A.M. Scheer

Vibrational Feshbach resonances in uracil and thymine

Sharp peaks in the dissociative electron attachment (DEA) cross sections of uracil and thymine at energies below 3 eV are assigned to vibrational Feshbach resonances (VFRs) arising from coupling between the dipole bound state and the temporary anion state associated with occupation of the lowest sigma* orbital. Three distinct vibrational modes are identified, and their presence as VFRs is consistent with the amplitudes and bonding characteristics of the sigma* orbital wave function. A deconvolution method is also employed to yield higher effective energy resolution in the DEA spectra. The site dependence of DEA cross sections is evaluated using methyl substituted uracil and thymine to block H atom loss selectively. Implications for the broader issue of DNA damage are briefly discussed.

Total dissociative electron attachment cross sections for molecular constituents of DNA

Total cross sections for the dissociative electron attachment process are presented for the DNA bases thymine, cytosine, and adenine and for three compounds used as surrogates for the ribose and phosphate groups, tetrahydrofuran, 3-hydroxytetrahydrofuran, and trimethylphosphate, respectively. Cross section magnitudes are obtained by observation of positive ion production and normalization to ionization cross sections calculated elsewhere using the binary-encounter-Bethe method. The average cross section of the three bases is 3-10 times smaller than the effective cross section per nucleotide reported for single strand breaks in surface-bound supercoiled DNA. Consequently, damage to the bases alone does not appear to account for the major portion of the strand breaks. The presence of an OH group on the ribose surrogate considerably enhances its cross section. Model compounds in which protonation or OH groups are used to terminate bonds may therefore display larger cross sections than in DNA itself.

Total dissociative electron attachment cross sections of selected amino acids.

Total dissociative electron attachment cross sections are presented for the amino acids, glycine, alanine, proline, phenylalanine, and tryptophan, at energies below the first ionization energy. Cross section magnitudes were determined by observation of positive ion production and normalization to ionization cross sections calculated using the binary-encounter-Bethe method. The prominent 1.2 eV feature in the cross sections of the amino acids and the closely related HCOOH molecule is widely attributed to the attachment into the -COOH pi* orbital. The authors discuss evidence that direct attachment to the lowest sigma* orbital may instead be responsible. A close correlation between the energies of the core-excited anion states of glycine, alanine, and proline and the ionization energies of the neutral molecules is found. A prominent feature in the total dissociative electron attachment cross section of these compounds is absent in previous studies using mass analysis, suggesting that the missing fragment is energetic H-.

An investigation of electron helicity density in bromocamphor and dibromocamphor as a source of electron circular dichroism

We investigate the causes of electron-circular dichroism (ECD) in bromocamphor and dibromocamphor, focusing specifi cally on the electron helicity density of the target. Using electron transmission spectroscopy (ETS) and quantum chemical calculations, we have observed and assigned temporary negative ion states of bromocamphor and dibromocamphor. Further calculations were conducted to determine the helicity densities of these compounds. Large helicity densities are found in the regions of large wavefunction amplitude of the normally unoccupied molecular orbitals responsible for resonances in the scattering cross sections. We relate our ETS assignments and helicity density results to the chiral asymmetry data observed in electron-circular dichroism experiments by the Munster group (Nolting et al 1997 J. Phys. B: At. Mol. Opt. Phys. 30, 5491). Our results support helicity density as a possible source of chiral asymmetry at certain resonance positions in bromocamphor and dibromocamphor.