A fluorescence probe such as pyrene is dissolved in various phospholipid dispersions and excited by means of a nanosecond-pulsed laser. The decay of the excited state is subsequently measured by monitoring the fluorescence at 400 nm. The lifetime of the pyrene-excited singlet state varies in different types of phospholipid dispersions and provides some measure of the structural organization of the hydrocarbon chain region in the micelle. The fluidities of lipid dispersion are measured by fluorescence depolarization of 2-methylanthracene. The temperature dependence of the lifetime of the pyrene-excited state indicates two transition points, ∼0, ∼55°C and ∼0, ∼39°C in the distearoyl and dipalmitoyl lecithin micelles, respectively. The rate of decay of the pyrene excited state increases in the presence of added quenchers which are dissolved primarily in the water phase, quenchers such as O2, iodide, and${\rm CH}_{3}{\rm NO}_{2}$. The quenching studies illuminate several factors which control the movement of the quencher into and across the micelle.

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