Carbon-Centered Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 101

mer, while the latter remains unreactive. For example, only 2-hydroxy-2-propyl
radical anion transfers electron to 4-bromobenzonitrile thereby inducing an ef-
ficient chain reaction. The ET reaction (19) is so fast that it is not the rate-limit-
ing step in this chain reaction (Fang et al. 1997).
Typically, the reactivity of the halogen substituent follows the sequence I > Br
> Cl (Lemmes and von Sonntag 1982), and three-electron-bonded adducts to
the halogen are potential intermediates (for similar intermediates see Chapts 5.2
and 7.4). This type of reaction is also given by 5IUra and 5BrUra (Chap. 10.7).
6.4
Reduction of Carbon-Centered Radicals by Electron Transfer
In the reduction of radicals by ET, simple carbanions are practically never formed,
and one-electron reduction of a carbon-centered radicals is only effective if the
electron can be accommodated by the substituent, e.g., a carbonyl group [reac-
tion (24), whereby upon electron transfer the enolate is formed (Akhlaq et al.
1987)]. Thus, in their reduction reactions these radicals react like heteroatom-
centered radicals despite the fact that major spin density is at carbon.
The mesomeric forms of the pyrimidine C (6)-adduct radicals may be writ-
ten with the free spin at a heteroatom and hence have as oxidizing properties.
Their yields have been determined with the help of a strong reductant, TMPD
(Chap. 10.3). Other convenient probes for monitoring oxidizing radicals are, for
example, ABTS and ascorbate (Wolfenden and Willson 1982; Bahnemann et al.
1983).
Radical cations are especially strong oxidants, since they are highly electron
deficient. They are intermediates in • OH-induced DNA strand breakage and are
capable of oxidizing a neighboring G (Chaps 12.4 and 12.9).