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Evidence for Lipid/Cholesterol Ordering in Model Lipid Membranes

Canay Ege ^*, Maria K. Ratajczak , Jaroslaw Majewski , Kristian Kjaer  and Ka Yee C. Lee ^*

^* Department of Chemistry, Institute for Biophysical Dynamics and James Franck Institute, and Department of Physics, The University of Chicago, Chicago, Illinois 60637; Lujan Neutron Scattering Center at Los Alamos Neutron Science Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545; and Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark

Correspondence: Address reprint requests and inquiries to Ka Yee C. Lee, E-mail: kayeelee{at}uchicago.edu.

It has been postulated that for a binary mixture of phospholipid and cholesterol, phospholipid/cholesterol complexes are formed. Using grazing incidence x-ray diffraction, we have obtained evidence for lipid/cholesterol ordering in model membranes. Scattering features consistent with the existence of lipid/cholesterol complexes persist to high surface pressures even though fluorescence microscopy suggests a homogeneously fluid phase. Contrary to pure phospholipid and cholesterol systems, the resulting lattice spacing, integrated scattering intensity, and coherence lengths of these complexes are almost independent of surface pressure. Furthermore, the single peak observed in these mixed systems is very broad, suggesting that the extent of order for a single scattering structure only persists over a few molecules. This observation is consistent with these complexes being dynamic structures.

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