Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 236

sugar moiety will also not exceed 10-15%. Product yields of the reaction of • OH
with dGuo and Ade are compiled in Tables 10.13 and 10.14. It is re-emphasized
that the material balance with respect to • OH formed under the given conditions
is very poor.
Based on a pulse-radiolytic study, it has been concluded that 17% of • OH add
to the C (8)-position of dGuo (see above). The ensuing reactions are complex and
seem to involve a number of equilibria,

β -fragmentation and 1,2-H-shift reac-

tions as depicted in reactions (94)−(104).

Final products then arise from subsequent reduction/oxidation (H-transfer) re-
actions. The C (8)- • OH-adduct undergoes rapid ring-opening [reaction (94), k =
2

10 5 s −1 ]. The C (8)- • OH-adduct and its ring-opened successor radical are re-
ducing radicals and readily react with Fe(CN) 6 3− or methylviologen. The oxida-
tion product of the ring-opened radical has not yet been identified, but the oxi-
dation product of the C (8)- • OH-adduct is a well-known product, 8-oxo-G. With
9MeGua, the formation of 8-oxo-9MeGua increases to about 20% in the presence
of Fe(CN) 6 3− [reactions (103) and (104); Candeias and Steenken 2000]. Interest-
ingly, the Fe(CN) 6 3− concentration required to oxidize fully the precursor radical
is much lower than suggested by the rate constant assigned to the ring-opening
process, and it has hence been assumed that reaction (94) must be reversible. The
yield of 8-oxo-G depends also on the pH. In the presence of Fe(CN) 6 3− , the maxi-
mum yield of G = 1.3

×

10 −7 mol J −1 is reached at an Fe(CN) 6 3− concentration of ×

10 −4 mol dm −3 in acid solution (von Sonntag and Schuchmann 2001; see also
Vieira et al. 1993). This G value corresponds to the yield of the C (8)- • OH-adduct,
i.e. there seem to be no side reactions that compete with this oxidation.
The ring-opened radical may also be reduced in, for example, disproportion-
ation reactions to yield FAPY-G [reactions (95) and (96)]. A 1,2-H-shift [reac-

5 ×