The temperature dependence of the fluorescence anisotropy of polar head group labeled fluorophores (i.e., N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)dipalmitoyl-L- alpha-phosphatidylethanolamine or N-(lissamine rhodamine B sulfonyl)dipalmitoyl-L-alpha-phosphatidylethanol- amine) incorporated into multiple phosphatidylethanolamine molecular species was parabolic, possessing minima (dr/dT = 0) that precisely correlated with the respective lamellar (L alpha) to hexagonal (HII) phase transition temperature of each species. The parabolic alterations in the thermotropic behavior of these fluorophores were due to increased motional constraints in the polar head group region during heating (dr/dT greater than 0), because significant alterations in the fluorescence lifetimes of these probes during the phase transition did not occur. The sensitivity inherent in identification of peak minima was exploited to determine the lamellar to hexagonal phase transition temperatures of several homogeneous molecular species of plasmenylethanolamine (e.g., the transition temperature of 1-O-(Z)-hexadec-1'-enyl-2-octadec-9'- enoyl-sn-glycero-3-phosphoethanolamine was 28 degrees C). Experiments using ethanolamine glycerophospholipids containing either an ester or a vinyl ether linkage at the sn-1 position demonstrated that introduction of the vinyl ether constituent increased the propensity of these species to assume the hexagonal phase. Collectively, these results identify and substantiate a new technique for the characterization of the lamellar to hexagonal phase transition in phospholipids that requires only small amounts of phospholipids present in dilute membrane suspensions.
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