Temperature dependence of the leakage of carboxy fluorescein from an inner aqueous phase of liposomes or mixed liposomes

Small unilamellar liposomes with an average diameter of about 500 Å containing carboxy fluorescein in an inner aqueous phase were prepared by 20 min sonication (60 W) of the aqueous dispersions of dimyristoyl-, dipalmitoyl-, or distearoyl phosphatidylcholine. The excess carboxy fluorescein in an outer aqueous phase was expelled by a Sepharose CL-4B column. The leaked carboxy fluorescein was analyzed successively by fluorescence spectroscopy at different temperatures. A rapid leakage was observed at the main phase transition temperature. On the other hand, little leakage was recorded below and above the phase transition temperature. The leakage was also observed around the main transition temperature when liposome suspensions were heated or cooled. Mixed liposomes, composed of two different lipids having different phase transition temperatures, showed a CF leakage at the lower transition temperature for the mixed liposomes. A good linear relationship was found between the transition temperature and the temperature where the fastest CF leakage was recorded for several liposomes. It is concluded that the leakage is not due to a simple increase in molecular motion at higher temperatures but is due to a transient structural disorder of the lipid packing induced by the phase separation.