Clarification of the ripple phase of lecithin bilayers using fully hydrated, aligned samples

Aligned samples of lipid bilayers have been fully hydrated from water vapor in a different type of x-ray chamber. Our use of aligned samples resolves issues concerning the ripple phase that were ambiguous from previous powder studies. In particular, our x-ray diffraction data conclusively demonstrate that, on cooling from the [Formula Presented] to the [Formula Presented] phase, both chiral and racemic samples of dipalmitoyl phosphatidylcholine (DPPC) exhibit phase coexistence of long and short ripples with a ripple wavelength ratio [Formula Presented] Moreover, the long ripple always forms an orthorhombic unit cell [Formula Presented] strongly supporting the possibility that these ripples are symmetric. In contrast, [Formula Presented] for short ripples was consistently different from [Formula Presented] implying asymmetric ripples. We continue to find no evidence that chirality affects the structure of rippled bilayers. The relative thermodynamic stability of the two types of ripples was investigated and a qualitative free energy diagram is given in which the long ripple phase is metastable. Finally, we suggest a kinetic mechanism, involving loss of water, that promotes formation of the metastable long ripple phase for special thermal protocols.