Oxygen transport parameter in membranes as deduced by saturation recovery measurements of spin-lattice relaxation times of spin labels.

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Abstract
Spin-lattice relaxation time (T1) measurements of nitroxide radical spin labels in membranes were made by using the saturation-recovery technique. Stearic acid and sterol-type labels were used as probes of dimyristoylphosphatidylcholine liposomes from 0.degree. C to 36.degree. C. In the absence of O2, the range of variation of T1 over all samples and conditions is about a factor of 3. Heisenberg exchange between O2 and spin labels is an effective T1 mechanism for the spin labels. The full range of variation of T1 in the presence of air is about a factor of 100. The O2 transport parameter W = .**GRAPHIC**. (air) - .**GRAPHIC**. (N2) is a useful new monitor of membrane fluidity that reports on translational diffusion of small molecules. The values of W change at the prephase and main phase transitions and vary in complex ways. Arguments are advanced that the data are indicative of anisotropic translational diffusion of O2.