Application of High-Pressure Electron Paramagnetic Resonance (EPR) Spectroscopy in Protein Science

High hydrostatic pressure at a few thousand bars has been found to offer an opportunity to probe protein conformational states which possess lower hydrostatic volumes and are often not well populated under the ambient pressure. An important contribution of Prof. Wayne Hubbell is to initiate EPR studies on proteins under elevated pressure via establishing the theoretical basis of high-pressure EPR, so that parameters from EPR measurements can be correlated to key protein structural factors, such as activation volume and molar compressibility. Prof. Hubbell also developed a series of experimental devices for the measurement of continuous wave (CW) and double electron-electron resonance (DEER) EPR under elevated pressure. These efforts have directly resulted in a series of high-pressure EPR studies on globular and transmembrane proteins. Via this review in the current Special Issue, we aim to briefly cover the most recent findings in high-pressure EPR and its applications in protein science. We will focus on what each technique can do in protein studies, how to use the technique to measure needed information, and what each measured parameter tells us. We will also discuss the combination of high-pressure EPR with other techniques and the future perspectives of the field. The ultimate goal is to broaden the application of high-pressure EPR and its combination with other high-pressure biophysical techniques to improve protein science research.