Evidence of the hydration force in gel phase lipid multibilayers

We present applied osmotic pressure vs. distance curves constructed from high-resolution neutron and X-ray diffraction scattering density profiles of dipalmitoyl phosphatidylcholine (DPPC) multibilayers in the gel L β ' phase. The data, obtained from three distinct regions of the lipid bilayer, show that at small interbilayer distances the repulsion observed in these bilayers can be accounted for by the so-called "hydration force". Also, a previous observation of a sharp upward break in the pressure-distance curve of highly ordered L c ' DPPC bilayers is not the result of steric repulsion but rather of a 2D → 3D phase transition. The data are consistent with a recent theoretical model which predicts that the repulsive forces in ordered phases ( e.g. , L β ' and L c ' phases) are dominated by the hydration force.