Simple lipids, chemistry

Lipids are defined as organic substances soluble in solvents regarded as fat-like, i.e., chloroform, carbon tetrachloride, benzene, diethyl ether, petroleum ether, etc., and which are related actually or potentially to esters of the long-chain fatty acids. A simple classification scheme which illustrates the role of these fatty acids in lipid chemistry is given in Table VI. It can be seen that most combined lipids consist of esters with long-chain alcohols, sterols, vitamins or, most commonly, glycerol and its substituted derivatives, viz., RCOOM where R = alkyl chain and M is an alcoholic moiety. 

A second source of information in the flavor field is journals scientific as well as trade journals. While flavor research appears in many scientific journals, journals that are particularly well respected and focus on flavor (by this text s definition) include the J. of Agricultural and Food Chemistry (American Chemical Society), Flavour and Fragrance J. (John Wiley and Sons) and Perfumer and Flavorist (Allured Pub. Corp.). As noted, many other journals publish flavor research, but these journals are typically general in scope (e.g., Food Chemistry, Z. Lebens. Unters. Forschung, J. Science Food Agriculture and J. Food Science) or focus on a com-modity/discipline (e.g.. Food Engineering, Cereal Chemistry, J. Dairy Science, and Lipid Chemistry) where flavor may also be relevant. The majority of these journals are now online, so retrieval of articles is simple if one s library/company has a subscription to this service. 

By the second half of the nineteenth century German chemists had established a dominant position in analytical and synthetic organic chemistry. Various simple sugars and aminoacids were being isolated and characterized, as well as more complex plant products. Studies on the composition of blood and the properties of hemoglobin were also well under way. The composition of lipid-rich components and the order of the different units within complex macromolecules, such as proteins and nucleic acids, could not however be resolved by techniques then available. 

A number of cationic lipids have been prepared using solid-phase methods [147— 159]. Along with the well-known advantages that solid-phase chemistry provide (e.g. mass action, simple purification, compatibility with microwave synthesis [ 160, 161]), the main reason to use this approach is that it facilitates parallel synthesis of libraries of compounds, allowing potential structure activity relationships to be rapidly determined by the systematic modification of the cationic lipid structure per domain. 

Application of data obtained from simple clean reaction systems in biological or chemical studies of heme catalysis also has its problems. Chemical model systems use chelators, model hemes, and substrate structures that are quite different from those existing in foods. Reaction sequences change with heme, substrate, solvent, and reaction conditions. Intermediates are often difficult to detect (141), and derivations of mechanisms by measuring products and product distributions downstream can lead to erroneous or incomplete conclusions. It is no surprise, then, that there remains considerable controversy over heme catalysis mechanisms. Furthermore, mechanisms determined in these defined model systems with reaction times of seconds to minutes may or may not be relevant to lipid oxidation being measured in the complex matrices of foods stored for days or weeks under conditions where phospholipids, fatty acid composition, heme state, and postmortem chemistry complicate the oxidation once it is started (142). Hence, the mechanisms outlined below should be viewed as guides rather than absolutes. More research should be focused on determining, by kinetic and product analyses, which reactions actually occur and are of practical importance in specific food systems. 

Up until 1977, the non-covalent polymeric assemblies found in biological membranes rarely attracted any interest in supramolecular organic chemistry. Pure phospholipids and glycolipids were only synthesized for biophysical chemists who required pure preparations of uniform vesicles, in order to investigate phase transitions, membrane stability and leakiness, and some other physical properties. Only very few attempts were made to deviate from natural membrane lipids and to develop defined artificial membrane systems. In 1977, T. Kunitake published a paper on A Totally Synthetic Bilayer Membrane in which didodecyl dimethylammonium bromide was shown to form stable vesicles. This opened the way to simple and modifiable membrane structures. Since then, organic chemists have prepared numerous monolayer and bilayer membrane structures with hitherto unknown properties and coupled them with redox-active dyes, porous domains and chiral surfaces. Recently, fluid bilayers found in spherical vesicles have also been complemented by crystalline mono-... 

It can be argued that this catalytic effect is only due to the increased solubility of both substrate and peptide catalyst due to the presence of the lipid bilayer. This may well be so, but it does not decrease at all the importance of the observation the presence of supramolecular aggregates brings about a significant catalytic effect that otherwise would have been not present. This hydrophobic catalysis can be obtained by simple physical means, without any enzymatic magic. Therefore we may have exerted an important role in the early chemistry. 

Folch J., Lees M., Stanley G.H.S., A Simple Method for the Isolation and Purification of Total Lipides from Animal Tissues, Journal of Biological Chemistry 226 (1957) 497-509. 

Karlsson, K.-A. 1970. On the chemistry and occurrence of sphingolipid long-chain bases. Lipids 5 6-43. Goni, F.M., Alonso, A. 2006. Biophysics of sphingolipids. I. Membrane properties of sphingosine, ceramides and other simple sphingolipids. Biochim. Biophys. Acta 1758 1902-1921. 

The term structure, as used in chemistry and biology, relates both to molecules and to molecular aggregates. A simple example is provided by water. We can deal with the structure of the H2O molecules and also with the way in which these molecules are associated in solid ice and in liquid water. A more complicated example is a biological membrane, where we face the problem of the molecular structures of protein, lipid, and other molecules, and also of the way these molecules are aggregated in the membrane. In this chapter we are concerned with both of these problems, but more attention is given to the structures of individual molecules. We deal with some of the experimental methods used for investigating structure and with some of the structural information which has been accumulated. 

It is by no means necessary to use natural lipids in order to form membranes. Modem bioorganic chemistry rather tends to develop new molecules, which allow production of membrane materials with properties unknown in nature (e.g., ultrathin asymmetrical membranes with different headgroups on the in- and outsides, polymeric membranes, and membranes that can be isolated and stored without water see Sec. 2.5). Table 2.2.5 reproduces a few useful artificial amphiphiles derived from simple fatty acids and fatty alcohols. 

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