Reflectivity from floating bilayers: can we keep the structural asymmetry?

To assess the structure of complex biomembranes, the use of asymmetric model systems is rare, due to the difficulty of realizing artificial membranes with desired heterogeneous composition and applicable for single membrane structural investigation. We developed an experimental model with a single macroscopic bilayer floating on top of another adhering to a silicon flat surface, prepared by Langmuir-Blodgett Langmuir-Schaefer technique, then investigated by neutron reflectivity. On the way to more complex systems, containing lipids of different nature, we tested whether a simple imposed asymmetry is kept in time and whether it can stand some standard experimental protocols commonly employed in treating model membranes. We focused on cholesterol, a basic component with a transverse distribution that is not symmetric in biomembranes, and may assume specific location in functional domains. So we forced different asymmetries in the "adhering + floating" bilayers system composed of phospholipids and cholesterol in bio-similar mole ratios. The neutron reflection accessible length-scale and its sensitivity, enhanced by the possibility to play with deuteration, allowed assessing the cross profile of the membrane and revealing that lipid redistribution can occur.