Reaction fields and solvent dependence of the EPR parameters of nitroxides: the microenvironment of spin labels

The sensitivity of nitroxide spin-label EPR to the polarity of aprotic environments arises from the reaction field produced by polarisation of the surrounding dielectric by the nitroxide electric dipole moment. The performances of three different reaction fields that have been proposed as improvements on the original Onsager model are compared for representative spin-label nitroxides in a range of apolar and dipolar aprotic solvents. Explicit allowance is made for the polarisability of the nitroxide, which effectively renormalises the reaction field but has been neglected in previous analyses of nitroxide hyperfine couplings when using the improved reaction fields. It is found that the model of Block and Walker, which incorporates an exponential dependence of the dielectric permittivity on inverse radial distance from the nitroxide, gives the best description of the solvent dependence of the isotropic (14)N-hyperfine couplings. These results should be useful not only for calibration of environmental polarity using homogeneous solvents, but also for transferring polarity scales and polarity profiles (e.g., in membranes) between different nitroxide spin labels (e.g., of the TEMPO and DOXYL variety).