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 X 10(6) - 1.3 X 10(9) dm3 mol-1 s-1) 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 less than 10(6) dm3 mol-1 s-1). These indole radicals were also found to be unreactive with oxygen (k less than 10(6) dm3 mol-1 s-1).