Percolation properties of two-component, two-phase phospholipid bilayers

Abstract

Temperature-composition phase diagrams of binary lipid mixtures under conditions of constant pressure and excess water show that component immiscibility is the rule rather than the exception in these systems. Phase immiscibility is particularly noted in the solid phase but several cases are known where immiscibility is apparent even in the fluid phase. Using the fluorescence recovery are photobleaching (FRAP) technique, we have examined the long-range translational diffusion (over several micrometers) of fluorescent lipid derivatives, soluble only in the fluid phase, in bilayers under conditions of temperature and composition where solid and fluid phases are co-existent. These experiments provide information regarding barriers to free diffusion of the reporter molecules in these systems. The barriers are solid phase domains that are impenetrable to the reporter molecules, and may either exist as a discontinuous (non-percolating) archipelago of solid phase ‘islands’ in as continuous (percolating) fluid phase ‘sea’, or as continuous (percolating) solid phase domain cluster with non-connected (non-percolating) ‘lakes’ of the fluid phase domains. The transition from one state to the other is the so-called ‘percolation threshold’. In FRAP experiments the former case is manifested as a reduction of the measured long-range diffusion coefficient (increased recovery time) with complete fluorescence recovery, whereas in the latter case both the apparent recovery times and the percent recoveries are altered. The position of the percolation threshold as a function of temperature and composition within the phase diagram permits a reasonably exact estimation of the mass fractions of the two phases in the system at this threshold and, with certain assumptions, an estimation of solid phase domain symmetry. The use of membrane-spanning fluorescent lipid probes also permits an evaluation of the degree of superposition of domains of the same phase across the lipid bilayer. Results obtained with bilayers prepared from several binary phophatidylcholine mixtures and one phospatidylcholine-cholesterol mixture will be discussed.

Key Words:

• Lipid bilayers
• phase separations
• percolation
• lateral diffusion
• FRAP