Delocalization of the Unpaired Electron in the Quercetin Radical: Comparison of Experimental ESR Data with DFT Calculations

In the antioxidant activity of quercetin (Q), stabilization of the energy in the quercetin radical (Q(•)) by delocalization of the unpaired electron (UE) in Q(•) is pivotal. The aim of this study is to further examine the delocalization of the UE in Q(•), and to elucidate the importance of the functional groups of Q for the stabilization of the UE by combining experimentally obtained spin resonance spectroscopy (ESR) measurements with theoretical density functional theory (DFT) calculations. The ESR spectrum and DFT calculation of Q(•) and structurally related radicals both suggest that the UE of Q(•) is mostly delocalized in the B ring and partly on the AC ring. The negatively charged oxygen groups in the B ring (3′ and 4′) of Q(•) have an electron-donating effect that attract and stabilize the UE in the B ring. Radicals structurally related to Q(•) indicate that the negatively charged oxygen at 4′ has more of an effect on concentrating the UE in ring B than the negatively charged oxygen at 3′. The DFT calculation showed that an OH group at the 3-position of the AC ring is essential for concentrating the radical on the C2–C3 double bond. All these effects help to explain how the high energy of the UE is captured and a stable Q(•) is generated, which is pivotal in the antioxidant activity of Q.