A spin label study of the effects of sterols on egg lecithin bilayers

Paramagnetic resonance of cholestane and three fatty acid probes is used to measure the effects of the addition of cholesterol, 7-dehydrocholesterol and ergosterol to egg phosphatidylcholine bilayers. At low concentrations we find that all three sterols effectively align the bilayers. However, concentrations of ergosterol above 15 mol% disorder and disrupt the bilayers. The observed behavior is explained in terms of a steric model in which the steroid nucleus organizes the bilayer and the bulky egosterol tail disorganizes the bilayer. The three fatty acid spin labels are used to probe the layers at different depths, and the data observed are in agreement with the normal presented.