  We have calculated the electron density maps of the ripple phase of
dimyristoylphosphatidylcholine (DMPC) and palmitoyl-oleoyl phosphatidylcholine
(POPC) multibilayers at different temperatures and fixed relative humidity. Our
analysis establishes, for the first time, the existence of an average tilt of
the hydrocarbon chains of the lipid molecules along the direction of the ripple
wave vector, which we believe is responsible for the occurrence of asymmetric
ripples in these systems.
