064710-8
Zheng et al.
J. Chem. Phys. 124, 064710 ͑2006͒
the formation of different transient anions at 6 and 10 eV
having different dissociation pathways. The direct-scattering
contribution to the yields necessarily involves no electron
transfer and direct electronic excitation on the basic unit is
responsible for bond cleavage. However, considering the
strong common dip in the yield functions of Figs. 5 and 6,
we do not expect such direct contributions to be very signifi-
cant in the yields of monomer and oligomer below 14 eV.
With the additional information of Fig. 9, we interpret
the small hump or shoulder in the yield functions of T, G, pT,
Gp, pAT, and pCAT as due to the formation of one or more
transient anions around 6 eV. This resonance always involves
the bases thymine and guanine and, with the exception of
Gp, occurs at a terminal nucleotide. No distinct features were
observed below 9 eV in the anion yield functions obtained
from electron scattering from solid films of these bases, but
in gaseous thymine the yield function for the formation of
bond. Shape resonances, which are known to be much
shorter lived beyond ϳ5 eV, are not expected to contribute
to dissociation, but would be responsible for electron transfer
to the phosphate group, leading to the formation of a local
shape or core-excited resonance on that group. The latter
transient anionic state could dissociate or decay by electron
emission, leaving the phosphate group in a dissociative elec-
tronic excited state; both processes would break the phos-
phodiester bond. The formation of such a core-excited reso-
nance on the phosphate group requires the transformation of
a single extra electron anion state ͑a shape resonance͒ into a
two-electron one-hole state ͑a core-excited resonance͒ by
electron transfer.
ACKNOWLEDGMENT
−
Financial support for this work was provided by the Ca-
nadian Institutes of Health Research ͑CIHR͒.
͑C4H5N2O͒−,͑C3H2NO͒−,͑C3H4N͒−,͑OCN͒−, and ͑CN͒
exhibits a maximum near 6 eV.7–13 Furthermore, as seen
from Fig. 8, a shoulder exists around 6 eV in the yield of
SSB from plasmid DNA, but not in the yield for DSB and H−
desorption. If the 6 eV transient anion leading to base release
arose from electron transfer to bases from a sugar or phos-
phate group, a resonance signal in the adenine and cytosine
yield functions would also be expected and the variation in
yields of products would not be as strong as that shown in
Fig. 9. Hence, as in the case of the 10 eV resonance, electron
transfer does not seem to be involved in the 6 eV resonant
N-glycosidic bond scission process.
1 L. Sanche, Mass Spectrom. Rev. 21, 349 ͑2002͒.
2 K. Aflatooni, G. A. Gallup, and P. D. Burrow, J. Phys. Chem. A 102,
6205 ͑1998͒.
3 S. Feil, K. Gluch, S. Matt-Leubner, P. Scheier, J. Limtrakul, M. Probst,
H. Deutsch, K. Becker, A. Stamatovic, and T. D. Märk, J. Phys. B 37,
3013 ͑2004͒.
4 S. Denifl, S. Ptasińska, G. Hanel, B. Gstir, M. Probst, P. Scheier, and T.
D. Märk, J. Chem. Phys. 120, 6557 ͑2004͒.
5 G. Hanel, B. Gstir, S. Denifl, P. Scheier, M. Probst, B. Farizon, M. Fari-
zon, E. Illenberger, and T. D. Märk, Phys. Rev. Lett. 9018, 8104 ͑2003͒.
6 R. Abouaf and H. Dunet, Eur. Phys. J. D 35, 405 ͑2005͒.
7 See, for example, S. Denifl, S. Ptasińska, M. Probst, J. Hrušák, P. Scheier,
and T. D. Märk, J. Phys. Chem. A 108, 6562 ͑2004͒.
8 M. I. Sukhoviya, I. A. Petruschko, and M. I. Shafranyosh, Book of Ab-
stract IX ECSBM ͑Prague, Czech Republic, 2001͒.
9 R. Abouaf, J. Pommier, and H. Dunet, Int. J. Mass. Spectrom. 226, 397
͑2003͒.
V. CONCLUSIONS
On the basis of product analysis, LEE irradiation of
GCAT gave nonmodified fragments containing a terminal
phosphate group, while those without a phosphate group
were negligible. Thus, the mechanism of phosphodiester
bond cleavage by 4–5 eV electrons involves cleavage of the
C–O bond rather than the P–O bond. The phosphodiester
bond cleavage essentially involves the formation of either a
C3 or C5 sugar radical and a phosphate anion. We also
10 S. Denifl, S. Ptasińska, M. Cingel, S. Matejcik, P. Scheier, and T. D.
Märk, Chem. Phys. Lett. 377, 74 ͑2003͒.
11 S. Gohlke, H. Abdoul-Carime, and E. Illenberger, Chem. Phys. Lett. 380,
595 ͑2003͒.
12 H. Abdoul-Carime, S. Gohlke, and E. Illenberger, Phys. Rev. Lett. 92,
168103-1 ͑2004͒.
13 S. Ptasińska, S. Denifl, V. Grill, T. D. Märk, P. Scheier, S. Gohlke, M. A.
Huels, and E. Illenberger, Angew. Chem., Int. Ed. 44, 1657 ͑2005͒.
14 H. Abdoul-Carime, J. Langer, M. A. Huels, and E. Illenberger, Eur. Phys.
J. D 35, 399 ͑2005͒.
Ј
Ј
reported the release of the four nonmodified nucleobases
from GCAT irradiated with 4–15 eV electrons. These analy-
ses corroborate the results previously obtained at a single
energy of 10 eV.36
15 M. A. Huels, L. Parenteau, M. Michaud, and L. Sanche, Phys. Rev. A 51,
337 ͑1995͒.
16 S. Ptasińska, S. Denifl, P. Scheier, and T. D. Märk, J. Chem. Phys. 120,
8505 ͑2004͒.
17 D. Antic, L. Parenteau, M. Lepage, and L. Sanche, J. Phys. Chem. 103,
6611 ͑1999͒.
As previously found in the experiments with plasmid
DNA, below 14 eV, the yield of LEE-induced damage prod-
ucts in GCAT is dominated by the formation of transient
anions located around 6 and 10 eV. Beyond 14 eV, direct
LEE impact is believed to contribute substantially to the
damage. The effect of the different mechanism of fragmen-
tation is seen in the modification of the repartition of the
strand breaks and base release to different positions along the
chain with electron energy. According to the present results,
electrons can transfer from the bases to the backbone but the
inverse does not occur to a substantial extent. Since base
release is observed, we conclude that both shape and core-
excited resonances are formed by electron attachment to the
bases. The latter resonance can retain the electron for a suf-
ficiently long period to allow dissociation of the N-glycosidic
18 D. Antic, L. Parenteau, and L. Sanche, J. Phys. Chem. B 104, 4711
͑2000͒.
19 A. R. Milosavljevic, A. Giuliani, D. Sevic, M.-J. Hubin-Franskin, and B.
P. Marinkovic, Eur. Phys. J. D 35, 411 ͑2005͒.
20 S.-P. Breton, M. Michaud, C. Jäggle, P. Swiderek, and L. Sanche, J.
Chem. Phys. 121, 11240 ͑2004͒.
21 X. Pan and L. Sanche, Phys. Rev. Lett. 94, 198104 ͑2005͒; ͑submitted͒.
22 H. Abdoul-Carime, S. Gohlke, E. Fischbach, J. Scheike, and E. Illen-
berger, Chem. Phys. Lett. 387, 267 ͑2004͒.
23 Y. Zheng, P. Cloutier, D. J. Hunting, J. R. Wagner, and L. Sanche, J. Am.
Chem. Soc. 126, 1002 ͑2004͒.
24 B. Boudaiffa, P. Cloutier, D. Hunting, M. A. Huels, and L. Sanche,
Science 287, 1658 ͑2000͒.
25 B. Boudaïffa, P. Cloutier, D. Hunting, M. A. Huels, and L. Sanche, Ra-
diat. Res. 157, 227 ͑2002͒.
26 M. A. Huels, B. Boudaïffa, P. Cloutier, D. Hunting, and L. Sanche, J. Am.
Chem. Soc. 125, 4467 ͑2003͒.
80.71.135.102 On: Sat, 24 May 2014 09:15:28