Peculiarities in the aggregative behaviour of gangliosides, glucosidic surfactant of biological origin

Glucosidic surfactants play an important role in living matter. An interesting class of them are the gangliosides which have a double-tail hydrophobic part, Like phospholipids, and a ramified oligosaccharide chain as polar head. The large hydrophobic moiety ensures very low CMC values, of the order of 10(-8) M. Phospholipids have a small head group compared with their hydrophobic: part. Therefore they form mainly bilayer-type structures in water. Gangliosides, instead, may form either micelles or bilayers depending on the length and conformation of the oligosaccharide chain. For long chains, micelles are formed with a large aggregation number and a long lifetime (many hours), while vesicles are found for short sugar chains. The presence of a large head group makes vesicle mechanical properties rather peculiar. For instance, the ganglioside GM3, which has the longest sugar chain compatible with bilayer-type structures, forms vesicles spontaneously in solution without supply of external energy. This means that the bending rigidity of the GM3 bilayer is very low, allowing very large thermal undulations. GM3 vesicles, made of a single surfactant, are then found to be in thermodynamic equilibrium with a small amount of large bilayer and multilamellar structures. Frustration of the single-component vesicle system is released if a second amphiphile, of the same type of the ganglioside GM3 but with a larger head group, is added. In this case, spontaneous-curvature readjustements of the two monolayers via demixing can give rise to a spontaneous curvature of the bilayer, energetically favouring vesicles over other structures. It is, in fact, observed experimentally that by adding increasing proportions of GM1 the large aggregates gradually diminish in number, until a pure vesicle solution is obtained for a GM1 to GM3 ratio of 35 to 65.