The effect of cholesterol on the viscosity of protein-lipid monolayers

The addition of cholesterol to a layer of lipids in a membrane structure is generally believed to result in an increase in the viscosity of the layer. We have shown that cholesterol and two other monolayer-forming lipids markedly decrease the viscosity of a serum albumin monolayer at the decane-water interface, a model membrane system. However, when the protein monolayer already has a surface active lipid component present, the effect of added cholesterol depends upon the other substance. When the albumin monolayer contains tristearin, added cholesterol increases the viscosity. When the lipid is octadecanol, cholesterol decreases the viscosity. The dependence of the change in interfacial viscosity due to cholesterol upon the original composition of the interfacial layer may be useful for characterizing the composition of layers of unknown composition, e.g. some natural membranes.