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

Abstract

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 N₃ radicals which oxidize AM-Trp to its N-centered radical (AM-TrpN˙). It is characterized by a strong absorption at 510 nm (ε = 1830 dm³ mol^–1 cm^–1). The bimolecular decay of the radicals (2k = 7.3 × 10⁸ dm³ mol^–1 s^–1) is not affected by O₂ [k(AM-TrpN˙ + O₂) < 10⁵ dm³ mol^–1; 2 × 10³ dm³ mol^–1 s^–1 deduced from other data]. In the presence of O₂, and when the majority of the OH radicals are converted with formate into superoxide radicals, O₂^˙–, decay of the AM-TrpN˙ radicals follows first-order kinetics [k(AM-TrpN˙ + O₂^˙–) = 1.2 × 10⁹ dm³ mol^–1 s^–1]. In O₂-saturated azide-containing solutions [G(AM-TrpN˙) = 2.9 × 10^–7 mol J^–1; G(O₂^˙–) = 3.3 × 10^–7 mol J^–1] AM-Trp is consumed with a G value of (2.9 ± 0.5) × 10^–7 mol J^–1, i.e. a restitution of AM-Trp by electron transfer from O₂^˙– to AM-TrpN˙, although thermodynamically possible [E⁷(O₂^˙–) = –0.33 V; E⁷(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 × 10^–7 mol J^–1) and two hydroperoxides (total G = 0.7 × 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.