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 dipalmitoylphosphatidylcholine bilayer vesicles are reported. Bangham-type, multilamellar vesicles exhibit two distinct phase transitions at 34.6 and 41.2 °C. On the other hand, single-lamellar spherical vesicles appear to exhibit a single transition at 37 °C. The single-lamellar vesleles are thermodynamically unstable below 27 °C and slowly transform into a multilamellar structure with a single phase transition of 41.2 °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.