5,7,3′,4′-Hydroxy substituted flavonoids reduce the heme of cytochrome c with a range of rate constants

Flavonoids are antioxidants known to be abundant in edible plants. Seven 5,7,3′,4′-tetrahydroxy substituted flavonoids representing each major flavonoid class were used as cytochrome c reductants to systematically investigate the redox role of their C-rings. Additional examples of flavonoids and benzenediols were investigated to better understand the role of the B-ring. Pseudo-first order rate constants (k obs )and apparent bimolecular rate constants (k app )values were calculated from spectroscopic measurements. Of the seven flavonoids tested, five yielded measurable observed reduction rate constants. Butein (a chalcone)had the highest apparent bimolecular rate constant (k app ), followed by taxifolin (a flavanonol), catechin (a flavanol), eriodictyol (a flavanone), and luteolin (a flavone). Quercetin (a flavonol)and cyanidin (an anthocyanidin), however, reduced cytochrome c but k app rate constants were unable to be calculated. Neither this trend nor trends in observed rate constants correlated with flavonoid pK a , solvent accessible surface area, polar surface area, reduction potential, antioxidant ability, resonance, or radical scavenging efficiency. Weak correlation, however, was found with degrees of freedom and the number of redox involved electrons. While some cytochrome c reduction rates have been reported, this study is the first to systematically investigate the role of the structure of the flavonoid C-ring across a full set of flavonoids with identical B-rings.