Translational Diffusion and Fluid Phase Connectivity in Multi-Component, Multi-Phase Lipid Bilayer Membranes

Abstract

We have used the fluorescence recovery after photobleaching technique (FRAP) to investigate the long range translational diffusion of a fluid-phase soluble, gel-phase insoluble fluorescent lipid derivative (NBD-dilauroylphosphatidylethanolamine or NBD-1-palmitoyl-2-oleoylphosphatidylethanolamine) in lipid bilayers made up of two or more lipid components as a function of temperature and chemical composition of the bilayers. The FRAP experiment gives information on two aspects of the diffusivity of the test particle (fluorescent probe): 1) the diffusion coefficient is related to the characteristic recovery time for the fluorescence after photobleaching, and 2) the freedom of diffusion in the membrane plane over long distances (H10μm) is related to the fraction of initial fluorescence recovered at ‘infinite’ time after photobleaching. For the case where solid and fluid phases co-exist in the plane of the lipid bilayer the two parameters taken together provide information on the connectivity of the fluid (or, reciprocally, the gel) phase domains and, when applicable, to what extent an archipelago of gel phase domains slows diffusion in the fluid phase. The results indicate that the gel phase in some cases forms highly elliptical domains, which at a very low mass fraction (≤20%) impede percolation of the fluid phase, whereas in others the gel phase domains are more centro-symmetric so that the fluid phase is continuous at a very low mass fraction (≤10%). A comparison of the experimental results with existing theoretical descriptions of diffusion in an archipelago becomes possible in systems of the latter type and was undertaken.