Effect of amino acids on formation of pigment precursors in garlic discoloration using UPLC–ESI-Q-TOF-MS analysis

The formation of pigment precursors was investigated systematically using green models containing arginine, lysine, alanine, glycine, serine, tyrosine, glutamic acid, and aspartic acid, respectively. Pigment precursor and blue pigments during garlic greening were investigated using ultra-performance liquid chromatography and tandem high-resolution mass spectrometry with a quadrupole time-of-flight analyser (UPLC–ESI-Q-TOF-MS). The intermediate products during the formation of a pigment precursor with a 3-oxopropenyl side chain were tentatively identified. N -substituted 3,4-dimethylpyrrole pigment precursors were found to react with 2-propenesulfenic acid first, and the propenesulfenic acid side-chain decomposed to a 3-oxopropenyl side chain, which provided a hypothesis of the formation pathway. Density functional theory calculations combined with the experimental results led to the conclusion that the primary amino group located on the α-carbon reacted with 3,4-dimethylbutanedithial S -oxide to form pigment precursors. In contrast, the amino group in the amino acid R group did not participate in pigment precursors formation. The R -groups of amino acids containing hydroxyls or carboxyl groups have influential roles in the stability of N -substituted 3,4-dimethylpyrrole pigment precursors. In contrast to a previous report, larger R-groups made a more outstanding contribution to the production of blue pigments.