Cyclodextrin-induced rearrangement of chain packing in dipalmitoylphosphatidylcholine/cholesterol mixed bilayers

Introduction The presence of cholesterol (chol) in biomembranes gives crucial influences on the biological functions; chol affects not only the protein activity but also the structure of the lipid bilayer. The addition of chol into the lipid bilayer brings froth extension of hydrocarbon chains of lipids, resulting in the increase in the bilayer thickness. In addition, it is known that the effects of chol on the lateral chain packing are different between in the disordered fluid phase and the ordered gel phase. When the rigid cholesterol molecules are added to the fluid phase membrane, they restrict the fluidity of lipid hydrophobic chains and promote the formation of the liquid ordered (LO) phase, which has been extensively studied as a model for the raft membrane [1]. On the other hand, when chol molecules are added to the gel phase membrane, they work as steric destabilizers, disturbing the tight chain packing in the membrane [2]. In order to examine the influences of chol on the membrane structures, cyclodextrin (cyd) has been extensively used in the monolayer system. The cyd molecule can extract chol from lipid/chol mixed membranes and change the structural organization of lipids in the membrane (e.g. modification of the liquid ordered phase). On the other hand, in the bilayer system much less information is available on the influence of cyd-mediated chol-extraction. In this study, we examined the cyd-induced change in lipid lateral packing in the dipalmitoylphosphatidylcholine (DPPC)/chol mixed single bilayer vesicle by wide-angle X-ray diffraction (WAXD).