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

<p>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 L_α to the P_β^′ phase, both chiral and racemic samples of dipalmitoyl phosphatidylcholine (DPPC) exhibit phase coexistence of long and short ripples with a ripple wavelength ratio λ_L/λ_S∼1.8. Moreover, the long ripple always forms an orthorhombic unit cell (γ_L=90°), strongly supporting the possibility that these ripples are symmetric. In contrast, γ_S 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.</p>