Cyanidin (CY), one of the most common anthocyanins, has a high radical-scavenging efficiency that can be compared with that of the flavone, quercetin (QU). Owing to the oxonium ion, CY exhibits diverse structures (isomers) in different (pH) environments. In this paper, to elucidate the high potency of CY as a radical scavenger, a combined density functional theory (DFT) method, labeled as (RO)B3LYP/6-311+G(2d,2p)//AM1/AM1, was employed to calculate the O-H bond dissociation enthalpies (BDEs), proton dissociation enthalpies (PDEs) and ionization potentials (IPs) of various CY isomers in acetonitrile and then the parameters were compared with those of QU. The comparable BDEs, PDEs and IPs of CY (in various forms) and QU account well for CY's high radical-scavenging activity in different environments. In addition, through estimating the contribution of each group to the BDEs and IP of the CY cation, some deeper insights into the antioxidant power of CY were obtained.
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