The ripple phase of phosphatidylcholines: effect of chain length and cholesterol.

Saturated phosphatidylcholine (PC) bilayers display a rippled surface in the temperature region between the pre- and main transitions. Ripple repeat distance was measured from freeze-fracture electron micrographs. All of the lipids examined (C13PC to C16PC; C14C16PC and equimolar C14PC/C16PC) showed a bimodal distribution of ripple repeat distances with the two dominant values being in the ratio of 1:2. Within this series, chain length was a weak determinant of the actual repeat distance. The introduction of increasing concentrations of cholesterol eliminated the bimodal distribution and led to the appearance of a single distribution of increasing repeat distance and decreasing amplitude. Ripples disappeared above a cholesterol concentration of 15 mol%. These observations are discussed within the framework of a model which links the genesis of the ripples (vertical displacement of lipid molecules) to the trans-gauche isomerization known to occur at the pre-transition.