title: Spin relaxation measurements using first-harmonic out-of-phase absorption EPR signals: rotational motion effects.
abstract: 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 (V. A. Livshits, T. Páli, and D. Marsh, 1998, J. Magn. Reson. 134, 113-123). The sensitivity of such V(1)(')-EPR spectra to molecular rotational motion is investigated here by spectral simulations for nitroxyl spin labels, over the entire range of rotational correlation times. Determination of the effective spin-lattice relaxation times is less dependent on rotational mobility than for other nonlinear CW EPR methods, especially at a Zeeman modulation frequency of 25 kHz which is particularly appropriate for spin labels. This relative insensitivity to molecular motion further enhances the usefulness of the V(1)(')-EPR method. Calibrations of the out-of-phase to in-phase spectral intensity (and amplitude) ratios are given as a function of spin-lattice relaxation time, for the full range of spin-label rotational correlation times. Experimental measurements on spin labels in the slow, intermediate, and fast motional regimes of molecular rotation are used to test and validate the method.
authors.affiliation: Centre of Photochemistry, Russian Academy of Sciences, 117427, Moscow, Russian Federation.
authors.forename: V A
authors.initials: VA
authors.lastname: Livshits
authors.forename: D
authors.initials: D
authors.lastname: Marsh
mesh_terms: D002138:Calibration; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005990:Glycerol / Q000737:chemistry; D008962:Models, Theoretical; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012710:Serum Albumin, Bovine; D013113:Spin Labels; D014216:Triacetoneamine-N-Oxyl; D014867:Water
chemical_list: D013113:Spin Labels; D014867:Water; D012710:Serum Albumin, Bovine; D014216:Triacetoneamine-N-Oxyl; D005990:Glycerol