Effects of fullerene guests on the stability of 1-palmitoyl-2-oleoylphosphatidylcholine liposomes

Membrane stability of extruded large unilamellar vesicles (LUV) formed by 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) containing fullerene C-60 or an amphiphilic fullerene derivative, 2-[2-(2-fulleropyrrolidin-1-ylethoxy)-ethoxy]-ethanol (FPE), has been investigated by spectrofluorimetrically monitoring the spontaneous release of entrapped carboxyfluorescein (CF). Under controlled conditions of temperature, osmolarity and pH, these guests increase the stability of the liposomal membrane as shown by the decrease in the rate of outflux of CF. The stability conferred to the POPC liposomes by C-60 and FPE has been compared with that conferred by the well-known stabilizing guest 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] ammonium salt (PEG). The addition of amphiphilic molecules, such as non-ionic surfactants, which intercalate into the membrane bilayer, and of sucrose or NaCl, which induce a hyposmotic stress, has been extensively investigated in order to get information on how to modulate the release of entrapped CF. This information could hopefully be useful in the formulation of new drug delivery systems as well as for getting a deeper understanding of the mechanisms of the formation/enclosure of channels through the membrane. The viscosity and the micropolarity of the membrane have been measured fluorimetrically by using pyrene as the probe. An interesting increase of the gel-liquid crystal phase transition temperature has been observed for POPC liposomes hosting C-60.