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 (e = 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.