E.S.R. Studies of nitroxide probes in lecithin-water systems

The molecular motion of a nitroxide free-radical probe in lecithin and lecithin-water systems has been studied as a function of temperature. In the lecithin-water system a significant decrease in the motion of the probe is observed on crystallisation of the hydrocarbon chains. At temperatures above the chain-melting point increase in the water content, up to about 25% water, produces an increase in the tumbling rate of the probe. At higher water content the probe motion remains approximately constant. Addition of cholesterol to the lecithin-water systems decreases the motion of the probe and decreases its solubility in the lipid.