Reaction of the superoxide radical with the N-centered radical derived from N-acetyltryptophan methyl ester

Hydroxyl radicals, solvated electrons and H atoms are generated by pulse radiolysis in aqueous solutions of N-acetyltryptophan methyl ester (AM-Trp). The solvated electrons are converted with N2O into further OH radicals and the latter with azide into N-3 radicals which oxidize AM-Trp to its N-centered radical (AM-TrpN'). It is characterized by a strong absorption at 510 nm (epsilon = 1830 dm(3) mol(-1) cm(-1)). The bimolecular decay of the radicals (2k = 7.3 x 10(8) dm(3) mol(-1) s(-1)) is not affected by O-2 [k(AM-TrpN' + O-2) < 10(5) dm(3) mol(-1); 2 x 10(3) dm(3) mol(-1) s(-1) deduced from other data]. In the presence of O-2, and when the majority of the OH radicals are converted with formate into superoxide radicals, O-2(.-), decay of the AM-TrpN' radicals follows first-order kinetics [k(AM-TrpN' + O-2(.-)) 1.2 x 10(9) dm(3) mol(-1) s(-1)]. In O-2-saturated azide-containing solutions [G(AM-TrpN') = 2.9 x 10(-7) mol J(-1); G(O-2(.-)) 3.3 x 10(-7) mol J(-1)] AM-Trp is consumed with a G value of (2.9 +/- 0.5) x 10(-7) mol J(-1), i.e. a restitution of AM-Trp by electron transfer from O-2(.-) to AM-TrpN', although thermodynamically possible [E-7(O-2(.-)) = -0.33 V; E-7(TrpN') = +1.0 V], must be of very little importance compared to an addition, This has been supported by a product study, The major products are the corresponding N-formylkynurenine (G 1.4 x 10(-7) mol J(-1)) and two hydroperoxides (total G = 0.7 x 10(-7) mol J(-1)) which to a large extent convert upon standing at room temperature into 1-acetyl-2-methoxycarbonyl-3-hydroxy-1,2,3,8,8-hexahydropyrroloindole (AM-HIP). The same products and product ratios are also formed, when singlet oxygen (from the irradiation of Rose Bengal with visible light) is reacted with AM-Trp suggesting that these two processes lead to the same (short-lived) hydroperoxidic intermediate.