Semiempirical models for biomembrane phase transitions and phase separations

Experimental studies are reviewed to construct a model for the hydrocarbon chain disordering in “fluid ” phospho-lipid bilayers. Together with approximate expressions for the interactions between neighboring molecules, this information is used to construct a simple statistical thermodynamic theory of the chain “melting ” based on the Bragg-Williams approxima-tion. Two unknown parameters in the theory, the interaction between neighboring chains in the “fluid ” state and the change in head group interaction associated with the chain “melting”, are determined for particular classes of phospholipids by ref-erence to experimental data. The theory is applied to predict the “melting ” temperatures of single-component bilayers and the phase diagrams of two-component bilayers. Possible extensions of this type of theory to rationalize other phenomena ob-served in biological membranes are discussed. I.