Phase properties of liquid-crystalline phosphatidylcholine/phosphatidylethanolamine bilayers revealed by fluorescent probes

The mixing properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolam ine (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 phosphatidylch oline (PC)/phosphatidylethanolamine (PE) (1:1) compared to PC matrix. When POPC was gradually replaced with POPE, the E/M ratio also increased suggest ing the enhanced lateral mobility or the lateral enrichment of PPC into dom ains or both. Evidences for the PE-induced domain formation were further pr ovided by resonance energy transfer between 2-(4,4-difluoro-5-methyl-8-boro -3a, 4a-diaza-s-indacene-3-dadecanoyl)-1-hexadecanoyl-sn-glycero-3-phosphoc holine and PPC, which was enhanced as a function of PE concentration, and b y the polarization of 1,6-diphenyl-1,3,5-hexatriene, In addition, PE reduce d free volume and polarity of lipid bilayers as measured by the emission fl uorescence of 1,2-bis PPC and 6-lauroyl-2-dimethylaminonaphthalene. When PO PE analogs with a methylated head group instead of normal POPE were used, t he diminished effect on the domain formation was shown in the order N-methy l PE > N,N-dimethyl PE. The results suggest that the mixing properties of P OPE and POPC are not random but that lipid domains of phospholipids are for med, (C) 1999 Academic Press.