The partial molar volume of water in biological membranes

A new algorithm is presented for interpreting the hydration dependence of x-ray diffraction measurements. The method assumes that the volume of the hydrocarbon phase of the lipid bilayer is not affected by hydration and that the volume expansion between bilayers at maximum hydration is caused by incorporation of water molecules whose partial molar volume is that of pure bulk water. These simple assumptions lead to a determination of the area expansion (and hence change in hydrocarbon-phase thickness) as a function of hydration. An analysis is made of x-ray data of the L alpha and L beta' phases of dimyristoyl phosphatidylcholine and the L alpha phase of egg phosphatidylcholine. The partial molar volume of water depends critically on the degree of lipid hydration and the presence of voids between the head groups of adjacent lipids. The calculated head-group spacings at minimum hydration are consistent with those obtained from neutron diffraction and indicate that the methyl groups of the choline are almost in contact with corresponding groups in the opposing bilayer. This calls into question the origin of the repulsive forces observed in dehydration experiments.