Biomedical Engineering Reference
In-Depth Information
Fig. 1.13
Yields of SSB and
DSB induced by 0-4.2 eV
electron impact on
supercoiled plasmid DNA
films [
31
]. The inset shows
the dependence of the
percentage of circular DNA
(i.e. SSB) on irradiation time
for a beam of 0.6 eV electrons
of 2 nA. (Reprinted with
permission from reference 31.
Copyright 2004 American
Physical Society)
1.4.4
Plasmid and linear DNA
The damage induced to plasmid and linear DNA films by LEE has been investigated
by (1) measuring ESD of anion radicals, (2) imaging the breaks by atomic force and
scanning tunneling microscopies and (3) analyzing, after bombardment, the change
of topology by gel electrophoresis. The results obtained are well described in the
literature and summarized in a previous review article [
26
]. In this section, we only
summarize the results of analysis of DNA damage by gel electrophoresis which are
the most relevant to radiosensitization.
LEE-induced damage to thin films of plasmid DNA below 4 eV was investigated
in UHV by Martin et al [
31
]. Exposure response curves were obtained at different
energies for SSB and DSB. The inset in Fig.
1.13
shows the measured dependence
of the percentage yields of circular (i.e., SSB) DNA on irradiation time for 0.6 eV
incident electrons. The other curves exhibit two peaks in the yield function of SSB
from plasmid DNA, with maxima of
.7:5 ˙ 1:5/ 10
3
SSB per incident electron, at electron energies of 0.8 and 2.2 eV, respectively
[
31
]. Since at such low energies electronic excitation is energetically impossible,
these peaks provide unequivocal evidence for the role of shape resonances in DNA
damage.
Martin et al. [
31
] compared the results in Fig.
1.13
with those obtained from
electron scattering from gaseous DNA base and sugar analogs [
92
]. The solid curve
in Fig.
1.13
, which reproduces in magnitude and line shape the SSB yield function,
is similar to that obtained by a model that simulates the electron capture cross-
section as it might appear in DNA owing to the
.1:0 ˙ 0:1/ 10
2
and
single-particle anion states
of the bases. In this simulation, the attachment energies were taken from electron
