Effects of steroid molecules on the dynamical structure of dioleoylphosphatidylcholine and digalactosyldiacylglycerol bilayers.

The ESR spectra of cholestane spin labels (CSL) in dioleoylphosphatidylcholine (DOPC) bilayers containing 20 wt% of cholesterol, 7-dehydrocholesterol, beta-sitosterol, stigmasterol and lanosterol exhibit a marked similarity, thus indicating that these steroids induced the same effects on the lipid bilayer over the temperature range 21-55 degrees C. The incorporation of these steroids into the DOPC bilayers enhances the orientational order of the CSL molecules at every temperature studied, but only induces a pronounced slow-down in their rotational motions at temperatures above 35 degrees C. Similar results were obtained in DOPC/ergosterol multilamellar liposomes, but the changes are now less pronounced than in the other five DOPC/steroid systems. In contrast, the addition of stigmasterol to digalactosyldiacylglycerol (DGDG) bilayers appears to increase the order parameter mean value of P2, without affecting the diffusion coefficients. Furthermore, the incorporation of 7-dehydrocholesterol to DGDG bilayers causes a large enhancement in the orientational order, but has only a small effect on D perpendicular of the CSL molecules. Importantly, this latter effect appears to be independent of temperature. The marked changes in the rates of the rotational motion brought about by the addition of steroids, contrasts with the lack of a significant effect of unsaturation on the bilayer dynamics reported by us previously (Korstanje et al. (1989), Biochim. Biophys. Acta 980, 225-233, and 982, 196-204).