Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 295

ribof lavin differs mechanistically from that of the other which mainly produce
O 2 ( 1

∆ g ).
These studies have been extended to the photooxidation of d(TpG) by ribo-
f flavin and methylene blue, and again Z and its precursor Iz have been character-
ized (Buchko et al. 1995b). When d(TpG) is photooxidized with benzophenone
or menadione, an additional product, <d(TpG)>, is formed [reactions (305) and
(305); Delatour et al. 1999]. It constitutes a tandem lesion.

The effects of the three sensitizers, menadione, benzophenone and ribof lavin
that have been investigated with respect to their reactivity towards Thd, dGuo
and d(TpG) are compared in Table 10.34. Ribof lavin is only capable of sensitiz-
ing dGuo and the Gua moiety of d(TpG), and thus does not give rise to <d(TpG)>.
In contrast, menadione does not sensitize dGuo or the Gua moiety of d(TpG).
Mechanistically, it is likely that in the first step to <dTpG)> the allylic Thy
radical is created. There are two possibilities: (a) ET from Thy to the sensitiz-
er and deprotonation of Thy • + or (b) H-abstraction of the allylic hydrogen by
the sensitizer. The authors favor route (a). Yet Thy • + should oxidize rapidly the
neighboring Gua moiety. Moreover, it is difficult to see, why excited menadione