INTERLAMELLAR WATERS IN DIMYRISTOYLPHOSPHATIDYLETHANOLAMINE-WATER SYSTEM AS STUDIED BY CALORIMETRY AND X-RAY-DIFFRACTION

The number of water molecules incorporated into the interlamellar regi on in a gel phase of dimyristoylphosphatidylethanolamine (DMPE)-water system containing up to about 40 g% water was estimated by techniques of calorimetry and X-ray diffraction. The calorimetric estimation base d upon enthalpy changes of deconvoluted ice-melting peaks revealed tha t bulk water existing outside lipid bilayers begins to appear although the gel phase is not fully hydrated. The gel phase showed a linear de pression of its transition temperature proportional to the amount of f reezable waters interposed between bilayers. For a fully hydrated gel phase, the numbers of non-freezable and freezable interlamellar waters estimated by calorimetric analysis were about 2.3 and 3.7 molecules p er lipid, respectively. The limiting, total number of interlamellar wa ters, 6 H2O/lipid, agreed with that estimated from both the X-ray diff raction data and the absolute specific volume for a DMPE molecule. Fur thermore, the analysis for the lamellar intensity data is also consist ent with the result of calorimetric analysis. (C) 1997 Elsevier Scienc e B.V.