PERCOLATION PROPERTIES OF 2-COMPONENT, 2-PHASE PHOSPHOLIPID-BILAYERS

Temperature-composition phase diagrams of binary lipid mixtures under conditions of constant pressure and excess water show that component i mmiscibility is the rule rather than the exception in these systems. P hase immiscibility is particularly noted in the solid phase but severa l cases are known where immiscibility is apparent even in the fluid ph ase. Using the fluorescence recovery are photobleaching (FRAP) techniq ue, we have examined the long-range translational diffusion (over seve ral micrometers) of fluorescent lipid derivatives, soluble only in the fluid phase, in bilayers under conditions of temperature and composit ion where solid and fluid phases are co-existent. These experiments pr ovide 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 (nonpercolating) archipelago of solid phase 'islands' i n as continuous (percolating) fluid phase 'sea', or as continuous (per colating) solid phase domain cluster with non-connected (non-percolati ng) 'lakes' of the fluid phase domains. The transition from one state to the other is the so-called 'percolation threshold'. In FRAP experim ents the former case is manifested as a reduction of the measured long -range diffusion coefficient (increased recovery time) with complete f luorescence recovery, whereas in the latter case both the apparent rec overy times and the percent recoveries are altered. The position of th e percolation threshold as a function of temperature and composition w ithin the phase diagram permits a reasonably exact estimation of the m ass fractions of the two phases in the system at this threshold and, w ith 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 phas e across the lipid bilayer. Results obtained with bilayers prepared fr om several binary phophatidylcholine mixtures and one phospatidylcholi ne-cholesterol mixture will be discussed.