Cholestane spin label study of filipin action on lipid planar multibilayers

EPR spectra of a cholestane probe dissolved in egg yolk lecithin and lecithin-cholesterol planar multibilayers were observed as a function of the filipin dose. The probe is structurally similar to cholesterol; its normal position when dissolved is with the long axis approximately along the bilayer normal. Both cholesterol-containing and cholesterol-free samples showed spectral components characteristic of bilayer fragmentation (tilted domains) which increased with dose. Furthermore, the cholesterol-free spectra indicated that some of the probe was frozen with the long molecular axis perpendicular to the slide normal. The frozen spectral component increased with dose. Spectra from a fatty acid probe did not have this feature. We interpret this as due to probe complexed with filipin (in place of cholesterol) in accordance with the filipin-cholesterol aggregate model of deKruijff and Demel. An ultraviolet study of filipin-probe interaction indicates that the probe is capable of complexing in just such a manner but has less affinity for the drug than cholesterol. Spectra from the cholesttane probe in liposomes were also observed.