Critical shape transitions of monolayer lipid domains.

Fluorescence microscopy can be used to visualize coexisting fluid phases in lipid monolayers composed of cholesterol and dipalmitoyl phosphatidylcholine under specified conditions of temperature, composition, and lateral pressure. At a critical composition of approximately 30 mol% cholesterol, decreasing the average molecular area below a(c) [unk]50 A(2) per molecule forces the binary mixture through a critical point, where the monolayer becomes homogeneous. At molecular areas approximately 10% above this critical area, we observe shape transitions from liquid domains with circular shapes to domains with less symmetrical shapes. Shape transitions and critical shape fluctuations can also be triggered with light, due to photochemical effects on the monolayer. Shape fluctuations of lipid domains can thus be used to sense chemical events at the air-water interface.