INFLUENCE OF HEISENBERG SPIN EXCHANGE ON CONVENTIONAL AND PHASE-QUADRATURE EPR LINESHAPES AND INTENSITIES UNDER SATURATION

The effects of the Heisenberg spin-exchange interaction on saturation-transfer EPR spectra are analyzed according to a simple analytical model which contains the essential features of anisotropic powder patterns and in which the saturation-transfer intensity is taken to be proportional to the effective spin-lattice relaxation time. Spectral diffusion induced by spin exchange is predicted to give rise to a large reduction in the overall spectral intensity, with relatively little change in the spectral lineshape. The effects at low exchange frequencies (of the order of or less than the intrinsic spin-lattice relaxation rate) are much greater than those on the saturation of the conventional EPR spectrum and are readily distinguishable from those of slow rotational diffusion due to the insensitivity of the spectral lineshape. The theoretical predictions are found to be in accord with experimental measurements on the spin-concentration dependence of the saturation-transfer EPR spectra of a spin-labeled phospholipid in model membrane systems. © 1992.