Fluorescence studies of the existence and functional importance of regular distributions in liposomal membranes

Liposomes containing rigid and bulky lipid molecules such as sterols and pyrene-labeled lipids can exhibit biphasic changes in membrane properties at several specific mole fractions predicted by the theory of lipid regular distributions (e.g., superlattices) in the plane of the membrane. This phenomenon has been observed in two-component as well as multicomponent liquid-crystalline liposomal membranes. The extent of sterol regular distribution plays a role in drug partitioning into membranes, the activities of surface-acting enzymes such as cholesterol oxidase and phospholipase A2, and in free-radical-induced sterol oxidation. This article summarizes the original fluorescence studies of lipid superlattices, reviews the recent findings in this area, and discusses the current controversial issues related to lipid regular distributions.