A calorimetric study of thermal hysteresis effects in ganglioside micelles

Light, X-ray and neutron scattering measurements clearly showed strong temperature-related variations in micellar mean aggregation number, accompanied by dramatic thermal hysteresis effects, for a variety of micelle-forming gangliosides. Gangliosides, sialic-acid-containing glycosphingolipids, are amphiphilic molecules of biological origin with a ceramide (a double-tailed hydrophobic part, like phospholipids) and an oligosaccharide chain as the headgroup. When temperature is varied in the range 30-55 °C (below the critical temperature Tc = 55 °C) strong hysteresis effects are observed, with the micellar size depending on the thermal history of the sample. This thermo-. tropic behaviour of ganglioside micelles has been attributed to the existence of different conformations of the bulky flexible saccharidic headgroup, each conformation hav-ing a slightly different geometric hindrance, and also to the temperature related change of the equilibrium between conformers in the micelle. In this paper, careful calorimetric measurements are presented which show that the collective properties of the hydrophobic core of ganglioside micelles also depend on the thermal history of the sample below the critical temperature Tc. The hydrophobic chains of the core undergo an order-disorder transition, like phospholipids bilayers, but at a temperature which depends on the micellar size. Measurements refer to the GM1 ganglioside, both in the natural composition and with a well-characterized hydrophobic part, namely C18 and C20 sphingosines.