Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 214

10 4 dm 3 mol −1 s −1 ], and thus the chain length becomes
dependent on the 1,3Me 2 Ura concentration.

[reaction (35), k = 1.2 ×

The same type of chain reaction is observed in the case of 1,3,6Me 3 Ura with one
minor difference: while with 1,3Me 2 Ura the only chain product is the glycol,
there are two chain products in 1,3,6Me 3 Ura, its glycol and 1,3,6-trimethyliso-
barbituric acid, formed in a ratio of 2:1 (Rashid et al. 1991). The latter is believed
to arise from the sulfate by an elimination of sulfuric acid. A deprotonation at
methyl does not take place. This is quite in contrast to the situation in 1,3Me 2 Thy
and other Thy systems discussed above.
A chain reaction is also apparent in the reaction of SO 4 • − with Thy, since the
quantum yield of Thy destruction exceeds = 2.0 in the photolysis of peroxodi-

sulfate in the presence of Thy (Sudha Sawargara and Adinarayana 2003).
In pulse radiolysis studies of Urd and its derivatives (but not with dUrd),
spectral changes are observed after the completion of the SO 4 • − reaction [ k =
3

Φ

10 5 s −1 ; Bothe et al. 1990] that are not typical for SO 4 • − reactions with py-
rimidines. On the basis of EPR experiments (Hildenbrand 1990; Catterall et al.
1992), these observations can be interpreted by an (overall) intramolecular H-
transfer giving rise to a radical at the sugar moiety. This requires that consider-
able amounts of Ura are released which is indeed observed (Fujita et al. 1988;
Aravindakumar et al. 2003; Table 10.4). Chain reactions occur as with the other
pyrimidine/peroxodisulfate systems. This increases the Ura yield beyond that
expected for a non-chain process, but when corrections are made for this by car-
rying out experiments at the very high dose rates of electron-beam irradiation, a

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