Volume constriction in a lipid-water liquid crystal using high-pressure x-ray diffraction

We report a measurement of the change in the total volume of a biological lipid-water system when water molecules are removed from the fully hydrated lyotropic liquid-crystal phase, where the water molecules are near a polar interface, to the surrounding bulk water. The commonly used assumption of linear additions of constituent volumes predicts that the water mole fractions of these lipid-water phases should be independent of hydrostatic pressure. The discrepancy between this prediction and our high-pressure x-ray-diffraction measurements is due to a decrease of 0.1 A${\mathrm{̊}}^3$ in the total volume of the system per molecule of water incorporated into the fully hydrated lipid-water aggregate from the bulk water.