The influence of different gangliosides (G(M1), G(D1a), G(T1b)) on the fluidity and surface dynamics of phosphatidylcholine small unilamellar vesicles was studied by electron paramagnetic resonance. 5- and 16-nitroxystearic acid, sounding respectively the region close to the surface and that close to the hydrophobic core of the vesicle, were employed as spin-label probes. The signals released by 5-nitroxystearic acid showed that the presence of gangliosides reduced the mobility of the hydrocarbon chains around the probe. The effect increased by increasing ganglioside concentration, and diminished from G(M1) to G(D1a) and G(T1b). The decrease of membrane fluidity was also monitored by the 16-nitroxystearic acid probe. On addition of Ca2+ the fluidity of ganglioside-containing vesicles (as signalled by the 5-nitroxystearic acid probe) promptly decreased, thereafter returning slowly to the original value. It is suggested that gangliosides cause strong side-side head group interactions on the bilayer surface - between ganglioside oligosaccharide chains and between ganglioside and phosphatidylcholine polar portions - which lead the lipid chains to assembly in a more rigid fashion. The influence of Ca2+ is interpreted as due to lateral phase separation in the vesicle membrane. This phenomenon can be related to the formation or stabilization of ganglioside clusters on the vesicle surface.
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