Pyrolysis of amino acids

The amino acid pyrolysis is relevant for protein pyrolysis because certain compounds in the pyrolysate are the same when the substance to be pyrolysed is the amino acid or a peptide formed from that specific amino acid (see Section 13.2). The main pyrolysis products of several amino acids are given in Table 12.1.2 [3,4]. 

The mechanisms of generating the compounds shown in Table 12.1.2 were already discussed in Section 3.5. The main mechanisms are the decarboxylation by CO2 elimination or the water elimination with the formation of a dipeptide and further of the diketopiperazines. By Strecker degradation, amino acids may also be converted to aldehydes. 

Other reactions are also possible. For example, R-H type compounds may be generated, and this is characteristic mainly for the aromatic amino acids. Smaller molecules, some unsaturated, are also formed by further decompositions. The compounds of the type R-CH=N-CH2-R and R-C=N-CH2-R are formed mainly from aliphatic amino acids (Ala, Val, Leu, lie). 

Besides the determination of major pyrolysis products for amino acids, a special issue is the formation of several mutagenic compounds (heterocyclic amines) during pyrolysis. These types of compounds were detected in traces in the pyrolysates of amino acids, and the finding is very important as the amino acids are components of proteins and are present in food. Some of these compounds isolated from pyrolysates performed at 550 C from several amino acids [5,6] are shown below  

3-amino 1.4-dimelhyl-5H-pyrido[4.3 b]indole (from triptophan), Trp-P-1 

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G. Chiavari, D. Fabbri and S. Prati, Gas chromatographic mass spectrometry analysis of products arising from pyrolysis of amino acid in the presence of hexamethyldisilazane, J. 

Thus, by the Maillard reaction in different browning systems of sugars and amino compounds, some mutagenic substances were formed, although their activities are quite weak compared with those formed by pyrolysis of amino acids. They were confirmed as intermediates and some of them were identified as furan, pyrrole, or thiazolidine derivatives formed from glucose and amino acids... 

Heterocyclic aromatic amines (HAA) are carcinogenic compounds that may occur in food. They are probably formed during cooking processes by the pyrolysis of amino acids and proteins. Since the mid 1990s, LC-MS plays a role in the analysis and characterization of these HAA. Both ESI-MS and APCl-MS are applied. A recent special issne of Journal of Chromatography B [82] emphasizes the analytical challenges related to HAA. The stractnres of the most abundant HAA in cooked food are shown in Figure 14.10. 

Pyrolysis of amino acids compared to ion fragments formation. 

Ouweland et al., 1978) or directly by the pyrolysis of amino acids (Fujimaki et al., 1969). Another important, if not the main, precursor of pyridines in roasted coffee is trigonelline (see Section 2.1.1.2), a product isolated by Goi ter (1910), identical to the product isolated from the seeds of Trigonella foenum-graecum. Viani and Horman (1974, 1976) identified 12 pyridinic compounds after pyrolysis of trigonelline, six of which have now been identified in roasted coffee (4-methylpyridine is noted as identified in coffee, but it is not present in the lists of quoted publications, and to our knowledge its identification has not yet been reported in the literature). The presence of four other alkyl derivatives and of two N-methylnicotinamides have not yet been confirmed in the flavor. The authors have also isolated two piperidylpyridines, 3-phenylpyridine and two of its methyl derivatives, as well as four unsubstituted and dimethyl-substituted dipyridyl compounds. 

Higman et al. (1647) also reported the generation of PAHs, phenols, pyridines, indole, quinoline, and other aromatic bases during the pyrolysis of amino acids and proteins from tobacco [see the review on pyrogenesis of smoke components by Chortyk and Schlotzhauer (722)]. The pyrolysis results reported by Higman et al. are summarized in Table IV.B-3. 

Pyrolysis and smoke studies of amino acids indicate that they are potential precursors of several nitrogen heterocyclic ring systems found in tobacco smoke. Proline has been shown to be efficiently converted to pyrrole upon pyrolysis (3219, 3724) and in addition has been proposed as a possible precursor of pyrocoll (2593, 4336). y-Amino acids and dicarboxy-lic amino acids are capable, under pyrolytic conditions, of forming 2-pyrrolidones (1967, 3079) and by a similar mechanism, A-amino acids can form 2-piperidones (1967, 3079). A side effect of the pyrolysis of amino acids is the formation of hydrogen cyanide. 

Johnson,W.R., J.C. Kang, andH.Wakeham Mechanisms of HCN formation from the pyrolysis of amino acids... 

Masuda, Y, K. More, and M. Kuratsune Studies on bladder carcinogens in the human environment. I. Naphthylamines produced by pyrolysis of amino acids ... 

Heckman Literature study of pyrolysis of amino acids 3240a. 

Smith, W.T. Jr and J.M. Patterson Pyrolysis of amino acids Proceedings, University of Kentucky Tobacco and Health Workshop (1969) 38. 

The CYP1A subfamily plays an integral role in the metabolism of two important classes of environmental carcinogens, PAHs, and arylamines (Tables 10.3 and 10.4) (22). The PAHs commonly are present in the environment as a result of industrial combustion processes and in tobacco products. Several potent carcinogenic arylamines result from the pyrolysis of amino acids in cooked meats and can cause colon cancer in rats. Environmental and genetic factors can alter the expression of this subfamily of these enzymes. 

Other products that form from the pyrolysis of amino acids are aldehydes, which contain one less carbon atom than the parent amino acid. This process occurs via an SNi deamination mechanism. Another process that can occur is side chain stripping involving chain homolysis. The result is the production of various saturated and unsaturated compounds. Many alpha-amino acids, when pyrolyzed, form an amine fragment, a nitrile fragment, and aldehydes that contain one less carbon atom than the parent amino acid. Also, a side stripping mechanism can result in various saturated and unsaturated compounds. ... 

Understanding the product formation from the pyrolysis of amino acids is the first step in comprehending pyrolysis behavior for more complex systems, such as proteins and nucleic acids, which comprise a large quantity of the material in biological systems. Later in this chapter, in Section 10.5, proteins are discussed in more detail. 

In view of their chemical structure, they can be classified into two main groups called thermic HCAs, IQ-type or amino-imidazoazaarenes and pyrolytic HCAs, non-lQ-type or amino-carbolines. Those of the first type are produced by Maillard reaction when mixtures of creat(ni)ne, amino acids, and sugars are heated at temperatures between 100°C and 300°C [32]. Those of the second type are mainly formed by the pyrolysis of amino acids and proteins at higher temperatures, above 300°C... 

The thermally-driven condensation polymerization of amino acids, as initially described by Harada and Fox (27,28) and summarized in the preceding section, has been criticized on account of the apparent side reactions that occur dirring the melt. This criticism is sutmnarized by Wills and Bada (47) in context of the history of chentical evolution research, but the gist of the problem lies with the pyrolysis of amino acids at high temperatures, which renders polymerizations more random than the desired synthetic route towards Unear polypeptides. 

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