Pyrene excimer fluorescence has been employed as a probe of the microviscosity of the hydrocarbon interior of a variety of phospholipid dispersions. The transition temperature, Tt, at which the nonpolar regions of these molecules undergo phase changes, was found to be 11, 24, 30, 41, and -3° for dilauroyl-, dimyristoyl-, 1-palmitoyl-2-palmitoleyl-, dipalmitoyl-, and egg phosphatidylcholine, respectively. These values agree closely with values obtained by other methods. The activation energy for the transacylation by lecithin:cholesterol acyltransferase is changed at a temperature which coincides with the transition temperature of the phosphatidylcholine acyl donor in the phosphatidylcholine:cholesterol vesicle. This dependence is consistent with a preferential association of the enzyme and phosphatidylcholine.
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