Phospholipid-cholesterol bilayers under osmotic stress.

Isothermal (27 degrees C) phase behavior of dimyristoyl phosphatidyl choline-cholesterol mixtures at various osmotic pressures and cholesterol contents was investigated by means of isothermal sorption microcalorimetry and (2)H-nuclear magnetic resonance. The calorimetric method allows for simultaneous measurement of the partial molar enthalpy and the chemical potential (the osmotic pressure) of water, thus providing an almost complete thermodynamic description of the sorption process. From the experimental results, the Pi(osm) - X(chol) and the ternary composition phase diagrams are constructed. We note that there are strong similarities between the Pi(osm) - X(chol) phase diagram and the previously reported T - X(chol) phase diagram at excess water. At high cholesterol contents a single liquid ordered (L(alpha)(o)) phase is present over the whole range of water contents, implying that this phase has a remarkable stability not only at decreasing temperature but also at increasing osmotic pressure. At low cholesterol contents, the microcalorimetric experiments confirm the extraordinary property of cholesterol not to cause any substantial melting point depression. One important conclusion in the present study is that the P(beta) phase can dissolve cholesterol more readily than the L(beta) phase and that the addition of cholesterol induces the P(beta) phase. Finally, the putative P(beta) - L(alpha)(o) periodic modulated structure is discussed.