Multilevel structuring of ganglioside-containing aggregates: from simple micelles to complex biomimetic membranes.

We revisit the structural investigation we performed over the years on gangliosides, biological amphiphiles typically found in the cell membranes of the nervous system of mammalians. Their molecular features, a large and charged saccharidic headgroup connected to a sticky and extended ceramide double tail, strongly dictate their aggregation properties and place ganglioside aggregates at the borderline between the curved world and the flatland. All along we found that unexpected interesting behaviours were induced by the hierarchical propagation of such extreme monomer properties, from the aggregate scale to the mesoscopic phases. In fact, even small changes in the monomer geometry or hindrance result in dramatic aggregate reshaping, due to collective amplification. Surface packing optimization requires preferential mutual orientation of headgroups, giving rise to trapped solid-disordered configurations. The interplay between interparticle and intraparticle interactions gives rise to unexpected behaviours and counterintuitive phase's landscape. In situ modification of monomer properties, operated by enzymatic digestion of aggregated ganglioside headgroups, either causes collective rearrangement or is overwhelmed by collective trapping, depending on their surface density. This aspect is interesting as gangliosides are not evenly distributed in cell membranes, but only in the outer leaflet, where they p]articipate in rafts, functional microdomains enriched in special lipids including cholesterol. We recently found that ganglioside GM1 forces a preferential distribution of cholesterol, constituting a collective structural pair across the membrane. In summary, ganglioside assemblies, through cooperativity, reach a structural complexity comparable or even bigger and more adaptive than that of a protein.