Used as a spin-labeling agent in electron paramagnetic resonance (EPR) spectroscopy to study protein dynamics and conformational changes. It reacts selectively with thiol groups (-SH) on cysteine residues, allowing site-specific attachment to proteins. This enables researchers to probe local environment, mobility, and structural changes in biomolecules. Also applied in biophysical studies of membrane proteins and nucleic acids, where its stability and sensitivity to motion provide insights into molecular interactions and folding processes.