six-membered ring system of the piperidine-type nitroxide radical (TEMPOL) reveals a more complex situation, i.e., a mixture of a σ-type with, presumably, an out-of-plane π-type complex, both present in comparable fraction in frozen solution. For TEMPOL, the DFT calculations failed to predict magnetic interaction parameters that are in good agreement with experiment, apparently due to the considerable flexibility of the nitroxide and hydrogen-bonded complex. The detailed information about nitroxide/solvent complexes is of particular importance for Dynamic Nuclear Polarization (DNP) and site-directed spin-labeling EPR studies that employ nitroxides as polarizing agents or spin labels, respectively.

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“…31 The interaction with the polarizing biradical would thus be short-lived, and the proton pool would get repeatedly polarized by different transient encounters with different biradicals; a certain ''pulsing'' of the effect will therefore occur spontaneously. 13,44,45 5. Analysis of the self-relaxation rates driving J-DNP Fig.…”

confidence: 99%

J-Driven dynamic nuclear polarization for sensitizing high field solution state NMR

Concilio

Kuprov

Frydman

2022

Phys. Chem. Chem. Phys.

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Spin density localization and accessibility of organic radicals affect liquid-state DNP efficiency

Abstract: We report a large variation in liquid DNP performance of up to a factor of about five in coupling factors among organic radicals used commonly as polarizing agents. A comparative...

Cited by 15 publications

References 48 publications

Influence of Rotational Motion of Nitroxides on Overhauser Dynamic Nuclear Polarization: A Systematic Study at High Magnetic Fields

Influence of Rotational Motion of Nitroxides on Overhauser Dynamic Nuclear Polarization: A Systematic Study at High Magnetic Fields

Hydrogen-Bonded Complexes of Neutral Nitroxide Radicals with 2-Propanol Studied by Multifrequency EPR/ENDOR

J-Driven dynamic nuclear polarization for sensitizing high field solution state NMR

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