Hexagonal phase forming propensity detected in phospholipid bilayers with fluorescent probes
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The fluorescence emission spectrum of N epsilon-dansyl-L-Lys undergoes a marked blue shift when incorporated from aqueous solution into phospholipid bilayers. This shift is greater for membranes composed of dipalmitoleoylphosphatidylcholine than for membranes of dipalmitoleoylphosphatidylethanolamine. With the latter but not the former lipid, the fluorescence emission from N epsilon-dansyl-L-Lys is markedly temperature-dependent. The marked temperature dependence of N epsilon-dansyl-L-Lys fluorescence in bilayers of dipalmitoleoylphosphatidylethanolamine is greatest as the sample is heated close to the bilayer to hexagonal phase transition temperature. The fluorescence emission properties of another probe of membrane surface hydrophobicity, Laurdan, also exhibit marked changes at temperatures just below the bilayer to hexagonal phase transition temperature. At these temperatures, the generalized polarization begins to increase rather than decrease with temperature, and the emission intensity decreases markedly. Such effects are not observed over the same temperature range with phosphatidylcholine. Thus, both dansyl-L-lysine and Laurdan provide probes to measure changes in the physical properties of membrane bilayers which occur when these bilayers are heated close to the temperature required for transition to the hexagonal phase.
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