9722818 Feigenson A goal of this work is to find the solubility limit of cholesterol in model and real biomembranes, above which cholesterol precipitates from the bilayer as a crystal. This project has four main components: (I) defined mixtures of cholesterol and phospholipids are used as models for biomembranes; (ii) novel methods, together with X-ray diffraction experiments, are developed that are sensitive to the formation of coexisting phases; (iii) Monte Carlo computer simulations are used in order to find the cholesterol and phospholipid interaction energy over the entire range of compositions; (iv) crystalline cholesterol monohydrate formation is probed in cultured mammalian cells. These experiments will enable construction of 3-component phase diagrams that describe the mixing of cholesterol with two different phospholipids. The theoretical part of this project develops a model to describe the microscopic interaction energy between cholesterol and its neighboring lipids. Guided by these model system studies, X-ray diffraction and electron and optical microscopy of cells in culture will determine cholesterol solubility limits in real biomembranes. The overall objective of this project is to define and measure excessive cholesterol content, for a given membrane. These basic studies will clarify mechanisms of cholesterol accumulation within cells and cellular signals for its uptake. ***
