We have used a lamellar phase made of a nonionic surfactant, dodecane and water, as a model membrane to investigate its interactions with macromolecular inclusions bringing together two membranes, i.e., acting as macromolecular snaps. In systems devoid of inclusions, the interlamellar distance depends on the total volume fraction of membranes Φ. We show that, in presence of a transmembrane protein, or of several de novo designed peptides of different length and composition, the lamellar phase undergoes a binding transition. Under such conditions, the interlamellar distance is no longer proportional to Φ⁻¹, but rather to the surface concentration of snaps within the membrane. It also appears that, in the presence of the hydrophobic segment of peptide snaps, the length of the inclusions must be at least equal to the hydrophobic length of the membrane to be active. Experimental results have been precisely fitted to a model of thermally stabilized membranes, decorated with snaps. However, in the presence of inclusions, the parameter describing the interactions between membranes, has to take into account the length of the inclusion to preserve good predictive capabilities.

The self-assembly of amphiphilic molecules into supramolecular aggregates involves a number of complex phenomena and forces. Recent developments of highly sensitive, densimetric and acoustic methods on small volume samples have provided novel sensitive probes to explore the physical properties of these complex fluids. We have investigated, by high precision densimetry and ultrasound velocimetry, reverse micelles of [sodium bis(2-ethylhexyl)sulfosuccinate] in oil (isooctane and decane), at increasing water concentration and at variable micellar volume fractions. The size of these spherical micelles has been determined by small angle x-ray scattering. Using these results, in the framework of the effective medium theory, we have developed a simple model of micellar compressibility, allowing the calculation of physical parameters (aggregation number, volume, and compressibility) of the surfactant monomolecular film as well as that of the micellar waters. In particular, we show that the central aqueous core designated as “free” water, located at a distance from the oil-water interacting interface, is twice as compressible as “bulk” water. One notable feature of this work is the influence of the nature of the oil on the above parameters.

Many experiments done on neutral lipid bilayers in pure water show weak repulsions. These weak forces prevent vesicles from adhering and are generally overcome by adding some salt in the aqueous medium. They also appear as stray repulsions in surface forces measurements made on lipid bilayers. Using a surface forces apparatus in pure water and in salt solution, we have measured the forces between two stearoyl-oleoyl-phosphatidyl-choline (SOPC) bilayers and between two dimiristoyl-phosphatidyl-ethanolamine (DMPE) bilayers. The results show that the repulsions are due to a small amount of negative charges coming from impurities in SOPC. This was quantitatively confirmed by electrophoretic measurements. There are 3 times less charges in the case of DMPE layers. The effect of these charges which is negligible at high salt concentration may significantly affect the adhesion energy and behaviour of neutral lipid bilayers between 0 and salt.

Many experiments have highlighted the existence of an unexpected long-range attractive force between two hydrophilic or hydrophobic monolayers, immersed in water or in nonaqueous solvents. The origin of this attraction has not been discovered yet. A mechanism responsible for these forces may be an interaction between ordered domains of the two close monolayers. The occurrence and characteristics of such domains are related to the nature of their constituting molecules and also their structural organization. We present here a molecular analysis, via molecular dynamics simulations, of ordered domains within a monolayer. We investigate the dependence between the characteristics of the domains and both the length of the molecules and the layer density.

The monolayer behavior of a lipid containing two unsaturated alkyl chains and a nucleoside derivative as polar headgroup has been investigated by the Langmuir technique. From the surface pressure vs. molecular area isotherm, the monolayer appears as a pure liquid-expanded phase and should be then considered as a two-dimensional liquid. However, grazing incidence X-ray diffraction experiments evidence a translational order that does not exist when the lipid headgroup is a choline moiety. Since unsaturated chains are expected to induce a fluid state of the monolayer at the temperature considered, this order is likely to originate from the natural tendency nucleosides have to establish among themselves -stacking interactions between the bases. The collected X-ray data are consistent with the geometrical requirements for bases stacking.