title: 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).
authors.affiliation: Department of Applied Chemistry, University of South Bohemia, Branišovská 31, 370 05 České Budějovice, Czech Republic.
authors.forename: Petra
authors.initials: P
authors.lastname: Kucerová
authors.forename: Roman
authors.initials: R
authors.lastname: Kubec
authors.forename: Petr
authors.initials: P
authors.lastname: Simek
authors.forename: Lukás
authors.initials: L
authors.lastname: Václavík
authors.forename: Jan
authors.initials: J
authors.lastname: Schraml
mesh_terms: D000490:Allium / Q000737:chemistry*; D002853:Chromatography, Liquid; D005511:Food Handling / Q000379:methods; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D010860:Pigments, Biological / Q000032:analysis* / Q000737:chemistry; D018517:Plant Roots / Q000737:chemistry*
chemical_list: D010860:Pigments, Biological