A method using nitroxide radical spin labels for determining both the isotropic rotational correlation time and the "rigid limit" value of the nitrogen hyperfine coupling along the pz orbital (Az) is described. The method requires measurement of the same sample under unchanged conditions at both X(9.5 GHz)- and Q(35 GHz)-bands, and is valid for rotational correlation times between 10-6 and 10-8 sec. In this context the "rigid limit" value of Az is in the absence of rotational diffusion but with fast restricted librational motions unchanged, and its measurement becomes a useful spectroscopic observable. Extension of the method to anisotropic motion, particularly as may occur in phospholipid bilayers, is discussed. © 1980.
Hyde, J. S., & Rao, K. V. S. (1980). Determination of the rotational correlation time and the rigid limit hyperfine coupling from the dependence of spin-label EPR spectra on microwave frequency. Journal of Magnetic Resonance (1969), 38(2), 313–317. https://doi.org/10.1016/0022-2364(80)90281-4