Lipid structures of the outer

Normal Lipid Structure of the Outer Stratum Corneum. 354... 

This chapter investigates alterations in the lipid structure of the outer SC that are induced by moisturizing ingredients and commercial moisturizing products. As a preface to this investigation, we also examine the normal variability in the lipid stmcture of the outer SC and how it is affected by factors such as age, level of visible dryness, and personal cleanser use. 

NORMAL LIPID STRUCTURE OF THE OUTER STRATUM CORNEUM... 

An improved understanding of the structure of the SC barrier is of interest for many reasons such as enhancing percutaneous penetration and, as discussed in this chapter, optimizing topical therapy for the treatment of dry or damaged skin. The results of this TEM work show that the lipid structure of the outer SC is quite variable. Typically, the intercellular spaces in the outer SC are considerably widened and filled with nonlamellar material. These data are consistent with earlier TEM studies13 14 and with an infrared spectroscopic study that found less structured lipids in the outer SC16 compared to the middle and inner regions. 

Since both the free and the bound forms of the lipoprotein are located exclusively in the outer membrane, there are two possible ways in which a superhelical assembly could interact with the outer membrane (1) The interaction could occur through the three fatty acids attached to the amino-terminal amino acid of the lipoprotein, as suggested by Braun. In this case, the hydrocarbon chains of the fatty acids stick out of the assembly and penetrate into the phospholipid bilayer of the outer membrane. Therefore, the protein part of the assembly protrudes from the inside surface of the outer membrane. This model would predict that the peptidoglycan layer should be at least 76 A apart from the outer membrane, which is not likely. (2) Alternatively, the whole assembled structure, with a height of 76 A, penetrates through the 75-A-thick outer membrane with hydrophobic interaction between the surface of the assembly and the lipid bilayer of the outer membrane. This arrangement is further stabilized by the three hydrocarbon chains at the amino-terminal end of the individual molecules, which could be flipped back over the helix and inserted into the bilayer (Fig. 14). In order to arrange the hydrocarbon chains as shown in Fig. 14, the side chains of two serine residues at the amino terminus are made to face upward, which makes the uppermost part of the assembly hydrophilic, as a part of the surface of the outer membrane. 

Fig. 14. Schematic illustration of the outer membrane structure. A superhelix made of six a-helices is shown to be inserted into the outer membrane and to span the full 75-A-thick membrane. The three hydrocarbon chains attached at the top of each molecule are flipped over, hanging down from the top, and are anchored in the lipid bilayer of the outer membrane. At the bottom (carboxyl-terminal ends of the lipoproteins) of the assembly, two molecules are linked to the peptidoglycan layer, as shown by small bars. The peptidoglycan layer is illustrated by rectangular blocks (for the glycan chains) and small bars (for the peptide portions) which are cross-linking the glycan chains. Phospholipids forming the lipid bilayer are shown by hydrophilic, open, circular heads and hydrophobic, hatched, long tails. Channel opening of 7- and 8-membered assemblies are also illustrated on the surface of the outer membrane.

The structure of the LH2 complex of R. acidophila is both simple and elegant (Figure 12.17). It is a ring of nine identical units, each containing an a and a P polypeptide of 53 and 41 residues, respectively, which both span the membrane once as a helices (Figure 12.18). The two polypeptides bind a total of three chlorophyll molecules and two carotenoids. The nine heterodimeric units form a hollow cylinder with the a chains forming the inner wall and the P chains the outer wall. The hole in the middle of the cylinder is empty, except for lipid molecules from the membrane. 

Fig. 7 Diagrammatic representation of the fluid mosaic model of the cell membrane. The basic structure of the membrane is that of a lipid bilayer in which the lipid portion (long tails) points inward and the polar portion (round head ) points outward. The membrane is penenetrated by transmembrane (or integral) proteins. Attached to the surface of the membrane are peripheral proteins (inner surface) and carbohydrates that bind to lipid and protein molecules (outer surface). (Modified from Ref. 14.)...

Figure 7.7 Structure of a generalized LPS molecule. LPS constitutes the major structural component of the outer membrane of Gram-negative bacteria. Although LPSs of different Gram-negative organisms differ in their chemical structure, each consists of a complex polysaccharide component, linked to a lipid component. Refer to text for specific details...

FIGURE 7-32 Bacterial lipopolysaccharides. (a) Schematic diagram of the lipopolysaccharide of the outer membrane of Salmonella ty-phimurium. Kdo is 3-deoxy-o-manno-octulosonic acid, previously called ketodeoxyoctonic acid Hep is L-glycero-D-mannoheptose AbeOAc is abequose (a 3,6-dideoxyhexose) acetylated on one of its hydroxyls. There are six fatty acids in the lipid A portion of the molecule. Different bacterial species have subtly different lipopolysaccharide structures, but they have in common a lipid region (lipid A), a core oligosaccharide, and an "O-specific" chain, which is the prin-... 

Figure 8-30 Structures of the lipopolysac-charides of the outer membrane of E. coli and S. typhimurium including the bilayer anchor lipid A. For structures of L-glycero-D-mannoheptulose, KDO, and colitose, see Fig. 4-15.

There is substantial history regarding the application of conventional vibrational spectroscopy methods to study the intact surface of skin, the extracted stratum corneum and the ceramide-cholesterol-fatty acid mixtures that constitute the primary lipid components of the barrier. The complexity of the barrier and the multiple phases formed by the interactions of the barrier components have begun to reveal the role of each of these substances in barrier structure and stability. The use of bulk phase IR to monitor lipid phase behavior and protein secondary structures in the epidermis, as well as in stratum corneum models, is also well established 24-28 In addition, in vivo and ex vivo attenuated total reflectance (ATR) techniques have examined the outer layers of skin to probe hydration levels, drug delivery and percutaneous absorption at a macroscopic level.29-32 Both mid-IR and near-IR spectroscopy have been used to differentiate pathological skin samples.33,34 The above studies, and many others too numerous to mention, lend confidence to the fact that the extension to IR imaging will produce useful results. 

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