Chemical Basis of DNA Sugar−Phosphate Cleavage by Low-Energy Electrons

Abstract

DNA damage by low-energy electrons (LEE) was examined using a novel system in which thin solid films of oligonucleotide tetramers (CGTA and GCAT) were irradiated with monoenergetic electrons (10 eV) under ultrahigh vacuum. The products of irradiation were examined by HPLC. These analyses permitted the quantitation of 16 nonmodified nucleobase, nucleoside, and nucleotide fragments of each tetramer resulting from the cleavage of phosphodiester and N-glycosidic bonds. The distribution of nonmodified products suggests a mechanism of damage involving initial electron attachment to nucleobase moieties, followed by electron transfer to the sugar-phosphate backbone, and subsequent dissociation of the phosphodiester bond. Moreover, virtually all the nonmodified fragments contained a terminal phosphate group at the site of cleavage. These results demonstrate that the phosphodiester bond breaks by a distinct pathway in which the negative charge localizes on the phosphodiester bond giving rise to nonmodified fragments with an intact phosphate group. Conversely, the radical must localize on the sugar moiety to give as yet unidentified modifications. In summary, the reaction of LEE with simple tetramers involved dissociative electron attachment leading to phosphodiester bond cleavage and the formation of nonmodified fragments.

Stable Valence Anions of Nucleic Acid Bases and DNA Strand Breaks Induced by Low Energy Electrons

“…Thus, these results demonstrate that cleavage of the phosphodiester bond by 4-15 eV electrons takes place via the formation of a sugar radical and a phosphate anion, as also demonstrated in the analysis of the products obtained from DNA bombardment with 10 eV electrons [6]. By using an X-ray secondary electron emission source, Cai et al [28] were able to directly compare DNA damage induced by high energy photons and LEEs under identical experimental conditions.…”

“…6 [26]. The probability of strand breaks at different sites along the backbone of GCAT is strongly dependent on site and electron energy [6,15,26,27], indicating that the nature and position of the base play a role in DNA damage, which seems to be an indirect evidence confirming that electron transfer from a base to phosphate is responsible for the SSBs formation. Furthermore, direct electron attachment to the phosphate groups should produce equal amounts of fragments for equivalent bonds.…”

“…Furthermore, direct electron attachment to the phosphate groups should produce equal amounts of fragments for equivalent bonds. This is clearly far from being the case [6,15,26,27] which implies that electron transfer from the bases to phosphate group is followed by the dissociation of the phosphodiester bond. Finally, electron transfer must account for the higher number of ruptured terminal phosphates observed for the GCAT tetramer irradiated with low energy electrons [6,15,26,27].…”

“…Reprinted with permission.) biological systems, most of the high energy radiation absorbed by living matter induces water radiolysis (generation of hydroxyl and hydrogen radicals) and the formation of secondary low-energy electrons (LEEs) [6]. LEEs are formed with the yield of ca.…”

Interaction of low-energy electrons with the purine bases, nucleosides, and nucleotides of DNA

“…Li et al 50,51 and Gu and co-workers [52][53][54][55][56][57][58][59] have carried out related electronic-structure studies of nucleoside ͑base+ sugar͒ and nucleotide ͑nucleoside + phosphate͒ neutrals, anions, and radicals with a view of elucidating the energetics of electron attachment and of cleaving both the phosphodiester C-O bonds of the backbone and the N-glycosidic base-sugar bonds. Condensedphase experiments on thymidine and a single-strand oligonucleotide have demonstrated that slow electrons do indeed break the C-O phosphate-sugar bonds as well as the C-N base-sugar bonds, [60][61][62] and C-O bond breaking was also found in gas-phase DA to a model phosphodiester. 63 A few electron-collision studies, experimental and theoretical, have looked at the individual backbone constituents, i.e., ribose or deoxyribose and a phosphate group, [64][65][66][67][68][69] and others have also been made of electron collisions with backbone analogs such as tetrahydrofuran, 29,65,[68][69][70][71][72][73][74][75][76][77][78][79] tetrahydrofurfuryl alcohol, 71,72,80,81 fructose, 79 and dibutyl phosphate.…”