“…The cancer preventive and therapeutic effects of quercetin have been demonstrated through in vitro as well as in vivo experimental findings [21,22]. As depicted in Scheme 1, Que consists of three phenolic rings (A, B, and C rings); it has three available sites for metal chelation including ortho-dihydroxyl (catechol) group of the B ring (site A), 5-hydroxy-4-keto group (site B), and 3-hydroxy-4keto group (site C), since both hydroxyl and keto groups of Que have the ability to form metal complexes [23][24][25][26][27][28][29][30][31][32][33][34][35]. Interestingly, it was observed that some biological properties of quercetin such as antioxidant, antibacterial, and antitumor activities, change with metal chelation.…”