Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 276

mediate by t BuOH under the given conditions. Thus, the suggested mechanism
must remain tentative. A way out of this dilemma would be if the radical anion
first protonated at a heteroatom and subsequently at C (8) (these are typical pu-
rine reactions, see above, and the latter step would be connected with such a
large KIE, see above). The resulting N (9) radical would have to undergo Br • loss
yielding the observed product Guo and the right isotope labeling. Elimination of
Br • in

β -fragmentation reactions is common.

10.8
Reactions of Sugar-Derived Radicals
in Nucleosides and Nucleotides

The bond dissociation energies of the various C-H bonds in the 2-deoxyribose
moiety have been calculated on the basis of N (1)-H BDE in dGuo (380 kJ mol −1 ;
Steenken et al. 2001). They are compiled in Chap. 4, Table 4.6.
The C (1 H. In H-

abstraction reactions, these positions are the most likely ones to be attacked be
reactive free radicals, while those bound to C (2

′ ) − H is the most loosely bound hydrogen, followed by C (4 ′ ) −

) are the most unlikely ones.
This argument only holds when other parameters, e.g. steric factors and aces-
sibility as in DNA (Chap. 12.2), can be neglected. In the ribo -series, the C (2

′

)-H
is, of course, again only loosely bound, and many striking differences between
the free-radical chemistry of 2

′

-deoxynuleosides and nucleosides and more
noticeably in the corresponding homopolymers may be due to this difference
(Chap. 11.2).

′

10.8.1
Isomerization at C (1 ′ ) and C (4 ′ )

The C (1 ) radical assumes a close to planar conformation. Upon subsequent re-

duction the

′

-isomers are likely to be formed in approximately equal
yields [reactions (248) and (249)].

α - and β