Monitoring structural transitions in IDPs by site-directed spin labeling EPR spectroscopy.

Electron paramagnetic resonance (EPR) spectroscopy is a technique that specifically detects unpaired electrons. EPR sensitive reporter groups (spin labels or spin probes) can be introduced into biological systems via site-directed spin labeling (SDSL). This is usually accomplished by cysteine-substitution mutagenesis followed by covalent modification of the unique sulfhydryl group with a selective nitroxide reagent. SDSL EPR spectroscopy has been shown to be a sensitive and powerful method to study structural transitions within intrinsically disordered proteins (IDPs). In this chapter, we provide a detailed experimental protocol for this approach and present a few examples of EPR spectral shapes illustrative of various mobility regimes of the spin probe, reflecting different protein topologies.