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Interaction of 0-15 eV electrons with DNA: Resonances, diffraction and charge transfer


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Title: Interaction of 0-15 eV electrons with DNA: Resonances, diffraction and charge transfer

Interaction of 0-15 eV electrons with DNA
Resonances, diffraction and charge transfer
The presented results represent the work of many
scientists especially Marc Michaud
Sylwia Ptasinska
Badia Boudaiffa Michael Huels Pierre
Cloutier Darel Hunting
Hassan Adoul-Carime Xiaoning Pan
Luc Parenteau Andrew Bass Frederick
Martin Yi Zheng
Wagner Xifeng Li
Michael Sevilla
Laurent Caron

This work was funded by
There are many processes driven by LEE, but why
are they important in relation to radiobiological
Secondary electrons (SE) are the most abundant
species produced by ionizing radiation. Most
of the energy distribution of SE is composed of
LEE The most probable energy lies below 10
eV. Cross sections for LEE damage to
biomolecules are large owing to the formation of
transient anions.
DNA and sub-units
What sort of damage is induced in DNA by LEE?
  • From LEE impact experiments on thin films of DNA
    and its basic constituents, we know that they
  • Base, phosphate and sugar modifications
  • Single strand breaks
  • Base release
  • Combination

Double strand breaks Multiple strand
breaks Crosslinks
What are the mechanisms of LEE damage?
Apparatus for productanalysis
"Electron stimulated desorption of H from thin
films of thymine and uracil" M.-A. Hervé du
Penhoat et al., J. Chem. Phys. 114, 5755 (2001).
LEE Damage to Plasmid DNA
M.A. Huels et al., J.A.C.S. 125, 4467 ( 2003)
H- desorption from films of linear and plasmid DNA
ESD of H-
  • Yield function similar to DSB damage
  • Anion yields from linear and supercoiled DNA are
    very similar
  • H- yield consistant with those of sub-units,
    especially Thymine and THF
  • ESD signal at low electron dose consistant with
    a one-step reaction

X. Pan et al., Phys. Rev. Lett. 90, 208102
Conclusions from comparisons of LEE stimulated
desorption of anion from random and oriented DNA
films and subunits of DNA
  • H arises from DEA to the bases with a minor
    contribution from the sugar
  • O arises from DEA to the phosphate group
  • OH arises from DEA to the protonated phosphate

Question Do strand breaks and other damages
occur via transient anion formation on the
Model Target System
Reference standards for fragment species obtained
using micrococcal nuclease (3 terminal
phosphate - GCp, GCAp) Phosphodiesteras (P1)
(5-phosphate pT, pAT, pCAT) alkaline
phosphatase to remove terminal phosphates
LEE irradiation of tetramers gave non-modified
fragments containing a terminal phosphate group
(A) while those without a phosphate group were
minor (B).
Proposed pathways of phosphodiester bond cleavage
of DNA by low-energy electrons
Carbon-centered sugar radical
phosphate anion
Alkoxyl anion phosphoryl radical
LEE damage to DNA Intro/Summary
  • DNA damage induced by LEE below 15 eV occurs
    principally by the formation of transient anions
    of the subunits. The contribution from direct
    scattering increases with energy.
  • Anion ESD yields of H arises from the bases
    with a small contribution from the backbone, O
    from the phosphate group, and OH from a
    protonated phosphate group. Other anions have
    been observed.
  • Anion ESD yields arise from DEA below 15 eV.
  • Two major pathways of LEE reactions in DNA
    cleavage of the N-glycosidic bond (base release)
    and the phosphodiester bond (strand break).
  • Phosphodiester bond breaks by C-O bond rather
    than P-O bond rupture.
  • Between 0-5 eV, SSB are produced with a cross
    section of about E-14 cm2 for 3,000 bp, similar
    values are found at 10 and 100 eV.

Sub-excitation energy electron damage to DNA
Barrios et al J. Phys. Chem. 106, 7991 (2002) -
Electron capture by cytosine and transfer to
dissociative C-O bond
Dablowska et al Eur. Phys. J. D 35, 429 (2005)
Proton transfer mechanism of DNA strand breaks
induced by excess electrons.
Li et al JACS 125, 13668 (2003) - Scission of 5
and 3 C-O bond by electron attachment.
Endothermic by 0.5 eV
Gu et al., Nucl. Acids Res. 1-8 (2007) (in press)
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Capture cross section of the bases vs single SB
Upper curve (Martin et al, Phys. Rev. Lett. 93,
068101 (2004)) From ETS data, sum of capture
cross sections for the four bases normalized to
the second peak of the DNA damage yield (full
squares) and shifted by 0.4 eV.
Lower curve (Denifl et al, Chem. Phys. Lett. 377,
74 (2003)) DEA cross section from
gaseous Thymine with no energy shift.
Electron transfer in DNA
  • LEE induced cleavage reactions greatly impeded
    next to the abasic site below 6 eV.
  • There is a shift of electron transfer to direct
    attachment from low to high electron energy.
  • Electron transfer of LEE occurs from base moiety
    to the sugar-phosphate backbone in DNA.

Percentage distribution of damage by sites of
cleavage, induced by 6, 10 and 15 eV electrons.
Xp was not detected by HPLC and the yield was
considered to lie below the detection
limit. Total damage SB base release 100
Yield functions GCAT vs GCXT
  • For strand break, a resonance shows at around 10
  • Presence of an abasic site greatly decreases the
    yield of strand break and base release in DNA
    (three times less).

On average 25 decrease for abasic Same results
for H- and O- desorption No diffraction Since
OH- and O- originate from the backbone, these
anions arise from e- transfer unless there is a
change in the resonance parameters
At higher energies, there is little coherence.
Thus, creation of an abasic site has little
effect on the branching ratios for electron
emission in the continuum or within DNA
At low energies, transfer within DNA becomes much
larger, but strongly depends on diffraction and
hence is considerably decreased by formation of
an abasic site

e (Eo)
e (EltltEo)
Neutral particle desorption from a single DNA
  • CN (black squares)
  • OCN and/or H2NCN (white circles)
  • H and H2 desorption also observed
  • Ratio CN/OCN is constant
  • Resonance structures superimposed in linearly
    increasing background

Isocyanic acid
H. Abdoul Carime et al., Surf. Sci. 451, 102
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Opinion of the presenter
Below 3-4 eV
  • Shape resonances have high cross-section and can
    lead to DEA (the only bond breaking process).
  • Electron transfer is high.
  • Above the energy threshold for electronic
  • Core excited shape resonances have a high cross
    section for decay into their parent neutral state
    and direct inelastic scattering may be
    significant. The magnitude of the DEA is not
    necessarily large compared to autoionization.
  • There is little coherent enhancement of the
    electron wavefunction at the primary impact

Proton transfer has to be re-examined in the
context of the present data and hypothesis
  • Are transient negative ions formed within the
    0-15 eV linked directly to stable anions of the
    bases or other SU?
  • If so how?
  • Possible mechanisms
  • Vibrational stabilization triggered by the change
    in DNA configuration by the extra charge. The
    extra energy (lt2eV) of the electron is dispersed
    in vibrational excitation of DNA and then
    transfered to the surrounding medium. Does not
    work for core-excited resonances.
  • Electron-emission decay of a core-excited shape
    resonance followed by vibrational stabilization.
  • Proton transfer stabilization. Neutralizes the
    anion charge while leaving a site with a ground
    state electron.
  • Superinelastic vibrational or electronic electron
    transfer. Lu, Bass and Sanche, Phys. Rev. Lett.
    88, 17601 (2002).

1.0 eV
0 eV
Superinelastic electron transfer
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Site of formation
O ????? O P - O
H (O18H) O