electron paramagnetic resonance EPR ESR epr EPR (also called ESR, Electron Spin Resonance) spectroscopy is analogous to NMR, but is based on the excitation of unpaired electrons instead of nuclei. Unpaired (single) electrons are only found in radicals and some metal ions (paramagnetic species); the EPR spectrum provides information about the environment and mobility of the paramagnetic species. The magnetic interaction of two paramagnetic centres in a protein can be used to calculate the distance between them; this allows studies of the movements and interactions of protein segments. In proteins without any intrinsic unpaired electrons it is possible to attach a radical probe (spin label). Stable nitroxide radicals can be bound to amino acid residues, in analogy with fluorescent probes. In combination with site directed mutagenesis this method is used in particular to study structure and assembly of membrane proteins, by measuring with EPR whether an amino acid is in a polar or non polar environment. MI:0042 PSI-MI electron resonance