Peroxyl Radicals - Free-Radical-Induced DNA Damage and Its Repair - page 151

Fig. 8.1. Pulse radiolysis of Ar/O 2 (9:1)-saturated aqueous solution of tetrachloroethene. Absorp-
tion spectrum of the trichlorovinylperoxyl radical taken after completion of the reaction. Inset
Rate of buildup of the trichlorovinylperoxyl radical as a function of the O 2 concentration. (Source:
Mertens and von Sonntag 1994, with permission)

burg et al. 2000). Only for the cyclohexadienyl radical, it is as low as 25 kJ mol − 1 ,
and this leads to noticeable reversibility (for DFT calculations see Naumov and
von Sonntag 2005). For the related pentadienylperoxyl radical a BDE of 56 kJ
mol − 1 has been determined for the gas phase (Zils 2000; Zils et al. 2001).
As a rule of thumb, peroxyl radicals absorb at shorter wavelength and have
lower and less structured absorptions than their parent radicals. There are only
a few exceptions: vinyl- and phenyl-type peroxyl radicals (Alfassi et al. 1994,
1995; Mertens and von Sonntag 1994; Fang et al. 1995b; Khaikin and Neta 1995)
as well as thiylperoxyl radicals (Jayson et al. 1971; Tamba et al. 1986; Zhang et
al. 1994) absorb in the near UV and visible, cf. Fig. 8.1, while their parent radi-
cals have barely any absorption in the wavelength region of interest (for a quan-
tum-chemical study of peroxyl radical absorption spectra see Naumov and von
Sonntag 2005).
In the present context of nucleic acid free-radical chemistry, such a strong
absorption in the visible of a vinylic peroxyl radical is observed in the case of the
uracil-5-peroxyl radical ( λ max = 570 nm; Mertens and von Sonntag 1994) which

plays a role in the free-radical chemistry of 5BrUra.
When the C

OO • bond is weak, the R • + O 2 /ROO • system becomes reversible
( k − 1 now being quite fast). To date, examples of reversibility at room temperature
has only been found for pentadienyl-type radicals (Chan et al. 1978; Pan and von
Sonntag 1990; Pan et al. 1993; Fang et al. 1995a). Such radicals are formed, when
a pentadienylic hydrogen is abstracted from polyunsaturated fatty acids or when
• OH adds to aromatic compounds (forming hydroxycyclohexadienyl radicals).
Hydroxycyclohexadienyl radicals absorb strongly near 310 nm (325 nm in the
case of the one derived from anisole, Fig. 8.2), while the corresponding peroxyl
radicals absorb only weakly at this wavelength. Due to the reversibility of the re-

−