Demystifying EPR: A Rookie Guide to the Application of Electron Paramagnetic Resonance Spectroscopy on Biomolecules

Electron Paramagnetic Resonance (EPR) spectroscopy, also known as Electron Spin Resonance (ESR) especially among physicists, is a strong and versatile spectroscopic method for investigation of paramagnetic systems, i.e. systems like free radicals and most transition metal ions, which have unpaired electrons. The sensitivity and selectivity of EPR are notable and intriguing as compared to other spectroscopic methods and approaches. As a qualitative method, EPR can detect species down to the nanomolar range. On the other hand, the specificity of the method stems from spectral features which directly depend on the types, distances, and relative orientations of the atoms in the neighborhood of the electron spin centers. In addition to structural information, EPR can be used to elucidate time dependent behavior of the studied system and it is applicable to systems of different size, ranging from small molecules to macromolecules. The following short review of general EPR methods is intended for an audience with little prior knowledge about EPR. It includes examples of suitable representative systems, techniques for the study of short lived paramagnetic species and even diamagnetic molecules, and introduces the reader to tools necessary for making sense of the spectra. This paper focuses only on continuous wave (cw) EPR and does not elaborate on the more advanced pulsed EPR methods.