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 pul se radiolysis in aqueous solutions of N-acetyltryptophan methyl ester (AM-Trp). The solvated electrons are converted with N2O into further O H radicals and the latter with azide into N-3 radicals which oxidize A M-Trp to its N-centered radical (AM-TrpN'). It is characterized by a s trong 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 fo rmate into superoxide radicals, O-2(.-), decay of the AM-TrpN' radical s 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-Tr pN') = 2.9 x 10(-7) mol J(-1); G(O-2(.-)) 3.3 x 10(-7) mol J(-1)] AM-T rp 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(Tr pN') = +1.0 V], must be of very little importance compared to an addit ion, This has been supported by a product study, The major products ar e 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 arbonyl-3-hydro xy-1,2,3,8,8-hexahydropyrroloindole (AM-HIP). The same products and pr oduct ratios are also formed, when singlet oxygen (from the irradiatio n of Rose Bengal with visible light) is reacted with AM-Trp suggesting that these two processes lead to the same (short-lived) hydroperoxidi c intermediate.