Phosphodiester bond rupture in 5′ and 3′ cytosine monophosphate in aqueous environment and the effect of low-energy electron attachment: A Car–Parrinello QM/MM molecular dynamics study

Secondary Electron Attachment-Induced Radiation Damage to Genetic Materials

Reactions of radiation-produced secondary electrons (SEs) with biomacromolecules (e.g., DNA) are considered one of the primary causes of radiation-induced cell death. In this Review, we summarize the latest developments in the modeling of SE attachment-induced radiation damage. The initial attachment of electrons to genetic materials has traditionally been attributed to the temporary bound or resonance states. Recent studies have, however, indicated an alternative possibility with two steps. First, the dipole-bound states act as a doorway for electron capture. Subsequently, the electron gets transferred to the valence-bound state, in which the electron is localized on the nucleobase. The transfer from the dipole-bound to valence-bound state happens through a mixing of electronic and nuclear degrees of freedom. In the presence of aqueous media, the water-bound states act as the doorway state, which is similar to that of the presolvated electron. Electron transfer from the initial doorway state to the nucleobase-bound state in the presence of bulk aqueous media happens on an ultrafast time scale, and it can account for the decrease in DNA strand breaks in aqueous environments. Analyses of the theoretically obtained results along with experimental data have also been discussed.

Electrophilic Properties of 2'-Deoxyadenosine···Thymine Dimer: Photoelectron Spectroscopy and DFT Studies

The anion radical of the 2'-deoxyadenosine···thymine (dAT^•-) pair has been investigated experimentally and theoretically in the gas phase. By employing negative-ion photoelectron spectroscopy (PES), we have registered a spectrum typical for the valence-bound anion, featuring a broad peak at the electron-binding energy (EBE) between ∼1.5 and 2.2 eV with the maximum at ∼1.9 eV. The measured value of the adiabatic electron affinity (AEA) for dAT was estimated to be ∼1.1 eV. Calculations performed at the M06-2X/6-31++G(d,p) level revealed that the structure, where thymine is coordinated to the sugar of dA by two hydrogen bonds, is responsible for the observed PES signal. The AEA_G and the vertical detachment energy of 0.91 and 1.68 eV, respectively, calculated for this structure reproduce the experimental values well. The role of the possible proton transfer in the stabilization of anionic radical complexes is discussed.

Solvation Effects on Dissociative Electron Attachment to Thymine

Ionizing radiation can excite the cellular medium to produce secondary electrons that can subsequently cause damage to DNA. The damage is believed to occur via dissociative electron attachment (DEA). In DEA, the electron is captured by a molecule in a resonant antibonding state and a transient negative ion is formed. If this ion survives against electron autodetachment, then bonds within the molecule may dissociate as energy is transferred from the electronic degrees of freedom into vibrational modes of the molecule. We present a model for studying the effect that transferring kinetic energy into the vibrational modes of a molecule in this way has on a DNA nucleobase. We show that when the base is in an aqueous environment, dissociation is affected by interactions with the surrounding water molecules. In particular, hydrogen bonding between the nucleobase and the solvent can suppress the dissociative channel.

Interactions between low energy electrons and DNA: a perspective from first-principles simulations

DNA damage caused by irradiation has been studied for many decades. Such studies allow us to better assess the dangers posed by radiation, and to increase the efficiency of the radiotherapies that are used to combat cancer. A full description of the irradiation process involves multiple size and time scales. It starts with the interaction of radiation-either photons or swift ions-and the biological medium, which causes electronic excitation and ionisation. The two main products of ionising radiation are thus electrons and radicals. Both of these species can cause damage to biological molecules, in particular DNA. In the long run, this molecular level damage can prevent cells from replicating and can hence lead to cell death. For a long time it was assumed that the main actors in the damage process were the radicals. However, experiments in a seminal paper by the group of Leon Sanche in 2000 showed that low-energy electrons (LEE), such as those generated when ionising biological targets, can also cause bond breaks in biomolecules, and strand breaks in plasmid DNA in particular (Boudaiffa et al 2000 Science 287 1658-60). These results prompted a significant amount of experimental and theoretical work aimed at elucidating the role played by LEE in DNA damage. In this Topical Review we provide a general overview of the problem. We discuss experimental findings and theoretical results hand in hand with the aim of describing the physics and chemistry that occurs during the process of radiation damage, from the initial stages of electronic excitation, through the inelastic propagation of electrons in the medium, the interaction of electrons with DNA, and the chemical end-point effects on DNA. A very important aspect of this discussion is the consideration of a realistic, physiological environment. The role played by the aqueous solution and the amino acids from the histones in chromatin must be considered. Moreover, thermal fluctuations must be incorporated when studying these phenomena. Hence, a special place in this Topical Review is occupied by our recent first-principles molecular dynamics simulations that address the issue of how the environment favours or prevents LEEs from causing damage to DNA. We finish by summarising the conclusions achieved so far, and by suggesting a number of possible directions for further study.

DFT reinvestigation of DNA strand breaks induced by electron attachment

The benchmark study of DFT methods on the activation energies of phosphodiester C3'-O and C5'-O bond ruptures and glycosidic C1'-N bond ruptures induced by electron attachment was performed. While conventional pure and hybrid functionals provide a relatively reasonable description for the C1'-N bond rupture, they significantly underestimate the energy barriers of the C-O bond ruptures. This is because the transition states of the later reactions, which are characterized by an electron distribution delocalized from the nucleobase to sugar-phosphate backbone, suffer from a severe self-interaction error in common DFT methods. CAM-B3LYP, M06-2X, and ωB97XD are the top three methods that emerged from the benchmark study; the mean absolute errors relative to the CCSD(T) values are 1.7, 1.9, and 2.2 kcal/mol, respectively. The C-O bond cleavages of 3'- and 5'-dXMP(•-), where X represents four nucleobases, were then recalculated at the M06-2X/6-31++G**//M06-2X/6-31+G* level, and it turned out that the C-O bond cleavages do not proceed as easily as previously predicted by the B3LYP calculations. Our calculations revealed that the C-O bonds of purine nucleotides are more susceptible than pyrimidine nucleotides to the electron attachment. The energies of electron attachment to nucleotides were calculated and discussed as well.

Excess electron interactions with solvated DNA nucleotides: strand breaks possible at room temperature

When biological matter is subjected to ionizing radiation, a wealth of secondary low-energy (<20 eV) electrons are produced. These electrons propagate inelastically, losing energy to the medium until they reach energies low enough to localize in regions of high electron affinity. We have recently shown that in fully solvated DNA fragments, nucleobases are particularly attractive for such excess electrons. The next question is what is their longer-term effect on DNA. It has been advocated that they can lead to strand breaks by cleavage of the phosphodiester C(3')-O(3') bond. Here we present a first-principles study of free energy barriers for the cleavage of this bond in fully solvated nucleotides. We have found that except for dAMP, the barriers are on the order of 6 kcal/mol, suggesting that bond cleavage is a regular feature at 300 K. Such low barriers are possible only as a result of solvent and thermal fluctuations. These findings support the notion that low-energy electrons can indeed lead to strand breaks in DNA.