Modulated phases of lipid membranes induced by sterol derivatives

Restricted Access.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