Liposomal Membranes. III. Permeation of Pyrene-labeled Lecithin into Matrix of Liposomal Bilayers

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Abstract
The permeation of a novel pyrene-labeled phosphatidylcholine, DPDL (1,2-bis[10-(1-pyrenyl)decanoyl]-sn-glycero-3-phosphatidylcholine), into the dipalmitoyl and egg phosphatidylcholine liposomes was studied. DPDL can adopt a sandwich conformation at the ground state, which comes in the fluorescence excimer emission. The fluorescent probe undergoes the initial rapid adsorption onto the liposome surfaces along with the instantaneous fluorescence enhancement followed by the relatively slow permeation into and diffusion in the liposomal bilayers. Through monitoring the relative quantum efficiency, Ie⁄Im (see text), during the permeation, suggestive information about the bilayer structure is obtained at least around the probe. The initial binding to and the subsequent diffusion in the membrane occur mostly as the monomeric form of the probe. At lower temperature without cholesterol the bilayer structure seems to be a lipid-separated conformation (Fig. 1 (c)) and above the chain-melting transition the bilayer seems to exist as a lipid-intimate conformation (Fig. 1 (b)). These results completely coincide with those obtained in our previous study on the codispersion of the probe with phospholipids (J. Sunamoto, et al., J. Am. Chem. Soc., 102, 1146 (1980)).

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