Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 224

On the other hand, the C (6)- • OH-adduct formed in reaction (61) has oxidiz-
ing properties (note its mesomeric form with the radical at oxygen; quantum-
mechanical calculations indicate that it is the most oxidizing radical among all
conceivable • OH- and • H-adducts of the nucleobases; Colson and Sevilla 1995).
The yield of this oxidizing radical can be determined with the help of a strong re-
ductant such as TMPD [reaction (63); the TMPD radical cation is monitored;

ε (565

nm) = 12,500 dm 3 mol −1 cm −1 ].

The allylic radical formed upon H-abstraction from the methyl group [reac-
tion (60)] has neither reducing nor oxidizing properties, and its yield may be
deducted from the difference of the sum of reducing and oxidizing radicals with
respect to the total • OH yield. A series of such experiments has been carried out
(Fujita and Steenken 1981; Al-Sheikhly and von Sonntag 1983; Hazra and Steen-
ken 1983), and their results are compiled in Table 10.7.
The yield of 1,4-benzosemiquinone anions in the presence of 1,4-benzosemi-
quinone (Hayon and Simic 1973; Simic and Hayon 1973; Bamatraf et al. 1998)
has been taken as a measure for the yield of reducing radicals formed upon • OH
attack. The values reported for Cyt (83% by Bamatraf et al. 1998, 75% by Simic
and Hayon 1973) and Thy (63% by Bamatraf et al. 1998, 35% by Simic and Hayon
1973, 41% by Hayon and Simic 1973) do not agree well with one another and only
approximately with the data in Table 10.7. The values for dCyd (75%) and Thd
(47%) (Bamatraf et al. 1998) are surprisingly low in comparison, especially since
nearly all sugar derived radicals should also react with 1,4-benzoquinone by re-
duction. However, in this context it is recalled that an addition which is the more