Phase Properties of Liquid-Crystalline Phosphatidylcholine/Phosphatidylethanolamine Bilayers Revealed by Fluorescent Probes

The mixing properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were examined in liquid-crystalline phase using fluorescent probes incorporated into lipid bilayers. The excimer to monomer (E/M) fluorescence ratio of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine (PPC) versus PPC concentration was higher for binary mixtures containing phosphatidylcholine (PC)/phosphatidylethanolamine (PE) (1:1) compared to PC matrix. When POPC was gradually replaced with POPE, the E/M ratio also increased suggesting the enhanced lateral mobility or the lateral enrichment of PPC into domains or both. Evidences for the PE-induced domain formation were further provided by resonance energy transfer between 2-(4,4-difluoro-5-methyl-4-boro-3a,4a-diaza-s-indacene-3-dodecanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine and PPC, which was enhanced as a function of PE concentration, and by the polarization of 1,6-diphenyl-1,3,5-hexatriene. In addition, PE reduced free volume and polarity of lipid bilayers as measured by the emission fluorescence of 1,2-bis PPC and 6-lauroyl-2-dimethylaminonaphthalene. When POPE analogs with a methylated head group instead of normal POPE were used, the diminished effect on the domain formation was shown in the order N-methyl PE >N,N-dimethyl PE. The results suggest that the mixing properties of POPE and POPC are not random but that lipid domains of phospholipids are formed.