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Pulsed EPR Dipolar Spectroscopy under the Breakdown of the High‐Field Approximation: The High‐Spin Iron(III) Case
Author(s) -
Abdullin Dinar,
Matsuoka Hideto,
Yulikov Maxim,
Fleck Nico,
Klein Christoph,
Spicher Sebastian,
Hagelueken Gregor,
Grimme Stefan,
Lützen Arne,
Schiemann Olav
Publication year - 2019
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201900977
Subject(s) - dipole , spin (aerodynamics) , electron paramagnetic resonance , field (mathematics) , chemistry , spectroscopy , relaxation (psychology) , nitroxide mediated radical polymerization , molecular physics , tensor (intrinsic definition) , spectral line , condensed matter physics , atomic physics , nuclear magnetic resonance , physics , quantum mechanics , mathematics , psychology , social psychology , radical polymerization , organic chemistry , pure mathematics , copolymer , thermodynamics , polymer
Pulsed EPR dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling and thus the distance between electron‐spin centers. To date, PDS measurements to metal centers were limited to ions that adhere to the high‐field approximation. Here, the PDS methodology is extended to cases where the high‐field approximation breaks down on the example of the high‐spin Fe 3+ /nitroxide spin‐pair. First, the theory developed by Maryasov et al. ( Appl. Magn. Reson . 2006 , 30 , 683–702) was adapted to derive equations for the dipolar coupling constant, which revealed that the dipolar spectrum does not only depend on the length and orientation of the interspin distance vector with respect to the applied magnetic field but also on its orientation to the effective g ‐tensor of the Fe 3+ ion. Then, it is shown on a model system and a heme protein that a PDS method called relaxation‐induced dipolar modulation enhancement (RIDME) is well‐suited to measuring such spectra and that the experimentally obtained dipolar spectra are in full agreement with the derived equations. Finally, a RIDME data analysis procedure was developed, which facilitates the determination of distance and angular distributions from the RIDME data. Thus, this study enables the application of PDS to for example, the highly relevant class of high‐spin Fe 3+ heme proteins.