On Domain Structure and Local Curvature in Lipid Bilayers and Biological Membranes

Abstract

The lateral lipid organization and the local surface curvature in small and giant bilayer vesicles of binary lipid mixtures were investigated. Mixtures of the following lipids are studied: cholestrin, di-alcyl-lecithins, dioleyl-lecithin and di-alcyl-phosphatidic acid. The latter is considered as being representative of charged natural lipids (e.g., phosphatidylserine or cardiolipin). Three different experimental methods are compared: the excimerfluorescence method, the spin label technique and the freeze fracture EM. The latter 2 methods yield information on the size of lipid precipitations. The surface curvature may be studied by EM. The essential experimental results are: mixtures of smectic phases of different symmetry undergo lateral phase separation; the phase separation leads to a domain-like lateral lipid organization; the domain structure is often accompanied by a variation in local curvature; and in membranes containing a charged lipid component, the domain structure may also be triggered by external charges (such as surface proteins). Two types of domains are observed: circular domains (diameters of the order of several hundred .ANG.) which occur in mixtures of non-tilted fluid and rigid phases, and elongated domains (width .apprx. 100 .ANG.) which are observed in mixtures of tilted and non-tilted phases. This domain pattern is characteristic for mixtures of lecithins and cholesterols. The domain structure is explained by combining the theory of spinodal decompositon of alloys with the essential result of the orientational elastic model of membranes. The domain size calculated from this model agrees well with the experimental result. The periodic ripple structure observed between the pre- and the main transition is explained by generalizing the concept of spinodal decomposition to include the separation of phases which are only distinguishable (e.g., by the tilt angle). The width of the domains in equilibrium is explained in terms of the spontaneous curvature of the decoupled monolayer of a bilayer. Good agreement with the experimental result is obtained. The ripple phase is only a special case of the surface induced domain structures in ordinary liquid crystals. The defect structure of the ripple phase was analyzed in terms of its symmetry. A symmetry rule is established which leads to a model of lipid orientation. Below the pretransition no periodic domain pattern is observed under normal conditions. But on cooling a bilayer very rapidly a defect structure reminiscent of a screw dislocation is observed. This is expected for coupled biaxial monolayers.