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Sphingomyelin-Cholesterol Domains in Phospholipid Membranes: Atomistic Simulation

Sagar A. Pandit ^*, S. Vasudevan ^*, S. W. Chiu , R. Jay Mashl , Eric Jakobsson  and H. L. Scott ^*

^* Department of Biological, Chemical, and Physical Sciences, Illinois Institute of Technology, Chicago, Illinois 60616; and Department of Molecular and Integrative Physiology, Department of Biochemistry, University of Illinois at Urbana-Champaign Programs in Biophysics, Neuroscience, and Bioengineering, National Center for Supercomputing Applications, and Beckman Institute, University of Illinois, Urbana, Illinois 61801

Correspondence: Address reprint requests to H. L. Scott, E-mail: scotth{at}iit.edu.

We have carried out an atomic-level molecular dynamics simulation of a system of nanoscopic size containing a domain of 18:0 sphingomyelin and cholesterol embedded in a fully hydrated dioleylposphatidylcholine (DOPC) bilayer. To analyze the interaction between the domain and the surrounding phospholipid, we calculate order parameters and area per molecule as a function of molecule type and proximity to the domain. We propose an algorithm based on Voronoi tessellation for the calculation of the area per molecule of various constituents in this ternary mixture. The calculated areas per sphingomyelin and cholesterol are in agreement with previous simulations. The simulation reveals that the presence of the liquid-ordered domain changes the packing properties of DOPC bilayer at a distance as large as 8 nm. We calculate electron density profiles and also calculate the difference in the thickness between the domain and the surrounding DOPC bilayer. The calculated difference in thickness is consistent with data obtained in atomic force microscopy experiments.

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