Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 277

When the starting material is the nucleoside, the reformation of the -form

cannot be monitored, but in the case of dAdo (Mariaggi et al. 1979) and dGuo
(Berger and Cadet 1985) the

β

-form as well as the pyranose forms have been
detected. To account for the latter, one has to assume that during the radical life-
time hydrolysis of the O -glycosidic linkage occurs [reaction (251)] and reclosure
to the pyranose forms takes place [reaction (252)] prior to a subsequent reduc-
tion [reactions (253) and (254)]. A rapid hydrolysis of the O -glycosidic linkage is
a well-documented process in the free-radical chemistry of disaccharides (Ko-
chetkov et al. 1965, 1968; Dizdaroglu and von Sonntag 1973; von Sonntag et al.
1976; Adam 1977; Zegota and von Sonntag 1977), but that the reverse reaction
must also be fast has no reported analogy in these systems.
The C (1

α

) radical can also be produced upon UV-photolysis of the correspond-
ing tert -butylketone (see below; Goodman and Greenberg 1996; Chatgilialoglu
et al. 2000). In the presence of H-donors such as thiols or 1,4-cyclohexadiene, the
dUrd-1

′

-deoxynucleo-
sides [cf. reactions (248) and (249)]. The rate constant of H-donation by thiols
varies only little (mercaptoethanol: 2.3

′ -yl radical is reduced into a 3:2 mixture of the β - and α -2 ′

10 6 ; cystein: 2.9 10 6 ; glutathione: 4.4

× ×

10 6 dm 3 mol −1 s −1 ) and so does the anomeric ratio of the product, dUrd (Chat-
gilialoglu et al. 2000).
In the case of dGuo an isomerization at C (4

×

) has also been observed. In a
preliminary communication this product had been assigned to the

′

-anomer

β