Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 208

From a pulse radiolysis study on the SO 4 • − -induced reactions of Thd (Deeble et
al. 1990), it has been concluded that the p K a of the Thd radical cation (deprot-
onation at N (3)) should be near 3.5, i.e. close to that at N (1) in Thy. It is noted
that also in the parent, Thy, the p K a values at N (1) and at N (3) are quite close. A
Fourier-transform EPR study using photoexcited anthraquinone-2,5-disulfonic
acid to oxidize Cyt and 1MeCyt shows that the radical cation of the former de-
protonates rapidly at N (1) while that of the latter deprotonates at the exocylic
amino group (Geimer et al. 2000). The EPR evidence for the formation of het-
eroatom-centered radicals by SO 4 • − in its reactions with some other pyrimidines
(Bansal and Fessenden 1978; Hildenbrand et al. 1989; Catterall et al. 1992) is in
agreement with a marked acidity of such radical cations. It is re-emphasized that
this conclusion does not require that radical cations are formed in the primary
step.
The primary exocyclic N -centered dCyd radical is only observed at very short
times and is converted into another radical which has been attributed (Naumov
et al. 2001) to its tautomer [reaction (8)]. This tautomerization has been calcu-
lated to be exothermic by 10.5 kJ mol − 1 .
Purines. The Gua radical cation, G • + , is an acid (p K a = 3.9; Willson et al. 1974;
Asmus et al. 1978) and readily deprotonates at a heteroatom. In water, the depro-
tonation product identified by pulse radiolysis is the N (1) radical [denoted as G • ;
equilibrium (9); Candeias and Steenken 1989].
The radical that is formed upon deprotonation at N (2), N (2)G • , is very close in
energy.