Thickening broken and critical fluctuation of phospholipid bilayers

It has been a puzzle that the phospholipid bilayers of a fluid phase fluctu ate critically and at the same time are weakly thickened in full hydration, when the temperature T is lowered toward the "main" transition [Chen et al ., 1997, Phys. Rev. Lett. 79, 4026]. The osmotic pressure Pi raises the tra nsitional temperature, and their relation defines a transitional line. Expl oring the fluid phase along the transition line down to the "main" transiti on point enables one to find out the answer for this puzzle. This study mea sured dilauroyl-phosphatidylcholine (DLPC) bilayers by lamellar x-ray diffr action along isotherms varying from 1 degrees to 35 degrees C as a function of Pi. The result shows that an intermediate phase of partial ordering in a chain up to a critical thickness DT emerges from the fluid phase along th e transition line. As the transition is close to the "main" transition poin t, the partial domain of the intermediate phase is water hydrolyzed so that the bilayer thickness is non-uniform, leading to a significant bilayer flu ctuation and a weaker bilayer thickening. This gives a new explanation for the critical swelling of phospholipid bilayers near the "main" transition.