Phase diagrams of phospholipid binary mixtures

Lipid bilayers are an important feature of biological membrane structure. The basic building blocks are the phospholipid molecules which possess a polar group and two nonpolar hydrocarbon chains. By means of the Landau phenomenological theory of phase transition we constructed a model to study the binary mixtures when the lipid components have the same head groups but the chain length is different or when the polar heads are different. When the mixture components have the same chain length but different polar heads (ethanolamine and choline head group are both in the mixtures) normal phase diagrams are obtained. Between the solidus and liquidus curves there is the mixed phase region. Below the solidus curve there is the pure gel phase while above the liquidus curve there is the liquid crystalline phase. The area of the mixed phase region decreases increasing the component chain length but no qualitatively differences exist between these phase diagrams. Drastic changes in the shape of phase diagrams are obtained if the polar heads are similar but the chain length difference is equal or higher than six. In this case the system is peritectic. Below the peritectic point a new mixed phase region appears, where the short chain rich liquid-crystalline state and short chain rich gel state coexist. A mixed gel phase region where short chain rich and long chain rich gel phase coexist, is too obtained. Peritectic system is derived too with choline/ethanolamine mixtures if the chain length of the ethanolamine molecule is the longer one and the difference is more than one. An eutectic system is obtained if the longer chain belongs to the lecithin molecules and the difference is more than one; azeotropic systems appear if the chain length difference is one. These results show that even a slight change in the lipid composition can change drastically the phase transition properties and then the functional properties of the lipid membranes.