Antioxidant Potential of Theaflavins. A Pulse Radiolysis Study

Spectral, acid−base, and redox properties of theaflavin radicals were studied by pulse radiolysis in aqueous solutions. Theaflavin radicals are generated by the azide radical one-electron oxidation of theaflavin, theflavin gallates A and B, and theaflavin digallate. Being relatively strong transient oxidant, N 3 • oxidizes more than one phenolic site in the complex polyphenols. The resulting mixture of phenoxyl radicals transforms via an intra molecular electron transfer to the hydroxycycloheptenone radical, with apparently lowest reduction potential. The neutral hydroxycycloheptenone radical is more aromatic than the parent compound, which reflects in high rate constant of the formation of the radical. The rate of the reaction of theaflavin with the superoxide radical at pH 7, k = 1 × 10 7 M - 1 s - 1, is an order of magnitude higher than that with epigallocatechin gallate (EGCG), k = 7.3 × 10 5 M - 1 s - 1, in spite of higher reduction potential of the theaflavin radicals ( E 7 = 0.53 V vs E 7 = 0.44 V for EGCG). Even the substitution in the adjacent benzene ring has relatively small effect on the electron density in the radical. Purpurogallin radical, with three hydroxy groups on the benzene ring, has p K r 1 = 4.7 and E 7 = 0.48 V, as compared to the theaflavin radical, with only two hydroxy groups on benzene, having p K r = 4.3 and E 7 = 0.51 V. The electron donating ability of theaflavins, which are major antioxidants in black tea, is quantitatively assessed on the basis of physicochemical characteristics of daughter radicals and their potential biological action discussed.