Mechanistic studies of tetramethylpyrazine formation under weak acidic conditions and high hydrostatic pressure

A significant enhancement of the tetramethylpyrazine (TMP) formation at high pressure was observed in the 3-hydroxy-2-butanone/ammonium acetate model system. In a water system, the activation volume of TMP formation under high pressure was found to be -6.82 mL/mol. A mechanism was proposed to elucidate the formation of TMP under a weak acidic condition and high hydrostatic pressure. Solvents such as propylene glycol (PG), glycerol, methanol, ethanol, propanol, and butanol were found to enhance TMP formation. Kinetic analyses indicated that TMP formation in aqueous, 80% PG, and ethanol systems followed pseudo-zero-order reaction kinetics. The activation energies were found to be 18.84 +/- 1.3, 14.19 +/- 7.1, and 13.09 +/- 4.7 kcal/mol, respectively. The intermediate of TMP formation was characterized as tetramethyldihdyropyrazine using gas chromatography-mass spectrometry. A N-15-labeled ammonium acetate/3-hydroxy-2-butanone model system was used to confirm the incorporation of a nitrogen atom in the molecule of tetramethyldihydropyrazine. Hydrogen accepters such as nicotinamide adenine dinucleotide and flavin adenine dinucleotide were found to increase TMP formation, and the formation of TMP from tetramethyldihydropyrazine through dehydrogenation was shown.