Biomedical Engineering Reference
In-Depth Information
H
-
/Thymine
N1
N3
C6
CH
3
H
-
/Thymine
deut
D
-
/Thymine
deut
0
2
4
6
8
10
12
14
Electron energy
Fig. 2.3
Ion yield of H
upon DEA to thymine (upper panel). Each peak corresponds to H
formation from a different site, which can be deduced from experiments with partially deuterated
or methylated thymine [
31
]. For example, the lower panel shows the H
and D
formed upon
DEA to thymine deuterated at the carbon positions [
32
]
yield indeed just showed the broad resonance at about 1.8 eV [
29
]. Moreover,
photoelectron spectroscopy with solvated uracil showed a dominance of valence
states compared to dipole states [
25
].
(ii) The situation is different for the H
channel, where the carbon sites are
available also for nucleobases embedded in the DNA-strand. Indeed, the ion
yield of H
resulting from hydrogen loss from the carbon positions resembled
in a remarkably way the quantum yield for strand breaks [
1
]. Thus one may
speculate that hydrogen abstraction from carbon positions may initiate double
strand breaks in DNA in addition to other pathways proposed recently [
35
],
which involves for example direct electron attachment to the phosphate group
at low energies [
36
].
Moreover, electron capture by an isolated nucleobase always leads to dissociation
or alternatively the TNI decays by electron emission. The question is then, if this
will be also the case if the nucleobase is surrounded by other molecules, or if excess
energy will be transferred to the neighbours by intermolecular vibrational redistri-
bution. Experiments with nucleobases embedded in helium droplets can answer this
question [
37
]. Indeed electron attachment experiments with nucleobases embedded
in helium droplets show a pronounced change of the anion abundances. In contrast to
the gas phase we can observe a parent anion on mass spectrometric timescales (few
s), and the fragmentation pattern changes in favour of the dehydrogenated parent
anion. While for the isolated molecule
/
is formed in considerable amounts
just below 4 eV, the abundance in the droplets is strongly increased above this energy
[
37
] as shown in Fig.
2.4
for adenine embedded in helium droplets. In contrast other
fragment anions, which are formed by cleavage of the ring, are suppressed in the
.
M-H
