Detection of lipid raft domains in neutral and anionic Langmuir monolayers and bilayers of complex lipid composition

In reference to the complexity and heterogeneity of cellular membranes, the structure and lateral ordering of lipid monolayers and bilayers composed of multi-component lipid mixtures have been investigated and compared in the present study. These complex model biomembrane systems represent valuable model systems, e. g. for studies of lipid-peptide interactions, where an integrated in situ multi-technique approach using both monolayer and bilayer techniques is required. A zwitterionic (3 components) and an anionic (5 components) heterogeneous model membrane system have been characterized that consist of saturated and unsaturated phospholipids as well as cholesterol. Lipid monolayers were analyzed by surface X-ray scattering techniques, and both the vertical structure (electron density profile) and the degree of in-plane ordering were determined as a function of surface pressure and temperature. The corresponding structure and lateral organisation of the bilayer membranes were characterized by atomic force microscopy. Both monolayers and bilayers reveal ordered domain formation, the monolayer ones being of much smaller size and different temperature stability, however. Furthermore, the charge density of the lipid monolayer has a drastic influence on the size of ordered domains as well as the intermolecular distances of the lipid molecules.