Flavonoid Deactivation of Ferrylmyoglobin in Relation to Ease of Oxidation as Determined by Cyclic Voltammetry

Abstract

Fourteen flavonoid aglycones, and the flavonoid glyco-side rutin, with redox potentials ranging from 0.20 (myricetin) to 0.83 V (chrysin) vs. NHE, as determined by cyclic voltammetry at 23°C in aqueous 50 mM phosphate, ionic strength 0.16 (NaCI) with pH = 7.4 and compared with redox potentials determined for four cinnamic acid derivatives, were all found to reduce ferrylmyoglobin, MbFe(IV)=O, to metmyoglo-bin, MbFe(III). Reaction stoichiometry depends strongly on the number of hydroxyl groups in the flavonoid B-ring. All compounds with 3′,4′-dihydroxy substitution reduce 2 equivalents of MbFe(IV)=O, whereas naringenin, hesperitin and kaempferol, with one hydroxyl group in the B-ring, reduce with a one-to-one stoichiometry. As studied spectrophotometrically under pseudo-first-order conditions with flavonoids in excess, rutin and apigenin react with MbFe(IV)=O with very similar and moderately high activation enthalpies of ΔH‡₂₉₈ = 69 ± 1kJ mol⁻¹ and ΔH‡₂₉₈ = 65 ± 3kJ mol-¹, respectively, and with positive activation entropies of ΔH‡₂₉₈ = 23 ± 4Jmol-¹ K⁻¹ and ΔS‡₂₉₈ = 13 ± 9Jmol⁻¹K-¹, respectively, in agreement with outer-sphere electron transfer as rate determining. For the fifteen plant polyphenols only qualitative relations exist between redox potential and rate constants rather than a linear free energy relationship (r² = 0.503), and especially the flavone apigenin was found more efficient as reducing agent. For the flava-nones, a linear relation (r² = 0.971) indicate that, in the absence of a 2,3 double bond, removal of the 4–carbonyl group or addition of a 3–hydroxy group only has minor effect on reactivity. The flavonols are the most efficient reducing agents, effectively reducing MbFe(IV)=O to MbFe(III) and establishing a steady state distribution between the flavonol and MbFe(III) and oxymyoglobin, MbFe(II)O₂. Oxidised flavonols reduces MbFe(III) to MbFe(II)0₂ very efficiently and much faster than the parent flavonol.

Key Words:

• Ferrylmyoglobin
• hypervalent iron
• flavonoids
• redox potentials
• cyclic voltammetry
• linear free energy relationship