Characterization of yellow pigments formed on reaction of 2-(1H-pyrrolyl)carboxylic acids with pyruvic acid in garlic greening model systems

Two new pyrrole derivatives 2-(1H-pyrrol-1-yl)succinic acid (P-Asp) and 2-(1H-pyrrol-1-yl)pentanedioic acid (P-Glu) were synthesized to study their effect on garlic greening, the structures of which are similar to that of a previously proposed pigment precursor for garlic greening. The puree of freshly harvested garlic bulbs turned green after being soaked in solutions of the two compounds. Also, it was found that yellow pigments can be produced by reacting the two model compounds with pyruvic acid at room temperature. Four major new yellow pigments from these two model systems were formed. Two of them named AUP-1 and AUP-2 produced from model system II consisting of P-Asp and pyruvic acid have the same molecular weight 417.1294 [M+H]+ but having different retention times in HPLC, suggesting that there are isomers with each other. Likewise, other two isomer compounds GUP-1 and GUP-2 from model system I containing P-Glu and pyruvic acid have molecular weight as 445.1586 [M+H]+. The molecular formula for AUP-1 and AUP-2 was C20H20N2O8 while the molecular formula for GUP-1 and GUP-2 was C22H24N2O8. All these results confirmed previous proposal that pyrrole derivatives acted as PP (pigment precursor) for Allium discoloration.