Hydrocarbon tails of fatty acids

Cholesterol modifies the fluidity of membranes. At temperatures below the T, it interferes with the interaction of the hydrocarbon tails of fatty acids and thus increases fluidity. At temperatures above the T, , it limits disorder because it is more rigid than the hydrocarbon tails of the fatty acids and cannot move in the membrane to the same extent, thus timiting fluidity. At... 

FIGURE 8.12 The effect of double bonds on the conformations of the hydrocarbon tails of fatty acids. Unsaturated fatty acids have kinks in their tails. 

The long hydrocarbon tails of fatty acids make them insoluble in water. Table 19.1 lists several different fatty acids and some common sources for each. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

SFC has played an important role in the extraction and isolation of fatty acids [355,356]. Underivatised fatty acids and methyl esters of fatty acids are surprisingly easy to elute using a bonded phase or a silica based packed column and pure C02, probably due to the long hydrocarbon tails on the molecules [357]. On the other hand, most aromatic and polysubstituted acids will not elute. Triglycerides with saturated fatty acids can be analysed faster with pSCF-ELSD than with GC-FID and do not require sample preparation [358]. Using... 

Figure 2. Simplified diagram shows packing patterns of fatty acids in the solid phase, (a) and (b) Hydrocarbon tails (straight lines) aligned at different angles to the line of the polar head groups (circles), (c) Head to tail packing, (d) Head to head packing.

Wood is the raw material of the naval stores industry (77). Naval stores, so named because of their importance to the wooden ships of past centuries, consist of rosin (diterpene resin acids), turpentine (monoterpene hydrocarbons), and associated chemicals derived from pine (see Terpenoids). These were obtained by wounding the tree to yield pine gum, but the high labor costs have substantially reduced this production in the United States. Another source of rosin and turpentine is through extraction of old pine stumps, but this is a nonrenewable resource and this industry is in decline. The most important source of naval stores is spent sulfate pulping liquors from kraft pulping of pine. In 1995, U.S. production of rosin from all sources was estimated at under 300,000 metric tons and of turpentine at 70,000 metric tons. Distillation of tall oil provides, in addition to rosin, neady 128,000 metric tons of tail oil fatty acids annually (78). 

The spreading of oil on the surface of water has been known since the eighteenth century and has been the subject of scientific investigation for over 100 years. Shortly before the turn of the century, Lord Rayleigh proposed that films consisted of monolayers of spherical molecules two decades later Langmuir and Harkins independently recognized that the formation of a monolayer and its structure were associated with the amphiphilic and rod-like nature of the molecules. They proposed, for example, that in monolayers of fatty acids, the carboxylic head group is immersed in the water surface while the hydrocarbon tail remains above the surface. 

The focus will be mainly on monolayers of fatty acids or alcohols and of phospholipids, because these are the simplest systems and have been the subject of the most recent research. These substances are similar in that their hydrophobic tails consist of long-chain hydrocarbons there are two chains per phospholipid and one per acid or alcohol, as shown in Table I. The chain lengths typically range from 13 to 20 carbons, the lower limit being... 

The fluid mosaic model of membrane structure pictures biological membranes that are composed of lipid bilayers in which proteins are embedded. Membrane lipids contain polar head groups and nonpolar hydrocarbon tails. The hydrocarbon tails of phospholipids are derived from saturated and unsaturated long-chain fatty acids containing an even number of carbon atoms. The lipids and proteins diffuse rapidly in the lipid bilayer but seldom cross from one side to the other. 

Among the diverse components of biomembranes, lipids are essential in structural aspects. Lipids are defined operationally as derivatives of fatty acids and their metabolites. As lipids are usually amphiphilic molecules with hydrophobic hydrocarbon tails and hydrophilic head groups (as shown in Figure 1), the bilayer structures of biomembranes are held with hydrophobic forces to the tails and heads of lipids. Another major component of biomembranes is proteins. The weight proportion of proteins in biomembranes is often more than that of lipids. As proteins are more rigid than lipid assemblies, specific interactions by biomembranes are often related to proteins. Further, the sterols contained in biomembranes play unique roles in their apolar regions. As the functionality of biomembranes arises from the diversity of their composition, various kinds of molecules have been studied for biomembrane modeling. 

An essential component of cell membranes are the lipids, lecithins, or phosphatidylcholines (PC). The typical ir-a behavior shown in Fig. XV-6 is similar to that for the simple fatty-acid monolayers (see Fig. IV-16) and has been modeled theoretically [36]. Branched hydrocarbons tails tend to expand the mono-layer [38], but generally the phase behavior is described by a fluid-gel transition at the plateau [39] and a semicrystalline phase at low a. As illustrated in Fig. XV-7, the areas of the dense phase may initially be highly branched, but they anneal to a circular shape on recompression [40]. The theoretical evaluation of these shape transitions is discussed in Section IV-4F. 

Figure 41-3. Diagrammatic representation of a phospholipid or other membrane lipid. The polar head group is hydrophilic, and the hydrocarbon tails are hydrophobic or lipophilic. The fatty acids in the tails are saturated (S) or unsaturated (U) the former are usually attached to carbon 1 of glycerol and the latter to carbon 2. Note the kink in the tail of the unsaturated fatty acid (U), which is important in conferring increased membrane fluidity.

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