Radical Formation in Pyrimidine Bases after X, Proton and α-Particle Irradiation

Abstract

Single crystals of anhydrous thymine (Ta) and cytosine monohydrate (Cm) were irradiated at room temperature using X rays, 20 MeV protons and 35 MeV alpha particles, and the relative distributions of the various radiation-induced stable radicals in the crystals were investigated. These two crystal systems were chosen because of systematic differences in their molecular packing and hydrogen-bonding network. The radicals stabilized in these systems have previously been identified and analyzed by several authors. Experimental EPR spectra could thus be reconstructed based on simulated benchmark spectra using a fitting procedure, yielding the relative amounts of the different radical species. It was found that the relative amounts of a given radical species varied with the type of radiation used, with differences being most prominent between alpha particles and protons or X rays. In Ta, an increased production of hydrogen addition or abstraction radicals was found after exposure to alpha particles. These radicals are believed to be formed predominantly from superexcited states, resulting from the higher density of ionizations along the track of the alpha particle. A corresponding reduction in radicals derived from proton transfer reactions was observed in comparison with exposure to protons or X rays. In Cm, the differences were smaller than for Ta. This is probably due to differences in the arrangement of the crystal lattice between the two systems, with Cm having an extended hydrogen-bonding network promoting fast proton transfers after ionization. Most interesting, however, was the observation that more than 50% of the radical population in Cm not could be accounted for by known radical species. The population with an unknown origin was represented with a broad singlet, having a line width of 1.5 mT and a g value of 2.0045, in the spectral reconstruction procedure.

EPR study of the free radicals in the single crystals of 2-thiothymine γ-irradiated at 300K

“…As can seen, hyperfine coupling tensors of the 7-yl radicals are similar in all thymine-based systems. The data presented in this article agree well with the previous findings [16,17,[24][25][26], indicating that substitution of oxygen with sulfur at C(2) in 2-thiothymine does not affect EPR features of the 7-yl radical and the values of the evaluated tensors. The calculated large values of the g-tensor elements (g max = 2.0077) of the 7-yl radical in 2-thiothymine is expected since the large values of the g-tensor were evaluated in irradiated single crystals of thymine monohydrate (g max = 2.0078) [16].…”

“…1. Such six-resonance line EPR spectrum is characteristic of three inequivalent and nonexchangeable a-proton interactions and it was unequivocally assigned to 7-yl radical, previously observed in all thymine derivatives studied so far [17,18,[24][25][26]. 7-yl radical is formed by a net hydrogen abstraction from the methyl group of 2-thiothymine, which causes that all of spin density in this radical is located only on two atoms -C(6) and C(7).…”

Electron Transfer in Amino Acid·Nucleic Acid Base Complexes: EPR, ENDOR, and DFT Study of X-Irradiated N-Formylglycine·Cytosine Complex Crystals

“…However, to obtain this simulation, and also those at all other orientations, a broad singlet structure with a line width of about 40 MHz (1.5 mT) had to be included. Similar singlet resonances have previously been observed in several crystalline cytosine systems and are so far only tentatively ascribed to various unidentified room-temperature cytosine radicals. The relative contributions of the various radicals typically were of the order 40% R3, 5−8% of each of R4 and R5, and 50% of the singlet.…”

Hydrogen-Atom Abstraction from the Adenine−Uracil Base Pair†

“…Along with positively and negatively charged species, various neutral radicals can also play an important role during the radiation-induced DNA damage process. − An example of this is the radicals arising from the homolytic C−H or N−H bond cleavage of the NABs. − Such radicals are generated either by direct abstraction of one hydrogen atom from the neutral NABs or by deprotonation of the oxidized (cationic) NABs. ,, For example, the radical generated by removal of a hydrogen atom from the methyl group of thymine is known to be readily oxidized to give modified nucleobases such as 5-(hydroxymethyl)uracil or 5-formyluracil. In addition, the recent studies of Greenberg and co-workers showed that this radical can also generate an interstrand cross-link in double-stranded DNA. − …”