J. Katsaras et al., Clarification of the ripple phase of lecithin bilayers using fully hydrated, aligned samples, PHYS REV E, 61(5), 2000, pp. 5668-5677
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 demonstrat
e that, on cooling from the L-alpha to the P-beta' phase, both chiral and r
acemic samples of dipalmitoyl phosphatidylcholine (DPPC) exhibit phase coex
istence of long and short ripples with a ripple wavelength ratio lambda(L)/
lambda(S) similar to 1.8 Moreover, the long ripple always forms an orthorho
mbic unit cell (gamma(L)= 90 degrees), strongly supporting the possibility
that these ripples are symmetric. In contrast, ys for short ripples was con
sistently different from 90 degrees, implying asymmetric ripples. We contin
ue to find no evidence that chirality affects the structure of rippled bila
yers. The relative thermodynamic stability of the two types of ripples was
investigated and a qualitative free energy diagram is given in which the lo
ng ripple phase is metastable. Finally, we suggest a kinetic mechanism, inv
olving loss of water, that promotes formation of the metastable long ripple
phase for special thermal protocols.