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Biophysical Journal 86:923-935 (2004)
© 2004 The Biophysical Society

Partitioning of Pyrene-Labeled Phospho- and Sphingolipids between Ordered and Disordered Bilayer Domains

Mirkka Koivusalo *, Joni Alvesalo *, Jorma A. Virtanen {dagger} and Pentti Somerharju *

* Institute of Biomedicine, Department of Biochemistry, University of Helsinki, Helsinki, Finland and {dagger} Department of Chemistry, Nanoscience Center, University of Jyväskylä, Jyväskylä, Finland

Correspondence: Address reprint requests to Pentti Somerharju, Institute of Biomedicine, Dept. of Biochemistry, University of Helsinki, Biomedicum, Room C205b, P. O. Box 63, 00014 University of Helsinki, Finland. Tel.: 358-9-191-25410; Fax: 358-9-191-25444; E-mail: pentti.somerharju{at}helsinki.fi.

Here we have studied how the length of the pyrene-labeled acyl chain (n) of a phosphatidylcholine, sphingomyelin, or galactosylceramide affects the partitioning of these lipids between 1), gel and fluid domains coexisting in bovine brain sphingomyelin (BB-SM) or BB-SM/spin-labeled phosphatidylcholine (PC) bilayers or 2), between liquid-disordered and liquid-ordered domains in BB-SM/spin-labeled PC/cholesterol bilayers. The partitioning behavior was deduced either from modeling of pyrene excimer/monomer ratio versus temperature plots, or from quenching of the pyrene monomer fluorescence by spin-labeled PC. New methods were developed to model excimer formation and pyrene lipid quenching in segregated bilayers. The main result is that partition to either gel or liquid-ordered domains increased significantly with increasing length of the labeled acyl chain, probably because the pyrene moiety attached to a long chain perturbs these ordered domains less. Differences in partitioning were also observed between phosphatidylcholine, sphingomyelin, and galactosylceramide, thus indicating that the lipid backbone and headgroup-specific properties are not severely masked by the pyrene moiety. We conclude that pyrene-labeled lipids could be valuable tools when monitoring domain formation in model and biological membranes as well as when assessing the role of membrane domains in lipid trafficking and sorting.




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