Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 212

From the competition of phosphate and water for the intermediate(s) formed by
SO 4 • − , it has been concluded that the rate constant of reaction with phosphate
must be 10 8 dm 3 mol −1 s −1 . The transformations of intermediates that play

a role in this system by OH − and by phosphate were followed by EPR techniques
(Hildenbrand et al. 1989; Geimer and Beckert 1999).
When the electron-donating methyl substituent is introduced at C (6), the
C (6)-phosphate-adduct radical is no longer stable on the EPR time scale, and

≥ 4 ×

only its hydrolysis product is observed [reaction (24); Behrens et al. 1988].
The reason for the greater stability of the phosphate radical-adduct compared
the sulfate radical-adduct is due to effects discussed in Chapter. 6.9.
It has been mentioned above that the pyrimidine radical cations are reason-
ably strong acids and rapidly deprotonate at a heteroatom. As all protonation/
deprotonation reactions at heteroatoms are reversible [e.g., equilibrium (22)],
the radical cations are regenerated upon reprotonation. Deprotonation at carbon
or reaction with water yield the final free-radical products. For the 1,3Me 2 Thy
system, where the deprotonation/reprotonation equilibria such as reaction (22)
fall away, reactions (25)-(28) have been postulated to account for the fact that in
the presence of O 2 1,3Me 2 5HOMeUra and 1,3Me 2 5(CHO)Ura [reaction (29)] are
formed in a combined yield of 80% of primary SO 4 • − radicals (Rashid et al. 1991).
The formation of these products has been taken as evidence that a free radical
cation must be an intermediate. It is, however, also possible that the allylic radi-
cal is formed in a concerted reaction HSO 4 − elimination. For such a process, a
six-membered transition state can be written.