Dicarbonyl, Strecker degradation

Strecker aldehyde are generated by rearrangement, decarboxylation and hydrolysis. Thus the Strecker degradation is the oxidative de-amination and de-carboxylation of an a-amino acid in the presence of a dicarbonyl compound. An aldehyde with one fewer carbon atoms than the original amino acid is produced. The other class of product is an a-aminoketone. These are important as they are intermediates in the formation of heterocyclic compounds such as pyrazines, oxazoles and thiazoles, which are important in flavours. 

The first group contains compounds produced in the early stages of the reaction by the breakdown of the Amadori or Heynes intermediates, and includes similar compounds to those found in the caramelisation of sugars. Many of these compounds possess aromas that could contribute to food flavour, but they are also important intermediates for other compounds. The second group comprises simple aldehydes, hydrogen sulphide or amino compounds that result from the Strecker degradation occurring between amino acids and dicarbonyl compounds. 

Several mechanisms have been proposed for the formation of pyrazines in food flavours [18, 23, 25], but the major route is from a-aminoketones, which are products of the condensation of a dicarbonyl with an amino compound via Strecker degradation (Scheme 12.3). Self-condensation of the aminoketones, or condensation with other aminoketones, affords a dihydropyrazine that is oxidised to the pyrazine. 

The Amadori compound may be degraded via either of two pathways, depending on pH, to a variety of active alcohol, carbonyl and dicarbonyl compounds and ultimately to brown-coloured polymers called melanoidins (Figure 2.31). Many of the intermediates are (off-) flavoured. The dicarbonyls can react with amino acids via the Strecker degradation pathway (Figure 2.32) to yield another family of highly flavoured compounds. 

Figure 1. Strecker degradation induced by (a) an a-dicarbonyl compound and (b)...

This important flavor compound was identified in the head-space volatiles of beef broth by Brinkman, et al. (43) and although it has the odor of fresh onions, it is believed to contribute to the flavor of meat. This compound can be formed quite easily from Strecker degradation products. Schutte and Koenders (49) concluded that the most probable precursors for its formation were etha-nal, methanethiol and hydrogen sulfide. As shown in Figure 5, these immediate precursors are generated from alanine, methionine and cysteine in the presence of a Strecker degradation dicarbonyl compound such as pyruvaldehyde. These same precursors could also interact under similar conditions to give dimethyl disulfide and 3,5-dimethyl-l,2,4-trithiolane previously discussed. 

Several mechanisms have been reported for pyrazine formation by Maillard reactions (21,52,53). The carbon skeletons of pyrazines come from a-dicarbonyl (Strecker) compounds which can react with ammonia to produce ot-amino ketones as described by Flament, et al. (54) which condense by dehydration and oxidize to pyrazines (Figure 6), or the dicarbonyl compounds can initiate Strecker degradation of amino acids to form ot-amino ketones which are hydrolyzed to carbonyl amines, condensed and are oxidized to substituted... 

The imidazole part of the IQ-compounds suggests creatinine as a common precursor. The remaining parts of the IQ-compounds could arise from Maillard reaction products, e.g., 2-methylpyri-dine or 2,5-dimethylpyrazine. These two compounds could be formed through Strecker degradation. In Maillard reactions, this is induced by a-dicarbonyl compounds derived from carbohydrates, which are thereby converted to pyrroles, pyridines, pyrazines, etc. (8). 

Figure 2.10. Strecker degradation of amino acids and a-dicarbonyls to form heterocyclic compounds. For glycine, R = H. Reprinted with permission from Wong, J. W., and Shib-amoto, T. (1996). Geotoxicity of the Maillard reaction products. In The Maillard Reaction. Consequences for the Chemical and Life Sciences, Ikan, R., ed., John Wiley Sons, Chichester, UK, 129-159.

Finally, heating of amino acids can produce volatiles Including aldehydes, amines and hydrogen sulfide. One minor, but Important, flavor generating pathway Involves the Strecker degradation of an amino acid as shown in Figure 2. In this reaction, an alpha amino acid reacts with an alpha dicarbonyl at an elevated temperature to produce an aldehyde (one carbon less than the amino acid) as well as an alpha amino ketone. These products can react further to yield Important heterocyclic aroma chemicals such as pyrazines, thlazoles, and dihydrofuranones. 

Pyridines and pyrroles can be formed in different pathways by Mail-lard reaction. The formation of 5-methyl pyrrole aldehyde and 6-methyl-3-pyridinole has been observed by Nyhammar et al (17) by the reaction of isotope labelled 3-deoxyosone with glycine. The 3-deoxy-hexosone represents an -dicarbonyl compound and in this way the Strecker degradation occurs. Another pathway is the reaction of fu-rans with ammonia. Under roast conditions, we have obtained primarily the corresponding pyrrole, whereas we found the corresponding py-... 

Another important aspect of the Maillard reaction involves the Strecker degradation of ot-amino acids. At elevated temperatures oe-dicarbonyl compounds, such as 3-deoxy glucosone, pyruvaldehyde, glyoxal, and dihydroascorbic acid will cause the degradation of an... 

Heterocyclic aroma compounds found in meat primarily arise from interactions between mono- and dicarbonyl compounds, H2S and ammonia. The carbonyl compounds are derived from the Maillard reaction, including Strecker degradation of amino acids, oxidation of lipids and aldolization reactions. H2S is produced by thermal degradation of sulfur amino acids and ammonia by amino acid pyrolysis. 

Heterocyclic compounds are dominant among the aroma compounds produced in the Maillard reaction, and sulfur-containing heterocyclics have been shown to be particularly important in meat-like flavors. In a recent review, MacLeod (6) listed 78 compounds which have been reported in the literature as possessing meaty aromas seven are aliphatic sulfur compounds, the other 71 are heterocyclic of which 65 contain sulfur. The Strecker degradation of cysteine by dicarbonyls is an extremely important route for the formation of many heterocyclic sulfur compounds hydrogen sulfide and mercaptoacetaldehyde are formed by the decarboxylation and deamination of cysteine and provide reactive intermediates for interaction with other Maillard products. 

The formation of pyrazines is generally linked to the Strecker degradation (see Chapter 2), in which the dicarbonyl reagent undergoes transamination, leading to an a-aminocarbonyl. Two molecules of this readily condense to a dihydropyrazine, as shown in Scheme 5.8. 

Cysteine can undergo the Strecker degradation, transamination, and -elimination, as shown by Tressl et al.247 using [1- or 6-13C]glucose (equimolar aqueous solution, 160 °C, 1.5 h). 2-Furylmethanethiol (T 0.005 ppb), very important in providing the aroma of roasted coffee and roasted meat, is formed as shown in Scheme 5.14 from [l-13C]glucose via the 3-deoxy-l,2-dicarbonyl, which loses... 

Pyrazines have been prepared by heating 1,2-dicarbonyl compounds with a-amino acids. Thus Rizzi (308) observed that under the conditions of the Strecker degradation, equimolar amounts of DL-valine (44) and butane-2,3-dione in refluxing bis(2-methoxyethyl) ether, diglyme, gave isobutyraldehyde, tetra-methylpyrazine (9%), and a mixture of cis- and trans-2-isopropyl-4,5-dimethyl-3-oxazoline (4%). He proposed a reductive amination mechanism in which butane-23-dione was converted to 2-aminobutan-3-one which underwent self-condensation to the pyrazine. Tetramethylpyrazine was also prepared when the same reactants were heated in dimethylformamide at 123° for 5 hours (and other pyrazines prepared similarly) (308a). 

Strecker [34] discovered that the reaction between amino acids (glycine, alanine, leucine) and the tricarbonyl compound alloxan yields CO2 and aldehydes. The Strecker degradation of amino acids occurs also with dicarbonyl compounds [35] including those that are formed in the course of the Maillard reaction, in particular deoxyglycosones and some of the smaller sugar fragments like 7 and diacetyl. Fig. 3.22 shows the reaction pathway that involves the formation of an imine 8, followed by decarboxylation and liberation of the resulting aminoketo compound and the Strecker aldehyde from the intermediate 10. Odour-active Strecker aldehydes which... 

Coffee flavors form during roasting from dicarbonyl compounds which derive from carbohydrates. The thermal degradation of hexoses is thought to be the preciu ors of fin ones like HDMF. The presence of alkylpyrazines affords the characteristic roast notes. These pyrazines are formed through Strecker degradation and the condensation of the resulting Strecker products (60). 

According to Carlin et al. (1986), the exact mechanism of oxazole formation is not known, despite the previous schemes proposed by Vitzthum and Werkhoff (1974a,b) and by Ohloff and Flament (1978). Formation pathways were proposed by Baltes and Bochmann (1987d) and Mottram (1991). For Vitzthum and Werkhoff (1974b), one pathway could be the decarboxylation of serine or threonine into ethanolamine or methylethanolamine condensation with an aldehydic compound into an oxazolidine, then oxidation into an oxazole unsubstituted or methylated on position 5 and bearing an alkyl or an acyl radical on position 2. Another pathway could be the condensation of amino acids with a-dicarbonyl compounds, followed by a Strecker degradation, formation of an a-amino ketone which, after acylation... 

Other possible mechanisms for the formation of pyrazines during smoking are possible. For instance, leaf carbohydrates could be degraded either through pyrolysis or Maillard reactions to form a-dicarbonyl compounds, which could, in turn, react with amino acids to undergo a Strecker degradation forming... 

The Strecker degradation involves the oxidative deamination and decarboxylation of a a-amino acid in the presence of a dicarbonyl compound. The products formed from this reaction are an aldehyde containing one less carbon atom than the original amino acid and an a-aminoketone (Table 9.2). The Strecker degradation of methionine and cystein is a source of sulfur-containing intermediates (e.g hydrogen sulfide and 2-methylthiopropanal = methional) [48]. 

Since n-glucosone is an a-dicarbonyl derivative, it reacts with a-amino acids along the Strecker degradation pathwayand may be converted into D-finctosamine (Scheme 16), although this reaction is unlikely to have any synfiietic value, on accoimt of a number of concomitant transformations. 

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