Effects of Cholesterol Concentration and Osmolarity on the Fluidity and Membrane Tension of Free-standing Black Lipid Membranes

Although the mechanical properties and compositions of lipid bilayer membranes can change upon deformation, the fundamental relations between the composition, membrane tension and fluidity of membranes with little curvature have not yet been studied. In the current study, the membrane tension and the diffusion coefficients of free-standing black lipid membranes (BLMs), based on 1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC), were observed by systematic control of the cholesterol concentration and the osmotic pressure with the laser-induced surface deformation (LISD) and fluorescence recovery after photobleaching (FRAP) techniques. When the osmotic pressure was raised and, therefore, the curvature became larger, both the membrane tension and the diffusion coefficients increased as well. On the other hand, when the cholesterol concentration was raised, the membrane tension increased whereas the diffusion coefficient decreased. The importance of the present results goes beyond this quantitative evaluation of the relation between the membrane tension and the fluidity, as it clarifies the changes in the fundamental properties of lipid bilayers upon natural fluctuations and perturbative deformation that were hitherto unknown.