Cholesterol effects on membrane structure and dynamics

The influence of cholesterol on the phase be¬haviour of phospholipid bilayers has been stu¬died using the fluorescence Generalized Polariz¬ation (GP) of 2-dimethylamino-6-lauroylnaph¬thalene (Laurdan). Cholesterol causes the decrease of both the polarity and the dipolar relaxa¬tion around Laurdan. The decrease of dipolar re¬laxation does not occur continuously with the increase of cholesterol concentration in the bilayer. Points of discontinuity depend on the tempera¬ture and on the composition of the phospholipid vesicles. Critical cholesterol concentrations are determined at about 15 mot % and 30 mot % with respect to phospholipids. These concentrations can indicate peculiar structural arrangements of the phospholipid-cholesterol mixture. The reduc¬tion of both the polarity and the dipolar relaxation can be related to the increase of the packing of the bilayer. In the liquid-crystalline phase, the increased order yields Laurdan GP values close to those obtained in the gel phase, and a phase transition cannot be detected. In vesicles com¬posed of gel and liquid-crystalline phase, at cho¬lesterol concentrations 15 mol% the wavelength dependence of GP value, characteristic of coex¬isting domains, cannot be observed. Conse¬quences on the possibility of observing domains coexistence in natural membranes, where cho¬lesterol is present at relatively high concentra¬tions, are discussed.