Osmotic threshold and water association for phospholipid gel phase bilayers

The hydration of dimyristoyl phosphatidylcholine (DMPC) bilayers at 10 degr ees C gel phase was studied. The samples were prepared in the form of plana r multilayers where the roughly parallel bilayers are separated by water la yers. Hydration of the samples was controlled by vapor osmotic pressure. Th e bilayer structure was measured by lamellar x-ray diffraction. With fine t uning in osmotic pressure, we discovered, for the first time, a threshold o f osmotic pressure in hydration. Below the threshold to full hydration, the cross sectional area per lipid molecule, or equivalently the bilayer hydro phobic thickness, is almost unchanged under the osmotic pressure. Also, whe n the osmotic pressure crosses over the threshold, the hydration rate shows a transitional change. As a result, the properties of water associated to the bilayer head group region, including the water content, water distribut ion and water binding energy, were determined. It turns out that the DMPC g el phase bilayer is bounded by one-water shell to head groups with binding energy less than the thermal energy by one order of magnitude. This kind of water association predicts that a gel phase bilayer, although close packed by lipids, is still capable of possessing partial disorders and certain fl uctuations.