[8c] Spirocyclohexyl moieties, meanwhile, offer some improvement in stability over tetramethyl substituents, but their most significant advantage lies in the excellent electron spin relaxation parameters exhibited by spirocyclic nitroxides (Figure ). The parameter that is most important to the resolution and range of distance measurements in the DEER experiment, known as the phase memory time (Tm), is related to transverse relaxation of the electron spin. Tm is negatively impacted by the rotation of methyl groups, a process that becomes particularly pronounced at temperatures over 70 K, due to coupling between the electron spin and the 1 H nuclear spin. Despite several efforts to develop nitroxide probes that display extended relaxation times at elevated temperatures, the combined requirement for low temperatures and reduction-resistant nitroxides limits the potential of EPR and other electron spin-based technologies in biological systems under physiological conditions.