Fluorescence characteristics of pyrene and phosphatidylethanolamine-bound pyrene incorporated into lipid vesicles...

We used the excimer/monomer ratio of pyrene (PY) and N-(1-pyrenesulfonyl)dipalmitoyl-L-alpha-phosphatidylethanolamine (DPPE-PY) fluorescence intensities (IE/IM), and the polarity ratio I/III to investigate the state of the polar head group region of small, unilamellar phosphatidylcholine vesicles (SUV-PC) solubilized in media of differing NaCl concentrations. PY or DPPE-PY excimer formation resulting from vesicles' collisions is not affected by the presence of monovalent ions. In addition, the ionic strength does not alter the dielectric environment in the neighborhood of PY incorporated into SUV-PC. Since IE/IM of both PY and DPPE-PY is insensitive to variations in the ionic strength, we conclude that the probes' diffusion in SUV-PC, and consequently the membrane fluidity, are independent of NaCl concentration at least up to 0.5 M. The vesicles' concentration in the aqueous solution was the only factor which induced a rise of IE/IM. To explain the results in the context of the transient-fusion model developed previously (G.P. L'Heureux and M. Fragata, Biophys. Chem. 30 (1988) 293) and the hypothesis of repulsive hydration forces, we postulate a heterogeneous distribution of dehydrated domains, or contact areas, on the outer surfaces of colliding vesicles.