STRUCTURE AND INTERACTIONS OF FULLY HYDRATED DIOLEOYLPHOSPHATIDYLCHOLINE BILAYERS

This study focuses on dioleoylphosphatidylcholine (DOPC) bilayers near full hydration. Volumetric data and high-resolution synchrotron x-ray data are used in a method that compares DOPC with well determined gel phase dipalmitoylphosphatidylcholine (DPPC). The key structural quant ity obtained is fully hydrated area/lipid A(0) = 72.2 +/- 1.1 Angstrom (2) at 30 degrees C, from which other quantities such as thickness of the bilayer are obtained. Data for samples over osmotic pressures from 0 to 56 atmospheres give an estimate for the area compressibility of K-A = 188 dyn/cm. Obtaining the continuous scattering transform and el ectron density profiles requires correction for liquid crystal fluctua tions. Quantitation of these fluctuations opens an experimental window on the fluctuation pressure, the primary repulsive interaction near f ull hydration. The fluctuation pressure decays exponentially with wate r spacing, in agreement with analytical results for soft confinement. However, the ratio of decay length lambda(fl) = 5.8 Angstrom to hydrat ion pressure decay length lambda = 2.2 Angstrom, is significantly larg er than the value of 2 predicted by analytical theory and close to the ratio obtained in recent simulations. We also obtain the traditional osmotic pressure versus water spacing data. Our analysis of these data shows that estimates of the Hamaker parameter H and the bending modul us K-c are strongly coupled.