Study of the reaction products of flavonols with 2,2‐diphenyl‐1‐picrylhydrazyl using liquid chromatography coupled with negative electrospray ionization tandem mass spectrometry

The products obtained after the reaction between flavonols and the stable free radical 2,2‐diphenyl‐1‐picrylhydrazyl (DPPH • ) in both methanol and acetonitrile were characterized using liquid chromatography coupled with negative electrospray ionization tandem mass spectrometry (LC/ESI‐MS/MS) and NMR spectroscopy. The flavonols studied were quercetin, kaempferol and myricetin. In methanol, two reaction products of oxidized quercetin were identified using LC/ESI‐MS/MS and NMR. Quercetin was oxidized through a transfer of two H‐atoms to DPPH • and subsequently incorporated either two CH 3 OH molecules or one CH 3 OH‐ and one H 2 O molecule giving the products 2‐(3,4‐dihydroxyphenyl)‐3,5,7‐trihydroxy‐2,3‐dimethoxy‐2,3‐dihydrochromen‐4‐one and 2‐(3,4‐dihydroxyphenyl)‐3,3,5,7‐tetrahydroxy‐2‐methoxy‐2,3‐dihydrochromen‐4‐one, respectively. LC/ESI‐MS/MS analysis revealed that in methanol, kaempferol and myricetin also gave rise to methoxylated oxidation products similar to that identified for quercetin. Kaempferol, in addition, also exhibited products where a kaempferol radical, obtained by a transfer of one H‐atom to DPPH • , reacted with CH 3 OH through the addition of CH 3 O • , yielding two isomeric products. When the reaction took place in acetonitrile, LC/ESI‐MS/MS analysis showed that both quercetin and myricetin formed stable isomeric quinone products obtained by a transfer of two H‐atoms to DPPH • . In contrast, kaempferol formed two isomeric products where a kaempferol radical reacted with H 2 O through the addition of OH • , i.e. similar to the reaction of kaempferol radicals with CH 3 OH. Copyright © 2004 John Wiley & Sons, Ltd.