Spin labels for ¹⁹F ENDOR distance determination: resolution, sensitivity and distance predictability

Area of Science:

• Biophysical Chemistry
• Structural Biology
• Spectroscopy

Background:

• 19F electron-nuclear double resonance (ENDOR) measures distances (0.7-2 nm) in biomolecules.
• This distance range is challenging for other methods like pulsed electron dipolar spectroscopy.
• The technique relies on spin labeling proteins with paramagnetic probes.

Purpose of the Study:

• To compare the performance of various spin labels for 19F ENDOR.
• To evaluate labels for accuracy, sensitivity, and geometric information in distance measurements.
• To provide guidance for selecting optimal spin labels in protein studies.

Main Methods:

• Proteins (GB1, ubiquitin) were site-specifically labeled with nitroxide, trityl (OXMA), Gd(III), and Cu(II) chelates.
• 19F ENDOR measurements were performed at W-band (95 GHz) for signal separation.
• Electron-nuclear distances were predicted and compared to experimental values.

Main Results:

• Gd(III) chelates exhibited the highest signal-to-noise ratio and closest distance predictions.
• The OXMA trityl label showed acceptable sensitivity but a reduced maximum measurable distance.
• Nitroxide and Cu(II) labels provided orientation selection for additional geometric insights.

Conclusions:

• Spin label choice significantly impacts 19F ENDOR performance.
• Gd(III) labels are recommended for high sensitivity and accurate distance measurements.
• The study offers practical guidance for optimizing 19F ENDOR experiments in structural biology.