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

摘要

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 (lo) phase, in which the lipid lateral diffusion was ∼5 times slower than for the lipids in the surrounding liquid disordered (ld) 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 lo domains, whereas cholesterol and dioleoylphosphatidylcholine were distributed in both lo and ld phases. The cholesterol concentration was found to be 10–20 mol % in the ld domain and 30–40 mol % in the lo domain. Comparison of these results with data from sphingomyelin-containing systems suggests that DPPC interacts more weakly with cholesterol than does sphingomyelin.