Maximizing the applicability of continuous wave (CW) Electron Paramagnetic Resonance (EPR): what more can we do after a century?

Abstract

Electron Paramagnetic Resonance/Electron Spin Resonance (EPR/ESR) spectroscopy is a powerful experimental approach solving challenging problems that are inaccessible to other experimental techniques. While pulsed EPR techniques and their applications have been the central focus of the field nowadays, methodology development and application of continuous wave (CW) EPR have never been undervalued. Although the importance of EPR in research has been recognized in many areas as indicated by the increasing number of EPR research articles published every year, the growth in the number of EPR research teams seems to not match the former, likely caused by the relatively high cost of modern pulsed EPR spectrometers. In this perspective, we aim to discuss the new areas and new directions of the application of CW EPR in combination with spin labeling that are slightly different from the classic/traditional applications of the CW EPR technique. Based on our experience, we show example applications of CW EPR on the interfaces of protein-confinement materials, protein-inorganic crystal lattices, protein-nanoparticles, and protein-polymers. Various structure and dynamics information can be resolved from the data analysis of CW EPR, leading to enhanced understanding of protein biophysics upon interaction with the synthetic nanostructures as well as possible guidance of hybrid materials development based on the combination of proteins and nanostructures. We also discussed the possibilities of even broadening the application spectrum of EPR spectroscopy. The relatively high affordability of CW EPR spectrometers (as compared to the pulsed ones) also facilitates the growth in EPR research groups worldwide.