Thermal hysteresis phenomena in micellar solutions of gangliosides : theory and experiments

Light scattering experiments show that micellar solutions of various gangliosides have a thermotropic behaviour which is irreversible with respect to temperature. Gangliosides, amphiphilic molecules of biological origin, form micelles in solution which decrease their average aggregation number upon heating. When a critical temperature is reached, any further cooling and heating cycle does not alter the micellar size. This process, not determined by a denaturation of the ganglioside molecule, is explained by considering the temperature-dependent coupling between micellar aggregation number and molecular conformation of ganglioside polar head groups. A simple model is proposed which assumes that the polar heads may exist in two different stable conformations, each of them with an energy dependent on its own internal structure and on the interactions with the surrounding heads at the micelle surface. The interconversion between the conformational minima is then described as a cooperative event, with a naturally emerging barrier due to collective effects which accounts for the experimentally observed irreversibilities. The model has some resemblances with the solidification process from an undercooled fluid, with the main difference that it deals with a confined two-dimensional system at the micellar surface.