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

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

In this report, we present solvent assisted excited-state proton transfer coupled to the deactivation of a photo-excited 2,2'-pyridylbenzimidazole bound to a single water molecule. Experimentally, the mass-selected 1 : 1 complex was probed using two-colour resonant two-photon ionization (2C-R2PI) and UV-UV hole-burning (HB) spectroscopy in a supersonically jet-cooled molecular beam. Computationally, three structural isomers were identified as the normal, the tautomer and the proton transfer product of the PBI-H₂O complex in the excited S₁ state using B3LYP-D4/def2-TZVPP and ADC(2) (MP2)/cc-pVDZ levels of theory. The most stable form in the ground state, i.e., the normal form, was identified using the excitation spectrum in the 30 544 to 30 936 cm^-1 region. The 2C-R2PI spectrum showed a sudden break-off above the 000 + 392 cm^-1 region, even though the Frack-Condon activity of the S₁ ← S₀ transition was measured beyond 000 + 1000 cm^-1 in the HB spectrum. The intensity of the bands associated with the excited state intermolecular vibrational modes near the break-off region was found to be drastically decreased, which indicates efficient quantum mechanical tunnelling along the hydrogen transfer coordinate. The sudden disappearance of the intermolecular vibrational modes in the spectrum revealed the existence of a deactivation channel in the PBI-H₂O complex near 392-450 cm^-1 above the 000 transition. The computational investigation predicted that the deactivation of the excited-state occurred via the intersection between the S₁ and S₀ states, which was associated with the proton transfer from the H₂O to the PBI molecule along the O(3)-H(4)→N(5) coordinate. The highest energy structure was identified as the point of intersection between the nπ* (S₂) and ππ* (S₁) states. The associated barrier height was experimentally determined to be 392-450 cm^-1, which showed a reasonable agreement with the calculated excited-state proton transfer barrier. Competing reaction channels such as dissociation and tautomerization were found to be highly energetically inaccessible.

Theoretical studies of the ultrafast deactivation mechanism of 8-oxo-guanine on the S1 and S2 electronic states in gas phase

The 8-oxo-deoxyguanosine is the most abundant specie of the DNA oxidative damage. Despite the deleterious effects such as gene mutation it may cause, the 8-oxodG was also reported to have beneficial effect such as repairing the nearby cyclobutane pyrimidine dimer (CPD) after photoexcitation. Due to its strong biological relevance, the photoinduced excited state dynamics behavior of 8-oxo-deoxyguanosine is of particular interest. In this work, a theoretical investigation by combination of complete active space self-consistent field (CASSCF) ab initio calculations and on-the-fly nonadiabatic dynamics simulations are implemented to provide intrinsic deactivation mechanism of its free base 8-oxoguanine after being excited to the S₁ and S₂ states. Two minimum energy conical intersections (MECIs) characterized by the C3-puckered motion with attractive chiral character are located, which contribute appreciably to the S₁ state deactivation process. When the system is being excited to the S₂ state directly, a S₂ → S₁ → S₀ two-step decay pattern is proposed. A nearly planar S₂/S₁ intersection plays a significant role in the S₂ → S₁ decay process. The subsequent S₁ state relaxation process is also dominated by the C3-puckered deformation motion. One decay time is estimated to be 704 fs, which compares well with the experimental observation of 0.9 ± 0.1 ps in solvents. Particular illustration is the fact that the MECIs configurations we located bear an exceptional resemblance with previous reported thymine, cytosine and guanine, suggesting that the current work could lend support for better understanding of the non-natural nucleobases and derivatives.

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

A study on the photocatalytic cycle of water splitting and coupled proton electron-wavepacket transfer (CPEWT) as key processes of the mechanism.

Collision induced charge separation in ground-state water splitting dynamics

The pathway of one-way electron–hole transfer induced by proton reciprocating motions, thereby realizing the collision induced ground-state charge separation.

Photorelaxation and Photorepair Processes in Nucleic and Amino Acid Derivatives

Understanding the fundamental interaction between electromagnetic radiation and matter is essential for a large number of phenomena, with significance to civilization.[...].

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

Benzopyridopyrimidine (BPP) is a fluorescent nucleobase analogue capable of forming base pairs with adenine (A) and guanine (G) at different sites. When incorporated into oligodeoxynucleotides, it is capable of differentiating between the two purine nucleobases by virtue of the fact that its fluorescence is largely quenched when it is base-paired to guanine, whereas base-pairing to adenine causes only a slight reduction of the fluorescence quantum yield. In the present article, the photophysics of BPP is investigated through computer simulations. BPP is found to be a good charge acceptor, as demonstrated by its positive and appreciably large electron affinity. The selective quenching process is attributed to charge transfer (CT) from the purine nucleobase, which is predicted to be efficient in the BPP-G base pair, but essentially inoperative in the BPP-A base pair. The CT process owes its high selectivity to a combination of two factors: the ionization potential of guanine is lower than that of adenine, and less obviously, the site occupied by guanine enables a greater stabilization of the CT state through electrostatic interactions than the one occupied by adenine. The case of BPP illustrates that molecular recognition via hydrogen bonding can enhance the selectivity of photoinduced CT processes.