Damage to Model DNA Fragments from Very Low-Energy (<1 eV) Electrons

Abstract

Although electrons having enough energy to ionize or electronically excite DNA have long been known to cause strand breaks (i.e., bond cleavages), only recently has it been suggested that even lower-energy electrons (most recently 1 eV and below) can also damage DNA. The findings of the present work suggest that, while DNA bases can attach electrons having kinetic energies in the 1 eV range and subsequently undergo phosphate-sugar O-C sigma bond cleavage, it is highly unlikely (in contrast to recent suggestions) that electrons having kinetic energies near 0 eV can attach to the phosphate unit's P=O bonds. Electron kinetic energies in the 2-3 eV range are required to attach directly to DNA's phosphate group's P=O pi orbital and induce phosphate-sugar O-C sigma bond cleavages if the phosphate groups are rendered neutral (e.g., by nearby counterions). Moreover, significant activation barriers to C-O bond breakage render the rates of both such damage mechanisms (i.e., P=O-attached and base-attached) slow as compared to electron autodetachment and to other damage processes.

Primary processes underlying the photostability of isolated DNA bases: Adenine

“…The problem of photostability mainly concerns DNA directly exposed to light. Hard radiation, however, easily penetrates the body and is known to generate slow secondary electrons that can subsequently attach to DNA, leading to single-and doublestrand breaks in the double helix (37)(38)(39)(40)(41)(42). Dissociative electron attachment of low-energy electrons (Ͻ3 eV) to adenine injects an electron into an empty * orbital and leads exclusively to dehydrogenation at the N9 position (40).…”

“…This could be tested in future TPRES experiments by attaching electron-withdrawing substituents to adenine at the N9 position. Work on low-energy electron attachment suggests that electrons attach either to the phosphate P ϭ O * orbital or directly to a * orbital of the nucleobase (39,43). Theoretical considerations show that electron attachment to a * orbital at the nucleobase can lead to rupture of the COO bond between the sugar and the phosphate (39,(44)(45)(46).…”

“…Work on low-energy electron attachment suggests that electrons attach either to the phosphate P ϭ O * orbital or directly to a * orbital of the nucleobase (39,43). Theoretical considerations show that electron attachment to a * orbital at the nucleobase can lead to rupture of the COO bond between the sugar and the phosphate (39,(44)(45)(46). We therefore speculate that an electron attached to the * orbital of adenine couples rapidly and efficiently to * at the N9 position in a manner similar to that demonstrated in the present communication for the case of photoexcitation.…”

UV degradation of deoxyribonucleic acid

“…Radiation induced damage in biomolecules is currently a hot topic in molecular physics since research has shown that irradiation with particle/photon energies below the ionizing potential can induce damage in deoxyribonucleic acid (DNA) [1].…”

(G–H) • –C and G–(C–H) • radicals derived from the guanine·cytosine base pair cause DNA subunit lesions

“…Yet little is known about the possible outcome of damage to the base pairs in terms of energetics and possible structural changes that can lead to strand breakage and therefore loss of genetic information. Sophisticated experimental techniques have been used to investigate the causes and effects of DNA damage and also to determine structural features, as well as delving into the electron-binding ability of the bases and base pairs (1)(2)(3)(4). Some of the more advanced experimental and theoretical methods have also been applied to model systems in both the gas and liquid phases to estimate and predict the physical properties and to study hydrogen bonding in purine and pyrimidine base pairs (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19).…”