DNA and Double-Stranded Oligonucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 389

Upon attack at C (2 ) and in the presence of O 2 , the sugar moiety is mainly modi-

fied into an erythrose unit (Sugiyama et al. 1993) which is an alkali-labile site
and undergoes a retroaldol reaction after deprotonation of H4

′

′ [reaction (45)],

but a Z-form type structure favors C (2 ′ )-hydroxylation.

Based on kinetic isotope effects, it has been suggested that the observed forma-
tion of the 2dRL lesion in the anaerobic photolysis of 5BrUra-containing duplex
DNA may not be caused by H1

-abstraction (Cook and Greenberg 1996). It has
been proposed that in the excited state ET occurs from a neighboring A to 5BrU-
ra [cf. reaction (40)]. Oxidation of the C (2

′

)-radical (formed by H-abstraction by
the 5-uracilyl radical) by A • + and subsequent 1,2-hydride shifts have been sug-
gested to lead to the observed products [reactions (46) and (47)].

′

When BrdUra- or IdUrd-substituted DNA is in contact with a protein, cross-link-
ing between DNA and the protein is observed upon UV-irradiation (Ogata and
Gilbert 1977; Babkina et al. 2002; Hicke et al. 1994). The chemistry of this in-
teresting reaction has not yet been elucidated. Interstrand DNA cross-links are
among the most potent inducers of sister chromatid exchanges (SCE), and hence
the high efficiency of SCE formation in the photolysis BrdUra-labeled DNA has
been tentatively attributed to cross-linking (Wojcik et al. 2003), and in the mean-
time this hypothesis has been experimentally substantiated (Wojcik et al. 2005).
In the case of sensitization of halouracil-containing DNA with ionizing ra-
diation, attention has been drawn to potential contributions of resonant electron
capture of low-energy electrons (Abdoul-Carine et al. 2001). The reaction of e aq −
with 8-Br-dGuo (Chap. 10.7) incorporated into DNA leads to a conversion of 8-
Br-G into G in high yields with Z-form DNA being more effective than B-form
DNA (Kimura et al. 2004).