ih-RIDME: a pulse EPR experiment to probe the heterogeneous nuclear environment

Area of Science:

• Biophysics
• Chemical Physics
• Spectroscopy

Background:

• Pulse Electron Paramagnetic Resonance (EPR) experiments probe nuclear spin baths near unpaired electrons.
• Intermolecular hyperfine relaxation-induced dipolar modulation enhancement (ih-RIDME) relies on hyperfine spectral diffusion.
• Characterizing heterogeneous nuclear arrangements is crucial for understanding complex molecular systems.

Purpose of the Study:

• To introduce the principles of ih-RIDME for analyzing heterogeneous nuclear spin systems.
• To extend the ih-RIDME data model for systems with varied local nuclear densities.
• To demonstrate ih-RIDME's application in characterizing structurally flexible macromolecules.

Main Methods:

• Quantitative description of hyperfine spectral diffusion kinetics.
• Development of an ih-RIDME data model for heterogeneous nuclear distributions.
• Application to a model spin-labeled macromolecule with unstructured domains.
• Conformational ensemble generation using Monte Carlo simulations.

Main Results:

• Demonstrated successful application of ih-RIDME to heterogeneous systems.
• Fitted distribution of local proton densities correlated with conformational ensemble.
• Discussion of spectral resolution, parameter determination, and noise influence on experimental data.
• Exploration of HYperfine Spectral Diffusion Echo MOdulatioN (HYSDEMON) sequences for improved signal-to-noise ratio.

Conclusions:

• ih-RIDME is a powerful tool for heterogeneity characterization and structure-based analysis in complex systems.
• The method accurately quantifies local nuclear densities in flexible macromolecules.
• Optimized pulse sequences enhance the practical utility of ih-RIDME experiments.