ABRUPT MODIFICATIONS OF PHOSPHOLIPID-BILAYER PROPERTIES AT CRITICAL CHOLESTEROL CONCENTRATIONS

The fluorescence generalized polarization (GP) of 2-dimethylamino-6-la uroylnaphthalene (Laurdan) reveals different effects of cholesterol on the phase behavior of phospholipid bilayers. Phospholipid vesicles co mposed of gel, liquid-crystalline, and coexisting domains of the two p hases have been studied at temperatures from 1 to 65 degrees C, withou t cholesterol and with cholesterol concentrations of 3-50 mol %. Laurd an GP measurements show the general effect of cholesterol of increasin g the molecular dynamics of the gel and of decreasing the molecular dy namics of the liquid-crystalline phase. In the liquid-crystalline phas e, the increased order yields Laurdan GP values close to those obtaine d in the gel phase. At cholesterol concentrations >15 mol % a phase tr ansition cannot be detected. Using the wavelength dependence of the ex citation and emission GP spectra we determine that differences between the two phospholipid phases cannot be detected. In particular, in ves icles composed of coexisting gel and liquid-crystalline phases the GP wavelength dependence characteristic of coexisting domains cannot be o bserved at cholesterol concentrations greater than or equal to 15 mol %. Cholesterol causes the decrease in both the polarity and the dipola r relaxation effects on the neighborhood of the fluorescent naphthalen e moiety of Laurdan. Probably because of a cholesterol-induced increas e in the bilayer packing, these effects do not occur continuously with the increase of cholesterol concentration in the bilayer. Cholesterol concentrations inducing higher Laurdan GP values have been determined at about 5, 10, 15, 30, and 45 mol % with respect to phospholipids. W e propose that the formation of ordered molecular microdomains at crit ical cholesterol concentrations can explain the occurrence of the obse rved discontinuities.