Excitation of Nucleobases from a Computational Perspective I: Reaction Paths

Photophysics of Nitro-Substituted Unnatural Nucleic Acid Base

The unnatural nucleic acid base (uNAB), 6-amino-3-methyl-5-nitropyridin-2(1H)one, often referred to as Z can form a base pair with the uNAB 2-aminoimidazo[1,2-a]-1,3,5-triazin-4(8H)-one (referred to as P) and is analogous to a guanine-cytosine (G-C) pair. However, it is well-known that the nonradiative decay pathway of the P-Z pair is significantly different from that of the G-C pair (Cui et al., Front. Chem. 2020, 8, 605117-605125). In this work, we study the excited state processes in Z using state-of-the-art multireference methods and dynamical techniques to ascertain the predominant nonradiative channels. We find that unlike in the natural NABs, the excited state processes in Z are driven primarily by the -NO2 group rotation. The electron-withdrawing effect of the -NO2 substituent plays a crucial role. We further ascertained that ultrafast deactivation channels are possible in Z and identified the stationary point geometries that are responsible for these channels.

Time-resolved x-ray spectroscopy of nucleobases and their thionated analogs

The photoinduced relaxation dynamics of nucleobases and their thionated analogs have been investigated extensively over the past decades motivated by their crucial role in organisms and their application in medical and biochemical research and treatment. Most of these studies focused on the spectroscopy of valence electrons and fragmentation. The advent of ultrashort x-ray laser sources such as free-electron lasers, however, opens new opportunities for studying the ultrafast molecular relaxation dynamics utilizing the site- and element-selectivity of x-rays. In this review, we want to summarize ultrafast experiments on thymine and 2-thiouracil performed at free-electron lasers. We performed time-resolved x-ray absorption spectroscopy at the oxygen K-edge after UV excitation of thymine. In addition, we investigated the excited state dynamics of 2-tUra via x-ray photoelectron spectroscopy at sulfur. For these methods, we show a strong sensitivity to the electronic state or charge distribution, respectively. We also performed time-resolved Auger-Meitner spectroscopy, which shows spectral shifts associated with internuclear distances close to the probed site. We discuss the complementary aspects of time-resolved x-ray spectroscopy techniques compared to optical and UV spectroscopy for the investigation of ultrafast relaxation processes.

Quantum mechanics/molecular mechanics studies on the excited-state decay mechanisms of cytidine aza-analogues: 5-azacytidine and 2'-deoxy-5-azacytidine in aqueous solution

The excited state properties and deactivation pathways of two DNA methylation inhibitors, i.e., 5-azacytidine (5ACyd) and 2'-deoxy-5-azacytidine (5AdCyd) in aqueous solution, are comprehensively explored with the QM(CASPT2//CASSCF)/MM protocol. We systematically map the feasible decay mechanisms based on the obtained excited-state decay paths involving all the identified minimum-energy structures, conical intersections, and crossing points driving the different internal conversion (IC) and intersystem crossing (ISC) routes in and between the 1ππ*, 1nπ*, 3ππ*, 3nπ*, and S0 states. Unlike the 1nπ* state below the 1ππ* state in 5ACyd, deoxyribose group substitution at the N1 position leads to the 1ππ* state becoming the S1 state in 5AdCyd. In 5ACyd and 5AdCyd, the initially populated 1ππ* state mainly deactivates to the S0 state through the direct 1ππ* → S0 IC or mediated by the 1nπ* state. The former nearly barrierless IC channel of 1ππ* → S0 occurs ultrafast via the nearby low-lying 1ππ*/S0 conical intersection. In the latter IC channel of 1ππ* → 1nπ* → S0, the initially photoexcited 1ππ* state first approaches the nearby S2/S1 conical section 1ππ*/1nπ* and then undergoes efficient IC to the 1nπ* state, followed by the further IC to the initial S0 state via the S1/S0 conical intersection 1nπ*/S0. The 1nπ*/S0 conical intersection is estimated to be located 6.0 and 4.9 kcal mol-1 above the 1nπ* state minimum in 5ACyd and 5AdCyd, respectively, at the QM(CASPT2)/MM level. In addition to the efficient singlet-mediated IC channels, the minor ISC routes would populate 1ππ* to T1(ππ*) through 1ππ* → T1 or 1ππ* → 1nπ* → T1. Relatively, the 1ππ* → 1nπ* → T1 route benefits from the spin-orbit coupling (SOC) of 1nπ*/3ππ* of 8.7 cm-1 in 5ACyd and 10.2 cm-1 in 5AdCyd, respectively. Subsequently, the T1 system will approach the nearby T1/S0 crossing point 3ππ*/S0 driving it back to the S0 state. Given the 3ππ*/S0 crossing point located above the T1 minimum and the small T1/S0 SOC, i.e., 8.4 kcal mol-1 and 2.1 cm-1 in 5ACyd and 6.8 kcal mol-1 and 1.9 cm-1 in 5AdCyd, respectively, the slow T1 → S0 would trap the system in the T1 state for a while. The present work could contribute to understanding the mechanistic photophysics and photochemistry of similar aza-nucleosides and their derivatives.

Ultrafast Intersystem Crossing Dynamics of 6-Selenoguanine in Water

Rationalizing the photochemistry of nucleobases where an oxygen is replaced by a heavier atom is essential for applications that exploit near-unity triplet quantum yields. Herein, we report on the ultrafast excited-state deactivation mechanism of 6-selenoguanine (6SeGua) in water by combining nonadiabatic trajectory surface-hopping dynamics with an electrostatic embedding quantum mechanics/molecular mechanics (QM/MM) scheme. We find that the predominant relaxation mechanism after irradiation starts on the bright singlet S₂ state that converts internally to the dark S₁ state, from which the population is transferred to the triplet T₂ state via intersystem crossing and finally to the lowest T₁ state. This S₂ → S₁ → T₂ → T₁ deactivation pathway is similar to that observed for the lighter 6-thioguanine (6tGua) analogue, but counterintuitively, the T₁ lifetime of the heavier 6SeGua is shorter than that of 6tGua. This fact is explained by the smaller activation barrier to reach the T₁/S₀ crossing point and the larger spin-orbit couplings of 6SeGua compared to 6tGua. From the dynamical simulations, we also calculate transient absorption spectra (TAS), which provide two time constants (τ₁ = 131 fs and τ₂ = 191 fs) that are in excellent agreement with the experimentally reported value (τ_exp = 130 ± 50 fs) (Farrel et al. J. Am. Chem. Soc. 2018, 140, 11214). Intersystem crossing itself is calculated to occur with a time scale of 452 ± 38 fs, highlighting that the TAS is the result of a complex average of signals coming from different nonradiative processes and not intersystem crossing alone.

On the Photostability of Cyanuric Acid and Its Candidature as a Prebiotic Nucleobase

Cyanuric acid is a triazine derivative that has been identified from reactions performed under prebiotic conditions and has been proposed as a prospective precursor of ancestral RNA. For cyanuric acid to have played a key role during the prebiotic era, it would have needed to survive the harsh electromagnetic radiation conditions reaching the Earth’s surface during prebiotic times (≥200 nm). Therefore, the photostability of cyanuric acid would have been crucial for its accumulation during the prebiotic era. To evaluate the putative photostability of cyanuric acid in water, in this contribution, we employed density functional theory (DFT) and its time-dependent variant (TD-DFT) including implicit and explicit solvent effects. The calculations predict that cyanuric acid has an absorption maximum at ca. 160 nm (7.73 eV), with the lowest-energy absorption band extending to ca. 200 nm in an aqueous solution and exhibiting negligible absorption at longer wavelengths. Excitation of cyanuric acid at 160 nm or longer wavelengths leads to the population of S_5,6 singlet states, which have ππ* character and large oscillator strengths (0.8). The population reaching the S_5,6 states is expected to internally convert to the S_1,2 states in an ultrafast time scale. The S_1,2 states, which have nπ* character, are predicted to access a conical intersection with the ground state in a nearly barrierless fashion (ca. ≤ 0.13 eV), thus efficiently returning the population to the ground state. Furthermore, based on calculated spin–orbit coupling elements of ca. 6 to 8 cm⁻¹, the calculations predict that intersystem crossing to the triplet manifold should play a minor role in the electronic relaxation of cyanuric acid. We have also calculated the vertical ionization energy of cyanuric acid at 8.2 eV, which predicts that direct one-photon ionization of cyanuric acid should occur at ca. 150 nm. Collectively, the quantum-chemical calculations predict that cyanuric acid would have been highly photostable under the solar radiation conditions reaching the Earth’s surface during the prebiotic era in an aqueous solution. Of relevance to the chemical origin of life and RNA-first theories, these observations lend support to the idea that cyanuric acid could have accumulated in large quantities during the prebiotic era and thus strengthens its candidature as a relevant prebiotic nucleobase.

The Excited State Dynamics of a Mutagenic Cytidine Etheno Adduct Investigated by Combining Time-Resolved Spectroscopy and Quantum Mechanical Calculations

Joint femtosecond fluorescence upconversion experiments and theoretical calculations provide a hitherto unattained degree of characterization and understanding of the mutagenic etheno adduct 3,N4-etheno-2'-deoxycytidine (εdC) excited state relaxation. This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The εdC fluorescence is modified with respect to that of the canonical base dC, with a 3-fold increased lifetime and quantum yield at neutral pH. This behavior is amplified upon protonation of the etheno ring (εdCH⁺). Quantum mechanical calculations show that the lowest energy state ππ*1 is responsible for the fluorescence and that the main nonradiative decay pathway to the ground state goes through an ethene-like conical intersection, involving the out-of-plane motion of the C5 and C6 substituents. This conical intersection is lower in energy than the ππ* state (ππ*1) minimum, but a sizable energy barrier explains the increase of εdC and εdCH⁺ fluorescence lifetimes with respect to that of dC.

Photophysical Deactivation Mechanisms of the Pyrimidine Analogue 1-Cyclohexyluracil

The photophysical relaxation mechanisms of 1-cyclohexyluracil, in vacuum and water, were investigated by employing the Multi-State CASPT2 (MS-CASPT2, Multi-State Complete Active-Space Second-Order Perturbation Theory) quantum chemical method and Dunning's cc-pVDZ basis sets. In both environments, our results suggest that the primary photophysical event is the population of the S11(ππ*) bright state. Afterwards, two likely deactivation pathways can take place, which is sustained by linear interpolation in internal coordinates defined via Z-Matrix scans connecting the most important characteristic points. The first one (Route 1) is the same relaxation mechanism observed for uracil, its canonical analogue, i.e., internal conversion to the ground state through an ethylenic-like conical intersection. The other route (Route 2) is the direct population transfer from the S11(ππ*) bright state to the T23(nπ*) triplet state via an intersystem crossing process involving the (S11(ππ*)/T23(nπ*))STCP singlet-triplet crossing point. As the spin-orbit coupling is not too large in either environment, we propose that most of the electronic population initially on the S11(ππ*) state returns to the ground following the same ultrafast deactivation mechanism observed in uracil (Route 1), while a smaller percentage goes to the triplet manifold. The presence of a minimum on the S11(ππ*) potential energy hypersurface in water can help to understand why experimentally it is noticed suppression of the triplet states population in polar protic solvent.

Ultrafast Dynamics of the Two Lowest Bright Excited States of Cytosine and 1-Methylcytosine: A Quantum Dynamical Study

The nonadiabatic quantum dynamics (QD) of cytosine and 1-methylcytosine in the gas phase is simulated for 250 fs after a photoexcitation to one of the first two bright states. The nuclear wavepacket is propagated on the coupled diabatic potential energy surfaces of the lowest seven excited states, including ππ*, nπ*, and Rydberg states along all the vibrational degrees of freedom. We focus in particular on the interplay between the bright and the dark nπ* states, not considering the decay to the ground electronic state. To run these simulations, we implemented an automatic general procedure to parametrize linear vibronic coupling (LVC) models with time-dependent density functional theory (DFT) computations and interfaced it with Gaussian package. The wavepacket was propagated with the multilayer version of the multiconfigurational time dependent Hartree method. Two different density functionals, PBE0 and CAM-B3LYP, which provide a different description of the relative stability of the lowest energy dark states, were used to parametrize the LVC Hamiltonian. Part of the photoexcited population on lowest HOMO-LUMO transition (π_Hπ_L^*) decays within less than 100 fs to a nπ* state which mainly involves a promotion of an electron from the oxygen lone pair to the LUMO (n_Oπ_L^*). The population of the second ππ* state decays almost completely, in <100 fs, not only to π_Hπ_L^* and to n_Oπ_L^* states but also to another nπ_L^* state involving the nitrogen lone pair. The efficiency of the adopted protocol allowed us to check the accuracy of the predictions by repeating the QD simulations with different LVC Hamiltonians parametrized either at the ground-state minimum or at stationary structures of different relevant excited states.

On the Intrinsically Low Quantum Yields of Pyrimidine DNA Photodamages: Evaluating the Reactivity of the Corresponding Minimum Energy Crossing Points

The low quantum yield of photoformation of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) adducts in DNA bases is usually associated with the presence of more favorable nonreactive decay paths and with the unlikeliness of exciting the system in a favorable conformation. Here, we prove that the ability of the reactive conical intersection to bring the system either back to the absorbing conformation or to the photoproduct must be considered as a fundamental factor in the low quantum yields of the mentioned photodamage. In support of the proposed model, the one order of magnitude difference in the quantum yield of formation of the cyclobutane thymine dimer with respect to the thymine-thymine (6-4) adduct is rationalized here by comparing the reactive ability of the seam of intersections leading respectively to the cyclobutane thymine dimer and the oxetane precursor of the thymine-thymine (6-4) adduct at the CASPT2 level of theory.

First-principles characterization of the singlet excited state manifold in DNA/RNA nucleobases

TD-DFT characterization of the high-energy singlet excited state manifold of the canonical DNA/RNA nucleobasesin vacuumis assessed against RASPT2 reference computations for reliable simulations of linear and non-linear electronic spectra.

Initial excited-state structural dynamics of 2′-deoxyadenosine

Purine nucleobases (adenosine and guanosine) are prone to spontaneous breaking of the nucleosidic bond to form abasic sites in both DNA and RNA. However, the purines also undergo photochemical reactions, including oxidation and cycloaddition, to form damage sites, albeit at lower photochemical quantum yields than the pyrimidines. In this study, we use ultraviolet resonance Raman spectroscopy to measure the initial excited-state structural dynamics in the nucleoside, 2′-deoxyadenosine. The resonance Raman-derived initial excited-state structural dynamics throughout the 260 nm La excited electronic state of adenine are found to be smaller in the nucleoside than in the previously reported 9-methyladenine nucleobase derivative, consistent with what is found for the pyrimidines thymine and uracil. Interestingly, resonance-enhanced vibrational modes in this electronic state also contain internal coordinates localized on the sugar, which may represent a different energy dissipation mechanism than in the pyrimidine nucleosides. The results will be discussed in terms of the initial excited-state photophysics and photochemistry of DNA and RNA.

Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation

In this article we characterise the radiationless decay of the first few electronic excited states of the cations of DNA/RNA nucleobases uracil and thymine, including the effects of dynamic electron correlation on energies and geometries (optimised with XMS-CASPT2). In both systems, we find that one state of ²n and another two of ²π⁺ character can be populated following photoionisation, and their different minima and interstate crossings are located. We find strong similarities between uracil and thymine cations: with accessible conical intersections suggesting that depopulation of their electronic excited states takes place on ultrafast timescales in both systems, suggesting that they are photostable in agreement with previous theoretical (uracil⁺) evidence. We find that dynamic electron correlation separates the energy levels of the "3-state" conical intersection (D₂/D₁/D₀)_CI previously located with CASSCF for uracil⁺, which will therefore have a different geometry and higher energy. Simulating the electronic and vibrational absorptions allows us to characterise spectral fingerprints that could be used to monitor these cation photo-processes experimentally.

Inverted energy gap law for the nonradiative decay in fluorescent floppy molecules: larger fluorescence quantum yields for smaller energy gaps

The non-radiative decay of substituted dicyano-distyrylbenzenes in solution increase with the Franck–Condon energy, being opposite to the conventional energy gap law.

Triplet photosensitization mechanism of thymine by an oxidized nucleobase: from a dimeric model to DNA environment

Nucleic acids are constantly exposed to external agents that can induce chemical and photochemical damage. In spite of the great advances achieved in the last years, some molecular mechanisms of DNA damage are not completely understood yet. A recent experimental report (I. Aparici-Espert et al., ACS Chem. Biol. 2018, 13, 542) proved the ability of 5-formyluracil (ForU), a common oxidatively generated product of thymine, to act as an intrinsic sensitizer of nucleic acids, causing single strand breaks and cyclobutane pyrimidine dimers in plasmid DNA. In the present contribution, we use theoretical methodologies to study the triplet photosensitization mechanism of thymine exerted by ForU in a model dimer and in DNA environment. The photochemical pathways in the former system are described combining the CASPT2 and TD-DFT methods, whereas molecular dynamics simulations and QM/MM calculations are employed for the DNA duplex. It is unambiguously shown that the 1n,π* state localised in ForU mediates the population of the triplet manifold, most likely the 3π,π* state centred in ForU, whereas the 3π,π* state localized in thymine can be populated via triplet-triplet energy transfer given the small energy barrier of <0.23 eV determined for this pathway.

Dynamics of the excited-state hydrogen transfer in a (dG)·(dC) homopolymer: intrinsic photostability of DNA

The intrinsic photostability of nucleic acids is intimately related to evolution of life, while its understanding at the molecular and electronic levels remains a challenge for modern science. Among the different decay pathways proposed in the last two decades, the excited-state hydrogen transfer between guanine-cytosine base pairs has been identified as an efficient non-reactive channel to dissipate the excess of energy provided by light absorption. The present work studies the dynamics of such phenomena taking place in a (dG)·(dC) B-DNA homopolymer in water solution using state-of-the-art molecular modelling and simulation methods. A dynamic effect that boosts the photostability of the inter-strand hydrogen atom transfers, inherent to the Watson-Crick base pairing, is unveiled and ascribed to the energy released during the proton transfer step. Our results also reveal a novel mechanism of DNA decay named four proton transfer (FPT), in which two protons of two adjacent G-C base pairs are transferred to form a biradical zwitterionic intermediate. Decay of the latter intermediate to the ground state triggers the transfer of the protons back to the guanine molecules recovering the Watson-Crick structure of the tetramer. This FPT process is activated by the close interaction of a nearby Na+ counterion with the oxygen atoms of the guanine nucleobases and hence represents a photostable channel operative in natural nucleic acids.

Excited State Decay Pathways of 2'-Deoxy-5-methylcytidine and Deoxycytidine Revisited in Solution: A Comprehensive Kinetic Study by Femtosecond Transient Absorption

Methylated cytosine is proved to have an important role as an epigenetic signal in gene regulation and is often referred to "the fifth base of DNA". A comprehensive understanding of the electronic excited state relaxation in cytosine and its methylated derivatives is crucial for revealing UV-induced photodamage to the biological genome. Because of the existence of multiple closely lying "bright" and "dark" excited states, the decay pathways in these DNA nucleosides are the most complex and the least understood so far. In this study, femtosecond transient absorption with different excitation wavelengths (240-296 nm) was used to study the relaxation of excited electronic states of 5-methylcytosine (5mC) and 2'-deoxy-5-methylcytidine (5mdCyd) in phosphate buffered aqueous solution and in acetonitrile solution. Two distinct nonradiative decay channels were directly observed. The first one is a several picosecond internal conversion channel that involves two bright ππ* states (ππ*₂ and ππ*₁) when ππ*₂ state is initially populated. The second channel contains the lower energy ππ*₁ state and a so far experimental unidentified long-lived state which exhibits a several nanosecond lifetime. The long-lived state can only be accessed by the initially excited ππ*₁ state. Inspired by this new discovery in 5mC and 5mdCyd, we revisited the decay of excited state of 2'-deoxycytidine (dCyd), revealing very similar decay pathways. Additionally, a well-known dark n_Oπ* state (carbonyl lone pair) with ∼30 ps lifetime is present in both decay channels in dCyd. With our detailed experimental results, we successfully reconcile the long history debate of cytosine excited state relaxation mechanism by pointing out that the reason for the complex dynamics under traditional 266 nm excitation is mixed signals from the above-mentioned two distinct decay pathways. Our findings lead to a dramatically different and new picture of electronic energy relaxation in 5mdCyd/dCyd and could help to understand photostability as well as UV-induced photodamage of these nucleotides and related DNAs.

Internal conversion and intersystem crossing pathways in UV excited, isolated uracils and their implications in prebiotic chemistry

The photodynamic properties of molecules determine their ability to survive in harsh radiation environments. As such, the photostability of heterocyclic aromatic compounds to electromagnetic radiation is expected to have been one of the selection pressures influencing the prebiotic chemistry on early Earth. In the present study, the gas-phase photodynamics of uracil, 5-methyluracil (thymine) and 2-thiouracil-three heterocyclic compounds thought to be present during this era-are assessed in the context of their recently proposed intersystem crossing pathways that compete with internal conversion to the ground state. Specifically, time-resolved photoelectron spectroscopy measurements evidence femtosecond to picosecond timescales for relaxation of the singlet (1)ππ* and (1)nπ* states as well as for intersystem crossing to the triplet manifold. Trapping in the excited triplet state and intersystem crossing back to the ground state are investigated as potential factors contributing to the susceptibility of these molecules to ultraviolet photodamage.

Photophysical properties of pyrrolocytosine, a cytosine fluorescent base analogue

The photophysical behavior of pyrrolocytosine (PC), a fluorescent base analogue of cytosine, has been investigated using theoretical approaches. The similarities between the PC and cytosine structures allow PC to maintain the pseudo-Watson-Crick base-pairing arrangement with guanine. Cytosine, similar to the other natural nucleobases, is practically non-fluorescent, because of ultrafast radiationless decay occurring through conical intersections. PC displays a much higher fluorescence quantum yield than cytosine, making it an effective fluorescent marker to study the structure, function, and dynamics of DNA/RNA complexes. Similar to 2-aminopurine, a constitutional isomer of adenine that base-pairs with thymine, PC's fluorescence is quenched when it is incorporated into a dinucleotide or a trinucleotide. In this work we examine the photophysical properties of isolated PC, microhydrated PC, as well as, complexes where PC is either base-stacked or hydrogen-bonded with guanine. Our results indicate that hydration affects the radiationless decay pathways in PC by destabilizing conical intersections. The calculations of dimers and trimers show that the radiative decay is affected by π stacking, while the presence of charge transfer states between PC and guanine may contribute to radiationless decay.

Photoinduced formation mechanism of the thymine–thymine (6–4) adduct in DNA; a QM(CASPT2//CASSCF):MM(AMBER) study

Out-of-plane puckering of the C_4′ atom forming a new CC bond seems essential for oxetane formation.

Insights into the deactivation of 5-bromouracil after ultraviolet excitation

5-Bromouracil is a nucleobase analogue that can replace thymine in DNA strands and acts as a strong radiosensitizer, with potential applications in molecular biology and cancer therapy. Here, the deactivation of 5-bromouracil after ultraviolet irradiation is investigated in the singlet and triplet manifold by accurate quantum chemistry calculations and non-adiabatic dynamics simulations. It is found that, after irradiation to the bright ππ* state, three main relaxation pathways are, in principle, possible: relaxation back to the ground state, intersystem crossing (ISC) and C-Br photodissociation. Based on accurate MS-CASPT2 optimizations, we propose that ground-state relaxation should be the predominant deactivation pathway in the gas phase. We then employ different electronic structure methods to assess their suitability to carry out excited-state dynamics simulations. MRCIS (multi-reference configuration interaction including single excitations) was used in surface hopping simulations to compute the ultrafast ISC dynamics, which mostly involves the 1nOπ* and 3ππ* states.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Resolving Ultrafast Photoinduced Deactivations in Water-Solvated Pyrimidine Nucleosides

For the first time, ultrafast deactivations of photoexcited water-solvated pyrimidine nucleosides are mapped employing hybrid QM(CASPT2)/MM(AMBER) optimizations that account for explicit solvation, sugar effects, and dynamically correlated potential energy surfaces. Low-energy S₁/S₀ ring-puckering and ring-opening conical intersections (CIs) are suggested to drive the ballistic coherent subpicosecond (<200 fs) decays observed in each pyrimidine, the energetics controlling this processes correlating with the lifetimes observed. A second bright ¹π₂π* state, promoting excited-state population branching and leading toward a third CI with the ground state, is proposed to be involved in the slower ultrafast decay component observed in Thd/Cyd. The transient spectroscopic signals of the competitive deactivation channels are computed for the first time. A general unified scheme for ultrafast deactivations, spanning the sub- to few-picosecond time domain, is eventually delivered, with computed data that matches the experiments and elucidates the intrinsic photoprotection mechanism in solvated pyrimidine nucleosides.

How Does Thymine DNA Survive Ultrafast Dimerization Damage?

The photodimerization reaction between the two adjacent thymine bases within a single strand has been the subject of numerous studies due to its potential to induce DNA mutagenesis and possible tumorigenesis in human skin cells. It is well established that the cycloaddition photoreaction takes place on a picosecond time scale along barrierless or low barrier singlet/triplet pathways. However, the observed dimerization quantum yield in different thymine multimer is considerable lower than might be expected. A reasonable explanation is required to understand why thymine in DNA is able to survive ultrafast dimerization damage. In this work, accurate quantum calculations based on the combined CASPT2//CASSCF/AMBER method were conducted to map the excited state relaxation pathways of the thymine monomer in aqueous solution and of the thymine oligomer in DNA. A monomer-like decay pathway, induced by the twisting of the methyl group, is found to provide a bypass channel to ensure the photostability of thymine in single-stranded oligomers. This fast relaxation path is regulated by the conical intersection between the bright S_CT(¹ππ*) state with the intra-base charge transfer character and the ground state to remove the excess excitation energy, thereby achieving the ground-state recovery with high efficiency.

Mechanism of the OH Radical Addition to Adenine from Quantum-Chemistry Determinations of Reaction Paths and Spectroscopic Tracking of the Intermediates

The OH radical is a well-known mediator in the oxidation of biological structures like DNA. Over the past decades, the precise events taking place after reaction of DNA nucleobases with OH radical have been widely investigated by the scientific community. Thirty years after the proposal of the main routes for the reaction of ^•OH with adenine ( Vieira , A. ; Steenken , S. J. Am. Chem. Soc. 1990 , 112 , 6986 - 6994 ), the present work demonstrates that the OH radical addition to C4 position is a minor pathway. Instead, the dehydration process is mediated by the A5OH adduct. Conclusions are based on density functional theory calculations for the ground-state reactivity and highly accurate multiconfigurational computations for the excited states of the radical intermediates. The methodology has been also used to study the mechanism giving rise to the mutagens 8-oxoA and FAPyA. Taking into account the agreement between the experimental data and the theoretical results, it is concluded that addition to the C5 and C8 positions accounts for at least ∼44.5% of the total ^•OH reaction in water solution. Finally, the current findings suggest that hydrophobicity in the DNA/RNA surroundings facilitates the formation of 8-oxoA and FAPyA.

Assessment of the Potential Energy Hypersurfaces in Thymine within Multiconfigurational Theory: CASSCF vs. CASPT2

The present study provides new insights into the topography of the potential energy hypersurfaces (PEHs) of the thymine nucleobase in order to rationalize its main ultrafast photochemical decay paths by employing two methodologies based on the complete active space self-consistent field (CASSCF) and the complete active space second-order perturbation theory (CASPT2) methods: (i) CASSCF optimized structures and energies corrected with the CASPT2 method at the CASSCF geometries and (ii) CASPT2 optimized geometries and energies. A direct comparison between these strategies is drawn, yielding qualitatively similar results within a static framework. A number of analyses are performed to assess the accuracy of these different computational strategies under study based on a variety of numerical thresholds and optimization methods. Several basis sets and active spaces have also been calibrated to understand to what extent they can influence the resulting geometries and subsequent interpretation of the photochemical decay channels. The study shows small discrepancies between CASSCF and CASPT2 PEHs, displaying a shallow planar or twisted ¹(ππ*) minimum, respectively, and thus featuring a qualitatively similar scenario for supporting the ultrafast bi-exponential deactivation registered in thymine upon UV-light exposure. A deeper knowledge of the PEHs at different levels of theory provides useful insight into its correct characterization and subsequent interpretation of the experimental observations. The discrepancies displayed by the different methods studied here are then discussed and framed within their potential consequences in on-the-fly non-adiabatic molecular dynamics simulations, where qualitatively diverse outcomes are expected.

Complete-active-space second-order perturbation theory (CASPT2//CASSCF) study of the dissociative electron attachment in canonical DNA nucleobases caused by low-energy electrons (0-3 eV)

Low-energy (0-3 eV) ballistic electrons originated during the irradiation of biological material can interact with DNA/RNA nucleobases yielding transient-anion species which undergo decompositions. Since the discovery that these reactions can eventually lead to strand breaking of the DNA chains, great efforts have been dedicated to their study. The main fragmentation at the 0-3 eV energy range is the ejection of a hydrogen atom from the specific nitrogen positions. In the present study, the methodological approach introduced in a previous work on uracil [I. González-Ramírez et al., J. Chem. Theory Comput. 8, 2769-2776 (2012)] is employed to study the DNA canonical nucleobases fragmentations of N-H bonds induced by low-energy electrons. The approach is based on minimum energy path and linear interpolation of internal coordinates computations along the N-H dissociation channels carried out at the complete-active-space self-consistent field//complete-active-space second-order perturbation theory level. On the basis of the calculated theoretical quantities, new assignations for the adenine and cytosine anion yield curves are provided. In addition, the π1 (-) and π2 (-) states of the pyrimidine nucleobases are expected to produce the temporary anions at electron energies close to 1 and 2 eV, respectively. Finally, the present theoretical results do not allow to discard neither the dipole-bound nor the valence-bound mechanisms in the range of energies explored, suggesting that both possibilities may coexist in the experiments carried out with the isolated nucleobases.

Photophysical deactivation pathways in adenine oligonucleotides

In this work we study deactivation processes in adenine oligomers after absorption of UV radiation using Quantum Mechanics combined with Molecular Mechanics (QM/MM). Correlated electronic structure methods appropriate for describing the excited states are used to describe a π-stacked dimer of adenine bases incorporated into (dA)20(dT)20. The results of these calculations reveal three different types of excited state minima which play a role in deactivation processes. Within this set of minima there are minima where the excited state is localized on one adenine (monomer-like) as well as minima where the excited state is delocalized on two adenines, forming different types of excimers and bonded excimers of varying but inter-related character. The proximity of their energies reveals that the minima can decay into one another along a flat potential energy surface dependent on the interbase separation. Additionally, analysis of the emissive energies and other physical properties, including theoretical anisotropy calculations, and comparison with fluorescence experiments, provides evidence that excimers play an important role in long-lived signals in adenine oligonucleotides while the subpicosecond decay is attributed to monomer-like minima. The necessity for a close approach of the nucleobases reveals that the deactivation mechanism is tied to macro-molecular motion.

From non-covalent binding to irreversible DNA lesions: nile blue and nile red as photosensitizing agents

We report a molecular modeling study, coupled with spectroscopy experiments, on the behavior of two well known organic dyes, nile blue and nile red, when interacting with B-DNA. In particular, we evidence the presence of two competitive binding modes, for both drugs. However their subsequent photophysical behavior is different and only nile blue is able to induce DNA photosensitization via an electron transfer mechanism. Most notably, even in the case of nile blue, its sensitization capabilities strongly depend on the environment resulting in a single active binding mode: the minor groove. Fluorescence spectroscopy confirms the presence of competitive interaction modes for both sensitizers, while the sensitization via electron transfer, is possible only in the case of nile blue.

Excimers and Exciplexes in Photoinitiated Processes of Oligonucleotides

A lot has been learned about the physical and chemical transformations that originate from the absorption of light by DNA, and computational chemistry has played a critical role in revealing the mechanisms of how these transformations occur. Nucleic acids consist of chromophores interacting via π stacking and hydrogen bonding. The fate of these systems after they absorb light is determined by the interplay and competition between pathways involving one chromophore or interacting chromophores. This Perspective highlights the role of π stacking in photophysical and photochemical processes in oligonucleotides and reveals the importance of excimers and exciplexes. Special types of excimers/exciplexes, characterized as bonded excimers/exciplexes, are also found to be important.

Multiconfigurational Second-Order Perturbation Theory with Frozen Natural Orbitals Extended to the Treatment of Photochemical Problems

A new flavor of the frozen natural orbital complete active space second-order perturbation theory method (FNO-CASPT2, Aquilante et al., J. Chem. Phys. 131, 034113) is proposed herein. In this new implementation, the virtual space in Cholesky decomposition-based CASPT2 computations (CD-CASPT2) is truncated by excluding those orbitals that contribute the least toward preserving a predefined value of the trace of an approximate density matrix, as that represents a measure of the amount of dynamic correlation retained in the model. In this way, the amount of correlation included is practically constant at all nuclear arrangements, thus allowing for the computation of smooth electronic states surfaces and energy gradients-essential requirements for theoretical studies in photochemistry. The method has been benchmarked for a series of relevant biochromophores for which large speed-ups have been recorded while retaining the accuracy achieved in the corresponding CD-CASPT2 calculations. Both vertical excitation energies and gradient calculations have been carried out to establish general guidelines as to how much correlation needs to be retained in the calculation for the results to be consistent with the CD-CASPT2 findings. Our results feature errors within a tenth of an eV for the most difficult cases and have been validated to be used for gradient computations where an up to 3-fold speed-up is observed depending on the size of the system and the basis set employed.

Quantum Mechanical Studies on the Photophysics and the Photochemistry of Nucleic Acids and Nucleobases

The photophysics and photochemistry of DNA is of great importance due to the potential damage of the genetic code by UV light. Quantum mechanical studies have played a key role in interpretating the results of modern time-resolved pump-probe spectroscopy, and in elucidating the main photoactivated reactive paths. This review provides a concise, complete picture of the computational studies carried out, approximately, in the past decade. We start with an overview of the photophysics of the nucleobases in the gas phase and in solution. We discuss the proposed mechanisms for ultrafast decay to the ground state, that involve conical intersections, consider the role of triplet states, and analyze how the solvent modulates the photophysics. Then we move to larger systems, from dinucleotides to single- and double-stranded oligonucleotides. We focus on the possible role of charge transfer and delocalized or excitonic states in the photophysics of these systems and discuss the main photochemical paths. We finish with an outlook on the current challenges in the field and future directions of research.

Direct Observation of Triplet-State Population Dynamics in the RNA Uracil Derivative 1-Cyclohexyluracil

Investigation of the excited-state dynamics in nucleic acid monomers is an area of active research due to the crucial role these early events play in DNA and RNA photodamage. The dynamics and rate at which the triplet state is populated are key mechanistic pathways yet to be fully elucidated. Direct spectroscopic evidence is presented in this contribution for intersystem crossing dynamics in a uracil derivative, 1-cyclohexyluracil. It is shown that intersystem crossing to the triplet manifold occurs in one picosecond or less in acetonitrile solution-at least an order of magnitude faster than previously estimated experimentally. Broadband transient absorption measurements also reveal the primary electronic relaxation pathways of the uracil chromophore, including the absorption spectra of the (1)ππ*, (1)nπ*, and (3)ππ* states and the rates of vibrational cooling in the ground and (3)ππ* states. The experimental results are supported by density functional calculations.

Intramolecular vibrational redistribution in the non-radiative excited state decay of uracil in the gas phase: an ab initio molecular dynamics study

We report a study of intramolecular vibrational distribution (IVR) occurring in the electronic ground state of uracil (S0) in the gas phase, following photoexcitation in the lowest energy bright excited state (Sπ) and decay through the ethylene-like Sπ/S0 Conical Intersection (CI-0π). To this aim we have performed 20 independent ab initio molecular dynamics simulations starting from CI-0π (ten of them with 1 eV kinetic energy randomly distributed over the different molecular degrees of freedom) and 10 starting from the ground state minimum (Franck-Condon, FC, point), with the excess kinetic energy equal to the energy gap between CI-0π and the FC point. The simulations, exploiting PBE0/6-31G(d) calculations, were performed over an overall period of 10 ps. A thorough statistical analysis of the variation of the geometrical parameters of uracil during the simulation time and of the distribution of the kinetic energy among the different vibrational degrees of freedom provides a consistent picture of the IVR process. In the first 0-200 fs the structural dynamics involve mainly the recovery of the average planarity. In the 200-600 fs time range, a substantial activation of CO and NH degrees of freedom is observed. After 500-600 fs most of the geometrical parameters reach average values similar to those found after 10 ps, though the system cannot be considered to be in equilibrium yet.

Deciphering the photochemical mechanisms describing the UV-induced processes occurring in solvated guanine monophosphate

The photophysics and photochemistry of water-solvated guanine monophosphate (GMP) are here characterized by means of a multireference quantum-chemical/molecular mechanics theoretical approach (CASPT2//CASSCF/AMBER) in order to elucidate the main photo-processes occurring upon UV-light irradiation. The effect of the solvent and of the phosphate group on the energetics and structural features of this system are evaluated for the first time employing high-level ab initio methods and thoroughly compared to those in vacuo previously reported in the literature and to the experimental evidence to assess to which extent they influence the photoinduced mechanisms. Solvated electronic excitation energies of solvated GMP at the Franck-Condon (FC) region show a red shift for the ππ(*) La and Lb states, whereas the energy of the oxygen lone-pair nπ(*) state is blue-shifted. The main photoinduced decay route is promoted through a ring-puckering motion along the bright lowest-lying La state toward a conical intersection (CI) with the ground state, involving a very shallow stationary point along the minimum energy pathway in contrast to the barrierless profile found in gas-phase, the point being placed at the end of the minimum energy path (MEP) thus endorsing its ultrafast deactivation in accordance with time-resolved transient and photoelectron spectroscopy experiments. The role of the nπ(*) state in the solvated system is severely diminished as the crossings with the initially populated La state and also with the Lb state are placed too high energetically to partake prominently in the deactivation photo-process. The proposed mechanism present in solvated and in vacuo DNA/RNA chromophores validates the intrinsic photostability mechanism through CI-mediated non-radiative processes accompanying the bright excited-state population toward the ground state and subsequent relaxation back to the FC region.

Electronic and structural elements that regulate the excited-state dynamics in purine nucleobase derivatives

The excited-state dynamics of the purine free base and 9-methylpurine are investigated using experimental and theoretical methods. Femtosecond broadband transient absorption experiments reveal that excitation of these purine derivatives in aqueous solution at 266 nm results primarily in ultrafast conversion of the S2(ππ*) state to the vibrationally excited (1)nπ* state. Following vibrational and conformational relaxation, the (1)nπ* state acts as a doorway state in the efficient population of the triplet manifold with an intersystem crossing lifetime of hundreds of picoseconds. Experiments show an almost 2-fold increase in the intersystem crossing rate on going from polar aprotic to nonpolar solvents, suggesting that a solvent-dependent energy barrier must be surmounted to access the singlet-to-triplet crossing region. Ab initio static and surface-hopping dynamics simulations lend strong support to the proposed relaxation mechanism. Collectively, the experimental and computational results demonstrate that the accessibility of the nπ* states and the topology of the potential energy surfaces in the vicinity of conical intersections are key elements in controlling the excited-state dynamics of the purine derivatives. From a structural perspective, it is shown that the purine chromophore is not responsible for the ultrafast internal conversion in the adenine and guanine monomers. Instead, C6 functionalization plays an important role in regulating the rates of radiative and nonradiative relaxation. C6 functionalization inhibits access to the (1)nπ* state while simultaneously facilitating access to the (1)ππ*(La)/S0 conical intersection, such that population of the (1)nπ* state cannot compete with the relaxation pathways to the ground state involving ring puckering at the C2 position.

Ultrafast Excited-State Deactivation of 8-Hydroxy-2'-deoxyguanosine Studied by Femtosecond Fluorescence Spectroscopy and Quantum-Chemical Calculations

The fluorescence properties of the 8-hydroxy-2'-deoxyguanosine (8-oxo-dG) in aqueous solution at pH 6.5 are studied by steady-state spectroscopy and femtosecond fluorescence up-conversion and compared with those of 2'-deoxyguanine (dG) and 2'-deoxyguanine monophosphate (dGMP). The steady-state fluorescence spectrum of 8-oxo-dG is composed of a broad band that peaks at 356 nm and extends over the entire visible spectral region, and its fluorescence quantum yield is twice that of dG/dGMP. After excitation at 267 nm, the initial fluorescence anisotropy at all wavelengths is lower than 0.1, giving evidence of an ultrafast internal conversion (<100 fs) between the two lowest excited ππ* states (Lb and La). The fluorescence decays of 8-oxo-dG are biexponential with an average lifetime of 0.7 ± 0.1 ps, which is about two times longer than that of dGMP. In contrast with dGMP, only a moderate dynamical shift (∼1400 vs 10,000 cm(-1)) of the fluorescence spectra of 8-oxo-dG is observed on the time scale of a few picoseconds without modification of the spectral shape. PCM/TD-DFT calculations, employing either the PBE0 or the M052X functionals, provide absorption spectra in good agreement with the experimental one and show that the deactivation path is similar to that proposed for dGMP, with a fast motion toward an energy plateau, where the purine ring keeps an almost planar geometry, followed by decay to S0, via out-of-the plane motion of amino substituent.