Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 266

ety to 5CH 2 OHCyt and 5CHOCyt in the absence of O 2 is observed but not yet
understood.
An additional product, <d(TpG)>, that is formed in the benzophenone and
menadione-sensitized oxidation of d(TpG), where the allylic Thy radical must
have added to the C (4) position of the neighboring Gua moiety, is discussed be-
low.

Purine-5 ,8-cyclonucleosides and -cyclonucleotides. With purine nucleosides/

nucleotides, cyclonucleosides/cyclonucleotides, cA and cG, have been observed
as products formed upon • OH-attack (Hagen et al. 1965; Keck et al. 1966; Raleigh
et al. 1976; Mariaggi et al. 1976; Raleigh and Blackburn 1978; Haromy et al. 1980;
Berger and Cadet 1983a; Raleigh and Fuciarelli 1985; von Sonntag 1994). Refer-
ence material has been synthesized (Romieu et al. 1999a), and a method for their
site-specific introduction into an ODN has been devised (Romieu et al. 1999c).
It is usually assumed that an H-abstraction at C (5

′

′ ) by • OH (see, however, be-

low) leads to the C (5 ) radical which adds to C (8) [e.g., reaction (221); for details

of this reaction see below, where experiments that lead to the specific generation
of this radical are discussed]. In a disproportionation reaction with other radi-
cals, the cyclonucleoside (cyclonucleotide) is formed [reaction (221)]. In compe-
tition, the C (5

′

′ ) radical may be oxidized giving rise to the 5 ′ -aldehyde [reaction

(220)].

With AMP as substrate, G (cA) strongly depends on the pH. This has originally
been explained by a variation of the rate constant of the H-abstraction reaction
by • OH from the C (5 )-position as a function of the charge at the base and the

′