A reevaluation of the ambident reactivity of the guanine moiety towards hydroxyl radicals.
ABSTRACT Radically different: Contrary to previous proposals, the main reaction of the HO(*) radical with guanosine or 2'-deoxyguanosine is the hydrogen abstraction from the NH(2) moiety to give a guanyl radical. This radical, characterized by a broad band in the visible region (around 610 nm), undergoes tautomerization to the most stable isomer.
Article: Tautomerism in the guanyl radical.[show abstract] [hide abstract]
ABSTRACT: Despite a few decades of intense study, a full description of tautomers of one-electron-oxidized guanine remains to be achieved. Here we show that two of these tautomers are produced by the protonation of an 8-haloguanine electron adduct. The rate constants for the reactions of hydrated electrons (e(aq)(-)) with a variety of 8-substituted guanine derivatives have been measured by a pulse radiolysis technique and correlated with both inductive and resonance components of the substituents. The fate of electron adducts was investigated by radiolytic methods coupled with product studies and addressed computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The reaction of e(aq)(-) with 8-haloguanosine or 8-halo-2'-deoxyguanosine produces the first observable transient species that decay unimolecularly (k = 1 x 10(5) s(-)(1) at 22 degrees C) to give the one-electron oxidized guanosine or 2'-deoxyguanosine. Theory suggests that the electron adducts of 8-bromoguanine derivatives protonated at C8 form a pi-complex, with the Br atom situated above the molecular plane, that is prompt to eject Br(-). The two short-lived intermediates, which show a substantial difference in their absorption spectra, are recognized to be the two purine tautomers (i.e., iminic 7 and aminic 3 forms). The spin density distributions of the two tautomers are quite different at the O6 and N10 positions, whereas they are very similar at the N3, C5, and C8 positions. The resonance structures of the two tautomers are discussed in some detail. B1B95/6-31+G calculations show also that the tautomerization from the iminic (7) to the aminic (3) arrangement is a water-assisted process.Journal of the American Chemical Society 11/2006; 128(42):13796-805. · 10.68 Impact Factor
- Journal of The American Chemical Society - J AM CHEM SOC. 04/2002; 117(43).
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ABSTRACT: The reaction of .OH with 2′-deoxyguanosine yields two transient species G4.-OH and G8.-OH, which have strongly different reactivity towards O2 or other oxidants or to reductants. G8.-OH reacts with oxygen with k=4×109 M−1 s−1; in the absence of oxygen it undergoes a rapid ring-opening reaction (k=2×105 s−1; see scheme). Both G8.-OH and its ring-opened successor are one-electron reductants. In contrast, G4.-OH has oxidizing properties; it undergoes a slower transformation reaction (k=6×103 s−1) to produce the even more strongly oxidizing (deprotonated) 2′-deoxyguanosine radical cation.Chemistry 02/2000; 6(3):475 - 484. · 5.93 Impact Factor