Addition of E⁻_aqand H Atoms to Hypoxanthine and Inosine and the Reactions of α-hydroxyalkyl Radicals with Purines. A Pulse Radiolysis and Product Analysis Study

Abstract

The reactions of hydrated electrons e-aq with hypoxanthine and inosine were followed using pulse radiolysis methods. In a neutral solution the electron adduct of inosine is immediately protonated at the heteroatoms of the purine ring by water (k >> 2.5 x 10(6)s-1) to give In(N,O-H).. These N,O-protonated intermediates have a single absorption maximum at 300 nm. In basic solution the protonation of the electron adduct of inosine by water leads to other intermediate products with an absorption maximum at 350 nm. These intermediates are believed to be the C-protonated electron adducts of inosine (In(N,O-H).). In (N,O-H). and In(C-H). differ strongly in their ability to reduce p-nitroacetophenone (PNAP). In(N,O-H). are strong reductants and reduce PNAP quantitatively to PNAP.-. Based on the pH dependence of PNAP.- yields, two types of tautomers of In(C-H). could be distinguished. One of the tautomers can reduce PNAP, albeit with slower rate than In(N,O-H)., the other tautomer has no reducing properties. The latter is the one with the higher pKa and therefore thermodynamically more stable. The absorption spectrum of the intermediates produced in the reaction of e-aq with hypoxanthine at neutral pH is very similar to that of In(N,O-H). with a maximum at 300 nm. However, no build-up at 350 nm was observed in basic solution as in the case of the electron adduct of inosine. The reaction of H atoms with inosine produces in basic solution intermediate radicals with the same absorption spectrum as the C-protonated electron adducts of inosine. It is suggested that both the reactions of e-aq and H. with inosine in basic solution produce the same radical, namely the H-adduct of inosine (In(C-H)) with the highest pKa. alpha-Hydroxyalkyl radicals were found to react very slowly with purine bases and nucleosides in neutral to basic solutions. In acidic solution their reactivity increases and a number of rate constants were determined by pulse radiolysis measurements at pH 0.4. The intermediates from the reaction of 2-hydroxy-2-propyl radicals with inosine could be observed pulse spectrometrically in neutral and in basic solutions. In basic solution this reaction leads to intermediates with the same absorption maximum at 350 nm as that of the H-adduct of inosine. Furthermore, the yield of acetone was found to increase strongly in basic pH.(ABSTRACT TRUNCATED AT 400 WORDS)

Reactions of Hydrated Electrons with Triazine Derivatives in Aqueous Medium

“…The k obs vs concentration plots obtained with all of the triazines have intercepts in the Y-axis (for a typical plot, see Figure 2) that corresponds to the self-decay of eaq , most likely due to its reaction with trace amount of impurities. Such intercepts in the case of the reactions of eaq have been reported in a number of earlier studies (35,36). The second-order rate constants obtained for T, TMT, AT, and CA are in the order of 10 9 dm 3 mol -1 s -1 , and for DHT, it is 10 8 dm 3 mol -1 s -1 (Table 1).…”

“…(5) T carbon atoms in the triazine ring (C-yl radicals). Such carboncentered radicals are known to be able to transfer one electron to reducing agents such as MV 2+ and pNAP (para-nitroacetophenone) (35,39).…”

“…The electron adducts of triazines are expected to be stronger bases and hence can be rapidly protonated by water (reaction 6). The protonation of T •- is therefore likely to be very fast as observed with purine electron adducts ( − ) where the rate of protonation at the heteroatom is of the order of 10 8 s -1 . Therefore, it is logical to assume that the triazine electron adduct gets protonated at nearly equal (or higher) rate due to the presence of three equivalent nitrogens in the triazine ring.…”

Radiation-induced reduction of quinoxalin-2-one derivatives in aqueous solutions

“…These are the sites where protonation by water will probably occur preferentially since protonation is likely to occur first at a heteroatom but ultimate protonation may take place at carbon atoms (Hissung et al, 1981). Since quinoxalin-2-ones being (to some extent) structurally similar to purines and hypoxanthines, it can be reasonably assumed that radical anions derived from them are also rapidly protonated as in the case of adenine Hissung et al, 1981;Visscher et al, 1987), guanine Nese et al, 1992), hypoxanthine (Aravindakumar et al, 1994), methyl derivative of xanthine (Rao et al, 1995), inosine (Aravindakumar et al, 1994), and 2'-deoxyinosine (Rao et al, 1996). This mechanism is apparently a general one for electron adducts of heterocyclic compounds Novais and Steenken, 1986).…”

Reaction of Hydroxyl Radicals with Azacytosines: A Pulse Radiolysis and Theoretical Study

“…The photomultiplier output was digitized with a 100 MHz storage oscilloscope interfaced to a computer for kinetic analysis. The details of the pulse radiolysis setup have been described elsewhere …”