Anthrylvinyl-labeled phospholipids as membrane probes: the phosphatidylcholine-phosphatidylethanolamine system

The phase behavior of mixtures of phosphatidylcholine (PC) with phosphatidylethanolamine (PE) identical or differing in their fatty acid composition has been investigated by using the steady-state fluorescence anisotropy of anthrylvinyl-labeled PC and PE (APC and APE) as well as of the non-lipid probe 1,6-diphenyl-1,3,5-hexatriene (DPH) to detect temperature-dependent changes in multilayer liposomes. APC, but not APE, was able to detect the pretransition of dimyristoyl-PC. The phospholipid probes APC and APE showed the main phase transition of their unlabeled disaturated analogues at temperatures almost identical with those revealed by differential scanning calorimetry, whereas the onset of the PE phase transition recorded by DPH was several degrees higher. In PC-PE mixtures with high content of PE the phase transitions shown by APC and APE were broader than those recorded by DPH. Comparison of phase diagrams constructed on the basis of fluorescence anisotropy and calorimetric data led to the conclusion that in biphasic PE and PC-PE systems DPH tends to partition into solid regions, whereas the anthrylvinyl-labeled phospholipids distribute more evenly between coexisting phases or prefer fluid domains. The use of anthrylvinyl phospholipid probes made it possible to demonstrate that PEs and PCs identical in their fatty acids are not miscible completely, not only below but also well above Tm of the higher melting component. Generally, APC and APE fluorescence anisotropy measurements correctly reflect headgroup-dependent phase segregations in mixtures of PC with PE, but may lead to ambiguous conclusions if demixing is caused by differences in the hydrocarbon chains.