Lamellar lipid-water structure

More commonly in lipids, however, a liquid-crystalline phase is formed in the presence of water, as shown in Fig. 8.9. Above a critical hydrocarbon chain melting temperature water penetrates the polar region and a lamellar lipid-water structure is formed with water layers alternating with lipid bilayers. 

Lamellar lipid-water mesophases (denoted as L or P) form alternating layers of lipid and water molecules. This quasi one-dimensional periodic structure exhibits diffraction patterns in the small-angle regime that are described by the equation... 

In binary mixtures of water, surfactants, or lipids the most common structure is the gyroid one, G, existing usually on the phase diagram between the hexagonal and lamellar mesophases. This structure has been observed in a very large number of surfactant systems [13-16,24—27] and in the computer simulations of surfactant systems [28], The G phase is found at rather high surfactant concentrations, usually much above 50% by weight. 

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

Lipid-water gel phases were previously regarded as metastable structures that are formed before separation of water and lipid crystals when the corresponding lamellar liquid crystal is cooled. New information on gel phases (see below) reveals that they can form thermodynamically stable phases with very special structural properties. This characteristic makes them as interesting as the lamellar liquid crystals from a biological point of view. 

Cubic lipid phases have a very much more complex architecture than lamellar and hexagonal phases. Their structural characteristics have been elucidated only very recently, and it has become clear that their subtleties are the key to a variety of biological problems. We will consider those subtleties in some detail. The three fundamental cubic minimal surfaces - the P-surface, the D-surface and the gyroid (or G-surface), introduced in Chapter 1, can all be foimd in cubic lipid-water phases. The lipid bilayer is centred on the surface with the polar heads pointing outwards. Water fills the labyrinth systems on each side of the surface. These cubic phases will be termed Cp, CD and CG/ respectively. It is likely that there are other more complex IPMS morphologies in cubic phases of lipid-water mixtures, as yet uncharacterised. 

Emulsifiers are necessary to allow water and lipids to combine. A surfactant is an amphiphilic molecule that has affinities for fats as well as water and that can be incorporated into lamellar lipid structures (e.g. cell walls). Surfactants increase the fluidity of the lipid structures by partitioning into the lipid membranes, as their lateral interactions with the membrane-forming lipids reduce the force of their attractive interaction. The mobility of the membrane lipids increases considerably in a similar manner to when a liquid crystal is converted into a gel. Finally, lipids can be seen to micellize or simply dissolve. Membranes lose their relative impermeability. See Figure 5.16. 

Non-lamellar lipid mesophases (Fig. 4) may also be identified by their characteristic small-angle diffraction pattern. The structure of the inverse hexagonal lipid-water mesophase (denoted as Hu) is based on cylindrical water rods, which are surrounded by lipid monolayers. The rods are packed in a two-dimensional hexagonal lattice with Bragg peaks positioned at... 

Two other structures, which together with the lamellar phase are the most important liquid-crystalline phases in lipid-water systems, are shown in Fig. 8.9. It should be pointed out that the classification of lipids into polar and non-polar is best defined from their interaction with water. Lipids which do not give lipid-water phases are thus non-polar whereas those forming aqueous phases are classified as polar lipids. 

It should be possible to incorporate polymer chains in various liquid crystalline systems which impose certain conformations on them. For example, if we have a lamellar phase of lipid + water, it may be possible to incorporate a hydrophilic polymer into the water layer, obtaining the structures in Fig. V.3. Assuming that the ternary system can be observed in certain phase diagrams, one might then crosslink the chains and wash out the lipid with suitable solvents. An unusual (and anisotropic) gel should result. ... 

In 1960, Luzzati et al. [3,4] found the so-called cubic phase between the hexagonal and lamellar phases in a lipid-water system. The cubic structure attributed to space group 1 3 consists of rods of finite length that join three by three to form two three-dimensional networks. 

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

Plants were probably the first to have polyester outerwear, as the aerial parts of higher plants are covered with a cuticle whose structural component is a polyester called cutin. Even plants that live under water in the oceans, such as Zoestra marina, are covered with cutin. This lipid-derived polyester covering is unique to plants, as animals use carbohydrate or protein polymers as their outer covering. Cutin, the insoluble cuticular polymer of plants, is composed of inter-esterified hydroxy and hydroxy epoxy fatty acids derived from the common cellular fatty acids and is attached to the outer epidermal layer of cells by a pectinaceous layer (Fig. 1). The insoluble polymer is embedded in a complex mixture of soluble lipids collectively called waxes [1], Electron microscopic examination of the cuticle usually shows an amorphous appearance but in some plants the cuticle has a lamellar appearance (Fig. 2). 

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