TIME-CORRELATED MODELS APPLICABLE TO REACTIONS IN RESTRICTED GEOMETRIES - PHOSPHOLIPID-VESICLES IN THEIR GEL AND LIQUID-CRYSTALLINE PHASES

The fluorescence decay profiles recorded with pyrene and with the fluorescence quenching process between diphenylhexatriene and 12-doxylstearic acid methyl ester in small unilamellar vesicles of dipalmitoylphosphatidylglycerol and L-alpha-phosphatidylglycerol have been analyzed with a model appropriate for reactions in restricted geometries. Measurements were made both in the gel and in the liquid crystalline phase, either by changing temperature or by changing the composition of a phospholipid mixture. We have found that in the gel phase, the reactions are more restricted, that is, they proceed more locally than in the liquid crystalline phase. The number of solubilization sites available to the reactants is much larger in the liquid crystalline phase than in the gel phase for pyrene, and it is much larger, tending to infinity, for the diphenylhexatriene-quencher couple. The probability of an immediate encounter between two reactants decreases from the gel to the liquid crystalline phase and as the temperature increases. In a short time scale of observation, the rate of the diffusion-controlled reaction increases with temperature and in going from the gel to the liquid crystalline phase. However, when the time scale is large, long-distance diffusion, even in the liquid crystal-line phase and at high temperature, is impossible.