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College Park, Maryland      June 6 - 10 , 2004

TP1: Influence of Cholesterol and Temperature on the Thickness of SOPC and DOPC Unilamellar Vesicles measured by Small-Angle Neutron Scattering

J. Pencer (Department of Chemical and Biomolecular Engineering, The Johns Hopkins University; NIST Center for Neutron Research)

Cholesterol-rich lateral domains or “rafts” are believed to serve a number of crucial roles in biological function. The formation of such domains in model systems has been shown to depend on the miscibility of lipids and cholesterol, and, in particular, on the degree of lipid chain unsaturation. There is evidence that the miscibility of cholesterol with different lipids is correlated with its influence on membrane bilayer properties such as fluidity, acyl-chain order parameters, and membrane thickness. Thus, measurements of cholesterol’s influence on lipid bilayer thickness should provide information regarding lipid – cholesterol interactions and miscibility.

In this study, small-angle neutron scattering (SANS) is used to characterize the influence of cholesterol on the thickness of large unilamellar vesicles (LUVs) composed of either stearoyloleoylphosphatidylcholine (SOPC) or dioleoylphosphatidylcholine (DOPC). It is found that the presence of ~ 40 % cholesterol in SOPC LUVs results in an apparent increase of membrane thickness of approximately 4.5 Å, while ~ 40 % cholesterol in DOPC LUVs results in an apparent thickness decrease of 1.5 Å. The thickness of SOPC and DOPC LUVs with and without cholesterol was also studied as a function of temperature. It was found that membrane thickness decreased linearly with increasing temperature for all membrane compositions examined. Our results suggest that there are significant differences in cholesterol’s interactions with SOPC and DOPC over physiologically relevant temperatures (from 25 °C to 55 °C). We comment on the implications of our results for the relative efficacy of SOPC and DOPC as non-raft components in raft – forming lipid mixtures.

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