Dependence of Saturation-Transfer EPR Intensities on Spin–Lattice Relaxation

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) on the microwaveH₁field, and on the effective relaxation times, were studied both theoretically and experimentally. Power-saturation curves and normalized integrated intensities (I_ST) of theV^′₂spectra 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 theV^′₂spectra were calculated from the Bloch equations incorporating the modulation and microwave fields (K. Halbach,Helv. Phys. Acta27, 259, 1954), and the results were fitted to the experimental data. The ST-EPR intensities depend approximately linearly on the effectiveT₁, but with a nonzero intercept. On the basis of the theoretical calculations and experimental correlations, relations betweenI_STandT₁are suggested that may improve precision in the application of this alternative form of ST-EPR spectroscopy to biological systems.