The behavior of water molecules in semi-crystalline bilayer structure of phosphatidylethanolamine DSC study

The crystalline phase of dimyristoylphosphatidylethanolamine (DMPE)-water system was obtained by annealing the gel phase at around -5 degrees C for periods up to 30 days. It was investigated by differential scanning calorimetry and negative-stain electron microscopy, particularly focusing on the behavior of water molecules. The crystalline phase showed a two-dimensional ribbonlike structure composed of regularly-stacked lamellae with an interlamellar spacing narrower than that of the gel phase. The conversion of the gel to crystalline phases on annealing was accompanied by a change in the bonding model of water molecules from a loosely-bound interlamellar water to a more loosely-bound water outside the lamellae. Ice-melting curves were deconvoluted using a computer program and different structures of water were estimated from enthalpy changes of each deconvoluted component. In accordance with a micrograph, only the loosely-bound water of one molecule of H2O per lipid was shown to be located between lamellae of the crystalline phase.