Water behavior in the conversion process of gel-to-subgel phase in dimyristoylphosphatidylethanolamine–water system as studied by DSC

The gel phase of dimyristoylethanolamine (DMPE)–water system was annealed at temperatures of −5 to +5 °C for periods of 2 weeks and was converted into a more stable state of the so-called L-subgel phase. To elucidate the role of water molecules in the conversion of the gel to the subgel phase, the ice-melting behavior for the DMPE–water samples of varying periods of the annealing was investigated by a differential scanning calorimetry (DSC). The ice-melting DSC curves were deconvoluted according to a computer program for multiple Gaussian curve analysis, and the numbers of freezable interlamellar and bulk water molecules were estimated from respective ice-melting enthalpies of the deconvoluted curves. At the annealing periods of 4 days, water molecules amounting to 3.5 H2O/lipid were observed to be further incorporated into regions between the bilayers, giving rise to the total number of freezable interlamellar water molecules 7 H2O/lipid which is fairly larger than that (4 H2O/lipid) estimated for the gel phase. Finally, the conversion of the gel-to-subgel phases completed at the annealing periods of 10 days. The resultant subgel phase showed the amount of nonfreezable interlamellar water larger by 1 H2O/lipid than that of the nonannealed gel phase. Furthermore, the present study indicated an essential role of the bulk water molecules, which fill an empty space produced at the lipid bilayer surface in the conversion process of the gel-to-subgel phases.