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ABSTRACT:Quercetin is one of the most potent antioxidants belonging to the flavonoids, which are a naturally occurring family of molecules. The optimized forms of the 3 -, 4 -, 3-, 5-, and 7-quercetin radicals were computed using ab initio procedure at the 6-31G * level. We have deduced that the 3 -, 4 -, and 3-radicals are the most energetically favored. The simulation of the H abstraction at the 4 -hydroxyl of quercetin by the free radical t-BuOO · (tert-butyl peroxyl) was conducted using the supermolecular model composed of quercetin in interaction with t-BuOO · . The mechanism of the scavenging of the free radical by quercetin is discussed on the basis of various AM1 and ab initio (level 6-31G * ) quantum molecular computations and the obtained frontier orbitals and total spin density. The analysis of the spin density around the obtained free radical 4 -quercetin indicates that the major density of spin α(↑) is concentrated in the B ring on the O4 oxygen; we observe also a slight delocalization of spins α(↑) and β(↓) on the B ring and on a part of the C ring. In conclusion, the antioxidant power of quercetin may be associated mainly with the B ring and a half of the C ring. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem 90: 888-902, 2002