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_α), lamellar gel/lamellar liquid-crystalline (L_β → L_α), and lamellar liquid-crystalline/lamellar gel (L_α → L_β) phase transitions proceed via coexistence of the initial and final phases with no detectable intermediates at scan rates 0.1 and 0.5°C/min. At constant temperature within the region of the L_β → L_α 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°C, has been observed in the cooling transition (L_α → L_β) 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_α 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.