Allium Discoloration: The Precursor and Formation of the Red Pigment in Giant Onion (Allium giganteum Regel) and Some Other Subgenus Melanocrommyum Species

Abstract

The precursor of the orange-red pigment formed upon wounding the bulbs of Allium giganteum (Allium subg. Melanocrommyum) was isolated and shown to be S-(2-pyrrolyl)cysteine S-oxide. In addition, two other pyrrolylsulfinyl derivatives were found in an extract from the bulbs, namely, 3-(2-pyrrolylsulfinyl)lactic acid and S-(3-pyrrolyl)cysteine S-oxide. Contrary to a previous report, the latter compound was shown not to serve as the precursor of the pigment, being in fact only an artifact formed during isolation. The formation of pyrrolyl-containing compounds following disruption of A. giganteum bulbs was studied by a combination of LC-MS, LC-NMR and DART-MS. It was found that S-(2-pyrrolyl)cysteine S-oxide is cleaved by a C-S lyase (alliinase) to yield 2-pyrrolesulfenic acid. Two molecules of the latter compound give rise to highly reactive S-(2-pyrrolyl) 2-pyrrolethiosulfinate which in turn converts into red 2,2'-epidithio-3,3'-dipyrrole (dipyrrolo[2,3-d:2',3'-e]-1,2-dithiin). Several other pyrrolyl-containing compounds were detected in A. giganteum for the first time, including S-methyl 2-pyrrolethiosulfinate, S-(2-pyrrolyl) methanethiosulfinate, di(2-pyrrolyl) disulfide, and S-(2-pyrrolyl) 2-pyrrolethiosulfonate. It can be concluded that the formation of the orange-red pigment in Allium subg. Melanocrommyum species, despite sharing several analogous features, is of a different nature than the pink discoloration of onion (A. cepa).