Delocalization of the Unpaired Electron in the Quercetin Radical: Comparison of Experimental ESR Data with DFT Calculations
2020
Zhengwen Li | Mohamed Moalin | Ming Zhang | Lily Vervoort | Alex Mommers | Guido R.M.M. Haenen
In the antioxidant activity of quercetin (Q), stabilization of the energy in the quercetin radical (Q<sup>•</sup>) by delocalization of the unpaired electron (UE) in Q<sup>•</sup> is pivotal. The aim of this study is to further examine the delocalization of the UE in Q<sup>•</sup>, 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<sup>•</sup> and structurally related radicals both suggest that the UE of Q<sup>•</sup> 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<sup>•</sup> have an electron-donating effect that attract and stabilize the UE in the B ring. Radicals structurally related to Q<sup>•</sup> 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<sup>•</sup> is generated, which is pivotal in the antioxidant activity of Q.
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