Figure 1.

(a) Backbone structure of commonly found flavonoids consisting of fused carbon rings and varying oxygen substituents. (b) Structure of quercetin and common metabolites found in circulation. Addition of a glucose-derived moiety at C-3 position gives rise to the quercetin-3-glucuronide form with a single hydroxyl group making up the aglycone form. Sulfation at the 3′- position represents the quercetin-3′-sulfate metabolite commonly found in plasma. Adapted from Harborne,² Rossi et al.,³ and Cao et al.⁴