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

1. Interaction of 0-15 eV electrons with DNA: Resonances, diffraction and charge transfer The presented results represent the work of many scientists especially: Marc MichaudSylwia Ptasinska Badia Boudaiffa Michael Huels Pierre Cloutier Darel Hunting Hassan Adoul-CarimeXiaoning Pan Luc Parenteau Andrew Bass Frederick MartinYi Zheng Richard Wagner Xifeng Li Michael Sevilla Laurent Caron This work was funded by:

2. DNA and sub-units Guanine Cytosine tetrahydrofuran (THF) 3-hydroxy- tetrahydrofuran Thymine a-tetrahydrofuryl alcohol Adenine +H2O

3. Electron Gun Rotatable Filament Sample Holder Electron Quadrupole Gate Lenses Mass Valve Spectrometer Oven Deflector Linear Transfer & Rotation -9 10 torr -10 10 torr Load - Lock Channeltron Ionizer & Ion Lenses (Preparation) Rotatable Main Chamber Custom Shutter Chamber Ion Lenses 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).

4. LEE Damage to Plasmid DNA M.A. Huels et al., J.A.C.S. 125, 4467 ( 2003)

5. 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.

6. 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

7. Gu et al., Nucl. Acids Res. 1-8 (2007) (in press)

8. [SU¯] (Eo) +diffraction SU=subunit base sugar phosphate water 1 3 2 [SU] [SU]* DEA + + e¯ (Eo) e¯ (E<<Eo) e¯c e¯t e¯t e¯c

9. 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.

10. 1 2 3 4 5 6 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

11. 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

12. 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).

13. 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

14. [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 1 3 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 2 [base]* [base] DEA + + e¯ (Eo) e¯ (E<<Eo) e¯c e¯t e¯c e¯t

15. 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).

17. 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

18. 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 (<2eV) 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)].

21. Site of formation | O ‌‌‌‌‌| O = P ─ O¯ H+ (O18H) | O |