Effect of lipid mixing on the permeability and fusion of saturated lecithin membranes

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Abstract
The effect of phospholipid mixing on the permeability properties of multilamellar lipid vesicles (MLV) was studied. In the solid state, dimyristoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DMPC/DPPC) vesicles exhibit ideal lipid miscibility; DMPC/distearoylphosphatidylcholine (DSPC) vesicles exhibit nonideal lipid miscibility at low DSPC molar fractions; DMPC/dibehenoylphosphatidylcholine (DBPC) vesicles exhibit lipid immiscibility in a large range of DBPC molar fractions. The rates of K+, ethylene glycol and water diffusion from these vesicles in the solid state were measured by photometric and electrometric techniques. The rate of solute diffusion, which is decreased monotonically, in DMPC/DPPC MLV, by increasing the molar fractions of DPPC, exhibits maxima at 0.2 molar fraction of DSPC in DMPC/DSPC MLV and at 0.4 molar fraction of DBPC in DMPC/DBPC MLV. The activation energy of the solute diffusion process abruptly decreases in approximately the same range of lipid molar fractions where nonideal lipid miscibility is present. The membrane pore radius is increased by increasing the lipid nonideal miscibility. The rate of vesicle size increase, measured by absorbance changes, is decreased monotonically in DMPC/DPPC monolamellar vesicles [small unilamellar lipid vesicles (SUV)] by increasing the molar fraction of DPPC. It exhibits a maximum in DMPC/DSPC SUV at 0.15 molar fraction of DSPC. A model was suggested in which the solute diffusion and the membrane fusion processes are controlled by fractures. The average width of the fractures is increased by increasing the lipid immiscibility.