Spin relaxation measurements using first-harmonic out-of-phase absorption EPR signals: Rotational motion effects

A recent survey of nonlinear continuous-wave (CW) EPR methods revealed that the first-harmonic absorption EPR signal, detected 90 degrees out of phase with respect to the Zeeman modulation (V'(1)-EPR), is the most appropriate for determining spin-lattice relaxation enhancements of spin labels CV. A. Livshits, T. Pali, and D. Marsh, 1998, J. Magn. Reson. 134, 113-123). The sensitivity of such V'(1)-EPR spectra to molecular rotational motion is inv estigated here by spectral simulations for nitroxyl spin labels, over the e ntire range of rotational correlation times, Determination of the effective spin-lattice relaxation times is less dependent on rotational mobility tha n for other nonlinear CW EPR methods, especially at a Zeeman modulation fre quency of 25 kHz which is particularly appropriate for spin labels. This re lative insensitivity to molecular motion further enhances the usefulness of the V'(1)-EPR method. Calibrations of the out-of-phase to in-phase spectra l intensity land amplitude) ratios are given as a function of spin-lattice relaxation time, for the full range of spin-label rotational correlation ti mes. Experimental measurements on spin labels in the slow, intermediate, an d fast motional regimes of molecular rotation are used to test and validate the method. (C) 2000 Academic Press.