Cholesterol Modifies Water Concentration and Dynamics in Phospholipid Bilayers: a Fluorescence Study Using Laurdan Probe

The effect of cholesterol on the gel, the liquid-crystalline, and mixed phospholipid phases has been studied using the fluorescence properties of 2-dimethylamino-6-lauroylnaphthalene (Laurdan). Laurdan sensitivity to the polarity and to the dynamics of its environment reveals that cholesterol affects phospholipid bilayers in the gel phase by expelling water and by increasing the amount of dipolar relaxation. In the liquid-crystalline phase, the effect of cholesterol is a reduction of both water concentration and amount of dipolar relaxation. Detailed studies of Laurdan excitation and emission spectral contours as a function of cholesterol concentration show that there are some cholesterol concentrations at which Laurdan spectral properties changes discontinuously. These peculiar cholesterol concentrations are in agreement with recent observations of other workers showing the formation of local order in the liquid-crystalline phase of phospholipids upon addition of phospholipid derivatives of pyrene. A local organization of phospholipids around cholesterol molecule seems to be produced by the presence of peculiar concentrations of cholesterol itself. This local organization is stable enough to be observed during the excited state lifetime of Laurdan of approximately 5-6 ns.