Time-resolved x-ray diffraction and calorimetric studies at low scan rates: II. On the fine structure of the phase transitions in hydrated dipalmitoylphosphatidylethanolamine.

The phase transitions of dipalmitoylphosphatidylethanolamine (DPPE) in excess water have been examined by low-angle time-resolved x-ray diffraction and calorimetry at low scan rates. The lamellar subgel/lamellar liquid-crystalline (L(c) --> L(alpha)), lamellar gel/lamellar liquid-crystalline (L(beta) --> L(alpha)), and lamellar liquid-crystalline/lamellar gel (L(alpha) --> L(beta)) phase transitions proceed via coexistence of the initial and final phases with no detectable intermediates at scan rates 0.1 and 0.5 degrees C/min. At constant temperature within the region of the L(beta) --> L(alpha) transition the ratio of the two coexisting phases was found to be stable for over 30 min. The state of stable phase coexistence was preceded by a 150-s relaxation taking place at constant temperature after termination of the heating scan in the transition region. While no intermediate structures were present in the coexistence region, a well reproducible multipeak pattern, with at least four prominent heat capacity peaks separated in temperature by 0.4-0.5 degrees C, has been observed in the cooling transition (L(alpha) --> L(beta)) by calorimetry. The multipeak pattern became distinct with an increase of incubation time in the liquid-crystalline phase. It was also clearly resolved in the x-ray diffraction intensity versus temperature plots recorded at slow cooling rates. These data suggest that the equilibrium state of the L(alpha) phase of hydrated DPPE is represented by a mixture of domains that differ in thermal behavior, but cannot be distinguished structurally by x-ray scattering.