The Hydroxyl Radical - Free-Radical-Induced DNA Damage and Its Repair - page 53

Fig. 3.1. Pulse radiolysis of N 2 O-saturated aqueous solutions of Cu 2+ . Consumption of H + as mea-
sured by conductometric changes after completion of the reaction. (Ulanski and von Sonntag 2000,
with permission)

Thus, in most • OH-induced oxidations short-lived adducts must be considered
as intermediates. A case in point in the realm of DNA free-radical chemistry is
the oxidation of guanine. From the above, it is evident that • OH, despite its high
reduction potential, cannot be directly used for the study of one-electron oxida-
tion reactions. However, one can make use of its high reduction potential by
producing other reactive intermediates [e.g., Tl(II); Chap. 10], which no longer
undergo an addition to double bonds or H-abstraction.

3.5
Detection of OH Radicals

When one looks for methods to detect • OH, one always has two keep in mind that
these radicals are very reactive, and in the presence of substrates their steady-
state concentrations are extremely low even at a high rate of • OH production.
The fact that • OH only absorbs far out in the UV region (Hug 1981) is thus not
the reason why an optical detection of • OH is not feasible. Electron paramag-
netic resonance (EPR) must also fail because of the extremely low steady-state
concentrations that prevail in the presence of scavengers. The only possibility to
detect their presence is by competition of a suitable • OH probe that allows the
identification of a characteristic product [probe product, reaction (41)]. When
this reaction is carried out in a cellular environment, the reaction with the probe
is in competition with all other cellular components which also readily react
with • OH [reaction (42)]. The concentration of the probe product is then given
by Eq. (43), where [ • OH ] is the total • OH concentration that has been formed in
this cellular environment and

η is the yield of the probe product per • OH that

has reacted with the probe.