Electrochemical quantification of the structure/antioxidant activity relationship of flavonoids

• CRAC assay simply and rapidly quantifies the reducing capacity of antioxidant compounds based on the consumption of a standard oxidizer (Ce4+).
• The electronic properties of studied flavonoid compounds and their antioxidant activity can be well correlated by this three variables: (i) polarizability, (ii) change at carbon 3’ (iii) change at carbon 5.
• Hierarchical antioxidant classification generated by CRAC assay: morin > kaempferol ≅ quercetin > fisetin > apigenin > luteolin > catechin > chrysin.
• Antioxidant hierarchy for groups present in the studied flavonoids compound generated by CRAC assay: OH(C2′C4′) > OH(C4′) ≅ OH(C3′C4′) > C2=C3 + 4-oxo > OH(C3,C5) +  4-oxo > OH(C3) + 4-oxo > OH(C5) + 4-oxo > OH(C3,C5).

Ceric Reducing/Antioxidant Capacity (CRAC) is an electrochemical test that has recently emerged as an alternative to the spectrophotometric tests employed in the determination of antioxidant capacity. CRAC simply and rapidly quantifies the reducing capacity of antioxidant compounds based on the consumption of a standard oxidizer (Ce4+). In this study, eight samples of flavonoids from three distinct groups were evaluated and showed the following antioxidant hierarchy: morin > kaempferol ≅ quercetin > fisetin > apigenin > luteolin > catechin > chrysin. This hierarchy is correlated with the behavior expected according to the structure/antioxidant activity relationship (SAR) of these polyphenolic compounds. Additionally, other correlations were established using SAR to explain the antioxidant behavior of the compounds with unrelated groups: OH(C2′C4′) > OH(C4′) ≅ OH(C3′C4′) > C2 = C3 + 4-oxo > OH(C3,C5) + 4-oxo > OH(C3) + 4-oxo > OH(C5) + 4-oxo > OH(C3,C5). Therefore, the use of these two tools together is very important for the study of the antioxidant behavior of flavonoids, contributing uniquely to the understanding of electronic transfer mechanisms involved in the antioxidant processes.