Analysis of the Antioxidant Capacities of Flavonoids under Different Spectrophotometric Assays Using Cyclic Voltammetry and Density Functional Theory

Flavonoids often show inconsistent antioxidant activities (AAs) depending on the assay used. The electrochemical properties of 14 flavonoid standards in cyclic voltammetry [area under anodic wave (Q) and oxidant peak potentials (E-pa)] and the structural parameters [bond dissociation enthalpy (BDE) and ionization potential (IP)] were investigated. They were compared with the results of four spectrophotometric assays, namely, diphenyl-1-picrylhydrazyl (DPPH), Folin-Ciocalteu reagent (FCR), ferric reducing ability of plasma (FRAP), Trolox equivalent antioxidant capacity (TEAC), to analyze the chemical reasons for the varying AAs of flavonoids under different assays. Using the cyclic voltammetry method, the AAs of the flavonoids in the DPPH, FCR, and FRAP assays were mainly determined by the ease of charge transferring in the first oxidation step. Meanwhile, the results of TEAC assays were primarily influenced by the amount of charge transfer in the multiple oxidation steps (MOS) of flavonoids. In the theoretical calculation, the BDE values of the selected flavonoids had considerably higher correlations with the results of the DPPH assay (r(2) = 0.89) compared with the other three assays, which indicates that the oxidant-scavenging reaction of the tested flavonoids in the DPPH assay is closer to a hydrogen atom transfer mechanism. Neither the IP values nor BDE values had satisfactory correlation with the AAs of the flavonoids in the TEAC assay (r(2) = 0.57, r(2) = 0.54, respectively), Therefore, complex reaction mechanisms underlie this method and appropriate structural descriptors for reflecting the AM of flavonoids based on MOS (e.g., TEAC values) need further investigation.