Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 213

The allylic Thy radical is observed by EPR in Thd (Catterall et al. 1992) and TMP
(Hildenbrand 1990). The identification of the C (6)-OH-5-yl radical by EPR sup-
ports the view (Deeble et al. 1990) that reaction with water competes with a de-
protonation at methyl. Due to the ready oxidation of the (reducing) C (5)-OH-6-yl
radicals by peroxodisulfate, this type of radical is only observed at low peroxo-
disulfate concentrations in these systems, i.e. the (oxidizing) C (6)-OH-5-yl
radicals may be correspondingly enriched (Schulte-Frohlinde and Hildenbrand
1989) (note that the nucleobases themselves are not oxidized at a reasonable rate
unless deprotonated; Moschel and Behrman 1974). These reducing C (5)-OH-6-
yl radicals are capable of reacting with peroxodisulfate and thus induce chain
reactions which in the case of 1,3Me 2 Ura shows some very interesting properties
(Schuchmann et al. 1987). It is nearly independent of the peroxodisulfate con-
centration, but shows a marked dependence on the 1,3Me 2 Ura concentration.
From this, it immediately follows that the mechanism is not characterized by the
reaction of the reducing C (5)-OH adduct radical with peroxodisulfate as the rate
determining step [reaction (30), k = 2.1 10 5 dm 3 mol −1 s −1 ], yielding exclusively

SO 4 2− , the glycol [via the carbocation and water, reaction (32)]. This would be
the trivial case of an SO 4 • − -induced chain reaction (cf. Schuchmann and von
Sonntag 1988; Ulanski and von Sonntag 1999). One rather has to consider that
in this reaction the 1,3Me 2 Ura carbocation, SO 4 • − and SO 4 2− are formed within
the cage [reaction (30)]. There, the carbocation may recombine with either of
the two anions or react with water [reactions (31)−(33)]. When the carbocation
reacts with SO 4 • − [reaction (33)], a new oxidizing species is formed which, how-
ever, is not as reactive as SO 4 • − . It propagates the chain with only a slow rate

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