Tautomerism in the Guanyl Radical

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 (eaq -) 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 eaq - with 8-haloguanosine or 8-halo-2‘-deoxyguanosine produces the first observable transient species that decay unimolecularly (k = 1 × 105 s-1 at 22 °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 π-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.

Chemical Radiation Studies of 8‐Bromo‐2′‐deoxyinosine and 8‐Bromoinosine in Aqueous Solutions

“…These values are very similar to the analogous reactions with 8-bromoadenine [2,3] and 8-bromoguanine [4,6] nucleosides. The reaction of an aqueous solution of 8 (1 mm) and tBuOH (0.25 m) at pH% 7 with e aq À in the absence of O 2 was complete within % 300 ns.…”

“…The reactions of hydrated electrons (e aq À ) with 8-bromoadenine [1][2][3] and 8-bromoguanine nucleosides [4][5][6] have recently been investigated by radiolytic methods. It was found that 8-bromo-2'-deA C H T U N G T R E N N U N G oxyA C H T U N G T R E N N U N G adenosine (1) captures electrons and rapidly loses a bromide ion to give the corresponding C8 radical 2.…”

“…[5,6] 8-Bromo derivatives 1 and 5 were also incorporated in a variety of single-or double-stranded oligonucleotides and G quadruplexes as a detection system for excess electrontransfer processes. [8][9][10][11] Interestingly, the 8-bromo-2'-deoxy-A C H T U N G T R E N N U N G adenosine moieties in a series of DNA hairpins containing a Abstract: The reactions of hydrated electrons (e aq À ) with 8-bromo-2'-deoxyinosine (8) and 8-bromoinosine (12) have been investigated by radiolytic methods coupled with product studies and have been addressed computationally by means of BB1K-HMDFT calculations.…”

Theoretical Modeling of Radiation‐Induced DNA Damage

“…þ . These questions have been treated by a number of authors employing both theory and experiment [142][143][144][145][146][147][148][149][150][151][152][153][154][155][156][157][158] without definitive conclusions. Recently, electron spin resonance (ESR) and the B3LYP/6-31G(d) level of theory have been used to identify the preferred site of deprotonation from guanine radical cation (G .…”

“…Similar conclusions were also drawn by Naumov and Sonntag 158 using UV-visible spectral properties and DFT study recently. Chatgilialoglu et al 155,156 also studied the tautomers of one-electron-oxidized guanosine using UV-visible spectral properties and the DFT method. Based on DFT results, they proposed the presence of a transient species G (N2-H) .…”