Spin-spin interactions between phosphatidylcholine nitroxide spin labels in fluid-phase dimyristoyl phosphatidylcholine bilayers have been studied in the concentration range 0.1-2.0 mol%. The total line broadening has been separated into its Gaussian and Lorentzian components. The Gaussian linewidth exhibits exchange narrowing of the unresolved proton hyperfine structure, with an approximate quadratic concentration dependence for the square of the linewidth, over this spin-label concentration range. The Lorentzian linewidth has a linear dependence on the spin-label concentration over the same range. The temperature dependence of the gradient of the Lorentzian broadening with respect to spin-label concentration is biphasic over the range 30-80°C, indicating a dominant contribution from magnetic dipole-dipole broadening at the lower temperatures. The contributions from the dipole-dipole and exchange interactions to the Lorentzian broadening have been separated by fitting the temperature dependence using an unconstrained optimization procedure and assuming a single effective activation energy for both diffusion-controlled processes. The lipid lateral diffusion coefficient has been determined from the concentration dependence of both the exchange broadening of the nitrogen hyperfine structure and the exchange narrowing of the proton hyperfine structure using a two-dimensional lattice diffusion model, and also from the concentration dependence of the dipole-dipole interaction using a three-dimensional Fick's law diffusion model. All three methods yield consistent values for the diffusion coefficient, although because of the approximate nature of some of the theoretical models the exchange broadening is expected to give the most reliable results. An experimental protocol for determining the actual spin concentration in the membrane sample is also presented. © 1987.
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