Resonant interactions of slow electrons with DNA constituents

Substituents' effect in electron attachment to epigenetic modifications of cytosine

Epigenetic modifications of cytosine have been found to influence differently in many processes in biological systems. In order to investigate the differences in electron attachment to different epigenetic modifications of cytosine, we reported the A″ component of the integral cross section of electron scattering by cytosine (C) and its epigenetic modifications 5-methylcytosine (5mC), 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC). Our results were obtained with the Schwinger multichannel method with pseudopotentials in the static-exchange (SE) and static-exchange plus polarization (SEP) approximations. In addition to the scattering results, we present electron attachment energies obtained through an empirical scaling relation for the five molecules. We observed three π^* resonances for C, 5mC, and 5hmC and four for 5fC and 5caC, in both SE and SEP approximations. The cross sections show that the π^* resonances of 5mC and 5hmC are located at higher energies than the resonances of C, while the resonances of 5fC and 5caC are located at lower energies. In order to investigate this shift in the resonances' positions, we analyzed the π^* lowest-lying orbitals and the electronic density over the molecules. Using the inductive and mesomeric effects, we were able to analyze the influence of each substituent over the molecule and on the resonances' positions.

Positron and electron scattering by glycine and alanine: Shape resonances and methylation effect

We report integral cross sections (ICSs) for both positron and electron scattering by glycine and alanine amino acids. These molecules differ only by a methyl group. We computed the scattering cross sections using the Schwinger multichannel method for both glycine and alanine in different levels of approximation for both projectiles. The alanine ICSs are greater in magnitude than the glycine ICSs for both positron and electron scattering, probably due to the larger size of the molecule. In electron scattering calculations, we found two resonances for each molecule. Glycine presents one at 1.8 eV, and another centered at around 8.5 eV, in the static-exchange plus polarization (SEP) approximation. The ICS for alanine shows one resonance at 2.5 eV and another at around 9.5 eV, also in SEP approximation. The results are in good agreement with most of the data present in the literature. The comparison of the electron scattering ICSs for both molecules indicates that the methylation of glycine destabilizes the resonances, shifting them to higher energies.

Electron Attachment to Microhydrated Deoxycytidine Monophosphate

DNA constituents are effectively decomposed via dissociative electron attachment (DEA). However, the DEA contribution to radiation damage in living tissues is a subject of ongoing discussion. We address an essential question, how aqueous environment influences the DEA to DNA. In particular, we report experimental fragmentation patterns for DEA to microhydrated 2-deoxycytidine 5-monophosphate (dCMP). Isolated dCMP was previously set as a model to describe mechanisms of DNA-strand breaks induced by secondary electrons and decomposes primarily by dissociation of the C-O phosphoester bond. We show that hydrated molecules decompose via dissociation of the C-N glycosidic bond followed by dissociation of the P-O bond. This significant change of the proposed mechanism can be interpreted by a reactive role of water in the postattachment dynamics. Comparison of the fragmentation with previous macroscopic irradiation studies suggests that the actual contribution of DEA to DNA radiation damage in living tissue is rather small.

Differential cross sections for electron-impact vibrational-excitation of tetrahydrofuran at intermediate impact energies

We report differential cross sections (DCSs) for electron-impact vibrational-excitation of tetrahydrofuran, at intermediate incident electron energies (15-50 eV) and over the 10°-90° scattered electron angular range. These measurements extend the available DCS data for vibrational excitation for this species, which have previously been obtained at lower incident electron energies (≤20 eV). Where possible, our data are compared to the earlier measurements in the overlapping energy ranges. Here, quite good agreement was generally observed where the measurements overlapped.

R-matrix calculation of low-energy electron collisions with uracil

R-matrix calculations on electron-uracil collisions are presented within the static exchange, static exchange plus polarization, and close-coupling approximations. Particularly as input for the close-coupling calculations, a series of target calculations is performed which considers low-lying singlet and triplet excited states of the uracil target. The scattering calculations find three low-lying shape resonances of (2)A(") symmetry and three higher-energy Feshbach resonances of (2)A(') symmetry. In both symmetries the precise resonance parameters are found to be sensitive to the treatment of polarization effects employed. Cross sections are presented for both elastic scattering and electronic excitation. Comparisons are made with energy-dependent, differential cross section measurements at 90 degrees angle and good agreement is found for scattering energies above 0.5 eV.