title: Double Schiff base adducts of 2,3-butanedione with glycine: formation of pyrazine rings with the participation of amino acid carbon atoms.
abstract: The 1,2-dicarbonyl compounds are well-known for their ability to undergo a one-to-one interaction with amino acids and generate aroma-active pyrazines through the Strecker reaction. An earlier publication reported the generation of tetrahydropyrazine moiety from the double addition of amino acids to 1,2-dicarbonyl compounds. To evaluate the potential of this intermediate to undergo oxidation and form pyrazines, a model system composed of glycine and 2,3-butanedione was evaluated under pyrolytic conditions at 250 °C, as well as under pressurized high-temperature conditions at 120 °C. These studies have indicated the unexpected formation of 2,3-dimethylpyrazine and 2,3,5-trimethylpyrazine in addition to the expected tetramethylpyrazine. Isotope-labeling studies using [¹³C-1]glycine (98%), [¹³C-2]glycine (99%), and [¹⁵N]glycine (98%) have shown that, as expected, tetramethylpyrazine was completely unlabeled, whereas 51% of 2,3-dimethylpyrazine incorporated two ¹³C-2 atoms from glycine, 20% incorporated one atom, and 29% was unlabeled. Furthermore, the label incorporation pattern in the major mass spectral fragment at m/z 67 indicated that the C-2 atoms originating from glycine reside in the ring system of 2,3-dimethylpyrazine. The formation of doubly labeled 2,3-dimethylpyrazine was rationalized through proposition of the double addition of glycine to 2,3-butanedione, and the formation of singly labeled isotopomer was justified by sequential Schiff base formation of 2-amino-butan-3-one first with the Strecker aldehyde and then followed by glycine. This pathway can also generate the double-labeled pyrazine. Finally, the unlabeled pyrazine was proposed to form through the Strecker reaction of 2,3-butanedione and its degradation product glyoxal with glycine. The proposed pathways were also consistent with the observed label distribution patterns of 2,3,5-trimethylpyrazine.
authors.affiliation: Department of Food Science and Agricultural Chemistry,  McGill University , 21111 Lakeshore, Sainte Anne de Bellevue, Quebec H9X 3V9, Canada.
authors.forename: Paula Vanessa
authors.initials: PV
authors.lastname: Guerra
authors.forename: Varoujan A
authors.initials: VA
authors.lastname: Yaylayan
mesh_terms: D000596:Amino Acids / Q000737:chemistry; D003931:Diacetyl / Q000737:chemistry*; D005998:Glycine / Q000737:chemistry*; D015416:Maillard Reaction; D015394:Molecular Structure; D011719:Pyrazines / Q000737:chemistry; D012545:Schiff Bases / Q000737:chemistry*
chemical_list: D000596:Amino Acids; D011719:Pyrazines; D012545:Schiff Bases; D003931:Diacetyl; D005998:Glycine