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Magnetic Resonance An interactive open-access publication of the Groupement AMPERE
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https://doi.org/10.5194/mr-2020-14
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/mr-2020-14
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Jul 2020

13 Jul 2020

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This preprint is currently under review for the journal MR.

Surprising absence of strong homonuclear coupling at low magnetic field explored by two-field NMR spectroscopy

Ivan V. Zhukov1,2, Alexey S. Kiryutin1,2, Ziqing Wang3, Milan Zachrdla3, Alexandra V. Yurkovskaya1,2, Konstantin L. Ivanov1,2, and Fabien Ferrage3 Ivan V. Zhukov et al.
  • 1International Tomography Center, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090, Russia
  • 2Novosibirsk State University, Novosibirsk, 630090, Russia
  • 3Laboratoire des Biomolécules, LBM, Département de chimie, École normale supérieure, PSL University, Sorbonne Université, CNRS, 75005 Paris, France

Abstract. Strong coupling of nuclear spins, which is achieved when their scalar coupling 2πJ is greater than or comparable to the difference δω in their Larmor precession frequencies in an external magnetic field, gives rise to efficient coherent longitudinal polarization transfer. The strong-coupling regime can be achieved when the external magnetic field is sufficiently low, as δω is reduced proportional to the field strength. In the present work, however, we demonstrate that in heteronuclear spin systems these simple arguments may not hold, since heteronuclear spin-spin interactions alter the δω value. The experimental method that we use is two-field NMR (Nuclear Magnetic Resonance), exploiting sample shuttling between a high field, at which NMR spectra are acquired, and low field, where strong couplings are expected, at which NMR pulses can be applied to affect the spin dynamics. By using this technique, we generate zero-quantum spin coherences by means of non-adiabatic passage through a level anti-crossing and study their evolution at low field. Such zero-quantum coherences mediate the polarization transfer under strong coupling conditions. Experiments performed with an 13C labelled amino acid clearly show that the coherent polarization transfer at low field is pronounced in the 13C-spin subsystem under proton decoupling. However, in the absence of proton decoupling, polarization transfer by coherent processes is dramatically reduced, demonstrating that heteronuclear spin-spin interactions suppress the strong coupling regime even when the external field is low. A theoretical model is presented, which can model the reported experimental results.

Ivan V. Zhukov et al.

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Ivan V. Zhukov et al.

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Short summary
We have studied spin dynamics in nuclear spin systems at low magnetic fields, where “strong coupling” among nuclear spins of the same kind, here C-13, is expected. Such conditions are known to be favorable for coherent polarization transfer. However, to our surprise, interactions to other nuclei, here protons, lead to breakdown of the strong coupling conditions. By using a two-field NMR approach, we are able to manipulate low-field spin dynamics and reintroduce strong coupling.
We have studied spin dynamics in nuclear spin systems at low magnetic fields, where “strong...
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