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

John Katsaras
Stephanie Tristram-Nagle, Carnegie Mellon University
Yufeng Liu, Carnegie Mellon University
R. L. Headrick, Cornell University
E. Fontes
P. C. Mason, National Research Council
John F. Nagle, Carnegie Mellon University

Abstract

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.