Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 252

Thus the rate of HO 2 • -elimination must be considerably slower (nearer to 1 s −1 ),
but still fast enough to play a role in DNA free-radical chemistry. Reaction (172)
is reminescent of the HO 2 • -elimination from -hydroxycyclohexadienylperoxyl

radicals (derived from the reaction of • OH with benzene in the presence of O 2 ;
Chap. 8.4). In this system, the reaction is much faster ( k = 800 s −1 ; Pan et al.
1993b); possibly due to the gain in energy in the course of the re-aromatiza-
tion].
In this context it is worth considering that in the free-radical chemistry of
DNA the C (6)- • OH-adducts radicals are certainly of a greater importance than
the C (6)-H-adduct radicals investigated here. If the benzene system is a good
guide HO 2 • -elimination from hydroxycyclohexadienylperoxyl radicals is notice-
ably slower than that from cyclohexadienylperoxyl radicals (Pan et al. 1993a),
i.e., the rate of HO 2 • -elimination from 5,6-dihydro-6-hydroxy-thymidine-5-per-
oxyl radicals may even be slower.
The Cyt electron adduct is rapidly protonated by water [reaction (174); t 1/2
< 200 ns, Hissung and von Sonntag 1979; t 1/2 < 20 ns, Visscher et al. 1988] most
likely at N (3) at or at the amino group (Symons 1990; Hüttermann et al. 1991;
Podmore et al. 1991; Barnes and Bernhard 1994).

β

The resulting neutral radical must have a p K a value 11, as shown by conduc-

tance measurements in pulse radiolysis (Hissung and von Sonntag 1979). The
absence of any noticeable changes in the absorption spectrum of the radical
derived from Cyd in the pH range 6-13 suggests that its p K a value is even >13
(Steenken et al. 1992). Like the situation in the corresponding Thy system, the
heteroatom-protonated species is not thermodynamically favored and subse-
quent (irreversible) protonation seems to occur at carbon [reaction (175)], albeit
with a rate constant (estimated at 2.5

≥

10 3 s −1 ; Nese et al. 1992) too slow to be ×

measured by pulse radiolysis.
For studies on the formation of the Ura-H • -adducts at O (4) and C (5) in the
gas phase see Syrstad et al. (2001) and Wolken and Turecek (2001).

Purines. With the purines, H • and e aq − react at close to diffusion-controlled rates.
Besides that, very little is known about the reactions of H • . However, H • and • OH
being both electrophilic radicals (Chaps 3.2 and 4.4), the position-specificity for
the addition of H • should be very similar to that of • OH.
The properties of the purine radical anions formed by their reaction with
e aq − resemble those of the Cyt radical anion. Because the p K a values of the het-
eroatom-protonated conjugate acid of the purine radical anions are very high,
the purine radical anion are rapidly protonated by water (Hissung et al. 1981a;
Visscher et al. 1987; von Sonntag 1991; Candeias and Steenken 1992a; Candeias
et al. 1992; Aravindakumar et al. 1994). For example, the dAdo radical anion