Electron-electron double resonance and saturation-recovery studies of nitroxide electron and nuclear spin-lattice relaxation times and Heisenberg exchange rates: lateral diffusion in dimyristoyl phosphatidylcholine.

Lateral diffusion constants of the stearic acid nitroxide radical spin label 2-(14-carboxytetradecyl)-2-ethyl-4,4-dimethyl-3-oxazolindinyl oxide in dispersions of dimyristoyl phosphatidylcholine have been measured. Electron-electron double resonance methods were used to determine the product of the bimolecular collision frequency and T1e, the electron spin-lattice relaxation time. T1e in turn was measured by the technique of saturation recovery. The theoretical model of Träuble and Sackmann was then used to relate the bimolecular collision frequencies to the diffusion constants. Results are in agreement with other methods. Lower spin-label concentrations than were used in previous electron paramagnetic resonance studies are needed (label-to-lipid ratio less than 0.5 mol%). Analysis of the data also yields values of the nitrogen nuclear spin-lattice relaxation time of the nitroxide moiety. These values are indicative of membrane fluidity.