Clustering of cholesterol in DMPC bilayers as indicated by membrane mechanical properties

Mechanical characteristics of bilayer lipid membranes (BLM) composed of dimyristoylphosphatidylcholine (DMPC) and cholesterol in the gel and liquid crystalline state were studied by measuring the modulus of elasticity in direction perpendicular to the BLM plane, E perpendicular. The value of E perpendicular varied nonmonotonically with the cholesterol concentration, with a maximum around c = 50 mol% cholesterol. E perpendicular of BLM in gel state was about 2 times higher than that measured for the liquid crystalline state but the shape of E perpendicular (c) curves was similar for both states of the membrane. This may be due to the formation of cholesterol clusters at c > 50 mol% in both phase states of BLM.