Role of Electron-Driven Proton-Transfer Processes in the Ultrafast Deactivation of Photoexcited Anionic 8-oxoGuanine-Adenine and 8-oxoGuanine-Cytosine Base Pairs

Abstract

It has been reported that 8-oxo-7,8-dihydro-guanosine (8-oxo-G), which is the main product of oxidative damage of DNA, can repair cyclobutane pyrimidine dimer (CPD) lesions when incorporated into DNA or RNA strands in proximity to such lesions. It has therefore been suggested that the 8-oxo-G nucleoside may have been a primordial precursor of present-day flavins in DNA or RNA repair. Because the electron transfer leading to the splitting of a thymine-thymine pair in a CPD lesion occurs in the photoexcited state, a reasonably long excited-state lifetime of 8-oxo-G is required. The neutral (protonated) form of 8-oxo-G exhibits a very short (sub-picosecond) intrinsic excited-state lifetime which is unfavorable for repair. It has therefore been argued that the anionic (deprotonated) form of 8-oxo-G, which exhibits a much longer excited-state lifetime, is more likely to be a suitable cofactor for DNA repair. Herein, we have investigated the exited-state quenching mechanisms in the hydrogen-bonded complexes of deprotonated 8-oxo-G− with adenine (A) and cytosine (C) using ab initio wave-function-based electronic-structure calculations. The calculated reaction paths and potential-energy profiles reveal the existence of barrierless electron-driven inter-base proton-transfer reactions which lead to low-lying S1/S0 conical intersections. The latter can promote ultrafast excited-state deactivation of the anionic base pairs. While the isolated deprotonated 8-oxo-G− nucleoside may have been an efficient primordial repair cofactor, the excited states of the 8-oxo-G−-A and 8-oxo-G−-C base pairs are likely too short-lived to be efficient electron-transfer repair agents.

A combined experimental and computational study on the deactivation of a photo-excited 2,2′-pyridylbenzimidazole–water complex via excited-state proton transfer

“…Additionally, an excited-state proton transfer can lead to the deactivation of the molecules, similar to the nucleobase pairs. 11,15,21,35 In this article, we have experimentally recorded the S 1 ' S 0 excitation spectrum to search for a barrier in the excited state hydrogen atom/proton transfer processes in the PBI-H 2 O complex. The reactions were computationally investigated to complement the experimental observations.…”

On the photocatalytic cycle of water splitting with small manganese oxides and the roles of water clusters as direct sources of oxygen molecules

“…However, PCET covers too broad a range to differentiate the precise phenomena and mechanisms. PCET has been widely studied, [9][10][11][12][13][14][15] but among these studies, those of Domcke and coworkers [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] are particularly relevant to the present work. They have extensively investigated static properties along the excited-state potential curves to consider mechanisms related to excited-state proton and/or electron transfer.…”

“…Their studies have also covered artificial photochemical water-splitting. [23][24][25][26][27][28][29][30][31][32] Water-splitting in this context is recognized as a photoinduced homolytic dissociation to obtain H and OH . This photochemical reaction eventually produces H 2 to store the photon energy in the form of chemical bonds, which can be regarded as artificial photosynthesis.…”

Mechanism Underlying the Nucleobase-Distinguishing Ability of Benzopyridopyrimidine (BPP)

“…The incorporation of modified nucleobases into DNA/RNA provides access to a range of chemical and optical functionalities, which are not available with only the five canonical nucleobases. Examples taken from the toolbox of molecular biology include unnatural base pairs, − fluorescent base analogues (FBAs), − and radiosensitizing agents. − Interestingly, modified nucleobases may also have played a key role in the early evolution of life: within the framework of the “RNA world” model of abiogenesis, − according to which all modern life originated from self-replicating RNA, it has been hypothesized that modified nucleobases were involved in the RNA catalysis of reactions, which are unlikely to have been feasible with catalysts based on the canonical nucleobases alone. − …”