Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 251

cals; Chap. 6.2). In contrast to a protonation at the heteroatom [reaction (164)],
protonation at carbon [reaction (165)] is connected with some conformational
changes, and thus the conversion into this thermodynamically more stable (for
DFT calculations see Naumov and von Sonntag, unpublished results) isomer
can only be observed on the pulse radiolysis time scale, when buffer is added to
speed up the protonation/deprotonation reactions (Das et al. 1984; Deeble et al.
1985; for rate constants see Table 10.20; for an EPR study see Novais and Steen-
ken 1986).
Together with this transformation, the redox properties of the radicals change
dramatically. While the radical anion and its O -protonated conjugate acid are
good reducing agents has the C (6) protonated tautomer oxidizing properties.
This is also ref lected in their reactions with O 2 . The former two give rapidly rise
to HO 2 • /O 2 • − thereby restoring the pyrimidine [reactions (167) and (168); Deeble
and von Sonntag 1987]. On the other hand, when O 2 reacts with the C (6) proton-
ated tautomer [reaction (169)], the pyrimidine chromophore is destroyed in the
subsequent reactions of the ensuing peroxyl radical. In the absence of O 2 , the
5,5

-dihydrodimer is formed [reaction (166); Ito et al. 2002].
The radical anion can also be repaired by polyphenols such as rutin or quer-
citin (Shi et al. 2000b; Zhao et al. 2003). Yet, this repair is not related to their phe-
nolic functions but is rather due to an ET to their carbonyl (f (flavone) functions.
Photolysis of 5,6-dihydro-5-selenophenyl-dTyd (Tallman et al. 1998) also af-
fords the C (6)H • -adduct, and in the presence of O 2 the corresponding peroxyl
radical [reactions (170) and (171)]. The latter may undergo HO 2 • -elimination
giving rise to Thd [reaction (172)] or, in the presence of tributyltinhydride, yields
5,6-dihydro-5-hydroxy-dTyd [reaction (173)]. The ratio of these two products
depends on the tributyltinhydride concentration, and from such data the ratio
of the rate constants of reactions (172) and (173) has been calculated at 1.3

′

×

10 −2 mol dm −3 .

65 s −1 assuming a reactivity
of tributyltinhydride similar to that of a thiol. For the latter a rate constant was
taken from the work of Schulte-Frohlinde et al. (1986) that later had to be revised
to a much lower value (Hildenbrand and Schulte-Frohlinde 1997; Lal et al. 1997).

The rate of reaction (173) has been estimated at

∼