Electron paramagnetic resonance (EPR) for investigating relevant players of redox reactions: Radicals, metalloproteins and transition metal ions

Electron paramagnetic resonance (EPR) spectroscopy is unique in providing robust information about free radicals, transition metal ions and metalloenzymes, which are crucial players in redox processes. EPR had a major role in advancing the redox biology field during the 20th century, but the interest in this methodology considerably decreased in recent years. Here, we discuss potential reasons for this decline as well as potential reasons for maintaining the mind open to the many possibilities brought by EPR and associated methodologies to the redox field. We present the fundamentals of EPR using pictorial images and minimal physicochemical language. We also present EPR derived methodologies developed to detect radical metabolites, that is, direct EPR of solutions (static and continuous-flow), direct EPR of frozen solutions, spin-trapping and spin-scavenging, showing examples and discussing the advantages and drawbacks of each one. Finally, we discuss the EPR spectra of metalloproteins and metal ion complexes of biological interest, which are more complex than those of radical metabolites in solution. In addition to introduce EPR methodologies to those new to the redox field, our goal is to show that these methodologies can contribute to advance the field.