Fatty acids general discussion

Gas liquid chromatography is still the method of choice for the routine separation and tentative identification of common milk fatty acids, as well as for the resolution of the less abundant and less common acids. Although several hundred fatty acids are listed here and elsewhere as being present in milk, we remind the reader that not all of these have been rigorously identified. Some of the pitfalls in qualitative and quantitative GLC of milk fatty acids are discussed by Jensen et al. (1967) and those of fatty acids in general by Ackman (1980). 

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. 

We Umit this section to a discussion of stereochemical studies that sought to demonstrate discriminating enantiomeric interactions in monolayers of simple surfactants having one hydrophobic chain of methylenes and, generally, a single chiral center. Work in this area includes derivatives of long chain fatty acids, alcohols, or esters whose chiral center is included in the methylene chain. 

Fatty acids are carboxylic acids containing an unbranched carbon chain and usually an even number of carbon atoms. Fatty acids do not occur freely in nature, but generally come from esters (esters are discussed later). A few common fatty acids and their sources are shown in Figure 15.12. Fatty acids are important in the production... 

Structural information of LB films has also been obtained from FTIR studies. In the carboxylate form of the fatty acid, the relative intensities of the vs(C02-) and va(C02-) signals are dependent on the orientation of the chain axis. The dipole moments of the vs(C02-) and va(C02 ) stretches are parallel to and perpendicular to the chain axis, respectively. In transmission mode the electric vector of the IR radiation interacts strongly with dipole moments parallel to the substrate. This means that in transmission mode the vs(C02-) will be most intense, and the va(C02-) the weakest, for films with the chain axis perpendicular to the substrate. The opposite is true for the FTIR-RA mode. There is general consensus that in M-FA films the chain axis is approximately perpendicular to the substrate while the protonated form of the acid after exposure to H2S has a tilt relative to the substrate. Further discussion of FTIR as an investigative tool into the reaction of M2+-FA films with dihydrogen chalcogenides is given in later sections. 

Two general classes of pheromone compound have been identified in moths, and these have some broad, although not uniform, associations with certain taxa. The polyene hydrocarbons and epoxides of various chain lengths are pheromones found in some subfamilies of the Geometridae and Noctuidae, and in the Arctiidae and Lymantridae (Millar, 2000). These compounds are probably derived from dietary Unoleic and linolenic acids. The other major class of pheromone compounds includes acetate, alcohols, and aldehydes, which are found in the Tortrici-dae, Pyralidae, Gelechiidae, Sessiidae, and Noctuidae. This class of compounds is derived from the insect s fatty acid synthesis pathway, with enzymatic modifications discussed above. Both classes of pheromone are broadly represented in the Noctuidae but are typically found in different subfamilies (Am et al., 1992,2003). 

Fatty acid chains may contain no double bonds—that is, be satu rated, or contain one or more double bonds—that is, be mono- or polyunsaturated. When double bonds are present, they are nearly always in the cis rather than in the trans configuration. (See p. 362 for a discussion of the dietary occurrence of cis and trans unsatu- rated fatty acids.) The introduction of a cis double bond causes the I tfatty acid to bend or "kink" at that position (Figure 16.3). If the fatly acid has two or more double bonds, they are always spaced at three carbon intervals. [Note In general, addition of double bonds decreases the melting temperature (Tm) of a fatty acid, whereas j increasing the chain length increases the Tm. Because membrane lipids typically contain LCFA, the presence of double bonds in some fatty acids helps maintain the fluid nature of those lipids.]... 

The most important use of neutron diffraction in the general field encompassed by this book is in the study of alternating layers of deuterated and undeuterated films. At the time of writing, three papers have appeared on this topic. They are by Buhaenko et al. [56], Grundy et al. [57] and Stroeve et at. [58]. In the latter study, alternate layers of deuterated and undeuterated fatty acids were deposited and studied by neutron diffraction. Subsequently this ordered structure was destroyed by thermal diffusion and the gradual loss of order was monitored. The ordered structure is only destroyed at temperatures well above ambient. Applications of neutron diffraction to the study of lipid films at the air/water interface will be discussed in Chapter 8. 

As mentioned, hydrolysis is the other important mechanism by which some lipids (glycerides and phosphoglycerides) degrade and can lead to a reduction in pH due to liberation of free fatty acids this was discussed in Chapter 10 (Part I Parenteral Application). This phenomenon is less important for oral formulations when compared to parenteral products, since the former generally have low amounts of water in the formulation. Hydrolysis could occur on storage if water is absorbed from or through the gelatin shell. 

Let us now briefly discuss the main enzymatic systems of mitochondria, which provide for primary dehydrogenation of substrates (tricarboxylic acid cycle dehydrogenase, fatty acid oxidation system, etc.) and proton and electron transfer from substrates to molecular oxygen (respiratory chain). For this purpose, let us use the general respiratory scheme (Figure 3.2), suggested by Lehninger [13]. 

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