Structure and fluctuations of a single floating lipid bilayer

Structure and fluctuations of a single floating lipid bilayer

J.Daillant, E. Bellet-Amalric, A. Braslau, T. Charitat, G. Fragneto, F. Graner, S. Mora, F. Rieutord, B. Stidder

A considerable amount of research has been devoted in the last two decades to amphiphilic monolayers and bilayers. One of the motivation for this constant interest is the description and modeling of physical properties of cell walls, an essential piece of bio-systems. In the constant quest for accurate and relevant models of such systems, researchers from CEA Saclay and co-workers (Laboratoire de Spectrométrie Physique (Grenoble, CNRS), ILL, Institut Charles Sadron (Strasbourg,CNRS)) have successfully carried out the study by X-ray scattering techniques of a single bilayer located in a solution above a wall covered with a bilayer. This experiment appears as a « textbook example » in the study of the model cell wall, as complications associated to peculiar interactions at the air-water, solid/solution interfaces or within a multilayer stack are avoided.

The system was built up by layer-by-layer deposition of the lipid monolayers (3 monolayers) atop an OTS- or layer- coated substrate with careful control of the transfer rate at each step.

Fig.1: Sketch of the system investigated: one bilayer is free to fluctuate in interaction with the other, bound to the solid substrate.

The system was studied using both X-ray reflectivity and off-specular scattering, hence probing both the average structure and the fluctuating structure. Reflectivity showed the full 4-layer structure while the off-specular scattering, mainly resulting from the fluctuating bilayer displayed bilayer features. From the scattering spectra as a function of the fluctuation length scale, the elastic parameters of the layer (surface tension, bending rigidity) and the bilayer-bilayer interaction potential (including hydration, entropic and van der Waals interaction) could be obtained.

Fig.2: Off-specular scattering as a function of exit angle

This study was performed as a function of temperature across the gel/fluid phase transition of the lipid layers where an order-of-magnitude change in the bending rigidity of the bilayer was found. Actual (i.e. lengthscale independent) surface tensions data were obtained and an upper limit for the bilayer wall interaction (ie a bilayer/bilayer interaction) have been established, well below expectations.

Fig.3: Bending rigidity  and surface tension  as extracted from off-specular scattering data, for C17 and C18 lipids. Note the drop of  at the gel/fluid transition.

These experiments have set up the grounds for systematic investigations of these model systems (e.g. as a function of chain length) with better control of the parameters and improved accuracy on the Helfrich parameters.

References

J. Daillant, E. Bellet-Amalric, A. Braslau, T. Charitat, G. Fragneto, F. Graner, S. Mora, F. Rieutord, B. Stidder, Structure and fluctuations of a single floating lipid bilayer, Proc. Nat. Acad. Sci. USA, 102 (33) (2005) 11639