Prediction of Thymine Dimer Repair by Electron Transfer from Photoexcited 8-Aminoguanine or Its Deprotonated Anion

Abstract

Electronic structure methods are used to estimate differences in reaction barriers for transfer of an electron from singlet ππ* excited 8-aminoguanine (A) or deprotonated 8-aminoguanine anion (A(-)) to a proximal thymine dimer site compared to barriers when ππ* excited 8-oxoguanine (O) or deprotonated 8-oxoguanine (O(-)) serve as the electron donor. It is predicted that the barrier for photoexcited A should be lower than for photoexcited O, and the barrier for photoexcited A(-) should be lower than for photoexcited O(-). Moreover, A, O(-), and A(-) are predicted to have ππ* excited states at energies near where O does, which allows them to be excited by photons low enough in energy to avoid exciting or ionizing any of DNA's bases. The origin of the differences in barriers is suggested to be the lower ionization potential of A compared to O and the lower electron detachment energy of A(-) compared to O(-). Because O and O(-) have been experimentally shown to produce thymine dimer repair, it is proposed that A and A(-) are promising repair agents deserving experimental study.

DNA Lesion Can Facilitate Base Ionization: Vertical Ionization Energies of Aqueous 8-Oxoguanine and its Nucleoside and Nucleotide

“…Qualitatively, the reason for the lowering of VIE compared to guanine is the presence of the electronegative oxygen atom in 8-OG, which provides additional electron density and thus destabilizes the part of the molecules from which ionization originates. We also mention that the present value of VIE for 8-OG is very close to the value of 7.1 eV calculated most recently for the same species using a similar approach . Finally, we have shown previously that in water the DNA environment has little effect on the VIE .…”

“…We also mention that the present value of VIE for 8-OG is very close to the value of 7.1 eV calculated most recently for the same species using a similar approach. 16 Finally, we have shown previously that in water the DNA environment has little effect on the VIE. 30 Our calculations thus support the hypothesis that not only free 8-OG but potentially also that produced in DNA could serve as a protective sink of the cationic hole upon prolonged exposure of DNA to ionizing radiation.…”

“…Further oxidation of 8-OG has been studied both by experimental , and theoretical groups. Additionally, there are experimental and computational studies of ionization, which may precede oxidation of guanine to 8-OG; however, the crucial knowledge of the vertical ionization energy (VIE) of 8-OG and its nucleoside and nucleotide in the context of the aqueous DNA environment has been missing, with only a single calculation on the parent species published most recently . The principle aim of the present study is to fill this gap by establishing computationally accurate VIEs of the aqueous 8-OG base and its nucleoside and nucleotide.…”