Vertical order of DPPC multilayer enhanced by cholesterol-induced ripple-to-liquid ordered (LO) phase transition: Synchrotron X-ray reflectivity study

We investigated the effect of cholesterol on the vertical order of spin-coated supported dipalmitoylphosphatidylcholine (DPPC) multilayers by using synchrotron X-ray reflectivity with customized X-ray liquid cell. As increasing mol% of cholesterol up to 10%, the lamellar spacing of the DPPC multilayer increases, implying that cholesterol induces a tilt-to-untilt transition. The sharpening of X-ray intensity upon cholesterol binding, reflected in the FWHM plot, suggests that cholesterol enhances the vertical ordering of the DPPC multilayer. In addition, fluorescence microscopy experiment confirms a gel-to-liquid ordered (LO) phase transition between 5 and 7% cholesterol. As a result, in low cholesterol regime, the DPPC multilayer is not in a simple gel phase but in a ripple phase where lipids are vertically disordered due to the differently tilted chains which leads to undulations. Therefore, we conclude that the cholesterol-induced ripple-to-LO phase transition enhances the vertical order of the DPPC multilayer. While most of works on membrane structure and cholesterol effect have focused on the physical properties of lipid membrane in lateral direction, this paper suggested a new physical aspect of studying vertical structures of membrane.