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Originally published as Biophys J. BioFAST on January 11, 2007.
doi:10.1529/biophysj.106.100107
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Biophysical Journal 92:2422-2433 (2007)
© 2007 The Biophysical Society

This is an Open Access article distributed under the terms of the Creative Commons-Attribution Noncommercial License (http://creativecommons.org/licenses/by-nc/2.0/), which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Investigation of Domain Formation in Sphingomyelin/Cholesterol/POPC Mixtures by Fluorescence Resonance Energy Transfer and Monte Carlo Simulations

Monica L. Frazier, Jenny R. Wright, Antje Pokorny and Paulo F. F. Almeida

Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, North Carolina 28403

Correspondence: Address reprint requests to Paulo F. F. Almeida, Dept. of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, NC 28403. Tel.: 910-962-7300; Fax: 910-962-3013; E-mail: almeidap{at}uncw.edu.

We have recently proposed a phase diagram for mixtures of porcine brain sphingomyelin (BSM), cholesterol (Chol), and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) on the basis of kinetics of carboxyfluorescein efflux induced by the amphipathic peptide {delta}-lysin. Although that study indicated the existence of domains, phase separations in the micrometer scale have not been observed by fluorescence microscopy in BSM/Chol/POPC mixtures, though they have for some other sphingomyelins (SM). Here we examine the same BSM/Chol/POPC system by a combination of fluorescence resonance energy transfer (FRET) and Monte Carlo simulations. The results clearly demonstrate that domains are formed in this system. Comparison of the FRET experimental data with the computer simulations allows the estimate of lipid-lipid interaction Gibbs energies between SM/Chol, SM/POPC, and Chol/POPC. The latter two interactions are weakly repulsive, but the interaction between SM and Chol is favorable. Furthermore, those three unlike lipid interaction parameters between the three possible lipid pairs are sufficient for the existence of a closed loop in the ternary phase diagram, without the need to involve multibody interactions. The calculations also indicate that the largest POPC domains contain several thousand lipids, corresponding to linear sizes of the order of a few hundred nanometers.




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