DEPENDENCE OF SATURATION-TRANSFER EPR INTENSITIES ON SPIN-LATTICE RELAXATION

The intensities of saturation-transfer EPR (ST-EPR) spectra from nitro xyl spin labels have proved a sensitive means for studying slow exchan ge processes (both Heisenberg spin exchange and physical/chemical exch ange) 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 absorp tion intensities detected in phase quadrature with the field modulatio n (V'(2) display) on the microwave a field, and on the effective relax ation times, were studied both theoretically and experimentally, Power -saturation curves and normalized integrated intensities (I-ST) 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 progr essive-saturation measurements of the double-integrated intensities of the conventional EPR spectra. Intensities of the V'(2) spectra were c alculated from the Bloch equations incorporating the modulation and mi crowave fields (K. Halbach, Helv, Phys. Acta 27, 259, 1954), and the r esults were fitted to the experimental data, The ST-EPR intensities de pend approximately linearly on the effective T-1, but with a nonzero i ntercept. On the basis of the theoretical calculations and experimenta l correlations, relations between I-ST and T-1 are suggested that may improve precision in the application of this alternative form of ST-EP R spectroscopy to b biological systems. (C) 1996 Academic Press, Inc.