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 theaflavi n, theaflavin gallates A and B, and theaflavin digallate. Being relati vely strong transient oxidant, N-3 . oxidizes more than one phenolic s ite in the complex polyphenols. The resulting mixture of phenoxyl radi cals transforms via an intramolecular electron transfer to the hydroxy cycloheptenone radical, with apparently lowest reduction potential. Th e neutral hydroxycycloheptenone radical is more aromatic than the pare nt compound, which reflects in high rate constant of the formation of the radical. The rate of the reaction of theaflavin with the superoxid e radical at pH 7, k = 1 x 10(7) M-1 s(-1), is an order of magnitude h igher than that with epigallocatechin gallate (EGCG), k = 7.3 x 10(5) M-1 s(-1), in spite of higher reduction potential of the theaflavin ra dicals (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 electr on density in the radical. Purpurogallin radical, with three hydroxy g roups on the benzene ring, has pK(r1) = 4.7 and E-7 = 0.48 V, as compa red to the theaflavin radical, with only two hydroxy groups on benzene , having pK(r) = 4.3 and E-7 = 0.51 V. The electron donating ability o f theaflavins, which are major antioxidants in black tea, is quantitat ively assessed on the basis of physicochemical characteristics of daug hter radicals and their potential biological action discussed.