Using fluorescence correlation spectroscopy, calorimetry, and Monte Carlo simulations, we studied diffusion processes in two-component membranes close to the chain melting transition. The aim is to describe complex diffusion behavior in lipid systems in which gel and fluid domains coexist. Diffusion processes in gel membranes are significantly slower than in fluid membranes. Diffusion processes in mixed phase regions are therefore expected to be complex. Due to statistical fluctuations the gel-fluid domain patterns are not uniform in space and time. No models for such diffusion processes are available. In this article, which is both experimental and theoretical, we investigated the diffusion in DMPC-DSPC lipid mixtures as a function of temperature and composition. We then modeled the fluorescence correlation spectroscopy experiment using Monte Carlo simulations to analyze the diffusion process. It is shown that the simulations yield a very good description of the experimental diffusion processes, and that predicted autocorrelation profiles are superimposable with the experimental curves. We believe that this study adds to the discussion on the physical nature of rafts found in biomembranes.

Supported lipid bilayers composed of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) and 1-palmitoyl-2-oleoyl-phosphatidylglycerol (POPG) were assembled by the vesicle fusion technique on mica and studied by temperature-controlled atomic force microscopy. The role of different physical parameters on the main phase transition was elucidated. Both mixed (POPE/POPG 3:1) and pure POPE bilayers were studied. By increasing the ionic strength of the solution and the incubation temperature, a shift from a decoupled phase transition of the two leaflets, to a coupled transition, with domains in register, was obtained. The observed behavior points to a modulation of the substrate/bilayer and interleaflet coupling induced by the environment and preparation conditions of supported lipid bilayers. The results are discussed in view of the role of different interactions in the system. The influence of the substrate on the lipid bilayers, in terms of interleaflet coupling, can also help us in understanding the possible effect that submembrane elements like the cytoskeleton might have on the structure and dynamics of biomembranes.

The usual biophysical approach to the study of biological membranes is that of turning to model systems. From these models, general physical principles ruling the lateral membrane structure can be obtained. A promising model system is the supported lipid bilayer (SLB) which could foresee the simultaneous investigation of the structure and physical properties of lipid bilayers reconstituted with membrane proteins. A complete exploitation of the model system to retrieve biologically relevant information requires an in-depth knowledge of the possible effect that experimental parameters could have on the behavior of the SLB. Here we used atomic force microscopy (AFM) to study the effect of different types of substrates on the behavior of SLBs as far as their main phase transition is concerned. We found that different substrates (mica and silicon oxide) can affect in dissimilar ways the interleaflet coupling of the bilayer, which might represent a sort of lipid signaling allowing communication between receptors on the extracellular leaflet and cytoplasmic components. By decreasing the interaction between the SLB and the substrate the interleaflet coupling is preserved independently of the bilayer preparation strategy. Moreover, we investigated by time-lapse AFM an isothermal phase transition induced by a pH change on a SLB. We established that the presence of a pH gradient across the bilayer can weaken the strength of the interleaflet coupling which is present in symmetrical pH conditions.