Theoretical study on microdomains in biomembranes

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In an effort to understand "rafts" in biological membranes, we propose phenomenological models for saturated and unsaturated lipid mixtures, and lipid plus cholesterol mixtures. We consider simple couplings between the local composition and internal membrane structure, and their influence on transitions between liquid and gel membrane phases. Assuming that the gel transition temperature of the saturated lipid is shifted by the presence of the unsaturated lipid, and that cholesterol acts as an external field on the chain melting transition, a variety of phase diagrams are obtained. Our results also apply to regions in the ternary phase diagram of lipid/lipid/cholesterol systems.We propose models for the first-order unbinding transition of lyotropic lamellae in surfactant solutions. The coupling between the surfactant volume fraction and the elastic degree of freedom is considered so that the net attractive interaction between the surfactant molecules is enhanced. The elastic degree of freedom can be either (i) a membrane elastic degree of freedom, or (ii) a bulk elastic degree of freedom. The phase behaviors of these two models are analyzed. For the both cases, the unbinding transition becomes first-order when the coupling is strong enough. We determine the associated preunbinding line which separates two lamellar phases having different repeat distances.