The intensities of saturation-transfer EPR (ST-EPR) spectra from nitroxyl spin labels have proved a sensitive means for studying slow exchange processes (both Heisenberg spin exchange and physical/chemical exchange) and weak interactions with paramagnetic ions, via the dependence on the effective spin-lattice relaxation rate (D. Marsh, Appl. Magn. Reson. 3, 53, 1992). The dependences of the second-harmonic EPR absorption intensities detected in phase quadrature with the field modulation (V′2 display) on the microwave H1 field, and on the effective relaxation times, were studied both theoretically and experimentally. Power-saturation curves and normalized integrated intensities (IST) of the V′2 spectra were determined as a function of the concentration of a spin-labeled phospholipid in lipid membranes and of the concentration of paramagnetic Ni2+ ions in the aqueous phase as a means of varying the effective relaxation times. The results were correlated with progressive-saturation measurements of the double-integrated intensities of the conventional EPR spectra. Intensities of the V′2 spectra were calculated from the Bloch equations incorporating the modulation and microwave fields (K. Halbach, Helv. Phys. Acta 27, 259, 1954), and the results were fitted to the experimental data. The ST-EPR intensities depend approximately linearly on the effective T1, but with a nonzero intercept. On the basis of the theoretical calculations and experimental correlations, relations between IST and T1 are suggested that may improve precision in the application of this alternative form of ST-EPR spectroscopy to biological Systems.
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