STRUCTURE OF THE STABLE AND METASTABLE RIPPLE PHASE OF DIPALMITOYLPHOSPHATIDYLCHOLINE

Dipalmitoylphosphatidylcholine (DPPC) dispersed in excess water forms a stable ripple phase upon heating from the gel phase and a metastable ripple phase P-beta' (mst) upon cooling from the liquid crystalline p hase. The X-ray diffraction pattern of P-beta' (mst) displays several reflections in the range from 1/25 to 1/2.8 nm(-1), which can all be i ndexed on a two-dimensional monoclinic lattice (space group p2) with a = 26.2, b = 8.63 nm and gamma = 107 degrees. In contrast to the stabl e ripple phase, which shows a saw-tooth like surface profile and an al most constant bilayer thickness, the electron density map of the metas table ripple phase shows an almost symmetric surface profile with a mo dulation length of 26.2 nm. The lipid bilayer thickness varies from 3. 9 to 4.4 nm, which most likely arises from a continuous periodic chang e of the tilt of the chains to the surface normal of between 30 and 30 degrees. A further important feature of the structure is the staggere d stacking of the bilayers with water pockets enclosed.