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

Citation
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
Citations number
58
Categorie Soggetti
Physics
Journal title
PHYSICAL REVIEW E
ISSN journal
1063651X → ACNP
Volume
61
Issue
5
Year of publication
2000
Part
B
Pages
5668 - 5677
Database
ISI
SICI code
1063-651X(200005)61:5<5668:COTRPO>2.0.ZU;2-Q
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 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.