Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 229

Table 10.9. γ -Radiolysis of N 2 O-saturated aqueous solutions of Ura. Products and their G
values (unit: 10 −7 mol J −1 ) at different pH values. (Idris Ali and Scholes 1980)

Product pH 2 pH 4 pH 6 pH 8

Dimers 1.35 3.3 4.38 1.65

5,6-Dihydro-5,6-dihydroxyuracil (Ug; cis and trans ) 0.68 0.32 0.30 0.18

5,6-Dihydro-5(and 6)-hydroxyuracil 0.31 0.44 0.29 0.18

Isobarbituric acid 0.29 0.12 0.52 0.16

Ura consumption 2.68 4.25 5.51 2.11

10.3.4
Products in the Absence of Oxygen

Pyrimidines. Detailed studies concerning the products that are formed upon
• OH-attack (radiolysis of N 2 O-saturated solutions) are available for Ura, Thy,
Cyt and 1,3Me 2 Ura (Tables 10.9-10.12).
In addition, the radiolysis of Ura in deoxygenated solutions (in the absence
of N 2 O) has also found attention (Infante et al. 1974; Shragge et al. 1974). Under
such conditions, however, not only the • OH-induced reactions play a role, but
also the electron-adduct radical with all the ensuing mechanistic complications
contributes to the products.
The N (1)

H of the nucleobases can take part in disproportionation reactions
leading to the formation of isopyrimidines (for their reactions see below). This
crucial hydrogen is not available in nucleosides which are a much closer model
for DNA. No detailed studies are available for pyrimidine nucleosides so far. A
compound which has no hydrogen at N (1) is 1,3Me 2 Ura. It still shows the major
aspects of the base moiety of Ura- and Thy-containing nucleosides but is much
more easy to handle analytically than nucleosides. Thus, results from a study on
the

−

-radiolysis of 1,3Me 2 Ura in N 2 O-saturated solutions (Table 10.12) may be a
good guide for the reactions that one might have to envisage in the nucleosides
and also in DNA. As has been discussed before, • OH adds mainly at C (5) (

γ

∼ 78%)

and to a lesser extent at C (6) ( 17%). Some H-abstraction at the methyls occurs

as well. The major reaction of the dominating C (6)-yl radical is its recombina-
tion [reaction (79)].

∼