Isotope Labeling Studies on the Origin of 3,4-Hexanedione and 1,2-Butanedione in an Alanine/Glucose Model System

Abstract

Although the importance of alpha-dicarbonyl compounds as reactive intermediates in the Maillard reaction and as precursors of heterocyclic and odor-active compounds is well-established, however, the detailed origin of many alpha-dicarbonyl compounds such as 3,4-hexanedione and 1,2-butanedione still remains unknown. Using glucose and glyoxal with labeled [(13)C-1]alanine, [(13)C-2]alanine, [(13)C-3]alanine, and [(15)N]alanine, the mechanism of their formation was investigated using the label incorporation pattern of the pyrazines derived through the Strecker reaction. Taking into account the non-oxidative mechanism of pyrazine formation, the data indicated that all of the ethyl-substituted pyrazines identified in the glyoxal/alanine model system incorporated C-2' and C-3' atoms of alanine, and not that of free acetaldehyde, as the ethyl group carbon atoms. This was achieved through spiking experiments using unlabeled acetaldehyde in the presence of labeled alanine. Furthermore, the data also indicated the occurrence of a chain elongation process of sugar-derived alpha-dicarbonyl compounds assisted by alanine. On the basis of the proposed mechanism, the glyoxal interaction with alanine through a decarboxylative aldol addition reaction can lead to the formation of 1,2-butanedione with the terminal ethyl carbon atoms originating from C-2' and C-3' atoms of alanine, and the similar interaction of 1,2-butanedione with a second molecule of alanine can lead to the formation of 3,4-hexanedione with both terminal ethyl carbon atoms originating from C-2' and C-3' atoms of alanine.

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“…Formaldehyde and acetaldehyde formed from Gly and Ala also participate in the chain elongation process of sugar-derived α-dicarbonyl compounds and these α-dicarbonyl compounds with ethyl and/or methyl substituents from amino acids may be involved in alkylpyrazine formation through oxidative or non-oxidative pathways. In the oxidative pathway, the dihydroalkylpyrazine intermediates readily oxidise to form the corresponding alkylpyrazines and this has been shown to be a major pathway (Chu & Yaylayan, 2009). Formaldehyde and acetaldehyde formed from Gly and Ala participate in the non-oxidative pathway of pyrazine formation and yield ethyl or methyl substituted alkylpyrazines upon dehydration of the alkylated site of dihydropyrazines.…”

“…Condensation of two α-amino carbonyl compounds produces a dihydropyrazine and subsequent oxidation yields a pyrazine (Shibamoto & Bernhard, 1977). Methyl substitution in pyrazines may originate from methylglyoxal (via 2-aminopropanal or 1-amino-2-propanone), 1-hydroxy-2-propanone (via 2-aminopropanal), 2,3-butanedione and 3hydroxy-2-butanone (via 3-amino-2-butanone) followed by oxidation of the corresponding dihydropyrazines (Chu & Yaylayan, 2009). Ethyl substitution may come from 2-oxobutanal, 2,3pentanedione and 3,4-hexanedione via this oxidative pathway (Chu & Yaylayan, 2009).…”

“…Methyl substitution in pyrazines may originate from methylglyoxal (via 2-aminopropanal or 1-amino-2-propanone), 1-hydroxy-2-propanone (via 2-aminopropanal), 2,3-butanedione and 3hydroxy-2-butanone (via 3-amino-2-butanone) followed by oxidation of the corresponding dihydropyrazines (Chu & Yaylayan, 2009). Ethyl substitution may come from 2-oxobutanal, 2,3pentanedione and 3,4-hexanedione via this oxidative pathway (Chu & Yaylayan, 2009). In addition to this oxidative pathway, nucleophilic attack by a dihydropyrazine at the carbonyl centre of formaldehyde or acetaldehyde, followed by the elimination of a water molecule, also yields alkyl substituted pyrazines in a non-oxidative way (Amrani-Hemaimi, Cerny, & Fay, 1995;Cerny & Grosch, 1994).…”

Double Schiff Base Adducts of 2,3-Butanedione with Glycine: Formation of Pyrazine Rings with the Participation of Amino Acid Carbon Atoms

“…In the above model system studied, the only expected pyrazine is the completely unlabeled tetramethylpyrazine because the only α-dicarbonyl present in the model system was unlabeled 2,3-butanedione. As mentioned above, not only were other pyrazines detected but they also exhibited various percentages of 13 C-2 atom incorporation patterns, as presented in Table 1. The formation of completely unlabeled pyrazines can only be explained by the degradation of 2,3-butanedione into glyoxal and pyruvaldehyde, the two unlabeled α-dicarbonyls needed for their formation (see Figure 2).…”

“…10 As was mentioned above, this pyrazine was formed in the 2,3butanedione/glycine model system with the contribution of C-2 atoms from glycine, where-isotope labeling studies have indicated the incorporation of one (20%) and two (50%) such carbon atoms from 13 C-2 glycine; similarly, 53% of trimethylpyrazine was also found to be formed from the contribution of multiple C-2 atoms from glycine (Table 2). The Strecker mechanism cannot justify the incorporation of 13 C-2 atoms into the ring system of the pyrazines. There is evidence from the mass spectral data of 2,3-dimethylpyrazine and 2,3,5trimethylpyrazine that the label incorporation occurred at the ring carbon atoms of the pyrazines (see Figure 3).…”