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The Condensing Effect of Cholesterol in Lipid Bilayers

Wei-Chin Hung ^*, Ming-Tao Lee , Fang-Yu Chen  and Huey W. Huang 

^* Department of Physics, Chinese Military Academy, Fengshan, Kaohsiung, Taiwan; National Synchrotron Radiation Research Center, Hsinchu, Taiwan; Department of Physics, National Central University, Chung-Li, Taiwan; and Department of Physics & Astronomy, Rice University, Houston, Texas

Correspondence: Address reprint requests to Dr. Huey W. Huang, Dept. of Physics & Astronomy, Rice University, Houston, TX 77251-1892. Tel.: 713-348-4899; Fax: 713-348-4150; E-mail: hwhuang{at}rice.edu; or Dr. Fang-Yu Chen, Dept. of Physics, National Central University, Chung-Li, Taiwan 32054. Tel.: 886-3-4227151, Ext. 65331; Fax: 886-3-4251175; E-mail: fychen{at}phy.ncu.edu.tw.

The condensing effect of cholesterol on phospholipid bilayers was systematically investigated for saturated and unsaturated chains, as a function of cholesterol concentration. X-ray lamellar diffraction was used to measure the phosphate-to-phosphate distances, PtP, across the bilayers. The measured PtP increases nonlinearly with the cholesterol concentration until it reaches a maximum. With further increase of cholesterol concentration, the PtP remains at the maximum level until the cholesterol content reaches the solubility limit. The data in all cases can be quantitatively explained with a simple model that cholesterol forms complexes with phospholipids in the bilayers. The phospholipid molecules complexed with cholesterol are lengthened and this lengthening effect extends into the uncomplexed phospholipids surrounding the cholesterol complexes. This long-range thickening effect is similar to the effect of gramicidin on the thickness of lipid bilayers due to hydrophobic matching.

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