Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 209

In fact, calculations show that in the gas phase N (2)G • is more stable than G •
(Mundy et al. 2002; Naumov et al., unpubl. results; in Wetmore et al. 1998a ap-
parently an energy-richer rotamer (local minimum) has been calculated). The
cross-over in stability from N (2)G • to G • would occur in a solvent having a DK
like MeOH, and in -irradiated crystals of dGMP and cyclo-GMP N (2)G • (and

not G • ) is the stable species formed upon deprotonation of G • + (Hole et al. 1987,
1989, 1992a,b; Close et al. 1985; Nelson et al. 1988). The reason for a change-over
in stability as a function of solvent (environment) polarity is the marked dif-
ference of the dipole moments of G • and N (2)G • which has been calculated at
5.5 Debye (in water; Naumov et al., unpubl. results). This may now also explain
the results obtained when GMP is oxidized in aqueous solution by photoexcited
tris(1,4,5,8-tetraazaphenanthrene)ruthenium(II) (Jaquet et al. 1995). After de-
protonation of G • + in the solvent cage, the resulting N (2)G • adds to the ligand of
the Ru-complex forming a well-characterized product. Since all these reactions
are believed not to occur in the bulk but in the solvent cage where the partners,
GMP and the Ru-complex with its aromatic ligands provide an environment
whose effective DK will be considerably below that of water, N (2)G • is the stable
species under such conditions.
At high pH, G • deprotonates further [equilibrium (10), p K a (G • ) = 10.8; Can-
deias and Steenken 1989]. G • + (p K a = 3.9) is a weaker acid than the protonated
parent (p K a = 2.4) but a stronger acid than its corresponding neutral parent (p K a
= 9.4). Based on the redox potential and the p K a value, the N (1)-H binding en-
ergy in dGuo has been calculated at 380 kJ mol −1 (Steenken et al. 2001).
Ade has a considerably higher reduction potential than Gua (Table 10.2), and
for this reason it is not as readily oxidized, even by strongly oxidizing radicals,
SO 4 • − excepted. The p K a of the dAdo radical cation, A • + , lies at less than 1 [equi-
librium (11); Steenken 1989; for gas phase data see Hwang et al. 1999].

γ