L beta '-->Lc' phase transition in phosphatidylcholine lipid bilayers: A disorder-order transition in two dimensions.

The structure of the Lc8 phase exhibited by hydrated dipalmitoylphosphatidylcholine ~DPPC! was recently determined by Raghunathan and Katsaras @Phys. Rev. Lett. 74, 4456 ~1995!# from x-ray diffraction studies on oriented multibilayers. Here, we reanalyze the powder diffraction data reported in the literature on a number of hydrated lipids possessing the phosphatidylcholine headgroup. As in DPPC, the Lc8 phase in all of these systems is found to be characterized by two-dimensional ordering of the lipid molecules on a superlattice of the hydrocarbon chain lattice. We also discuss the influence of headgroup interactions on the structure of this phase. @S1063-651X~96!00310-8#