Compounds Formed from Amino Acids

Amino acids may also undergo thermal degradation, which is almost always coupled with some other food components, particular sugars. The major types of volatile compounds formed from amino-sugar interactions include Strecker degradation aldehydes, alkyl pyrazines, alkyl thiazolines and thiazoles and other heterocycles [35, 36]. As the subject has mainly relevance for baked and roasted vegetable food products, this subject will not be discussed in further detail. 

Finally amino acids are precursors for some branched aliphatic compounds such as 2-methyl-1-butanol and 3-methyl-1 -butanol that are formed during the amino acid catabolism [20]. 

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Proteins are large compounds formed from amino acids linked together. 

The naturally occurring amino acids are very polar, and cannot be separated as the free compounds by GC at a temperature below decomposition. If the polar groups in the molecule are chemically modified to produce a more volatile derivative a suitable temperature is then possible. Weinstein (25) reviews all the various derivatives which may be formed from amino acids and the GC conditions necessary to separate them. In actual practice only three derivatives are in popular use. These include the N-heptafluorobutyryl n-propyl ester derivatives, the N-trimethyl-silyl ether derivatives, and the n-trifluoroacetyl n-butyl ester derivatives. 

Knowledge about the chemical structure of the antioxidative MRP is very limited. Only a few attempts have been made to characterize them. Evans, et al. (12) demonstrated that pure reductones produced by the reaction between hexoses and secondary amines were effective in inhibiting oxidation of vegetable oils. The importance of reductones formed from amino acids and reducing sugars is, however, still obscure. Eichner (6) suggested that reductone-like compounds, 1,2-enaminols, formed from Amadori rearrangement products could be responsible for the antioxidative effect of MRP. The mechanism was claimed to involve inactivation of lipid hydroperoxides. 

Proteins are ubiquitous nitrogen-bearing polymeric molecules formed from amino acids. Thus, not only do they constitute hormones and the myriad enzymes and enzyme inhibitors that control all body reactions including cell metabolism, but they also constitute such substances as antigens, which serve to stimulate the immune system. Not only are proteins involved in the light against already-established cancers, but in the body s resistanee to cancer in the first place. Nitrogen compounds, it seems, are everywhere, for better or worse. 

Fusel oil unpleasant-tasting side product of alco-hohc fermentation, consisting mainly of amyl, isoamyl, isobutyl and propyl alcohols. The compounds are formed from amino acids, in particular leucine, isoleucine and tyrosine by deamination and decarboxylation. Tyrosol, which is formed from tyrosine, is a component of beer. 

Amino acids react with carbonyl compounds, forming azomethines. If the carbonyl compound has an electron-withdrawing group, e.g., a second carbonyl group, transamination and decarboxylation occur. The reaction is known as the Strecker degradation and plays a role in food since food can be an abundant source of dicarbonyl compounds generated by the Maillard reaction (cf. 4.2.4.4.7). The aldehydes formed from amino acids Strecker aldehydes) are aroma compounds (cf. 5.3.1.1). The ninhydrin reaction is a special case of the Strecker degradation. It is an important reaction for the quantitative determination of... 

Biogenic amines are formed from amino acids by the action of carboxy-lyases (decarboxylases containing as a cofactor pyridoxal 5 -phosphate), or arise from amino adds and carbonyl compounds by the action of transaminases (see Section 8.2.10.1.2). The so-called endogenous biogenic amines are the products of metabohsm and at low concentrations are natural components of almost aU foods. Exogenous biogenic amines are formed in foods as a result of microbial contamination and fermentation processes. 

A potentially toxic alkaline gas which is formed from amino acids or urea by (1) intestinal bacteria, or (2) metabolic activities of cells. Normally, the accumulation of ammonia in the body is prevented by enzymes which convert the ammonia to safer compounds such as urea or amino acids. However, ammonia intoxication is a common occurrence in certain liver diseases. 

Aldehydes account for about 2% of the relative chromatographic peak area in both series of extracts [32]. Short chain aldehydes can be formed from amino acids during cheese ripening via Strecker degradation [34]. In DHS samples, linear and branched-chain saturated aldehydes as well as two aromatic derivatives were identified. Unsaturated aldehydes were isolated in the SDE samples, together with long-chain compounds, such as tetradecanal, pentadecanal, and hexa-decanal. Long-chain aldehydes derive from fatty acids by an a-oxidation mechanism [35]. In the SDE extracts of all 21 samples analyzed, mass spectral data... 

Polar organic compounds such as amino acids normally do not polymerize in water because of dipole-dipole interactions. However, polymerization of amino acids to peptides may occur on clay surfaces. For example, Degens and Metheja51 found kaolinite to serve as a catalyst for the polymerization of amino acids to peptides. In natural systems, Cu2+ is not very likely to exist in significant concentrations. However, Fe3+ may be present in the deep-well environment in sufficient amounts to enhance the adsorption of phenol, benzene, and related aromatics. Wastes from resinmanufacturing facilities, food-processing plants, pharmaceutical plants, and other types of chemical plants occasionally contain resin-like materials that may polymerize to form solids at deep-well-injection pressures and temperatures. 

Alkaloids are compounds that contain nitrogen in a heterocyclic ring and are commonly found in about 15-20% of all vascular plants. Alkaloids are subclassified on the basis of the chemical type of their nitrogen-containing ring. They are formed as secondary metabolites from amino acids and usually present a bitter taste accompanied by toxicity that should help to repel insects and herbivores. Alkaloids are found in seeds, leaves, and roots of plants such as coffee beans, guarana seeds, cocoa beans, mate tea leaves, peppermint leaves, coca leaves, and many other plant sources. The most common alkaloids are caffeine, theophylline, nicotine, codeine, and indole... 

True alkaloids derive from amino acid and they share a heterocyclic ring with nitrogen. These alkaloids are highly reactive substances with biological activity even in low doses. All true alkaloids have a bitter taste and appear as a white solid, with the exception of nicotine which has a brown liquid. True alkaloids form water-soluble salts. Moreover, most of them are well-defined crystalline substances which unite with acids to form salts. True alkaloids may occur in plants (1) in the free state, (2) as salts and (3) as N-oxides. These alkaloids occur in a limited number of species and families, and are those compounds in which decarboxylated amino acids are condensed with a non-nitrogenous structural moiety. The primary precursors of true alkaloids are such amino acids as L-ornithine, L-lysine, L-phenylalanine/L-tyrosine, L-tryptophan and L-histidine . Examples of true alkaloids include such biologically active alkaloids as cocaine, quinine, dopamine, morphine and usambarensine (Figure 4). A fuller list of examples appears in Table 1. 

Reduction of a S-aminoketone resulting from the addition of an equivalent of a glycinate anion on ethyl difluoro- or trifluoroacetate is stereoselective and leads to ethyl di- or trifluorothreoninate threo (syn). Release of the acid, performed by saponification, is accompanied by a partial epimerization into an alio compound. However, the amino acids are obtained in enantiopure forms by using a lipase. . It s important to note that (25, 35)-difluorothreonine exhibits activity toward the growth of leukemia cell hnes comparable to 5-fluorouracil. ... 

In addition to the 20 commonly occurring a-amino acids, a variety of other amino acids are found in minor amounts in proteins and in nonprotein compounds. The unusual amino acids found in proteins result from modification of the common amino acids. In a few cases these amino acids are incorporated directly into the polypeptide chains during synthesis. Most frequently the amino acid is modified after incorporation. The unusual amino acids found in nonprotein compounds are extremely varied in type and are formed by a number of different metabolic pathways (see chapter 21). 

Several of the smaller volatile compounds formed from the catabolism of products of primary proteolysis (e.g., amino acids) can be determined by GC. The development of capillary columns and interfacing GC with MS has noticeably increased the sensitivity of this analysis. Over 200 volatile compounds have been identified in Cheddar cheese. A list of several of these compounds can be found elsewhere (Fox et ah, 2004a Singh et ah, 2003). The instrumental techniques available for the characterization of cheese aroma were also discussed recently (Le Quere, 2004 Singh et al., 2003). 

Proteins are polymers produced from amino acids that are joined by peptide linkages (amine-group carboxylic acid bonds). These compounds form the bulk of living tissue. Enzymes, those very selective and powerful catalysts, are also proteins. Enzymes may contain a group based on a metal atom, as may molecules from other classes of compounds (hemoglobin, chlorophyll, etc.). Some proteins serve special functions, such as hemoglobin, an oxygen carrier. 

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