Nitroxide spin labels and EPR spectroscopy: a powerful association for protein dynamics studies

Site-Directed Spin Labelling (SDSL) technique consists in the attachment of a paramagnetic label onto a specific position of a protein (or other bio-molecules) and in its subsequent study by Electron Paramagnetic Resonance (EPR) spectroscopy. In particular, continuous wave EPR (cw-EPR) spectra can detect the local conformational dynamics of proteins under various conditions. Moreover pulse-EPR experiments on doubly spin-labelled proteins allow measuring distances between spin centres in the 1.5-8 nm range, providing information about the global structure. This review focuses on SDSL-EPR spectroscopy as a structural biology tool to investigate protein using nitroxide labels. The versatility of this spectroscopic approach for protein structural characterization has been demonstrated through the choice of recent studies. The main aim is to provide a general overview of the technique, in particular for non-experts, in order to spread the applicability of this technique in various fields of structural biology.