Modulated phases of lipid membranes induced by sterol derivatives

Abstract

We have studied the effect of two secosteroids, namely cholecalciferol (vitamin D₃) and ergocalciferol (vitamin D₂), on the structure and phase behavior of dipalmitoyl phosphatidylcholine (DPPC) membranes using X-ray diffraction. Temperature–composition phase diagrams of both the systems have been determined from diffraction data of aligned multilayers at 98% relative humidity. The two phase diagrams are found to be almost identical, with the appearance of four distinct modulated phases as a function of secosteroid concentration below the main-transition of the lipid. Three of these phases are characterized by periodic height modulation of the bilayers, whereas the one that occurs at high secosteroid concentrations has a periodic thickness modulation. As far as we know, this is the first report on the thickness modulated phase of lipid bilayers.