Q-Band Double Quantum Coherence ESR for Sensitive Nitroxide-Based Distance Measurements

Area of Science:

• Biophysics
• Electron Spin Resonance (ESR) Spectroscopy
• Structural Biology

Background:

• Pulsed dipolar spectroscopy (PDS) has advanced, enabling measurements at physiological concentrations and in complex biological systems.
• Electron spin resonance (ESR) is a powerful technique for probing molecular structure and dynamics.
• Double quantum coherence (DQC) is a specific PDS method that can provide distance information.

Purpose of the Study:

• To establish Q-band double quantum coherence (DQC) electron spin resonance (ESR) as a practical technique for distance measurements in doubly nitroxide-labeled proteins.
• To optimize DQC ESR parameters for efficient signal generation and high signal-to-noise ratios.
• To assess the impact of experimental conditions, such as pulse durations and deuteration levels, on DQC signal sensitivity and measurable distance range.

Main Methods:

• Utilized Q-band DQC ESR spectroscopy on doubly nitroxide-labeled proteins.
• Investigated the effect of different π pulse durations (8, 12, and 16 ns) on double quantum transition excitation.
• Analyzed the field dependence of the DQC signal to probe g-tensor orientation and assessed the impact of varying deuteration levels on sensitivity.

Main Results:

• Demonstrated efficient excitation of the double quantum transition at Q-band using 8 ns π pulses, with strong signals also obtained using 12 and 16 ns pulses.
• Achieved signal-to-noise ratios greater than 100 in 11 minutes for 50 μM protein samples with a 1.5 μs dipolar evolution time.
• Showed that DQC signal sensitivity is enhanced by deuteration, extending dipolar evolution times and measurable distances, with negligible intermolecular contributions up to 100 μM.

Conclusions:

• Q-band DQC ESR is a highly sensitive and practical technique for distance measurements in dual nitroxide-labeled proteins.
• The method offers deep dipolar modulations, minimal background decay at micromolar concentrations, and improved sensitivity with deuterium incorporation.
• Q-band nitroxide DQC is positioned as a broadly accessible technique for pulsed ESR distance measurements in structural biology.