Behavior of ice-water transition in dimyristoylphosphatidylethanolamine-water system

Differential scanning calorimetric (DSC) studies on the ice-water transition were performed for gel, metastable crystalline, and stable crystalline phases of a fully hydrated dimyristoylphosphatidylethanolamine (DMPE)-water system. We found that the ice-melting behavior differs among these three phases. Endothermic peaks due to the ice-melting were observed at temperatures below 0 degrees C for the all phases. The starting temperature of ice-melting increases in the following order: gel (-35 degrees C) < metastable crystalline (-20 degrees C)< stable crystalline (-10 degrees C) phases. The numbers of water molecules interacting with a DMPE headgroup were estimated to be 6, 7, and 2 for the gel, metastable crystalline and stable crystalline phases, respectively, from the transition enthalpies of the ice-water transitions. In addition, X-ray diffraction analyses revealed that the thickness of interlamellar water region is also different among the three phases and is less than ca. 0.8 nm for the all phases. Based on these results, the interaction of freezable bound water molecules and DMPE headgroups is discussed from the viewpoint of the relation between ice-melting temperatures and interbilayer hydration forces. (C) 1998 Elsevier Science B.V.