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Magnetic Resonance An interactive open-access publication of the Groupement AMPERE
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Preprints
https://doi.org/10.5194/mr-2020-19
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/mr-2020-19
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Aug 2020

13 Aug 2020

Review status
A revised version of this preprint was accepted for the journal MR and is expected to appear here in due course.

High sensitivity Gd3+-Gd3+ EPR distance measurements that eliminate artefacts seen at short distances

Hassane El Mkami1, Robert I. Hunter1, Paul A. S. Cruickshank1, Michael J. Taylor1, Janet E. Lovett1, Akiva Feintuch2, Mian Qi3, Adelheid Godt3, and Graham M. Smith1 Hassane El Mkami et al.
  • 1SUPA, School of Physics and Astronomy, University of St Andrews, St Andrews KY16 9SS, UK
  • 2Department of Chemical Physics, Weizmann Institute of Science, Rehovot, Israel
  • 3Faculty of Chemistry and Center of Molecular Materials (CM2), Bielefeld University, Universitätsstraße 25, 33615 Bielefeld, Germany

Abstract. Gadolinium complexes are attracting increasing attention as spin labels for EPR dipolar distance measurements in biomolecules and particularly for in-cell measurements. It has been shown that flip-flop transitions within the central transition of the high spin Gd3+ ion can introduce artefacts in dipolar distance measurements, particularly when measuring distances less than 3–4 nm. Previous work has shown some reduction of these artefacts through increasing the frequency separation between the two frequencies required for the Double Electron-Electron Resonance (DEER) experiment. Here we use a high power (1 kW), wideband, non-resonant, system operating at 94 GHz to evaluate DEER measurement protocols using two rigid Gd(III)-rulers, consisting of two [GdIII(PyMTA)] complexes, with separations of 2.1 nm and 6.0 nm, respectively. We show that by avoiding the |−1/2⟩ → |1/2⟩ central transition completely, and placing both the pump and the observer pulses on either side of the central transition, we can now observe apparently artefact-free spectra and narrow distance distributions, even for a Gd-Gd distance of 2.1 nm. Importantly we still maintain excellent signal-to-noise ratio and relatively high modulation depths. These results have implications for in-cell EPR measurements at naturally occurring biomolecule concentrations.

Hassane El Mkami et al.

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Hassane El Mkami et al.

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High sensitivity Gd3+- Gd3+ EPR distance measurements that eliminate artefacts seen at short distances Hassane El Mkami, Robert I. Hunter, Paul A. S. Cruickshank, Michael J. Taylor, Janet E. Lovett, Akiva Feintuch, Mian Qi, Adelheid Godt, and Graham M. Smith https://doi.org/10.17630/96ab76ee-38f4-468f-9ea8-e947f638261f

Hassane El Mkami et al.

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Latest update: 01 Dec 2020
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Short summary
Through a series of DEER measurements on two Gd-rulers, with Gd-Gd distances of 2.1 and 6.0 nm, we show that artefacts commonly observed when measuring short distances can be eliminated by avoiding excitation of the central transition by both the pump and observer pulses. By using a wideband induction mode sample-holder at 94 GHz, we demonstrate that high quality DEER measurements will become possible using Gd spin labels at sub-uM concentrations, with implications for in-cell DEER measurements.
Through a series of DEER measurements on two Gd-rulers, with Gd-Gd distances of 2.1 and 6.0 nm,...
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