Formation of Reactive Free Radicals in an Aqueous Environment - Free-Radical-Induced DNA Damage and Its Repair - page 7

they will cause further ionizations in the very neighborhood of their formation.
Considering these mechanisms of energy absorption, it is understood why high-
energy electrons (1−10 MeV) and

γ -rays (60Co- γ -rays: 1.3 MeV, 137Cs- γ -rays:

0.5 MeV) have practically the same LET, and thus results obtained by γ -radioly-

sis can be compared with those from electron-beam irradiation.
Ionizing radiation is absorbed by the components of a given mixture approxi-
mately proportionally to the contributions by weight of the various components.
Thus, when an aqueous DNA solution containing 500 mg dm −3 DNA is

γ -irradi-

ated, ∼ 99.5% of the energy of the ionizing radiation is absorbed by the water and

only 0.5% by DNA. Under these conditions, the overwhelming contribution of

free-radical damage to DNA will thus be caused by the free radicals generated by
the radiolysis of water, and only a negligible part will arise from the absorption
of the energy of the ionizing radiation by DNA itself (for the situation in a cel-
lular environment see Chap. 12).
When the energy loss of high-energy electrons is absorbed by water,

∼

100 eV
of energy is deposited per ionization event (on average), whereby a water radi-
cal cation and an electron is generated [reaction (1)]. The latter may still con-
tain enough energy to cause further ionizations in the very near neighborhood.
These areas, containing a number of ionization and occasionally also electronic-
excitation events [reaction (2)], are called spurs. In the case of sparsely ioniz-
ing radiation these spurs do not overlap. In densely ionizing radiation, however,
they indeed do forming cylinders of spurs called tracks, wherefrom some

∼

-rays
(medium-energy electrons) may branch off (for the effects of high-LET radiation
see, e.g., Swiatla-Wojcik and Buxton 1998).

δ

(1)

(2)

The water radical cation, produced in reaction (1), is a very strong acid and im-
mediately loses a proton to neighboring water molecules thereby forming • OH
[reaction (3)]. The electron becomes hydrated by water [reaction (4), for the
scavenging of presolvated (Laenen et al. 2000) electrons see, e.g., Pimblott and
LaVerne (1998); Pastina et al. (1999); Ballarini et al. (2000); for typical reactions
of e aq − , see Chap. 4]. Electronically excited water can decompose into • OH and
H • [reaction (5)]. As a consequence, three kinds of free radicals are formed side
by side in the spurs, • OH, e aq − , and H • . To match the charge of the electrons, an
equivalent amount of H + are also present.

H 2 O • + • OH + H +

→ (3)

e − + n H 2 O e aq − → (4)

H 2 O* • OH + H • (5)

→

Since a spur can contain more than one free-radical pair, there is always the pos-
sibility that they interact with one another [ cf. the higher yields of e aq − and • OH