The thermotropic behavior of dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine large unilamellar vesicles containing ganglioside GM1of homogeneous long chain base composition has been studied by high-sensitivity differential scanning calorimetry and fluorescence spectroscopy. At neutral pH and in the absence of Ca2+, the thermotropic behavior of these systems is independent of the ganglioside chain length composition. The presence of Ca2+ at concentrations higher than 5 mM induces ganglioside phase separation in a manner dependent upon the length difference between the ganglioside long chain base and the phosphatidylcholine acyl chains. The analysis of the chain length dependence of the thermotropic behavior suggests that the driving force for ganglioside phase separation is not a Ca2+-induced cross-bridging of the ganglioside head group but a passive ganglioside exclusion from Ca2+-perturbed phosphatidylcholine-rich regions within the bilayer. Experiments with native ganglioside GM1, primarily a mixture of C18:1 and C20:l long chain bases, indicate that the individual components of the mixture maintain their characteristic behavior within the lipid bilayer matrix. These results, together with the presence of a phase transition in native GM1micellar dispersions, absent in purified C18:1 or C20:l ganglioside micelles, strengthen the idea of a possible role of chain length composition in the modulation of ganglioside function.
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