SULFATE-ION-INDUCED SLOW FUSION OF DIOCTADECYLDIMETHYLAMMONIUM BROMIDE VESICLES

Addition of equimolar sodium sulfate to vesicles, prepared from 4.0 x 10(-4) M dioctadecyldimethylammonium bromide (DODAB), in their gel state at room temperature slowly increased the mean hydrodynamic diameters, D(H), of the aggregates from 80 to 740 nm over several days. Fluorescence resonance energy transfer efficiency in DODAB vesicles prepared from 4.0 x 10(-4) M DODAB, 0.8 mol % of N-(7-nitrobenz-2-oxa-1,3-diazol-4-oyl)phosphatidylethanolamine (donor), and 0.8 mol % of N-(lissamine Rhodamine B sulfonyl)phosphatidylethanolamine (acceptor) was also found to decreases slowly upon dilution by equal volumes and equal concentrations of DODAB vesicles and Na2SO4. Thus, the slow increase of D(H) following Na2SO4 addition corresponds to fusion rather than to nonproductive adhesion of the DODAB vesicles. Monitoring the fluorescence relaxation times, lifetimes, steady-state anisotropies, and average angles of distribution of diphenylhexatriene (fluorescence probe sensitive to its microenvironment) in DODAB vesicles undergoing fusion revealed systematic and parallel fluctuations of different and independently obtained parameters which were attributed to the subtle changes (flattening, adhesion, destabilization, inverse micelle or lipidic intramembranous particle formation, and merging of adjacent bilayers through ellipsoid shape to the final spherical structure) accompanying the fusion of the vesicle ensemble.