Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 262

perature EPR experiments (Wang et al. 1997). Under such conditions the allylic
radicals adds to the C (6)-position [reaction (211)], but there is no evidence for
C (1

H abstraction. Thus, this observation may be of some importance regard-
ing potential DNA-DNA cross-linking reactions.

′ ) −

In contrast, with dCMP as a substrate that lacks the weakly-bound allylic hydro-
gens, H 2 NCOCH 2 • abstracts the C (1

H which is the most weakly-bound among
the hydrogens of the sugar moiety (Wang et al. 1997).
Addition of the (reducing) CO 2 • − radical to Thy (products not fully eluci-
dated) shows that this radical behaves similar to other alkyl radicals, and reduc-
tion of Thy is certainly not its only reaction (Wada et al. 1982). Similar studies
on Thd yielded the 5,5

′ ) −

10 −7 mol J −1 ) and H 2 Thd ( G =

′ -dihydrodimer ( G = 0.75 ×

10 −7 mol J −1 ; Nishimoto et al. 1983b). This does not yet fully account for the
total CO 2 • − yield (G

1.4 ×

10 −7 mol J −1 ), but is a strong indication that reduction
of Thd by CO 2 • − takes place in competition to addition.
The thiyl radical (RS • ) is electrophilic and hence with pyrimidines its pre-
ferred site of attack is C (5) and to a lesser extent C (6) (Jellinek and Johns 1970;
Varghese 1973, 1974; Shetlar and Hom 1987). Addition of RS • to C

≈ 6 ×

− C double

10 5
s -1 (Woijik et al. (2005); see also Chap. 7.4], and for fixing the RS • -adduct a fast
subsequent reaction is required [reaction (213)].

10 7 dm 3 mol -1 s -1 , k -212 bonds is highly reversible [reaction (212), k 212 ≈ ≈

Considering that the equilibrium concentration of the C (6)-yl radical should be
low, the major reaction should be a dimerization of RS • , but data confirming this
are not available.
In reaction (214), the addition of RS • to the C (6) position is followed by a
very rapid and irreversible cyclopropyl carbinyl radical rearrangement [reac-
tion (215); Carter et al. 2000]. The resulting radical is comparatively long-lived
and can be reduced by the thiol [reaction (216)]. Thus in this example, the minor
pathway is detected, while the other, being reversible, remains unobserved.