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 Lα to the Pβ′ phase, both chiral and racemic samples of dipalmitoyl phosphatidylcholine (DPPC) exhibit phase coexistence of long and short ripples with a ripple wavelength ratio λL/λS∼1.8. Moreover, the long ripple always forms an orthorhombic unit cell (γL=90°), strongly supporting the possibility that these ripples are symmetric. In contrast, γS for short ripples was consistently different from 90°, 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.