Repair of tryptophan radicals by antioxidants

Oxidizing free radicals with redox potential greater than 1 V generate indole radicals, R-Ind^⨥ and R-Ind·, as in tryptophan. These resonance-stabilized free radicals can be repaired efficiently with electron donors (k = 5 × 10⁶ − 1.3 × 10⁹ dm³ mol⁻ s⁻) such as ascorbate, N,N,N′,N′-tetramethyl-p-phenylenediamine dihydrochloride (TMPD), and phenolic antioxidants. Sulfhydryl compounds, which are good H-atom donors, were found to be relatively unreactive (k < 10⁶ dm³ mol⁻¹s⁻¹). These indole radicals were also found to be unreactive with oxygen (k < 10⁶ dm³ mol⁻¹ s⁻¹).