Natural polyphenols are largely distributed in the plant kingdom (e.g., flowers, fruit and vegetables). Polyphenols are π-conjugated compounds containing OH-phenolic groups and most of them (e.g., flavonolignans) exhibit free radical scavenging properties and protective effects. Their antioxidant effect is often related to other biological activities. Over the past years quantum-chemical methods have appeared promising to support experimental data, allowing a comprehensive understanding at the atomic scale. Within the density functional theory (DFT) formalism, the role of the different OH groups of polyphenols in the antioxidant activity can be thoroughly investigated. Mainly the calculated O-H bond dissociation enthalpies (BDEs) of the different OH groups correlate and rationalize experimental results as obtained by the DPPH scavenging assay and voltammetry. The spatial spin density distribution is another secondary quantum descriptor of the antioxidant action: the more delocalized the spin density in the phenoxy radical (formed after hydrogen abstraction from the polyphenol antioxidant), the lower the BDE, and the higher the weight of the corresponding OH group in the antioxidant activity. The absence or presence of the 2,3-double bond in flavonolignan derivatives is a crucial structural feature related to the antioxidant activity of silybin, silychristin, silydianin and their dehydroderivatives.
This work was supported by grant no. L01204 National program of Sustainability I, grant OPVK CZ.1.07/2.3.00/20.0057 from Czech Ministry of Schools, Youth and Sports, and by the French embassy, which financially supports my cotutelle Ph.D. program.