TOPOLOGY OF GEL-PHASE DOMAINS AND LIPID MIXING PROPERTIES IN PHASE-SEPARATED 2-COMPONENT PHOSPHATIDYLCHOLINE BILAYERS

The influence of the lipid mixing properties on the lateral organizati on in a two-component, two-phase phosphatidylcholine bilayer was inves tigated using both an experimental (fluorescence recovery after photob leaching (FRAP)) and a simulated (Monte Carlo) approach. With the FRAP technique, we have examined binary mixtures of 1-stearoyl-2-capryl-ph osphatidylcholine/1, ,2-distearoyl-phosphatidylcholine (C18C10PC/DSPC) , and 1-stearoyl-2-capryl-phosphatidylcholine/1 ,2-dipalmitoyl-phospha tidylcholine (C18C10PC/DPPC). Comparison with the 1,2-dimyristoyl-phos phatidylcholine/ 1,2-distearoyl-phosphatidylcholine (DMPC/DSPC) previo usly investigated by FRAP by Vaz and co-workers (Biophys. J., 1989, 56 :869-876) shows that the gel phase domains become more effective in re stricting the diffusion coefficient when the ideality of the mixture i ncreases (i.e., in the order C18C10PC/DSPC-->C18C10PC/DPPC-->DMPC/DSPC ). However, an increased lipid miscibility is accompanied by an increa sing compositional dependence: the higher the proportion of the high-t emperature melting component, the less efficient the gel phase is in c ompartmentalizing the diffusion plane, a trend that is best accounted for by a variation of the gel phase domain shape rather than size. Com puter-simulated fluorescence recoveries obtained in a matrix obstructe d with obstacle aggregates of various fractal dimension demonstrate th at: I)for a given obstacle size and area fraction, the relative diffus ion coefficient increases linearly with the obstacle fractal dimension and 2) aggregates with a lower fractal dimension are more efficient i n compartmentalizing the diffusion plane. Comparison of the simulated with the experimental mobile fractions strongly suggests that the frac tal dimension of the gel phase domains increases with the proportion o f high-temperature melting component in DMPC/DSPC and (slightly) in C1 8C10PC/DPPC.