Structure of the stable and metastable ripple phase of dipalmitoylphosphatidylcholine

Abstract

Dipalmitoylphosphatidylcholine (DPPC) dispersed in excess water forms a stable ripple phase upon heating from the gel phase and a metastable ripple phase P_β′ (mst) upon cooling from the liquid crystalline phase. The X-ray diffraction pattern of P_β′ (mst) displays several reflections in the range from 1/25 to 1/2.8 nm⁻¹, which can all be indexed on a two-dimensional monoclinic lattice (space group p2) with a=26.2, b=8.63 nm and γ=107°. In contrast to the stable ripple phase, which shows a sawtooth like surface profile and an almost constant bilayer thickness, the electron density map of the metastable ripple phase shows an almost symmetric surface profile with a modulation 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 change of the tilt of the chains to the surface normal of between 30 and 40 degrees. A further important feature of the structure is the staggered stacking of the bilayers with water pockets enclosed.