Pretransition-ripples in bilayers of dipalmitoylphosphatidylcholine: undulation or periodic segments? A freeze-fracture study.

Freeze-fracture analysis of ripple structures of 1,2-dipalmitoylphosphatidylcholine bilayers leads to the conclusion that the asymmetric ripple is the basic structure formed by periodic segments with different tilt direction. The molecules are tilted by about 30 degrees from the bilayer normal but arranged in two positions. Symmetric ripples are also formed by an alternation in tilt direction of the segments but the succession is more complex. A ridge in their valleys or a cleft at their crests may indicate structures formed or deformed during preparation (replication, etching). The freeze-fracture method reveals transition structures in ripple formation which are helpful in interpretation, but does not support a model consisting of an undulation of the bilayer by periodic fluid-like and gel-like domains.