Electronic relaxation mechanism of 9-methyl-2,6-diaminopurine and 2,6-diaminopurine-2'-deoxyribose in solution

Prolonged ultraviolet exposure results in the formation of cyclobutane pyrimidine dimers (CPDs) in RNA. Consequently, prebiotic photolesion repair mechanisms should have played an important role in the maintenance of the structural integrity of primitive nucleic acids. 2,6-Diaminopurine is a prebiotic nucleobase that repairs CPDs with high efficiency when incorporated into polymers. We investigate the electronic deactivation pathways of 2,6-diaminopurine-2'-deoxyribose and 9-methyl-2,6-diaminopurine in acetonitrile and aqueous solution to shed light on the photophysical and excited state properties of the 2,6-diaminopurine chromophore. Evidence is presented that both are photostable compounds exhibiting similar deactivation mechanisms upon the population of the S1 (ππ* La ) state at 290 nm. The mechanism involves deactivation through the C2- and C6-reaction coordinates and >99% of the excited state population decays through nonradiative pathways involving two conical intersections with the ground state. The radiative and nonradiative lifetimes are longer in aqueous solution compared to acetonitrile. While τ1 is similar in both derivatives, τ2 is ca. 1.5-fold longer in 2,6-diaminopurine-2'-deoxyribose due to a more efficient trapping in the S1 (ππ* La ) minimum. Therefore, 2,6-diaminopurine could have accumulated in significant quantities during prebiotic times to be incorporated into non-canonical RNA and play a significant role in its photoprotection.

The tautomer-specific excited state dynamics of 2,6-diaminopurine using resonance-enhanced multiphoton ionization and quantum chemical calculations

2,6-Diaminopurine (2,6-dAP) is an alternative nucleobase that potentially played a role in prebiotic chemistry. We studied its excited state dynamics in the gas phase by REMPI, IR-UV hole burning, and ps pump-probe spectroscopy and performed quantum chemical calculations at the SCS-ADC(2) level of theory to interpret the experimental results. We found the 9H tautomer to have a small barrier to ultrafast relaxation via puckering of its 6-membered ring. The 7H tautomer has a larger barrier to reach a conical intersection and also has a sizable triplet yield. These results are discussed relative to other purines, for which 9H tautomerization appears to be more photostable than 7H and homosubstituted purines appear to be less photostable than heterosubstituted or singly substituted purines.

Solvent assisted excited-state deactivation pathways in isolated 2,7-diazaindole-S1-3 (S = Water and Ammonia) complexes

The role of solvent molecules in the deactivation of photo-excited 2,7-diazaindole (DAI) - (H₂O)_1-3 and DAI - (NH₃)_1-3 complexes were computationally investigated. An excited-state proton transfer (ESPT) path from the solvent to the DAI molecule was followed using the TD-DFT-D4 (B3LYP) level of theory. The computed potential energy profile of ESPT process has shown intersection between ππ* and nπ* states facilitated via relative stabilization of the nπ* state with decreasing N(7)-H_b bond length. The ESPT process, starting from the DAI-S_n (ππ*) state, crosses through a barrier ranging from 27 to 36 kJmol^-1 for water complexes and 26-30 kJmol^-1 for ammonia complexes. The energy of the excited state was rapidly decreased with a shorter N(7)-H_b bond length. Subsequently, a significant trend of finding a second intersection between the ground and the excited state was observed for all the complexes. The results firmly suggested a significant deactivation channel of excited azaindole derivatives. In the present system, two competing channels, ESPT and ESHT, were found to be energetically accessible. The energy barriers associated with the ESPT barriers for DAI-(H₂O)_1-3 complexes are similar to the ESHT barrier, depicting equal dominance of both processes. The increased basicity of the N(7) atom in the excited state resulted a facile ESPT process from the water to N(7) of the DAI molecule. However, DAI-(NH₃)_1-3 complexes show clear preference for ESHT over ESPT process owing to its higher gas-phase pK_a value making it a poor proton donor. The above systems can be used as a model to computationally and experimentally investigate the competing radiative and deactivation pathways of photo-excited solvated complexes of N-H-bearing bio-relevant molecules via proton and hydrogen transfer reactions.

2-Oxopurine Riboside: A Dual Fluorescent Analog and Photosensitizer for RNA/DNA Research

There is significant interest in developing suitable nucleoside analogs exhibiting high fluorescence and triplet yields to investigate the structure, dynamics, and binding properties of nucleic acids and promote selective photosensitized damage to DNA/RNA, respectively. In this study, steady-state, laser flash photolysis, time-resolved IR luminescence, and femtosecond broad-band transient absorption spectroscopies are combined with quantum chemical calculations to elucidate the excited-state dynamics of 2-oxopurine riboside in aqueous solution and to investigate its prospective use as a fluorescent or photosensitizer analog. The Franck-Condon population in the S₁ (ππ*) state decays through a combination of solvent and conformational relaxation to its minimum in 1.9 ps. The population trapped in the ¹ππ* minimum bifurcates to either fluoresce or intersystem cross to the triplet manifold within ca. 5 ns, while another fraction of the population decays nonradiatively to the ground state. It is demonstrated that 2-oxopurine riboside exhibits both high fluorescent (48%) and significant triplet (between 10% and 52%) yields, leading to a yield of singlet oxygen generation of 10%, making this nucleoside analog a dual fluorescent and photosensitizer analog for DNA and RNA research.

Excited state dynamics of 2'-deoxyisoguanosine and isoguanosine in aqueous solution

Photostability is thought to be an inherent property of nucleobases required to survive the extreme ultraviolet radiation conditions of the prebiotic era. Previous studies have shown that absorption of ultraviolet radiation by the canonical nucleosides results in ultrafast internal conversion to the ground state, demonstrating that these nucleosides efficiently dissipate the excess electronic energy to the environment. In recent years, studies on the photophysical and photochemical properties of nucleobase derivatives have revealed that chemical substitution influences the electronic relaxation pathways of purine and pyrimidine nucleobases. It has been suggested that amino or carbonyl substitution at the C6 position could increase the photostability of the purine derivatives more than the substitution at the C2 position. This investigation aims to elucidate the excited state dynamics of 2'-deoxyisoguanosine (dIsoGuo) and isoguanosine (IsoGuo) in aqueous solution at pH 7.4 and 1.4, which contain an amino group at the C6 position and a carbonyl group at the C2 position of the purine chromophore. The study of these derivatives is performed using absorption and emission spectroscopies, broadband transient absorption spectroscopy, and density functional and time-dependent density functional levels of theory. It is shown that the primary relaxation mechanism of dIsoGuo and IsoGuo involves nonradiative decay pathways, where the population decays from the S₁(ππ*) state through internal conversion to the ground state via two relaxation pathways with lifetimes of hundreds of femtoseconds and less than 2 ps, making these purine nucleosides photostable in aqueous solution.

Excited state dynamics of 7-deazaguanosine and guanosine 5'-monophosphate

Minor structural modifications to the DNA and RNA nucleobases have a significant effect on their excited state dynamics and electronic relaxation pathways. In this study, the excited state dynamics of 7-deazaguanosine and guanosine 5'-monophosphate are investigated in aqueous solution and in a mixture of methanol and water using femtosecond broadband transient absorption spectroscopy following excitation at 267 nm. The transient spectra are collected using photon densities that ensure no parasitic multiphoton-induced signal from solvated electrons. The data can be fit satisfactorily using a two- or three-component kinetic model. By analyzing the results from steady-state, time-resolved, computational calculations, and the methanol-water mixture, the following general relaxation mechanism is proposed for both molecules, L_b → L_a → ¹πσ^*(ICT) → S₀, where the ¹πσ^*(ICT) stands for an intramolecular charge transfer excited singlet state with significant πσ^* character. In general, longer lifetimes for internal conversion are obtained for 7-deazaguanosine compared to guanosine 5'-monophosphate. Internal conversion of the ¹πσ^*(ICT) state to the ground state occurs on a similar time scale of a few picoseconds in both molecules. Collectively, the results demonstrate that substitution of a single nitrogen atom for a methine (C-H) group at position seven of the guanine moiety stabilizes the ¹ππ^* L_b and L_a states and alters the topology of their potential energy surfaces in such a way that the relaxation dynamics in 7-deazaguanosine are slowed down compared to those in guanosine 5'-monophosphate but not for the internal conversion of ¹πσ^*(ICT) state to the ground state.

Revealing the switching mechanisms of an off–on–off fluorescent logic gate system

We showed that TICT and PET were responsible for the off–on–off switching mechanisms of a fluorescent molecular logic gate.