PECULIARITIES IN THE AGGREGATIVE BEHAVIOR OF GANGLIOSIDES, GLUCOSIDICSURFACTANTS OF BIOLOGICAL ORIGIN

Glucosidic surfactants play an important role in living matter. An int eresting class of them are the gangliosides which have a double-tail h ydrophobic part, Like phospholipids, and a ramified oligosaccharide ch ain as polar head. The large hydrophobic moiety ensures very low CMC v alues, of the order of 10(-8) M. Phospholipids have a small head group compared with their hydrophobic: part. Therefore they form mainly bil ayer-type structures in water. Gangliosides, instead, may form either micelles or bilayers depending on the length and conformation of the o ligosaccharide chain. For long chains, micelles are formed with a larg e aggregation number and a long lifetime (many hours), while vesicles are found for short sugar chains. The presence of a large head group m akes vesicle mechanical properties rather peculiar. For instance, the ganglioside GM3, which has the longest sugar chain compatible with bil ayer-type structures, forms vesicles spontaneously in solution without supply of external energy. This means that the bending rigidity of th e GM3 bilayer is very low, allowing very large thermal undulations. GM 3 vesicles, made of a single surfactant, are then found to be in therm odynamic equilibrium with a small amount of large bilayer and multilam ellar structures. Frustration of the single-component vesicle system i s 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 favourin g vesicles over other structures. It is, in fact, observed experimenta lly that by adding increasing proportions of GM1 the large aggregates gradually diminish in number, until a pure vesicle solution is obtaine d for a GM1 to GM3 ratio of 35 to 65.