A calorimetric and fluorescent probe study of the gel-liquid crystalline phase transition in small, single-lamellar dipalmitoylphosphatidylcholine vesicles

The results of a calorimetric and fluorescent probe study of the thermotropic behavior of various types of dispersions of dipalmitolphosphatidylcholine bilayer vesicles are reported. Bangham-type, multilamellar vesicles exhibit tow distinct phase transitions at 34.6 and 41.2 degrees C. On the other hand, single-lamellar spherical vesicles appear to exhibit a single transition at 37 degrees C. The single-lamellar vesicles are thermodynamically unstable below 27 degrees C and slowly transform into a multilamellar structure with a single phase transition of 41.2 degrees C. These transformed structures resemble, but are not identical with, Bangham-type vesicles. An experimentally testable thermodynamic and kinetic model based upon these results is developed.