The orientational order and dynamics of diphenylhexatriene molecules incorporated in planar multibilayers of unsaturated lipids were studied by angle-resolved flourescence depolarization at room temperature. Bilayer systems of the phosphatidylcholines soybean PC, dioleoyl PC and egg PC as well as the galactolipid digalactosyl diacylglycerol (DGDG) were used in the experiments. The depolarization ratios of diphenylhexatriene in these systems exhibited a weak angular dependence, in marked contrast to those observed from diphenylhexatriene in multibilayers of saturated phosphatidylcholines (Vos, M.H., Kooyman, R.P.H. and Levine, Y.K. (1983) Biochem. Biophys. Res. Commun. 116, 462–468). The results indicate that the diphenylhexatriene molecules possess a low degree of orientational order. However, the dynamics of the reorientational motions are inconsistent with the assumption that the diphenylhexatriene molecules possess effective cylindrical symmetry about their long axes. The fluorescence depolarization experiment is sensitive to the rotational motions about the long axes as the absorption and emission moments of diphenylhexatriene molecules are not mutually parallel. It is suggested that diphenylhexatriene molecules in bilayers of unsaturated lipids undergo slower reorientational motions around their long molecular axes than in bilayers of saturated phosphatidylcholines. A further conclusion of this study is that the ‘wobbling-in-cone’ model provides a poor description of the behaviour of diphenylhexatriene molecules in lipid bilayers
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