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Lateral Diffusion Coefficients of Separate Lipid Species in a Ternary Raft-Forming Bilayer: A Pfg-NMR Multinuclear Study

Greger Orädd ^*, Philip W. Westerman  and Göran Lindblom ^*

^* Department of Chemistry: Biophysical Chemistry, Umeå University, Umeå, Sweden; and Department of Biochemistry and Molecular Pathology, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio, USA

Correspondence: Address reprint requests to Göran Lindblom, Tel.: 46-90-7865228; Fax: 46-90-786- 7779; E-mail: goran.lindblom{at}chem.umu.se.

By isotopical labeling lipid lateral diffusion coefficients for each of the membrane constituents, including cholesterol, have been measured by ¹H, ²H, and ¹⁹F pulsed field gradient NMR spectroscopy in macroscopically oriented lipid bilayers. This provides a means of obtaining detailed dynamic and compositional information in raft-forming lipid bilayers without introducing foreign molecules into the systems. The raft systems studied contained dioleoylphosphatidylcholine/dipalmitoylphosphatidylcholine (DPPC)/cholesterol at the molar ratios of 42.5:42.5:15 and 35:35:30 in excess water. At temperatures below 30°C the raft system forms large (>1µm) domains of a liquid ordered (l_o) phase, in which the lipid lateral diffusion was 5 times slower than for the lipids in the surrounding liquid disordered (l_d) phase. Within each domain all lipid species showed the same diffusion coefficient, despite the very different structures of cholesterol and phospholipids. DPPC partitions exclusively into the l_o domains, whereas cholesterol and dioleoylphosphatidylcholine were distributed in both l_o and l_d phases. The cholesterol concentration was found to be 10–20 mol % in the l_d domain and 30–40 mol % in the l_o domain. Comparison of these results with data from sphingomyelin-containing systems suggests that DPPC interacts more weakly with cholesterol than does sphingomyelin.

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