The Role of Secondary Electrons in Radiation Damage - Radiation Damage in Biomolecular Systems - page 51

transfer experiments [ 26 ]. In free electron attachment to an isolated nucleobase
a dipole bound anion formed is vibrationally excited and thus unstable towards
autodetachment. However, a possible doorway exists by coupling of the dipole
bound state with the dissociative ¢ (N1-H) state, i.e. by tunnelling of the N1

/ [ 27 ]. Thus the sharp peaks (at 0.7 eV, 1.1 eV and
1.47 eV for thymine; at 0.8 eV, 0.92 eV and 1.15 eV for adenine, respectively)
shown in Fig. 2.2 represent vibrational progressions of various stretching modes
as assigned in the figure (vibrational Feshbach resonances [ 28 ]). The rather broad
resonances (at 1.8 eV for thymine and 1.44 eV and 2.21 eV for adenine, respectively)
may arise from vibronic coupling of a non-dissociative and a dissociative state but in
this case a valence state is involved as intermediate (for example the

hydrogen to form a . M-H

2

resonance

¢ (N3-H) state in the case of thymine) [ 27 ].
In line with the experimental results another important aspect of this assignment
is that the cleavage of bonds can be selectively chosen by the initial electron energy.
All gas phase experiments with partially deuterated or methylated nucleobases
clearly showed that no hydrogen loss from the carbon positions of the molecules
occurs below 4 eV [ 22 , 29 , 30 ]. Instead only hydrogens from nitrogen positions are
involved and the loss from N1 and N3 site could be distinguished (see Fig. 2.2 ).
This site selectivity is even more dramatic in the H channel. The corresponding ion
yield is shown in Fig. 2.3 and consists of a series of partially overlapping resonances
at about 5.5 eV, 6.8 eV, 8.5 eV and 10 eV [ 31 , 32 ]. Due to the low electron affinity
of H .0:75

coupling with the

the endothermicity of H eV / formation is higher than compared to

/ and all sites become only thermodynamically accessible above 4-5 eV [ 31 ].
By measurements with partially deuterated or methylated nucleobases one can show
that each of the 4 resonances is formed by hydrogen loss from different position
of the thymine molecule as assigned in Fig. 2.3 . Thus we have here the case of a
true bond and site selective process, which is not only a consequence of different
threshold energies for the various isomers. Instead it might be related to particular
electronic structures of the associated transient negative ions formed at the different
resonance energies [ 31 ].
In addition to DEA arising from the cleavage of a single N-H or C-H bond,
DEA to nucleobases show many more fragmentation channels which are also
accompanied by dissociation of the ring structure. These processes mainly occur
above 5 eV and as shown in [ 18 ] they considerably contribute to the total cross
section for negative ion formation. For example, CNO is the dominant fragment
anion for thymine at these energies [ 33 ].
From these experiments three main conclusions for the description of radiation
damage of biological matter can be deduced:

. M-H

(i) The sharp peaks of the dehydrogenated parent anion may not be likely present
in the ion yield for complex biomolecules since they are formed by hydrogen
loss from the N1 position. In the DNA framework the 1N-H bond is replaced
by the glycosidic bond to the sugar molecule. The experimental proof of
this prediction has been given by DEA to the nucleoside thymidine [ 34 ]and
moreover, thymine methylated at the 1N position, where the / ion . M-H