The dynamics of lipid hydrocarbon chains in phosphatidylcholine (dimyristoyl- or dipalmitoyl-) and cholesterol/dimyristoylphosphatidylcholine membranes were investigated by nanosecond time-resolved fluorescence depolarization measurements on a lipophilic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene embedded in the membranes. In the pure lipid membranes, both the range (amplitude) and the rate of the wobbling motion of the probe increased sigmoidally with temperature reflecting the thermotropic phase transition of the lipid. The rise in the rate slightly preceded the increase in the range, suggesting that the fluctuation of lipid chains is activated to a high level before the ordered array of chains melt into the liquid-crystalline phase. Above the transition temperature, incorporation of cholesterol resulted in a dramatic decrease in the range of wobbling motion while the rate remained high. Below the transition, on the other hand, cholesterol had little effect on the range, whereas the rate was greatly increased. These effects of cholesterol are remarkably similar to the effects of cytochrome oxidase on lipid chain dynamics (Kinosita, K., Jr., Kawato, S., Ikegami, A., Yoshida, S. and Orii, Y. (1981) Biochim. Biophys. Acta 647, 7-17).
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