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 forinvestigation of paramagnetic systems, i.e. systems like free radicals and most transition metalions, which have unpaired electrons. The sensitivity and selectivity of EPR are notable andintriguing 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 themethod stems from spectral features which directly depend on the types, distances, and relativeorientations of the atoms in the neighborhood of the electron spin centers. In addition tostructural information, EPR can be used to elucidate time dependent behavior of the studiedsystem and it is applicable to systems of different size, ranging from small molecules tomacromolecules. The following short review of general EPR methods is intended for anaudience with little prior knowledge about EPR. It includes examples of suitable representativesystems, techniques for the study of short lived paramagnetic species and even diamagneticmolecules, and introduces the reader to tools necessary for making sense of the spectra. Thispaper focuses only on continuous wave (cw) EPR and does not elaborate on the more advancedpulsed EPR methods.