Lipid double layer

Further investigations were carried out at lipid double layers and at phospholipids of membranes. Lipid-lipid and lipid-protein interactions were recognized by diazirine labeling (79PNA2595). 

Peripheral membrane proteins only touch the surface of the membrane integral membrane proteins penetrate the lipid double layer - they are partly or fully embedded in it. 

Experiments with liposomes, which are more complex stmctures than micelles, point in the same direction. Liposomes consist of one (or several) lipid double layers (see Sect. 10.2), ordered concentrically around an aqueous interior. Blocher et al. (2000) carried out polycondensation reactions of amino acids and peptides using l-palmitoyl-2-oleoyl-.vn-glycero-3-phosphocholine liposomes hydrophobically activated amino acids gave chain lengths of up to 29. The linkage of dipeptides (e.g., H-Trp-Trp-OH) to give Trps-OH, i.e., an octapeptide, was also carried out using liposomes. Thus, a second possibility, as well as adsorption on mineral surfaces, was available for polymer formation via micelles and liposomes. 

Liposomes are the closest approach to biomembranes they are spherical structures having an aqueous interior and one or several lipid double-layers (fig. 8). 

We have formulated the reaction on the example of ubiquinone, also called coenzyme Q, which is particularly abundant in mitochondria. Several workers, furthermore, have proposed the participation of vitamin E quinone, of vitamin K (Martins), and — in plants—of plastoquinone. These quinones are grouped with the fat-soluble vitamins, but they will be discussed in Chapt. XIV because of their isoprenoid side chains. Their formulas will be shown there, too. Being lipids they are bound as mitochondrial lipoproteins or are at least associated with the lipoproteins. The quinones may be thought of as being incorporated in an orderly arrangement in the protein-lipid double layers of mitochondria. 

In certain tissues, fats are also structural components, although in relatively small amounts compared to the more important fat-like phosphatides, sterols, etc. And in most membrane material they participate in the construction of the protein-lipid double layers (Chapt. XIX-3). 

Double bonds are found in the lipids that compose cell membranes. Membranes are composed of complex mixtures of lipids and the complexity of the mixture enables membranes to produce their responsive properties. Membrane lipids are all amphiphilic and have a polar head group in combination with a lipophilic tail which causes in an aqueous medium the typical lipid double layer. Figure 2.19 shows some typical membrane lipids. The phospholipids are triesters of glycerol where Rj may be a saturated or unsaturated fatty add, R2 is usually an unsaturated fatty add and R3 is one of a number of polar head groups. The lipophilic fatty acid tails of the molecules are composed of saturated and unsaturated hydrocarbon chains between 14 and 24 carbons long. Unsaturated lipids may contain cis or trans double bonds. Unsaturated lipids tend to orientate themselves as shown for the trans monounsaturated fatty add shown in Figure 2.19. 

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