Neutrons for rafts, rafts for neutrons.

The determination of the structure of membrane rafts is a challenging issue in biology. The selection of membrane components both in the longitudinal and transverse directions plays a major role as it determines the creation of stable or tunable platforms that host interactions with components of the outer environment. We focus here on the possibility to apply neutron scattering to the study of raft mimics. With this aim, we realized two extreme experimental models for the same complex membrane system (phospholipid : cholesterol : ganglioside GM1), involving two of the characteristic components of glycolipid-enriched rafts. One consists of a thick stack of tightly packed membranes, mixed and symmetric in composition, deposited on a silicon wafer and analyzed by neutron diffraction. The other consists of a free floating individual membrane, mixed and asymmetric in composition in the two layers, studied by neutron reflection. We present here results on the ganglioside-cholesterol coupling. Ganglioside GM1 is found to force the redistribution of cholesterol between the two layers of the model membranes. This causes cholesterol exclusion from compositionally symmetric ganglioside-containing membranes, or, alternatively, asymmetric cholesterol enrichment in raft-mimics, where gangliosides reside into the opposite layer.