Phase transitions in a 4:1 molar ratio mixture of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE) in excess water have been investigated using time resolved X-ray diffraction at the Daresbury Synchrotron Laboratory, United Kingdom. Phospholipids subjected to temperature scans of 10°C/min or temperature jumps of approx 5°C/s were found to undergo order-disorder phase transitions at previously reported temperatures. A fully hydrated sample with a 4:1 molar ratio of DPPC:DPPE produced a single phase transition, characteristic of the gel to liquid crystal bilayer transition, at approx 40°C when heated at a scan rate of 10°C/min. Upon cooling at 10°C/min, Lβ′-phase stability occurred at 35°C. The Lα to Lβ, transition was characterized by the coexistence of broad acyl chain peaks diagnostic for each phase with the eventual formation of a sharp peak at 0.41 nm. The transition from the Lα to the Lβ′ form in the lipid mixture requires that this miscibility be unaffected by the rate of the cooling process and proceeds by the nucleation of small domains of Lβ′-phase lipids and the eventual growth to larger domains before the transition is complete. Under rapid heating at approx 5°C/s the gel to liquid crystal bilayer phase transition of this mixture was observed to require approx 7.5 s after 40°C was observed.
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