Nucleobases, Nucleosides and Nucleotides - Free-Radical-Induced DNA Damage and Its Repair - page 207

possibly very short-lived, intermediates. Yet, concerted release of sulfate ion
and a proton would lead to the same heteroatom-centered radicals without free
radical cations as intermediates (Aravindakumar et al. 2003). Once the hetero-
atom-centered radicals are formed, their protonation will lead to the formation
of radical cations. Thus, such intermediates may play a role in the subsequent
chemistries even if not formed in the primary step. This may, for example, be the
reason for the formation of the allylic radical in the reaction of SO 4 • − observed
with Thy (Deeble et al. 1990).
Br 2 • − can also be used to oxidize good electron donors, but at least with the
pyrimidines its rate of reaction is too slow to be of any importance. Instead, deg-
radation may occur by Br 2 , the product of the disproportionation of Br 2 • − , as has
been shown for Thd (Cadet et al. 1983b).

10.2.2
p K a Values of Radical Cations and Heteroatom-Centered Radicals

Usually, radical cations have much lower p K a values than their parent com-
pounds. A typical examples is phenol, whose p K a value is at 10 while that of its
radical cation is at

2 (Dixon and Murphy 1976). Thus in this case, ionization
causes an increase in acidity by 12 orders of magnitude. It is hence expected that
also the nucleobase radical cations should be much stronger acids than their
parents. This has indeed been found in all systems where equilibrium conditions
are established, and the consequences for DNA base pairs have been discussed
(Steenken 1992).

−

Pyrimidines. Photoexcited anthraquinone-2,6-disulfonate undergoes ET with
Thy and its methyl derivatives as indicated by Fourier transform EPR (Geimer
et al. 1997). These pyrimidine radical cations deprotonate at N (1) thereby giving
rise to the corresponding N -centered radicals [reaction (6)].

This view is been confirmed by an electrochemical product study (Hatta et al.
2001) that is discussed below. The p K a value of the Thy radical cation has been
determined at 3.2 (Geimer and Beckert 1998). When the position at N (1) is sub -
stituted by a methyl group and deprotonation of the radical cation can no lon-
ger occur at this position, deprotonation occurs at N (3) (Geimer and Beckert
1999; for spin density calculations using density functional theory (DFT) see
Naumov et al. 2000). This N (3) type radical is also produced upon biphotonic
photoionization of N (1)-substituted Thy anions [reaction (7)] in basic 8 molar
NaClO 4− D 2 O glasses which allowed to measure their EPR spectra under such
conditions (Sevilla 1976).