Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

Radiation-induced bioradicals: physical, chemical and biological aspects
properties of the irradiated medium and of the radiation. In covalently bonded systems,
such as organic and biological systems, a large proportion of the ionised and excited
molecules react or dissociate with the formation of free radicals, concentrated in tracks
distributed in a manner similar to their parent molecules. The electrons produced by the
absorption of radiation are rapidly thermalised and consequently solvated in most liquid
media. The duration of these processes is very short (10 -15 s).
By the end of the energy deposition an irradiated molecular system contains ions,
electrons, excited molecules and free radicals. As we will see further, these species will
in general be the same in a particular material, regardless of the type or energy of the
radiation. All ionising radiations will therefore give rise to qualitatively similar
chemical effects. The quantitative differences of chemical effects of distinct radiations
stem from the different spatial distributions of the reactive species.
The chemical reactivity of positive and negative ions is not high, but if they
recombine with other ions they can form free radicals. Because of their unpaired
electron free radicals are very reactive. Their reactions are usually very fast, in
particular the radical-radical reactions, so radicals play a predominant role during this
stage. Radicals have been shown to be transient intermediates also in many nonradiation-induced
chemical reactions. However, the initial high concentration of
radiation-induced radicals along the radiation tracks can lead to a completely different
radical behaviour as compared to systems where the radicals are more uniformly
distributed. During the initial stage there exists an enormous variety of intermediates
leading to a complicated set of reactions (Klassen 1987). It is the production of this
large number of free radicals that accounts for the fact that high-energy radiation is
much more effective in inducing chemical changes than, for example, an equivalent
amount of thermal energy. It is also the basis of the great variety of applications of
radiation-induced processes (Woods 1994).
About 10 -12 s after the initial events, any radicals that have not reacted within the
tracks, have diffused from these and become essentially homogeneously distributed in
the medium. Chemical changes in the material being irradiated are generally the result
of further free radical reactions, that are completed within approximately 1 ms in
gaseous and liquid systems. In solids the reactions proceed much slower, due to the
reduced mobility of the free radicals. Trapped radicals may be detected even weeks or
months after irradiation.
When ionising radiation is absorbed in living material, there is a possibility that it
will act directly on critical targets in the cell. The molecules may be ionised or excited,
thereby initiating a chain of events that leads to biological change and cell death if the
change is critical. In contrast to this direct effect, radiation may also interact with other
atoms or molecules in the cell, particularly water, to produce free radicals which can
diffuse far enough to reach and damage the DNA.
Radiation effects to living species (e.g. loss of viability, sterility, cancer, and genetic
damage) can occur over longer time scales, from a few hours to many years, depending
on the irradiation conditions. In each case, however, the changes to the living system
are the result of the chemical changes brought about in the first fractions of a second
after irradiation.
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