Biomedical Engineering Reference
In-Depth Information
Thus, secondary electrons are the most abundant of all secondary species (electrons,
ions, radicals) produced by this interaction. Another remarkable fact is their kinetic
energy distribution, which shows that the vast majority is formed in the energy
region below 100 eV with its maximum pointing towards zero eV [
4
,
5
]. Another
additional source for low energy electrons may be intermolecular Coulombic decay
which is a kind of non-local autoionization process [
6
]. Thereby a single hole in
an inner shell (produced by the initial high energy radiation) is replaced by two
vacancies in the outer valence shells of two adjacent molecules in a molecular
framework and an additional free electron with low kinetic energy. Independent
of the way of formation, it is then the action of such low energy electrons in
driving severe damage to the DNA which can lead to mutagenic, genotoxic and other
potential DNA lesions. It was supposed that this direct DNA damage leads to about
1/3 of the overall damage of the genome of a living cell, while 2/3 is ascribed to
indirect damage from free radicals produced by energy deposited in water molecules
(a cell consists of 70%-80% of water) and other biomolecules surrounding DNA [
7
].
However, recent results from experiments with prehydrated electrons (formed by
the radiolysis of water) challenged the convential notation of indirect DNA damage
ascribed mainly to action of the OH radical [
8
,
9
]. Prehydrated electrons are weaker
bound than hydrated electrons and have much higher quantum yield than hydrated
electrons and OH radicals [
8
].
Placing emphasis now on direct damage, laboratory experiments with plasmid
DNA like the experiments of the Sanche group are therefore crucial for determining
quantum yields for cell damage by low energy electrons. Another important aspect
turned out to be the electron energy dependence of DNA strand breaks which give a
hint on the underlying molecular mechanism. The measurements showed a resonant
behaviour instead of a monotonic increase above the threshold energy which would
be characteristic for ionization. Thus rather dissociative electron attachment has to
be considered as the decisive step because such resonant behaviour is characteristic
for this process. Thus it was proposed that DNA damage starts with electron
attachment to a site of DNA with subsequent decay of the transient negative ion
formed [
1
,
2
].
2.2
Electron attachment to biomolecules in the gas phase
Motivated by the electron irradiation experiments with plasmid DNA a large number
of elastic and inelastic electron scattering experiments with simple biomolecules
have been carried out. While other chapters of this topic cover the theoretical
description of electron induced radiation damage (see also for example Refs. [
10
-
12
]), electron scattering experiments with thin biomolecular films of different
complexity [
3
,
13
], or experiments with other projectiles like for example atomic
multiply charged ions [
14
], the focus of the present topic chapter lies on electron
attachment experiments with isolated biomolecules and biomolecularclusters. In
this case high vacuum conditions provide well-defined conditions for electron
