Deprotonated guanine radical cation

Scheme 6 Deprotonation of a guanine radical cation with and without a complementary base...

The guanine radical cation is a weak acid = 3.9) [22]. Therefore deprotonation will depend on the environment. Bachler and Hildenbrand [38] have studied the guanine oxidation product in aqueous solution of 5 -dGMP. The best fit to their EPR spectra seems to be from the radical cation (Gua " ). 

Some experiments have been performed on guanine molecules that were originally protonated at N7. Subsequent electron loss by this molecule leads to deprotonation at N7 yielding Gua(N7 + H, N7—H), which is equivalent to the guanine radical cation. The experimental results from this guanine cation have p(C8) = 0.18, p(N2) = 0.17, and p(N3) = 0.28 [40]. 

In double-stranded DNA, electron abstraction from the guanine radical cation can be associated with an extremely fast shift of the N1 proton to its Watson-Crick partner cytosine (Scheme 2a) [9]. The equilibrium constant for the protonation of C (pfCa=4.3) with the concomitant deprotonation of G estimated from the pK values of the free nucleosides, is about 2.5 [49]. Within these constraints, the guanine radical should retain some radical cation character [82] and the complete deprotonation of G would require a base pair opening event occurring on a millisecond timescale [74]. An alternative mechanism of G deprotonation is the release of the N2 proton (Scheme 2b). This mechanism was experimentally established for 1-methyl-guanosine conductometric results showed that in neutral solutions, the radical cation of this nucleoside rapidly deprotonates with the formation of the neutral radical [48]. Although the exact mechanism of the G deprotonation in double-stranded DNA requires further clarification, electron abstraction... 

In neutral aqueous solutions, the ultimate product of one-electron abstraction from guanine is the guanine neutral radical. In DNA, this radical is formed via the deprotonation of the guanine radical cation arising, e.g., from hole localization or directly via proton-coupled electron transfer from guanine to an appropriate electron acceptor. The G(-H) radicals do not exhibit observable reactivities with molecular oxygen (fc<10 s ) [93]. 

The first one electron oxidation produces a radical cation on the sugar phosphate (SP +). The radical cation subsequently deprotonates yielding a neutral carbon centered radical SP(-H). The second oxidation involves an electron transfer from SP(-H) to a nearby guanine radical cation G +. This step requires that the hole on the guanine have some mobility. It is known that a hole located on guanine at 4K is mobile, with a range of ca. 10 base pairs [76], The result of this second oxidation is a a deoxyribose carbocation SP(-H)+. 

Experiments with methyl guanine (27), in which the acidic proton of the radical cation is exchanged by a methyl group, support this explanation [22]. With this base in a mismatch situation (strand 25) the hole transfer becomes efficient again because a deprotonation cannot occur (Fig. 15). 

It is not clear what the structure of one-electron oxidized guanine is in DNA. The amino-deprotonated product observed in 5-dGMP does not seem to fit parameters of the oxidation species observed in DNA. Recently, Reynisson and Steenken have proposed that the one-electron oxidized species found in ds DNA is the radical cation [41]. 

Analysis of the transient absorption spectra recorded at 100 ns allows for the determination of the prompt relative yields of the G /G(-H) radicals, I g (<100 ns). This yield is probably due, in part, to the oxidation of the guanines by the 2AP + radical cations [11]. Assuming that the 2AP radical cations decay only via the deprotonation of 2AP (rate constant ku) and hole transfer from 2AP + to guanine (fct), the prompt yield Og may be expressed as follows ... 

Deprotonation of the dG radical cations in double-stranded DNA is evident from the EPR spectra of guanine radicals recorded in neutral solutions at room temperature [80, 81]. The EPR signal assigned to the neutral G(-H) radical derived from the deprotonation of G shows the singlet with g 2.004 and half width at a half height -0.8 mT. However, the deprotonation rate of G cannot be estimated from the conventional EPR spectra, and further time-resolved EPR studies with laser pulse generation of the radicals are required to address this problem. 

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