Continuous lipidic cubic phase

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

The polymerization of one or more components of a lyotropic liquid crys in such a way as to preserve and fixate the microstructure has recently been successfully performed. This opens up new avenues for the study and technological application of these periodic microstructures. Of particular importance are the so-called bicontinuous cubic phases, having triply-periodic microstructures in which aqueous and hydrocarbon components are simultaneously continuous. It is shown that the polymerization of one of these components, followed by removal of the liquid components, leads to the first microporous polymeric material exhibiting a continuous, triply-periodic porespace with monodisperse, nanometer-sized pores. It is also shown that proteins can be immobilized inside of polymmzed cubic phases to create a reaction medium allowing continuous flow of reactants and products, and providing a natural lipid environment for the proteins. 

The minimal surface description naturally reveals the infinite lipid bilayer nature of cubic phases, viz. the fact that a single bilayer with no selfintersections can separate two continuous water regions. If we consider models of these cubic lipid-water phases, the structures of the Cp (or Cd) phases look like water globules separated by bilayers and fused in four (or three) lateral directions, respectively. Such a structure is not consistent with the earlier rod description. 

Several details, described by Bassot and coworkers [73, 76], such as the presence of a continuous membrane, and especially its bicontinuity, are consistent with our cubic membrane model. Indeed, similarities between the structure of cubic phases in lipid-water systems (c/. Chapters 4 and 5) and that of the photosome membrane system have been discussed by Bassot and coworkers [77]. These particularly well developed cubic membranes have been exhaustively studied surpassed only by studies of the PLB cubic membrane. 

Fig. 24. a) Schematic illustration of the "stretching" of water channel junctions during the continuous transformation between the D and G cubic phases, which occur with no disruption of the bilayer topology. A junction of four water channels in the Qu° phase is converted into two three-way junctions in the Qu° phase, b) Possible mechanism of membrane fusion the monolayers of two apposed lipid bilayers mix to form a stalk intermediate that expands radially to a trans monolayer contact (TMC), leading to rupture as a result of curvature and interstitial stresses and finally to the formation of a fusion pore. 

The three dimensional (3D) cubic V2 phases are arranged as single continuous lipid curved bilayers forming a eomplex network containing two non-intersecting water channels [90]. Three different bicontinuous cubic nanostructures (a family of closely related phases) have been identified in the literature. They have a primitive (P), a gyroid (G), or a diamond (D) infinite periodic minimal surface (IPMS) [88, 89]. 

Lee et al. demonstrated the synthesis of nanostructured cubic polymer gels by copolymerization of dienyl substituted lipids [66]. No phase transitions, or changes in dimensions, were observed with temperature changes for the polymerized sample. Furthermore, the polydomain square lattice of the gel was visualized by TEM of ultramicrotomed samples after extraction of the template (Fig. 7). In contrast, copolymerization of monoacylglycerol and 1,2-diacylglyc-erol in a cubic lyotropic state did not result in a continuous gel structure. Linear polymer chains were obtained instead, and the cubic morphology was destroyed by addition of organic solvent [67]. Similar polymerizations in the inverted... 

In the presence of water, surfactants and lipids give rise to a variety of phases referred to as lyotropic phases or mesophases.i The most important of these phases are the lamellar, hexagonal, cubic micellar, and cubic bicontinuous phases denoted by L, H and V, and Q, respectively (see Figure 1.11 in Chapter 1). The subscripts 1 or 2 attached to these phase symbols indicate that the phase is direct (water continuous) or inverse (discontinuous water domains). Many other lyotropic phases have been identified that differ from the main ones by the state of the alkyl chain (crystalline or disordered) and of the head group arrangement (ordered or disordered). In the particular case of the lamellar phase, additional variations come from the possible different orientations adopted by the alkyl chains with respect to the plane of the lamellae (angle of tilt of the chain) and also from the state of the surface of the lamellae that can be planar or rippled. Numerous detailed descriptions have been given for the equilibrium state of the various phases that surfactants and lipids can form in the presence of water. 

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