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Updated: May 12, 2025
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Site Directed Spin Labeling and EPR Spectroscopic Studies of Pentameric Ligand-Gated Ion Channels
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Using PELDOR to count spins on multi-nitroxides
Matthias Bretschneider1, Burkhard Endeward1, Jörn Plackmeyer1
1Institute of Physical and Theoretical Chemistry and Center of Biomolecular Magnetic Resonance, Goethe University Frankfurt am Main, Germany.
Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|April 30, 2025
Summary
Pulsed electron-electron double resonance (PELDOR) can count coupled nitroxide spins in molecules up to six. Accuracy is limited by pulse calibration, but dipolar defocusing effects are minimized with specific pulse types for distances over 2 nm.
Area of Science:
- Magnetic Resonance Spectroscopy
- Physical Chemistry
- Supramolecular Chemistry
Background:
- Pulsed electron-electron double resonance (PELDOR) is a powerful technique for measuring distances between electron spins.
- Accurately determining the number of coupled spins in multi-spin systems is crucial for understanding molecular structure and dynamics.
- Previous methods for spin counting using PELDOR modulation depth have limitations in complex systems.
Purpose of the Study:
- To investigate the accuracy and limitations of using PELDOR modulation depth for counting coupled spins in multi-nitroxide molecules.
- To identify the key factors affecting the precision of spin counting using this technique.
- To assess the applicability of PELDOR for spin counting in systems with varying numbers of coupled spins.
Main Methods:
- Synthesis of multi-nitroxide molecules with 2-6 coupled spins.
- Application of pulsed electron-electron double resonance (PELDOR) experiments at Q-band frequencies.
- Utilized broadband sech/tanh and short rectangular pump pulses to analyze modulation depth suppression effects.
Main Results:
- The reproducibility of pump pulse excitation efficiency is the primary limitation for accurate spin counting of larger spin numbers.
- Modulation depth suppression effects were avoided for intramolecular spin distances > 2 nm using specific pulse shapes.
- Transverse relaxation times were independent of spin number, but primary Hahn echo signal intensity decreased significantly with increasing spin count due to dipolar defocusing.
Conclusions:
- PELDOR modulation depth is a viable method for counting coupled nitroxide spins, potentially applicable up to hexameric complexes.
- Careful calibration of pump pulse excitation efficiency is critical for accurate spin counting, especially in systems with many spins.
- Dipolar defocusing effects reduce echo intensity and accuracy, necessitating optimized pulse sequences and experimental conditions.
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