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College Park, Maryland      June 6 - 10 , 2004

T1-B2 (9:00 AM): Detection by Small-Angle Neutron Scattering of Lateral Segregation or "Rafts" in Lipid Cholesterol and Lipid Ergosterol Mixtures in Large Unilamellar Vesicles

J. Pencer (Dept. of Chemical and Biomolecular Engineering, The Johns Hopkins University; NIST Center for Neutron Research), S. Krueger (NIST Center for Neutron Research), R. M. Epand (Biochemistry Department, McMaster University)

Small-angle neutron scattering (SANS) measurements are used here to identify and characterize lateral heterogeneities or "rafts" in three component mixtures of lipids and cholesterol or ergosterol in 50 nm diameter unilamellar vesicles. By using deuterated lipid as the saturated lipid component in mixtures of lipids and cholesterol, and appropriate contrast conditions, we are able to enhance the contrast between heterogeneous membranes and the medium. This allows us to detect the onset of "raft" formation on cooling of homogeneously mixed membranes, and to measure the form factor of heterogeneous membranes. We show evidence, for the first time using SANS, that nanoscopic domains form in unilamellar vesicles of a size several orders of magnitude smaller than similar vesicles used in fluorescence microscopy studies. It is found that, while "rafts" form in 1:1:1 mixtures of DOPC:DPPC:cholesterol and ergosterol, there is no such evidence for similar heterogeneities in 1:1:1 SOPC:DPPC:cholesterol. Furthermore, it is found that ergosterol has a greater ability to stabilize "rafts" than cholesterol.

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