EPR spectroscopy of nitroxide spin probes

In this article, we will introduce the main chemical and spectroscopic properties of nitroxides. These paramagnetic non-endogenous probes have been widely used in EPR spectroscopy in the last decade due to their high stability and simple spectral fngerprint, which provides a wealth of qualitative and quantitative information about their microscopic environment under almost unrestricted experimental conditions. Nitroxides can be covalently or noncovalently introduced into a variety of different materials to monitor viscosity, local dynamics, pH, polarity, H-bond networks, transition temperatures, and distances toward other nitroxide probes. In general, these small probes minimally perturb the system under investigation, and being the unique paramagnetic centers in an otherwise diamagnetic sample, they provide unequivocal information. Here we will focus on their exquisite sensitivity to report molecular motions within defned 'EPR timescales' and spin-spin interactions via changes in their spectral lineshape. Additionally, we will discuss some methods to monitor polarity and formation of H-bonds in their microenvironment. © 2017 John Wiley &Sons, Ltd.