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Asymmetric Structural Features in Single Supported Lipid Bilayers Containing Cholesterol and G_M1 Resolved with Synchrotron X-Ray Reflectivity

Christian Reich ^*, Margaret R. Horton , Bärbel Krause , Alice P. Gast , Joachim O. Rädler ^* and Bert Nickel ^*

^* Department für Physik, Ludwig-Maximilians-Universität, Munich, Germany; Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts; European Synchrotron Radiation Facility, Grenoble, France; and Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania

Correspondence: Address reprint requests to Christian Reich. E-mail: creich{at}alumni.tum.de.

The cell membrane comprises numerous protein and lipid molecules capable of asymmetric organization between leaflets and liquid-liquid phase separation. We use single supported lipid bilayers (SLBs) to model cell membranes, and study how cholesterol and asymmetrically oriented ganglioside receptor G_M1 affect membrane structure using synchrotron x-ray reflectivity. Using mixtures of cholesterol, sphingomyelin, and 1,2-dioleoyl-sn-glycero-3-phosphocholine, we characterize the structure of liquid-ordered and liquid-disordered SLBs in terms of acyl-chain density, headgroup size, and leaflet thickness. SLBs modeling the liquid-ordered phase are 10 Å thicker and have a higher acyl-chain electron density (_chain = 0.33 e^–/ų) compared to SLBs modeling the liquid-disordered phase, or pure phosphatidylcholine SLBs (_chain = 0.28 e^–/ų). Incorporating G_M1 into the distal bilayer leaflet results in membrane asymmetry and thickening of the leaflet of 4–9 Å. The structural effect of G_M1 is more complex in SLBs of cholesterol/sphingomyelin/1,2-dioleoyl-sn-glycero-3-phosphocholine, where the distal chains show a high electron density (_chain = 0.33 e^–/ų) and the lipid diffusion constant is reduced by 50%, as measured by fluorescence microscopy. These results give quantitative information about the leaflet asymmetry and electron density changes induced by receptor molecules that penetrate a single lipid bilayer.