Lipids phase behavior

In aqueous solution, single lipids behave as lyotropic liquid crystals, forming a variety of different phases. We can observe the micellar, lamellar, and hexagonal lyotropic phases as a function of lipid concentration, although micellar phases are less likely in a system with two flexible chains (due to their cylindrical shape see Section 3.6). Alternatively, lipids at low concentrations will tend to form bilayer shells, either multilamellar or unilamellar vesicles. These phases correspond directly to the surfactant phases that we discovered in Chapter 3. Lipid molecules are composed 

FIGURE 6.3 Molecular structures for some examples of common lipids found in the human body top, sphingomyelin, [(2S,3R,4E)-2-acylaminooctadec-4-ene-3-hydroxy-l-phosphocholine] middle, a phospholipid with one polyunsaturated chain (DHA) and bottom, cholesterol, a sterol also classified as a lipid. 

FIGURE 6.4 Cartoon models depicting the organization of lipids in cross section in aqueous solution for (a) a lipid bilayer and (b) the hexagonal phase. Individual lipids are shown in a simplified form here as a hydrophilic head group with two hydrophobic tails.  

In addition to their lyotropic properties, Hpids can also behave as thermotropic liquid crystals in a similar fashion to the rod-like liquid crystal molecules described in Chapter 2. In aqueous solution, the primary organization of the lipid phase is lyotropic however, the internal structural organization of the bilayer will vary as a function of temperature. In thinking about these thermotropic Hpid phases, it is helpful to consider the lipid to be in the lamellar phase, then to vary the temperature of this phase and look at the effects of temperature on molecular ordering within the bilayer. Lyotropic organization itself is also dependent on temperature, but more weakly so. For the following discussion, assume that the Hpids are organized in a lamellar phase composed of stacked Hpid bilayers. 

FIGURE 6.5 Cartoon model demonstrating the differences in chain packing between lipid bilayers in the gel phase Lp- (left) and the liquid crystalline phase Lg (right). Cross sections only are shown. 

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M. W. De Jager, G. S. Gooris, M. Ponec, and J. A. Bouwstra. Lipid mixtures prepared with well-defined synthetic ceramides closely mimic the unique stratum corneum lipid phase behavior. J. Lipid Res. 46 2649-2656 (2005). 

This article discusses some micellar and liquid crystalline phases with nonionic substances, water, and hydrocarbons and some factors are delineated for their association phenomena. Lipid phase behavior has an extremely important direct influence on certain biological phenomena (Chapter 10) and is treated in Chapter 4. The treatment here is limited... 

Koynova R, Brankov J, Tenchov B (1997) Modulation of lipid phase behavior by kosmo-tropic and chaotropic solutes - experiment and thermodynamic theory. Eur Biophys J Biophys Lett 25 261-274... 

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. 

Each of the studies mentioned so far is fundamentally correlative, with lipid-phase behavior and water-loss rates being measured with different animals. Lipid composition and physical properties can vary substantially within species, so a close correlation between these parameters is not necessarily expected. One possible explanation is a file drawer problem of the type alluded to by Lighton (1998) results that conflict with the... 

We know the most about cuticular hydrocarbons, because they are abundant and because it is relatively easy to isolate and identify them. They are also the most hydrophobic lipid components, and so should provide the best barrier to water-loss. -Alkanes isolated from insect cuticles typically have chain lengths of 20-40 carbons. These can be modified by insertion of cis double bonds, or addition of one or more methyl groups. Relatively polar surface lipids include alcohols, aldehydes, ketones and wax esters (see Chapter 9). Given this diversity, is it possible to predict lipid phase behavior (and, by extension, waterproofing characteristics) from composition alone If so, a large body of literature would become instantly interpretable in the context of water balance. Unfortunately, this is not the case. 

To better understand the structure, function, and dynamics of the endogenous lipid matrix of the stratum corneum intercellular space some general principles of lipid phase behavior, dynamics, and structural organization may represent a useful starting point. Further follows a short overview of some basic physico-chemical principles that may be of relevance for stratum corneum lipid research, followed by a presentation of the new technique cryo-transmission electron microscopy of fully hydrated vitreous skin sections and how this technique recently has been applied to the study of the structural organization and formation of the lipid matrix of the stratum corneum intercellular space. 

Humphrey, K.L., and Narine, S.S. (2005). Lipid phase behavior. In Fat Crystal Networks, A.G. Marangoni (ed.). Marcel Dekker, New York. 

Krill, S. L., Knutson, K. and Higuchi, W. I. Ethanol effects on the stratum comeum lipid phase behavior. Biochimica et Biophysica Acta 7772(2) 273-280, 1992. 

Skin stratum corneum lipid phase behavior... 

Changes in pH modulate the lipid phase behavior as a consequence of protonation/deprotonation of the lipid headgroups, which results in a change of the surface charge of the membrane (77). They also modify the surface polarity and hydration. Typically, protonation decreases lipid hydration and increases the main transition temperature (53). The effects of pH titration on the chain-melting transition temperature of dimyristoyl phospholipids is illustrated in Fig. 3e, which shows that single protonation increases the melting transition temperature by about 5-15° C. 

LIPID PHASE BEHAVIOR 2.1. Nature of the Liquid Phase... 

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