Transition of a liquid crystalline phosphatidylcholine bilayer to the gel phase in a vesicle reduces the internal aqueous volume

The liquid crystalline to gel phase transition in phospholipid bialyers is associated with a marked reduction in the area per phospholipid molecule. Geometric considerations based on published data suggest that this decrease in molecular area is accompanied by a reduction in the internal aqueous volume trapped within a unilamellar bilayer vesicle. This volume reduction, which depends upon the shape of the vesicle, is shown to be between 23 and 60 percent. We have observed a 25 to 30 percent reduction in the internal aqueous volume of unilamellar vesicles about 700 Å in diameter formed from dipalmitoylphosphatidylcholine using the self-quenching of 6-carboxyfluorescein trapped within this compartment.