"Semiempirical" models for biomembrane phase transitions and phase separations.

Experimental studies are reviewed to construct a model for the hydrocarbon chain disordering in “fluid” phospholipid 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 approximation. 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 reference 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 observed in biological membranes are discussed.