Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 237

tion (98)], typical for heteroatom-centered radicals, may also contribute to the
formation of 8-oxo-G [reaction (100)] and FAPY-G [reactions (99) and (97)]. In
Table 10.13, the formation of two isomers is reported. With dAdo, considerable
isomerization and even the hydrolysis of FAPY-A is observed (see below). Equi-
librium (97) would not account for the isomerization of β -FAPY-G into α -FAPY-

G. Thus it is likely that this isomerization occurred during work-up.
In this context it is worth mentioning that there is evidence (Candeias and
Steenken 2000) that neither G • + nor G • react with water at an appreciable rate ( k
= < 0.1 s − 1 ) forming the C (8)- • OH-adduct, that is, the precursor of 8-oxo-G. This
seems to be in contradiction to a later suggestion by this group that at least the
reaction of G • + with water is highly exothermic (306 kJ mol −1 ; Reynisson and
Steenken 2002) and may undergo this reaction, [reaction (57)] quite readily un-
less kinetically disfavored.
8-oxo-G is typically synthesized according to Kasai and Nishimura (1984) by
an Udenfriend-type reaction (Chap. 2.5), but in this synthesis a steady-state con-
centration of only about 6% (with respect to remaining dGuo) is reached (Wager,
Schuchmann and von Sonntag, unpubl.). Mere competition for • OH would result
in a much higher yield. 8-oxo-G is much more readily oxidized than dGuo itself
(see above), and in the sequence of events there must be an oxidant other than
• OH which is responsible for this low yield. Despite a more recent study of this
reaction (Hofer 2001), mechanistic details remain unresolved.
The C (8)- • OH-adduct of Ade and its derivatives shows the same reaction pat-
tern [reactions (105)−(115)] as discussed above for Gua and its derivatives.

There are only minor differences. Its yield is higher ( 37%). With

N 6 , N 6 Me 2 dAdo, the ring opening reaction (105) occurs at a rate of 9.5

∼ 30 −

10 4 s −1 ,
and its rate is enhanced by OH − (Vieira and Steenken 1987a,b). The yield of

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