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


Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstr. 12, 53115, Bonn, Germany


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.