DNA and Double-Stranded Oligonucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 430

Fig. 12.17. Radical structures assigned in γ -irradiated solid DNA

and ET to DNA occurs from the hydration shell. Some • OH are also formed and
trapped in this hydration layer (Becker et al. 1994; Mroczka and Bernhard 1993)
and may be scavenged by a spin trap upon warming up (Ohshima et al. 1996; La
Vere et al. 1996). In dry DNA, T and C, and to some extent A, are the trapping sites
for the electron and G for the hole (Wang et al. 1994; see also Sevilla et al. 1991).
More than ten chemical structures have been assigned (Fig. 12.17; Weiland and
Hüttermann 1998; Yan et al. 1992). They mainly consist of the one-electron-re-
duced pyrimidines (in different states of protonation). The one-electron-reduced
T converts by protonation at C (6) (Chap. 10.4). H atom scavenging by the purines
or protonation of their electron adducts at C (8) yields an N -centered radical. The
purines largely serve as the sink for the hole. Detected H-abstraction sites were
attributed to the sugar moiety and the methyl group of T. The latter radical may
also be produced by the deprotonation of the T radical cation (Chap. 10.2). This
has been put on a quantitative basis, and it has been suggested that at early times
the electrons are captured by T (30-35%) and C (20-28%), while among the radi-
cal cations G (26-28%) and A (8-17%) dominate (Yan et al. 1992). ET from C • − to
T and subsequent protonation at C (6) occurs at a later stage.
Upon

γ -irradiation of dry DNA unaltered bases are set free with a G value of

10 −7 mol J −1 for Ade, Cyt and Thy, and 0.12 10 −7 mol J −1 for Gua in about 0.2 × ×

10 −7 mol
J −1 (Gua) in its presence (Swarts et al. 1992). Hydration of the DNA lowers these
values somewhat. The precursor of these unaltered bases are sugar radicals, i.e.
largely sugar radical cations. These are strong oxidants, and since G is the best
electron donor among the nucleobases, it is likely that the G-bound sugar radical
cation may effectively be reduced by the neighboring G (Sect. 12.10). This must

10 −7 mol J −1 (Ade, Cyt and Thy) and 0.25 the absence of O 2 , and 0.4 × ×