Peroxyl Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 152

Fig. 8.2. Pulse radiolysis of N 2 O-saturated aqueous solutions of anisole. Spectrum of the anisole-
derived hydroxycyclohexadienyl radicals. Inset shows their decay in the presence of 15% O 2 and
formation of the “plateau”. (Source: Fang et al. 1995a, with permission)
action, the absorbance of the hydroxycyclohexadienyl radicals does not decay to
the low level that would be given by the absorption of the remaining hydroxycy-
clohexadienylperoxyl radicals alone, but settles a higher "plateau" value (Fig. 8.2,
inset). The height of this "plateau" decreases with increasing O 2 concentration.
Subsequent to the rapid attainment of this near-equilibrium situation, the
radicals decay more slowly by both unimolecular and bimolecular processes
(discussed below). For this simple case, it can be shown that when the decay
process is much slower than the rates of the forward and reverse reactions, the
observed rate constant for the disappearance of R • is given by the expression k obs
= k forward [O 2 ] + k reverse . When k obs of the decay of the hydroxycyclohexadienyl
radicals is plotted as a function of the O 2 concentration, the slope represents the
rate constant of the forward reaction, and the intercept that of the reverse reac-
tion (Fig. 8.3).
Stability constants of a number of differently substituted hydroxycyclohexa-
dienyl peroxyl radicals have been obtained from such data (for a compilation see
von Sonntag and Schuchmann 1997).
The thiyl/thiylperoxyl radical system is also reversible [reaction (2); Tamba
et al. 1986; Zhang et al. 1994], although the thiylperoxyl radicals are stabilized
by an intramolecular charge transfer (Razskazovskii et al. 1995). The evalua-
tion of the equilibrium constant is not straightforward because the thiylperoxyl
radical undergoes a thermal but also a light-induced (Sevilla et al. 1990a) re-
arrangement into the much more stable sulfonyl radical [reaction (3)] which is
subsequently converted into the corresponding peroxyl radical [reaction (4)]. In
addition, equilibrium (5) has to be taken into account, although this equilibrium
lies largely on the side of the thiyl radical (Zhang et al. 1994; for DFT calculations
see Naumov and von Sonntag 2005).