Sterol ordering effects and permeability regulation in phosphatidylcholine bilayers. A comparison of ESR spin-probe...

The effects of a series of sterols on molecular order and motion in bilayers formed from egg lecithin and dicetylphosphate were examined and correlations between order and data on permeability to 22Na+ were sought. Electron spin resonance spectra were observed for probes intercalated both in multilamellar dispersions where the effects of motion and orientation are difficult to separate, and in planar multibilayers where the degree of molecular order may be measured even in the presence of slow probe motion. It was concluded from the planar multi-bilayer data that sterols which increase the degree of ordering of lipid molecules decrease 22Na+ permeability, and that sterols which have the opposite effect on order increase permeability. All the sterols tested lead to decreased rates of motion of the probes. This effect obscures the correlation between order and permeability using data from dispersions.