ENERGETICS OF A HEXAGONAL LAMELLAR HEXAGONAL-PHASE TRANSITION SEQUENCE IN DIOLEOYLPHOSPHATIDYLETHANOLAMINE MEMBRANES

The phase diagram of DOPE/water dispersions was investigated by NMR and X-ray diffraction in the water concentration range from 2 to 20 water molecules per lipid and in the temperature range from -5 to +50-degrees-C. At temperatures above 22-degrees-C, the dispersions form an inverse (H(II)) phase at all water concentrations. Below 25-degrees-C, an H(II) phase occurs at high water concentrations, an L(alpha) phase is formed at intermediate water concentrations, and finally the system switches back to an H(II) phase at low water concentrations. The enthalpy of the L(alpha)-H(II)-phase transition is +0.3 kcal/mol as measured by differential scanning calorimetry. Using P-31 and H-2 NMR and X-ray diffraction, we measured the trapped water volumes in H(II) and L(alpha) phases as a function of osmotic pressure. The change of the H(II)-phase free energy as a function of hydration was calculated by integrating the osmotic pressure vs trapped water volume curve. The phase diagram calculated on the basis of the known enthalpy of transition and the osmotic pressure vs water volume curves is in good agreement with the measured one. The H(II)-L(alpha)-H(II) double-phase transition at temperatures below 22-degrees-C can be shown to be a consequence of (i) the greater degree of hydration of the H(II) phase in excess water and (ii) the relative sensitivities with which the lamellar and hexagonal phases dehydrate with increasing osmotic pressure. These results demonstrate the usefulness of osmotic stress measurements to understand lipid-phase diagrams.