Decomposition of Amino Acids

Studies of model systems have revealed that protein cleavage and degradation of side chains, rather than formation of protein networks, are the preferred reactions when the water content of protein/lipid mixtures decreases. Several examples of the extent of losses of amino acids in a protein in the presence of an oxidized lipid are presented in Table 3.37. The strong dependence of this loss on the nature of the protein and reaction conditions is obvious. Degradation products obtained in model systems of pure amino acids and oxidized lipids are described in Table 3.38. 

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Lee, C., and C. Cronin. 1982. The vertical flux of particulate organic nitrogen in the sea decomposition of amino acids in the Peru upweUing area of the equatorial Atlantic. Journal of Marine Research 40 227-251. 

E. J. Mclntee, R. P. Remmel, R. F. Schinazi, T. W. Abraham, C. R. Wagner, Probing the Mechanism of Action and Decomposition of Amino Acid Phosphomonoester Amidates of Antiviral Nucleoside Prodrugs , J. Med. Chem. 1997, 40, 3323-3331. 

Fig. 4.5. Rhodium(II) acetate-catalyzed decomposition of amino acid-derived diazoamides...

Amides and Nitriles. Primary amines or ammonia, from the thermal decomposition of amino acids, can be acylated by carboxylic acids to produce amides (30). 

The major precursors in meat flavors are die water-soluble components such as carbohydrates, nucleotides, thiamine, peptides, amino acids, and the lipids, and Maillard reaction and lipid oxidation are the main reactions that convert these precursors in aroma volatiles. The thermal decomposition of amino acids and peptides, and the caramelization of sugars normally require temperatures over 150C for aroma generation. Such temperatures are higher than those normally encountered in meat cooking. During cooking of meat, thermal oxidation of lipids results in the formation of many volatile compounds. The oxidative breakdown of acyl lipids involve a free radical mechanism and the formation of... 

It is probable that the oxyacids formed in die organism come from the decomposition of amino-acids, in consequence of the intervention of a special amidase. 

Another important reaction is decomposition of amino acid derived ketosamines in the presence of oxygen, which is catalysed by transition metal ions. Products of this decomposition are glycos-2-uloses. For example, n-fructosamine yields D-arahino-hexos-2-ulose and in addition to other products, this reaction produces the corresponding Strecker aldehyde (Figure 4.88). 

To study the photocatalytic decomposition of amino acids by TiOi, Dolamic and Buergi modulated UV light on and off while flowing l-G1u and L-Asn over the catalyst surface in an ATR-IR cell [22], The demodulation of the data with PSD removed any signals not oscillating with the UV light frequency, allowing them to observe cyanide on the catalyst surface, which was not observed in other more conventional studies. 

The melting points of the derivatives of a number of amino acids are collected in Table 111,132. Most a-amino acids decompose on heating so that the melting points would be more accurately described as decomposition points the latter vary somewhat with the rate of heating and the figures given are those obtained upon rapid heating. 

The deterruination of amino acids in proteins requires pretreatment by either acid or alkaline hydrolysis. However, L-tryptophan is decomposed by acid, and the racemi2ation of several amino acids takes place during alkaline hydrolysis. Moreover, it is very difficult to confirm the presence of cysteine in either case. The use of methanesulfonic acid (18) and mercaptoethanesulfonic acid (19) as the protein hydroly2ing reagent to prevent decomposition of L-tryptophan and L-cysteine is recommended. En2ymatic hydrolysis of proteins has been studied (20). 

Amino acids have high melting or decomposition points and are best examined for purity by paper or thin layer chromatography. The spots are developed with ninhydrin. Customary methods for the purification of small quantities of amino acids obtained from natural sources (i.e. l-5g) are ion-exchange chromatography (see Chapter 1). For general treatment of amino acids see Greenstein and Winitz [The Amino Acids, Vols 1-3, J.Wiley Sons, New York 1961] and individual amino acids in Chapters 4 and 6. 

The crude product is dissolved in five times its weight of water, and after clearing with a little Norite the solution is diluted with one and one-half volumes of 95 per cent alcohol. The product separates in well-formed, snow-white crystals, and after standing for several days in an ice chest is collected with suction on a Buchner funnel. The yield of purified histidine monohydrochloride is 75-80 g. (Note 5). The compound melts at 251-2520, with decomposition. The amino acid is not race-mized by the procedure employed, and shows the characteristic optical activity, [a]n6° = +8.00, in the presence of three moles of... 

Fragmentation of amino acid-derived spirophosphorane 128 has been analyzed using field desorption (FD), El, and Cl mass spectrometry <1997RCM1825, 1997CCL629>. In spiro-crypta cyclophosphazene derivatives 129, the major decomposition pathway involved the initial cleavage of a P-Cl bond rather than cleavage of an exocyclic P-N bond as is normally seen for cyclophosphazenes <2004JST139>. 

Amino Acid Dating Techniques depend on the "rates of hydrolysis reactions of proteins and racemization, epimerization, and decomposition reactions of amino acids [they have] been applied to the age-dating of fossil bone, teeth, and shell. Activation energies range from near 20 kcal per mole for hydrolysis reactions to around 30 kcal per mole for racemization... 

The effect of temperature on the rate of racemization of amino acids in fossils was investigated and the implications of the findings on fossil dating were analyzed313. The high rate of conversion of L-aspartic acid into its D-isomer, observed in uncontaminated bone samples taken from catacombs in Rome (IV century BC) was attributed to collagen decomposition due to the humidity of the catacombs314. 

As to the origins of the major N compounds identified, it is possible that at least a portion of some of these compounds are pyrolysis products of amino acids, peptides, proteins, [18] and porphyrins (a component of chlorophyll), [19] or originate from the microbial decomposition of plant lignins and other phenolics in the presence of ammonia. [20] Of considerable interest are the identifications aromatic and aliphatic nitriles. Nitriles can be formed from amines with the loss of 2 H2, from amides with the loss of H20, and also by reacting n-alkanoic acid with NH3. [21] The detection of long-chain alkyl- and dialkyl-nitriles points to the presence in the soil or SOM of long-chain amines... 

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