Cholesterol-induced modulated phase in phospholipid membranes

Abstract

We report the observation of a cholesterol-induced modulated phase (P_β) in dipalmitoyl phosphatidylcholine bilayers. It occurs below the main transition of the lipid at cholesterol concentrations of around 15 to 20 mol % and is distinct from the ripple (P_β') phase found in between the main and pretransitions at lower cholesterol concentrations. An electron density map of this phase, constructed from X-ray diffraction data from oriented multilayers, shows that the bilayers in this phase have a one-dimensional periodic height modulation with an amplitude of about 2.5 Å. A partial phase diagram of the system deduced from diffraction data is in broad agreement with earlier studies.