Formation of Reactive Free Radicals in an Aqueous Environment - Free-Radical-Induced DNA Damage and Its Repair - page 13

Fig. 2.1. Sonolysis of Ar-saturated water at 321 Hz and an intensity of 170 W kg − 1 . Yields of I 2 ( • ),
H 2 O 2 ( ◊ ) and I 2 plus H 2 O 2 ( ∆ ) as a function of low concentrations of iodide ( left ) and as a function of
high concentrations of iodide ( right ). Source: von Sonntag et al. (1999), with permission

the quality used in medicine seems not to have major damaging effects on the
cellular DNA (Harder 1981).

2.4
Thermolysis and Photolysis

When a given bond is very weak, it may already be cleaved thermally at room
temperature. Otherwise, elevated temperatures may be required for the reac-
tion to proceed at a sufficiently high rate to be followed experimentally with-
in a reasonable time. The necessary activation energy may also be provided
by electronic excitation, but also bonds with relatively strong BDEs may be
cleaved upon photolysis. Thus, photolysis is a very convenient way of creating
free radicals and other reactive intermediates in aqueous solution. Some sys-
tems are discussed below. Since photochemical bond cleavage can be very se-
lective, it allows us to generate very specifically a given radical in DNA model
systems and to study mechanistic details of radicals that are also created by
very reactive radicals such as • OH only in low yields. The great value of this
technique is the possibility to incorporate such photolabile precursors even
into dsODNs.

2.4.1
Thermolysis of Peroxides

The O O bond in peroxides and hydroperoxides is often rather weak. Pernitrous

acid already decomposes readily at room temperature (see below), and the de-
composition of peroxodisulfate at 70 °C is used in technical processes, but H 2 O 2

−