Excited State Relaxation of Neutral and Basic 8-Oxoguanine

Abstract

8-Oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dGuo) is one of the most common forms of DNA oxidative damage. Recent studies have shown that 8-oxo-dGuo can repair cyclobutane pyrimidine dimers in double-stranded DNA when photoexcited, making its excited state dynamics of particular interest. The excited state lifetimes of 8-oxo-dGuo and its anion have been previously probed using transient absorption spectroscopy; however, more information is required to understand the decay mechanisms. In this work, excited state potential energy surfaces for the neutral and deprotonated forms of the free base, 8-oxoguanine (8-oxo-G), are explored theoretically using multireference methods while the nucleoside is experimentally studied using steady-state fluorescence spectroscopy. It is determined that the neutral species exhibits ultrafast radiationless decay via easy access to conical intersections. The relatively long lifetime for the anion can be explained by the existence of sizable barriers between the Franck-Condon region and two S1/S0 minimum energy conical intersections. A Strickler-Berg analysis of the experimentally measured fluorescence quantum yields and lifetimes is consistent with emission from ππ* excited states in line with theoretical predictions.

Exploration of the Singlet O2 Oxidation of 8-Oxoguanine by Guided-Ion Beam Scattering and Density Functional Theory: Changes of Reaction Intermediates, Energetics, and Kinetics upon Protonation/Deprotonation and Hydration

“…In sum, our conformation search not only reproduced the neutral, − ,, protonated, and deprotonated − , 8-oxoG conformers reported in the literature, but identified many new structures. On the basis of their populations calculated at 298 K, [8-oxoG + H] + _1, [8-oxoG + H] + _2·W67, [8-oxoG – H] − _1, [8-oxoG – H] − _1·W67 represent dominant reactants in gas-phase reactions, and were used as starting structures in construction of reaction PESs.…”

Absorption Characteristics and Quantum Yields of Singlet Oxygen Generation of Thioguanosine Derivatives

“…Photophysical and photochemical properties of 2a, 3a and their nucleosides (2b and 3b) have not been documented although their synthetic studies were reported (20)(21)(22)(23). 8-Oxoguanine (with a carbonyl group at 8-position of the purine ring and also known as an oxidation photoproduct of guanine (4a)) was reported to exhibit a redshifted absorption band relative to its 6-oxo-analogs (4a and 4b) (24)(25)(26). In addition to 1a and 1b, thiocarbonyl-modified pyrimidine bases (such as thiolated-uracil and thiolated-thymine) have a strong absorption maximum at longer wavelength than their respective unthiolated nucleobases (27)(28)(29)(30)(31).…”

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

“…It is firmly established that the photo-excited state of neutral 8-oxo-G has a sub-picosecond lifetime in aqueous solution, while deprotonated 8-oxo-G − exhibits a much longer fluorescence lifetime of 43 ps [ 26 ]. The drastic shortening of the excited-state lifetime of 8-oxo-G − relative to neutral 8-oxo-G has been explained by either CIs intrinsic to guanine, which are more easily accessible in the neutral than in the anionic form [ 57 ], or by an EDPT reaction along the H-bond between guanine and ribose in 8-oxo-guanosine, which is available in the neutral form, but not in the anionic form [ 23 ]. It has been speculated that the long lifetime of anionic 8-oxo-G − should be favorable for repair by electron transfer in the excited state, while the very short excited-state lifetime of neutral 8-oxo-G should be detrimental in this respect [ 26 ].…”

“…Correspondingly, the latter shows a significant quantum yield for fluorescence [ 26 ]. More recent fluorescence up-conversion and theoretical studies led to the conclusion that neutral 8-oxo-G exhibits an ultrafast radiationless decay via two CIs which are accessible by certain out-of-plane deformations of guanine, whilst the longer lifetime of anionic 8-oxo-G − was attributed to the existence of sizable barriers along the reaction paths connecting the Franck-Condon region to the S 1 /S 0 CIs [ 57 ]. This mechanism was also explored by Changenet-Barret et al for the neutral form [ 58 ].…”