Reorientational dynamics in lipid vesicles and liposomes studied with ESR: effects of hydration, curvature and unsaturation

Electron spin resonance experiments were carried out on 3-doxyl-5 alpha-cholestane spin-label (CSL) molecules embedded in multilamellar liposomes and small unilamellar vesicles (SUVs) of palmitoyloleoylphosphatidylcholine (POPC), dioleoylphosphatidylcholine (DOPC) and dilinoleoylphosphatidylcholine (DLPC). The experimental spectra were analyzed by a numerical solution of the stochastic Liouville equation. Effects of temperature, presence of unsaturated bonds and high bilayer curvature on the dynamic behaviour of the lipid molecules were studied. Our results, combined with results from planar multibilayers with a varying hydration rate (Korstanje et al. (1989) Biochim. Biophys. Acta 980, 225-233), give a consistent picture of the orientational order and rotational dynamics of CSL molecules embedded in lipid matrices with various geometrical configurations. Increase of hydration or temperature reduces molecular ordering and increases molecular dynamics. In highly curved vesicle configurations, SUVs, molecular order is found to be lower than in multilamellar liposomes. In contrast, rotational motion is not affected by increase of curvature. In all lipid configurations studied, increase of the number of unsaturated bonds in the fatty acid chains reduces molecular ordering. We find, however, no effect of unsaturation on the rotational mobility of the CSL probe molecules. These results clearly show that changes in molecular orientational order and reorientational dynamics have to be considered separately, and that they are not necessarily correlated as implied by the common concept of membrane fluidity. Comparing our results with data from a motional narrowing analysis shows that the latter approach seriously overestimates the rate of molecular reorientation.