Layers cubic phases

Figure B3.6.4. Illustration of tliree structured phases in a mixture of amphiphile and water, (a) Lamellar phase the hydrophilic heads shield the hydrophobic tails from the water by fonning a bilayer. The amphiphilic heads of different bilayers face each other and are separated by a thin water layer, (b) Hexagonal phase tlie amphiphiles assemble into a rod-like structure where the tails are shielded in the interior from the water and the heads are on the outside. The rods arrange on a hexagonal lattice, (c) Cubic phase amphiphilic micelles with a hydrophobic centre order on a BCC lattice.

Bicontinuous cubic phases have not, to date, been accounted for using SSL theory. The OBDD phase has been shown to be unstable with respect to lam and hex phases (Likhtman and Semenov 1994 Olmsted and Milner 1994a,b). As discussed above, it now appears that the OBDD was a misidentified gyroid phase however, SSL calculations for the gyroid structure have not been performed as yet. A perforated layer structure was found to be unstable by Fredrickson (1991), using SSL theory following Semenov s method. 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

For polycatenar hydrogen bonded complexes with fluorinated chains at both ends (e.g., 138,139, see Fig. 36) formation of columnar phases was observed [246]. However, compound 137, having a branched Rp-chain at one end and three RH-chains at the other has a sequence of three distinct phases in the unusual sequence Cub-Col-SmA-Iso. For the SmA phase of compound 137 a structure with intercalated aromatic cores and RF-chains and separated layers of the hydrocarbon chains was proposed. At lower temperature, when incompatibility rises and the aromatics and Rp-chains disintegrate, all three components form their own layers. However, this produces interface curvature and a columnar phase with square lattice is formed. On further cooling a transition to a cubic phase with Im3m lattice takes place which is most likely of the bicontinuous type [262]. This leads to the unusual phase sequence Cubv-Col-SmA where the positions of the Cubv and Col phases are exchanged with respect to the usually observed phase sequences. The Col-Cub transition at lower temperature could be the result of the decreased conformational disorder of the terminal chains which reduces the steric frustration and hence reduces the interface curvature. 

In a related series of compounds with 5-heptyl-2-phenylpyrimidines replacing the 4 -cyanbiphenyls of compounds 208-210, the sequence SmC-SmA-Colrec was observed on increasing the number of phenylpyrimidine units [367]. This indicates an even stronger distortion of layer curvature by these rod-like units as hexagonal columnar and cubic phases are removed and replaced by a rectangular columnar ribbon phase. 

Fig. 11 AFM micrograph of the (110) surface of the cubic phase of compound 36c showing a one-layer step.

These cubic phases, isomorphous with skutterudite C0AS3, are polyanionic compounds related to pyrite. The cations are also octahedrally coordinated by anions only. The layers of octahedra are similar to those in pyrite but the octahedra are tilted in a different way so that square-planar anion rings are formed (see Fig. 7). The coordination tetrahedra... 

Tin(II) oxide exists in several modifications. The commonest is blue-black tetragonal tin(II) oxide, formed by alkaline hydrolysis of a tin(II) salt it has a layer lattice with square pyramidal coordination at tin and equal Sn-O distances (Sn- Sn distances between Sn atoms in adjacent layers are 3.70 A, close to the values in pSn). Tin(II) oxide is amphoteric and dissolves in aqueous acid and aUcahes. Tin(II) hydroxide, Sn(OH)2, has been obtained from MesSnOH and SnCl2. Tin(II) sulfide has a structure with parallel zigzag Sn S chains, which are connected by short interchain Sn- S contacts this gives a basic pyramidal [SnSs] unit. Tin(II) selenide (mp 861 °C) exhibits one phase which is isomorphous with SnS and a second cubic phase with the NaCl lattice the latter is also taken up by tin(II) telluride. 

When a bicontinuous cubic lipid-water phase is mechanically fragmented in the presence of a liposomal dispersion or of certain micellar solutions e.g. bile salt solution), a dispersion can be formed with high kinetic stability. In the polarising microscope it is sometimes possible to see an outer birefringent layer with radial symmetry (showing an extinction cross like that exhibited by a liposome). However, the core of these structures is isotropic. Such dispersions are formed in ternary systems, in a region where the cubic phase coexists in equilibrium with water and the L(x phase. The dispersion is due to a localisation of the La phase outside cubic particles. The structure has been confirmed by electron microscopy by Landh and Buchheim [15], and is shown in Fig. 5.4. It is natural to term these novel structures "cubosomes". They are an example of supra self-assembly. 

We close this survey of cell membranes with a remarkable observation that adds support to this novel picture of cytomembrane shape. In Chapter 4 (section 4.13), it was noted that many bacteria are shrouded in a mesh-like protein coat, which often displays a regular, crystallographic form. The most exotic examples of bacteria are the thermophilic archaebacteria, that thrive at temperatures between 70°-105°C, in sulfur-rich hot-springs and mud holes. (So anachronistic are these single-celled organisms, that they are sometimes taxonomically classified as a distinct Kingdom.) It appears that the dimensions of the protein layers in species of these bacteria, Solfolobus solfataricus, are in "precise epitaxial coincidence" with the lattice parameters of a bicontinuous cubic phase, formed in excess water with the membrane lipids predominant in this organism in vitro) [140]. Such a coincidence is indeed difficult to reconcile with the usual notion of a flat, neutral, cytomembrane, whose sole function is to support the real stuff of life, the proteins. 

---