Headgroup and chain melting transition in dispersed bilayers of GM3 ganglioside

We report on wide angle X-ray scattering (WAXS) and density experiments performed on the ganglioside GM3 in water solution. As compared to what usually found in lipid assemblies, GM3 bilayers show a degree of cooperativity feebler in the region of the lipid chains and stronger in the region of the headgroups. The collected WAXS spectra indicate the existence of a strong degree of order on the molecular scale, which we propose to be due to a highly constrained and symmetric disposition of the trisaccharide charged headgroups of GM3 at the bilayer surface. It is the first time that such an order in the headgroup region is seen in dispersed lipid assemblies, in normal pressure conditions. Experiments were carried out with no added salt, in the semidilute range of concentration, 5-15% bw, and in the range of temperature from 15 to 60°C. Parallel SAXS experiments indicate that on the scale of the bilayer cross section the same structure is preserved from the very dilute (0.1%) to the semidilute range of concentration.