Nitroxide spin labels are extensively used in EPR for distance measurements and many polarizing agents are based on nitroxides. More recently they are also used in Overhauser DNP measurements (ODNP) to study surface hydration dynamics of larger (membrane) proteins. Although the article is already a bit older, it is a nice review of spin labels and their use in EPR spectroscopy.
Bordignon, E., EPR Spectroscopy of Nitroxide Spin Probes, in eMagRes. 2017, John Wiley & Sons, Ltd. p. 235-254.
http://dx.doi.org/10.1002/9780470034590.emrstm1513
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 fingerprint, 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 defined ‘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.