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

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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
Richard
Wagner Xifeng Li
Michael Sevilla
Laurent Caron

This work was funded by
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There are many processes driven by LEE, but why
are they important in relation to radiobiological
damage?
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.
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DNA and sub-units
H2O
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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
  produce
• 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?
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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).
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LEE Damage to Plasmid DNA
M.A. Huels et al., J.A.C.S. 125, 4467 ( 2003)
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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
(2003).
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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
  group

Question Do strand breaks and other damages
occur via transient anion formation on the
subunits?
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Model Target System
G C A T
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
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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
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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.

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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
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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.
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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.

X
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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
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Yield functions GCAT vs GCXT

• For strand break, a resonance shows at around 10
  eV.
• Presence of an abasic site greatly decreases the
  yield of strand break and base release in DNA
  (three times less).

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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
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base
(Eo)
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
diffraction
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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
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base
base
DEA


e (Eo)
e (EltltEo)
ec
et
ec
et
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Neutral particle desorption from a single DNA
strand

• 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
(2000).
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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
  excitation
• 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
  energy.

Proton transfer has to be re-examined in the
context of the present data and hypothesis
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• 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).

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H2O
ER
Eo
?Eph
E
1.0 eV
0 eV
Superinelastic electron transfer
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Site of formation
O ????? O P - O
H (O18H) O