Formation of Reactive Free Radicals in an Aqueous Environment - Free-Radical-Induced DNA Damage and Its Repair - page 17

of 10 −4 mol dm −3 in E. coli (Keyer and Imlay 1997)]. This will release iron ions
that then may undergo Fenton reactions (see below). Unfortunately, our knowl-
edge of the cellular steady-state concentrations of the various intermediates that
we are concerned with here may not be sufficiently well established to reach final
conclusions in this dispute.
In vivo, peroxynitrite may be intercepted by various cellular agents which
will keep its steady-state low (Table 2.4). Not all these interceptors, however, re-
act with peroxynitrite to non-reactive products. For example, carbon dioxide
enhances tyrosine nitration and thiyl radical formation. Myeloperoxidase also
enhances tyrosine nitration, and in the reactions with GSH and albumin thiyl
radicals are formed (for details see Arteel et al. 1999).
This reduction of the lifetime of peroxynitrite by cellular components will, at
least to a certain extent, protect DNA against the attack of this oxidant. The reac-
tions of peroxynitrite with DNA and its model systems are discussed in Chap-
ters 10 and 12.

2.4.2
Photolysis of Peroxides

Peroxides absorb in the UV range and are readily decomposed upon photoexcita-
tion yielding two oxygen-centered radicals. Although the excited state is dissocia-
tive, the free-radical yield is never unity because of cage recombination reactions
(Crowell et al. 2004). For example, the quantum yield of • OH-formation in the pho-
tolysis of H 2 O 2 is only 1.0, i.e., the efficiency is only 0.5 (Legrini et al. 1993; Yu and
Barker 2003). The peroxide chromophore is only weak [H 2 O 2 or S 2 O 8 2− :

(254 nm)

ε