BEHAVIOR OF ICE-WATER TRANSITION IN DIMYRISTOYLPHOSPHATIDYLETHANOLAMINE-WATER SYSTEM

Differential scanning calorimetric (DSC) studies on the ice-water tran sition were performed for gel, metastable crystalline, and stable crys talline phases of a fully hydrated dimyristoylphosphatidylethanolamine (DMPE)-water system. We found that the ice-melting behavior differs a mong these three phases. Endothermic peaks due to the ice-melting were observed at temperatures below 0 degrees C for the all phases. The st arting temperature of ice-melting increases in the following order: ge l (-35 degrees C) < metastable crystalline (-20 degrees C) < stable cr ystalline (-10 degrees C) phases. The numbers of water molecules inter acting with a DMPE headgroup were estimated to be 6, 7, and 2 for the gel, metastable crystalline and stable crystalline phases, respectivel y, from the transition enthalpies of the ice-water transitions. In add ition, X-ray diffraction analyses revealed that the thickness of inter lamellar 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 i nteraction of freezable bound water molecules and DMPE headgroups is d iscussed from the viewpoint of the relation between ice-melting temper atures and interbilayer hydration forces. (C) 1997 Elsevier Science B. V.