Biomedical Engineering Reference
In-Depth Information
nucleic acids by low-energy electrons (LEE) with energies between 0 and 20 eV
[
1
,
4
-
8
,
12
,
18
,
20
,
30
,
33
-
36
,
38
,
39
,
57
] produced by ionising radiation.
The mechanism of DNA-LEE interaction is important because the low-energy
secondary electrons are the most abundant radiolysis species generated following
the impact of high-energy radiation [
12
,
31
,
32
] and therefore highly pertinent to
issues such as radiation damage and the development of radio therapy. Nucleic acids
can be ionised and damage produced through the dissociation of the anion when
the electron energy is higher than the ionisation threshold for DNA (between 7.85
and 9.4 eV, as measured for the DNA bases [
22
]). If the electron energy is lower,
damage can still be generated, but through a negative anion-mediated mechanism,
which starts with the capture of the electron in a molecular resonance, followed by
the transfer of energy and electron density towards a weak bond that subsequently
ruptures.
There are many controversial issues that concern the location of the initial capture
site [
8
,
40
], the dynamics of the metastable anion generated by electron capture
(called transient molecular anion), and the identification of the final bond that
ruptures [
5
-
7
,
23
,
27
-
29
,
41
,
57
].
There is a wide agreement that the electron capture is mainly due to the DNA
and RNA bases, because these molecules have extended aromatic systems. The
scattering electron can temporarily be captured by an unoccupied
orbital giving
rise to a shape resonance [
1
,
5
,
25
]. When scattering is connected with electron
excitation, Feshbach or core-excited resonances can occur. It has been suggested
that electron attachment to the phosphate group also contributes to DNA strands
breaks [
5
,
6
,
28
,
37
].
Simons and co-workers [
5
-
7
] performed model calculations which showed
that electrons with energies of about 1.0 eV can attach to a base to form a
anion, which then can break a
bond connecting the phosphate to a sugar group.
Li et al. [
21
] performed calculations on a sugar-phosphate-sugar model system using
the ONIOM layer method and found that the activation barrier for bond rupture of
the anion's phosphate-sugar C-O bond is only 0.5 eV, indicating that very-low-
energy electrons can induce DNA strand breaks. Berdys at al. [
6
,
7
] found that near
zero energy, electrons may not easily attach directly (i.e. vertically) to the phosphate
units, but can produce a metastable P
anion above 2 eV.
Resonances were observed by Pan et al. [
27
,
28
] in linear and super-coiled DNA.
They also observed desorption of H
as the result of temporary capture of electrons
by the bases, with a small contribution from a core-excited resonance on the sugar
group, OH
desorption by the localisation of electrons on the protonated form of the
phosphate group, and production of O
D
O
via the temporary localisation of electrons
double bond of the phosphate group. Pan and Sanche [
29
] measured
dissociative-electron attachment (DEA) to the monosodium salt of phosphoric acid,
Na
2
PO
4
, in the condensed phase, confirming DNA damage can be induced by low-
energy electrons. A single broad peak whose maximum fell at 8.8, 8.0 or 7.3 eV,
depending on whether the anion detected was H
,O
,orOH
, was observed.
K onig et al. [
19
] measured DEA spectra for the dibuthyl and triethyl phosphate ester,
and observed a variety of anionic fragments. Using dibuthyl phosphate they found
on the
