The behavior of water molecules associated with structural changes in phosphatidylethanolamine assembly as studied by DSC

The bilayer packing properties of dimyristoylphosphatidylethanolamine (DMPE), a hydrogen-bonding lipid, in aqueous medium were investigated from the thermal behavior associated with the lipid transition and ice-melting of frozen water. DMPE gel phase was obtained by cooling its liquid crystal phase converted into a stable crystal phase, most likely a three-dimensional crystal constituted of closely-stacked bilayers, by a specified two-step annealing treatment. On heating, the crystal phase transformed directly into the liquid crystal phase at 6 degrees C higher temperature (56.4 degrees C) and with a three times larger enthalpy (19.6 kcal mol(-1)) than the gel-to-liquid-crystal phase transition. Estimation of the enthalpy change associated with the ice-melting peaks of the present system indicated that the conversion of the gel into crystal phases on annealing involves a change in water structure from a loosely-bound water (3 H2O) to bulk free water caused by formation of lipid intermolecular hydrogen-bonding between adjacent bilayers.