[Formula Presented]→[Formula Presented] phase transition in phosphatidylcholine lipid bilayers: A disorder-order transition in two dimensions

The structure of the [Formula Presented] 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 [Formula Presented] 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.