Enhanced Sensitivity to Slow Motions Using 15N-Nitroxide Spin Labels

The rotational motion of an iSN nitroxide in diisobutylphthalate solutions at varied temperatures has been studied for correlation times from 1Om8 to > 10m3 sec. Data from both conventional (first-harmonic, in-phase) and saturation-transfer (second-harmonic, out-of-phase) EPR are reported. For many spectral parameters, including linewidth, position of outer extrema, and ratios of spectral amplitudes in saturation transfer spectra, the 15N-nitroxide signals are sensitive to the same range of motions as are **N-nitroxide spectra, but the i5N data offer the convenience of enhanced signal amplitude. In addition, the peak-to-peak separation of the low-field signal of 15N nitroxides in the conventional V, display has a smooth dependence on rates of motion from 10’ to 10’ set and is measured in the most intense regions of the spectrum.