Evidence of entropic contribution to "hydration" forces between membranes.

The main purpose of this work was to measure the forces between phospholipid membranes, where out-of-plane motions of membrane surface (undulations as well as "protrusions") are suppressed. With the help of small-angle and wide-angle x-ray diffraction out-of-plane and in-plane structure of polymeric phospholipid membranes of 1,2-bis(11, 13) tetradecadienoyl-snglycero-3-phosphocholine (DTDPC) was studied. Small-angle x-ray diffraction study confirms the previous conclusions that polymeric DTDPC membranes are rigid due to high level of polymerization in the central part of hydrophobic region. The measured via osmotic stress method forces between membranes show a dramatic decrease of membrane hydration in comparison with that of usual phospholipid membranes. It does not exceed two water layers between membranes. The repulsive forces decay quickly with the decay length of 0.7 Å. Wide-angle x-ray diffraction showed the in-plane packing of lipid molecules similar to that of the usual phospholipid membranes in liquid phase. It means that hydration of polymeric DTDPC membranes cannot be explained in the framework of "hydration" (polarization) hypothesis of the origin of short-range repulsive forces between membranes. Thus, if out-of-plane motions of membrane surface are "switched off", the repulsive forces dramatically decrease. This, alongside with the recent neutron scattering study of out-of-plane and in-plane membrane structure, shows that entropic repulsive forces between phospholipid membranes are dominating, at least, at the intermembrane distances larger than a few angstroms. [ABSTRACT FROM AUTHOR]