Ternary mixtures of a high-melting lipid, a low-melting lipid, and cholesterol are known to form domains of a liquid-ordered and a liquid-disordered phase in bilayer membranes. We prepare giant vesicles from a sphingomyelin/dioleoylphosphocholine/cholesterol mixture and then examine them using fluorescence microscopy. NBD-labeled lipid and BODIPY-labeled cholesterol are used to identify the phase domains of the membrane. A vesicle with only two domains, one in a liquid-ordered and one in a liquid-disordered phase, is chosen because of its simple geometry, for convenient comparison of the experimental results with the theoretical predictions. A microinjector is used to gradually decrease and/or increase the volume of the vesicles by changing the osmolarity of the sugar solution. The relevant energy terms of the membrane mechanics are the elastic energies of the domains and the energy of the domain boundary. The elastic energy of the membrane domains can be described by two terms: the bending energy and the Gaussian bending energy. The energy of the domain boundary is proportional to its length. At the boundary between the domains a contact angle is taken into consideration. Then, in order to obtain values for the lateral tension and the contact angle, the areas of the domains and the characteristic dimensions of the shape are determined for different volumes. The best fits were obtained for a line tension of 6+/-3 pN and a contact angle of 1.4+/-0.3 rad.
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