Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 290

(Greenstock and Dunlop 1973). The adducts are still radicals. As discussed in
Chap. 6.3, they may decompose unimolecularly by releasing the sensitizer radical
anion or undergo

-fragmentation yielding the nitroso compound and an oxyl
radical (Nese et al. 1995). Although in its outcome the former reaction is equiva-
lent to an ET, the branching (competition) occurs now at the level of the adduct.
Quinones are also strong oxidants, and their potential as radiation sensitiz-
ers has been investigated on the model level. Again, ET competes with addition
(Simic and Hayon 1972, 1973; Hayon and Simic 1973). With carbon-centered
radicals, a C

β

C bond is formed in the addition reaction, and subsequent reac-
tions are different (von Sonntag et al. 2004) from the adducts formed with nitro
compounds.

−

10.10
Irradiation in the Solid State

Experiments in the solid state, especially EPR studies, have supplemented our
knowledge in the area of the free-radical chemistry of nucleobases and related
compounds considerably. Here, only some salient points can be mentioned, and
for more detailed information the reader is referred to the excellent reviews that
have appeared on this topic (Wyard and Elliott 1973; Bernhard 1981; Hütter-
mann 1982, 1991; Close 1993; Sevilla and Becker 1994; Becker and Sevilla 1997)
and to the topic by Box (1977).
Mainly two types of experiments have been carried out, -irradiation in the

solid state (mainly single-crystals) and in glassy matrixes at low temperatures. In
addition,

γ

-irradiated solids may be dissolved in water containing spin traps, and
the spin-trapped radicals identified by EPR afterwards (cf. Mossoba et al. 1981;
Spalletta and Bernhard 1982; Zhang et al. 1983). While irradiation in a glassy ma-
trix is very much related to the situation that prevails in aqueous solutions, that is,
the radicals generated in the matrix react with the added substrates, irradiation in
the solid state may cause radical formation via two different processes: ionization
processes (formation of radical cation is and radical anions) and the decomposi-
tion of electronically excited states (which could also be formed upon recombina-
tion of a radical cation and an electron). It has often been tried to disentangle these
two primary processes, but, of course, such assignments are not always straight-
forward. Sometimes, a confusing terminology persists which calls the electron-
loss centers radical cations and the electron-gain centers radical anions, irrespec-
tive of their protonation state (i.e. charge sign).
Whenever H • -adducts are observed, they must not necessarily have H • as pre-
cursor. Protonation of the radical anion must always be considered as an alter-
native/additional route. In crystals, the radical cations may serve as the proton
source, while in frozen aqueous solutions the solvent will provide the proton.

γ

Frozen solutions. In frozen aqueous solutions, the additive may not precipitate
but accumulate in ice-free areas in a rather uncontrolled way. Upon irradiation,
there are only a few radicals from the radiolysis of ice that reach the solute, and
radical formation can occur by direct absorption of the energy of ionizing radia-