RIDME spectroscopy on high-spin Mn²⁺ centers

Area of Science:

• Biophysics
• Structural Biology
• Biomolecular NMR Spectroscopy

Background:

• Pulsed electron paramagnetic resonance (EPR) dipolar spectroscopy is vital for measuring distances (1.5-10 nm) in biological macromolecules.
• High-spin Mn²⁺ ions are biologically compatible spin probes for EPR distance measurements, but their use in dipolar spectroscopy is underexplored.

Purpose of the Study:

• To explore the utility of high-spin Mn²⁺ as spin probes in pulsed EPR dipolar spectroscopy.
• To compare the performance of PELDOR (DEER) and RIDME experiments using Mn²⁺ probes at high frequencies (W- and J-band).

Main Methods:

• Performed W-band (94 GHz) and J-band (263 GHz) pulsed electron electron double resonance (PELDOR/DEER) and relaxation-induced dipolar modulation enhancement (RIDME) experiments.
• Utilized a bis-MnDOTA model system with Mn²⁺ spin probes.
• Applied Tikhonov regularization analysis, incorporating higher harmonics of the dipolar coupling frequency.

Main Results:

• Obtained distance measurements consistent with predictions for the bis-MnDOTA model system.
• RIDME experiments demonstrated significantly higher modulation depth compared to PELDOR/DEER.
• Observed and accounted for higher harmonics of the dipolar coupling frequency due to Mn²⁺ relaxation and multi-component background functions.

Conclusions:

• Pulsed EPR dipolar spectroscopy with Mn²⁺ probes is effective for structural and dynamic studies of biomacromolecules.
• RIDME is a promising technique for distance measurements in biological samples using Mn²⁺ probes due to its higher sensitivity.
• Advanced analysis methods are necessary to accurately interpret data involving high-spin probes and complex relaxation effects.