L(BETA')-]L(C') PHASE-TRANSITION IN PHOSPHATIDYLCHOLINE LIPID BILAYERS - A DISORDER-ORDER TRANSITION IN 2 DIMENSIONS

The structure of the L(c') phase exhibited by hydrated dipalmitoylphos phatidylcholine (DPPC) was recently determined by Raghunathan and Kats aras [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 possess ing the phosphatidylcholine headgroup. As in DPPC, the L(c') phase in all of these systems is found to be characterized by two-dimensional o rdering of the lipid molecules on a superlattice of the hydrocarbon ch ain lattice. We also discuss the influence of headgroup interactions o n the structure of this phase.