Analysis of Nitroxide-Based Distance Measurements in Cell Extracts and in Cells by Pulsed ESR Spectroscopy

Area of Science:

• Biophysics
• Spectroscopy
• Cellular Biology

Background:

• Pulsed Electron Paramagnetic Resonance (EPR) spectroscopy, specifically double electron-electron resonance (DEER), offers unique insights into protein structures within native cellular environments.
• However, the in-cell milieu poses significant challenges to the stability of nitroxide radicals, commonly used as spin labels in DEER experiments.
• Understanding and mitigating nitroxide radical degradation is crucial for reliable in-cell EPR measurements.

Purpose of the Study:

• To systematically investigate the causes of signal loss in in-cell DEER experiments.
• To evaluate the effectiveness of an oxidizing agent in preserving nitroxide radical integrity within cells.
• To assess the stability and diffusion dynamics of proteins labeled with nitroxides in a cellular context.

Main Methods:

• Utilized pulsed Electron Spin Resonance (ESR) spectroscopy, including double electron-electron resonance (DEER) and continuous wave (CW) spectroscopy.
• Examined nitroxide radical decay pathways, specifically nitroxide decay and side-chain cleavage, under in-cell conditions.
• Employed an oxidizing agent to enhance nitroxide radical lifetime and performed DEER measurements on doubly nitroxide-labeled GB1 protein over time.

Main Results:

• Identified nitroxide side-chain cleavage as a significant contributor to DEER signal loss, more pronounced in cellular environments than in cell extracts.
• Demonstrated that an oxidizing agent can extend the lifetime of nitroxide radicals in cells.
• Monitored local spin concentrations to determine the timescale for molecular diffusion of GB1 protein within the cellular milieu.

Conclusions:

• Nitroxide side-chain cleavage is a critical factor limiting the application of in-cell DEER spectroscopy.
• Oxidizing agents show promise for improving the stability of nitroxide labels, enabling longer measurements.
• The study provides insights into protein diffusion dynamics within cells, crucial for understanding cellular processes.