Guanine cation

Whereas other experimental methods have been used to obtain values of kti no other method provides values of k-t or equilibrium data. There are, however, several important limitations of our method. First, the method is restricted to relatively fast hole transport processes that can compete with charge recombination of the Sa -G+ radical ion pair (Fig. 6). This precludes the use of strong acceptors which can oxidize A as well as G (Fig. 2a). We find that hole transport cannot compete with charge recombination in such systems, even when a charge gradient is constructed which should favor hole transport [35]. Second, the method is unable to resolve the dynamics of systems in which return hole transport, k t, is very slow (<104 s-1) or systems in which multiple hole transport processes occur. Third, since the guanine cation radical cannot be detected by transient spectroscopy, the method is dependent upon the analysis of the behavior of Sa-. In section 3.4 we de-... 

Therefore, the N(9) radical should be more stable than the N(6) one. That is why both radicals coexist in the system and both N(9) and N(6) deprotonations take place. In the case of the guanine cation-radical, the presence of the carbonyl group in the pyridazine ring brings about two additional effects Deprotonation infringes on this ring exclusively, and double deprotonation leads to the formation of a distonic anion-radical. Scheme 1.25 depicts the differences mentioned. Adhikary et al. (2006) substantiated it experimentally (ESR and UV) and theoretically (B3LYP). 

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]. 

Close DM, Sagstuen E, Nelson WH (1985) ESR study on the guanine cation. J Chem Phys 82 4386-4388... 

Hole EO, Sagstuen E, Nelson WH, Close DM (1992b) The structure of the guanine cation ESR/EN-DOR of cyclic guanosine monophosphate single crystals after X irradiation at 10 K. Radiat Res 129 1-10... 

Isotopic substitution (13C,2D,15N) can often be used to resolve complicated spectra. For example, if a labile proton is replaced with a deuterium, the proton isotropic hyperfine coupling is reduced by a factor 6.51. This technique was used in the adenosine study discussed in Section 18.3.5.1, and in the study of the guanine cation (Section 18.3.5.3). EPR spectra of a normal crystal of guanine HCl H20, and of the same crystal grown from DC1 D20 are shown in Figure 18-11. Most of these techniques are easy to try. If they don t succeed however, or if one is interested in measuring small hyperfine couplings, then one needs to consider the ENDOR technique. 

Crystals of 2 -Deoxyguanosine 5 -Monophosphate Tetrahydrate Disodium Salt (5 -dGMP) have a neutral guanine base. In the solid-state, oxidation of 5 -dGMP at 10 K leads to deprotonation at the exocyclic nitrogen which is characterized by p(C8) = 0.175 and p(N2) = 0.33 [37], The same radical was detected in crystals of 3, 5 -cyclic guanosine 5 -monohydrate. In this second study, the N3 spin density was determined to be 0.31 [38], These two studies then provide a detailed description of the amino deprotonated guanine cation G(N2-H) ... 

Figure 18-12. Structure of the guanine cation with spin densities p(N2) = 0.168 and p(C8) = 0.182. (Reprinted with permission from ref. [36], J. Chem. Phys. (1985) American Physical Society.)...

The guanine cation is a weak acid (pKa = 3.9) [5], Therefore deprotonation will depend on the environment. Bachler and Hildenbrand have studied the guanine oxidation product in aqueous solution of 5 -dGMP [53], The best fit to their EPR spectra seems to be from the radical cation (guanine remains protonated at Nl). 

A new paper by Adhikary et al. [61] has also looked at the deprotonated states of the guanine cation. This paper first revisits the calculated stabilities of G(N1-H) vs. G(N2-H) Their calculations agree with those of Mundy et al. [57] discussed above that G(N2-H) is more stable than G(N1-H) in a non-hydrated environment. However when discrete waters of hydration are added, G(N1-H) +7H20 is more stable than G(N2-H) 7H20. This paper is complimented with simulations of the EPR spectra that are obtained from experimentally determined hyperfine couplings. 

CT, and possible C4 and C5 sugar radicals have been observed in irradiated hydrated DNA at 77K [66], The CT sugar radical was reported by Razskazovskii et al. in a DNA double helix [69]. The C1 was also produced in double stranded DNA at 77 K by photoexcitation of the guanine cation radical [70], The C3 radical was reported to be 4.5% of the total radical yield in the duplex (d(CTCTCGAGAG)), x-irradiated and observed at 4 K [71], It is very likely that the DNA simulations could be improved with the inclusion of a small percentage of these typical sugar radicals. 

Scheme 20-2. The numbering scheme and prototropic equilibria of one-electron oxidized guanine cation radical (G +), the mono- deprotonated species, (G(N1-H) and G(N2-H), in syn and anti- conformers with respect to the N3 atom) and the di- deprotonated species, G(-2H) . (Reprinted with permission from ref. [189], J. Phys. Chem. (2006) American Chemical Society.)...

Adhikary A, Kumar A, Becker D, Sevilla MD. (2006) The guanine cation radical Investigation of deprotonation states by ESR and DFT. J Phys Chem B 110 24171-24180. 

At higher temperatures, ionic radicals are not expected to be stable, but rather these species protonate or deprotonate to form neutral, secondary radical products. As mentioned, the first of these products, T(C6H), was identified as evolving from the thymine anion in early ESR studies. Later, the decay of the guanine cation was predicted to be related to the growth of G(N1) [73,74]. T(CH2) has also been observed in highly hydrated DNA samples [73]. Evidence exists that the cytosine anion is stabilized by protonation at N3 at 77 K [75]. Additionally, in thymine deuterated DNA samples, a deuteron has been determined to add to the C6 position of the cytosine anion [73]. Despite the fact that the types of products observed are diverse, these products were each observed in different samples. 

Adhikary A., Malkhasian A.Y. S., Collins S., Koppen J., Becker D., Sevilla M.D., UVA-Visible photoexcitation of guanine cation radicals produces sugar radicals in DNA and model structures, Nuc. Acid Res., 2005,33,5553-5564. 

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