DOMAIN FORMATION STUDIED BY FLUORESCENCE RESONANCE ENERGY TRANSFER AND MONTE CARLO SIMULATIONS

ABSTRACT

Domain formation in mixed vesicles of sphingomyelin, cholesterol, and phosphatidylcholine or phosphatidylserine has been examined using resonance energy transfer between two fluorescent probes incorporated in the membrane. Changes in energy transfer reflect changes in domain sizes, which has been confirmed using Monte Carlo computer simulations. With increasing percentages of phosphatidylcholine, its domains of phosphatidylcholine were found to become larger in number or area and energy transfer increased. Interaction parameters between lipid components of the membrane were found using Monte Carlo simulations. The interaction free energy between phosphatidylcholine (or phosphatidylserine) and cholesterol or sphingomyelin was found to be approximately 250 cal/mol. The interaction between sphingomyelin and cholesterol was found to be approximately -250 cal/mol. These interaction free energies are all small compared to thermal energy. In addition, results have shown that energy transfer is possible across the membrane, from donor probes in one monolayer to acceptors in the apposing monolayer. Domains of sphingomyelin and cholesterol tend to be located opposite to domains of phosphatidylcholine in the bilayer.