Formation of Reactive Free Radicals in an Aqueous Environment - Free-Radical-Induced DNA Damage and Its Repair - page 10

ionization quantum yield increases with increasing photon energy and changes
between 7.8 and 9.3 eV with an almost exponential dependence from

= 0.019
to 0.22 (Bartels and Crowell 2000). Moreover, with high-intensity lasers (>10 11 W
m −2 ) non-trivial processes that lead to the photolysis/ionization of water by two-
photon absorption via virtual states take place (Nikogosyan and Angelov 1981;
Reuther et al. 1996; Görner and Nikogosyan 1997).
With substrates other than water, the photoionization may occur at photon
energies <7 eV as a monophotonic process. With DNA in aqueous solution, pho-
toionization is observed at 193 nm as a monophotonic process (Gurzadyan and
Görner 1992; Candeias et al. 1992) (Chap. 11). With high-intensity lasers, bipho-
tonic processes with quanta of lower energy (e.g., at

Φ

= 266 nm) also give rise
to photoionization. As a consequence, the ratio of the typical UV DNA dam-
ages, e.g., pyrimidine dimers, decrease with increasing laser intensity, while the
free-radical-induced products such as DNA SSBs and DNA cross-links increase
(Zavilgelsky et al. 1984).
In general, ionizing radiation produces in DNA and its model systems a large
number of different radicals through the action of • OH, e aq − an H • . Yet in cer-
tain cases, one can also generate a given radical quite specifically by making
use of the ready splitting of the C

λ

Br bond by e aq − , for example in 6-bromo-5-
hydroxydihydrothymine or in 5BrUra (Chap. 10).

−

2.3
Ultrasound

One of the phenomena observed with ultrasound is cavitation. Ultrasound cre-
ated in a liquid is ref lected at the liquid/gas interface and standing waves or
f fluctuating areas of sound nodes and antinodes develop. In the antinodes, the
high-pressure and low-pressure phases change with frequency (frequencies used
range typically between 20 and 1000 kHz). In the negative-pressure phase, small
gas (e.g., air) bubbles may draw in some more gas and water vapor from the sur-
rounding thereby growing to resonance size. In the compression phase (acceler-
ated by the surface tension of the liquid), these gas bubbles heat-up adiabatically
to several thousand degree (Flint and Suslick 1991; Tauber et al. 1999a; Rae et al.
2005). The contents of the gas bubble can decompose and give rise to free radi-
cals and other reactive intermediates (Henglein 1987, 1993; Suslick 1990).
Besides small gas bubbles, other nucleation sites (e.g., at minute dust par-
ticles) may give rise to the cavitation phenomenon. Normally, the surface tension
of water is too high to allow the formation of water vapor bubbles at the relatively
small negative pressures created by the sonic field. However, at the surface of
the dust particles the surface tension of water may be sufficiently low to create a
water vapor bubble in the sonic field and thus start the cavitation process.
The oscillating gas bubbles are a continuous source of free-radicals as long as
they remain in the antinode area of the sonic field, but one has also to envisage
a catastrophic collapse that generates in addition to the free radicals a number
of smaller bubbles. These serve as further nucleation sites for subsequent cavita-
tion processes.