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Zhou Q, Hao Y, Jie J, Wang S, Xia Y, Yang C, Liu L, Fang WH, Su H. Dual Functionality of 6-Methylthioguanine: Synergistic Effects Enhancing the Photolability of DNA Nucleobases. JACS AU 2024; 4:441-453. [PMID: 38425924 PMCID: PMC10900203 DOI: 10.1021/jacsau.3c00575] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/28/2023] [Revised: 01/05/2024] [Accepted: 01/05/2024] [Indexed: 03/02/2024]
Abstract
A small chemical modification of the nucleobase structure can significantly enhance the photoactivity of DNA, which may incur DNA damage, thus holding promising applications in photochemotherapy treatment of cancers or pathogens. However, single substitution confers only limited phototoxicity to DNA. Herein, we combine femtosecond and nanosecond time-resolved spectroscopy with high-level ab initio calculations to disentangle the excited-state dynamics of 6-methylthioguanine (me6-TG) under variable wavelength UVA excitation (310-330 nm). We find that double substitution of nucleobases (thionation and methylation) boosts the photoactivity by introducing more reactive channels. Intriguingly, 1nNπ*, rather than 1nSπ*, acts as the doorway state engendering the formation of the long-lived reactive triplet state in me6-TG. The 1nNπ* induces a low spin-orbit coupling of 8.3 cm-1, which increases the intersystem crossing (ISC) time (2.91 ± 0.14 ns). Despite the slowed ISC, the triplet quantum yield (ΦT) still accounts for a large fraction (0.6 ± 0.1), consistent with the potential energy surface that favors excited-state bifurcation to 1nNπ*min (3.36 ± 0.15 ps) rather than 1ππ*min (5.05 ± 0.26 ps), such that the subsequent ISC to triplet via 1nNπ*min constitutes the main relaxation pathway in me6-TG. Although this ΦT is inferior to its single-substituted predecessor 6-thioguanine (6-TG, 0.8 ± 0.2), the effect of thionation in synergy with methylation opens a unique C-S bond cleavage pathway through crossing to a repulsive 1πσ* state, generating thiyl radicals as highly reactive intermediates that may invoke biological damage. This photodissociation channel is extremely difficult for conventional nucleobases. These findings demonstrate the synergistic effects of double functionality substitution in modulating excited-state dynamics and enhancing the photolabile character of DNA nucleobases, providing inspirations for the rational design of advanced photodynamic and photochemotherapy approaches.
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Affiliation(s)
| | | | - Jialong Jie
- College of Chemistry, Beijing Normal University, Beijing 100875, China
| | - Shuo Wang
- College of Chemistry, Beijing Normal University, Beijing 100875, China
| | - Ye Xia
- College of Chemistry, Beijing Normal University, Beijing 100875, China
| | - Chunfan Yang
- College of Chemistry, Beijing Normal University, Beijing 100875, China
| | - Lihong Liu
- College of Chemistry, Beijing Normal University, Beijing 100875, China
| | - Wei-Hai Fang
- College of Chemistry, Beijing Normal University, Beijing 100875, China
| | - Hongmei Su
- College of Chemistry, Beijing Normal University, Beijing 100875, China
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Chang XP, Zhao G, Zhang TS, Xie BB. Quantum mechanics/molecular mechanics studies on mechanistic photophysics of cytosine aza-analogues: 2,4-diamino-1,3,5-triazine and 2-amino-1,3,5-triazine in aqueous solution. Phys Chem Chem Phys 2023; 25:7669-7680. [PMID: 36857660 DOI: 10.1039/d2cp05639a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/15/2023]
Abstract
The excited-state properties and photophysics of cytosine aza-analogues, i.e., 2,4-diamino-1,3,5-triazine (2,4-DT) and 2-amino-1,3,5-triazine (2-AT) in solution have been systematically explored using the QM(MS-CASPT2//CASSCF)/MM approach. The excited-state nonradiative relaxation mechanisms for the initially photoexcited S1(ππ*) state decay back to the S0 state are proposed in terms of the present computed minima, surface crossings (conical intersections and singlet-triplet crossings), and excited-state decay paths in the S1, S2, T1, T2, and S0 states. Upon photoexcitation to the bright S1(ππ*) state, 2,4-DT quickly relaxes to its S1 minimum and then overcomes a small energy barrier of 5.1 kcal mol-1 to approach a S1/S0 conical intersection, where the S1 system hops to the S0 state through S1 → S0 internal conversion (IC). In addition, at the S1 minimum, the system could partially undergo intersystem crossing (ISC) to the T1 state, followed by further ISC to the S0 state via the T1/S0 crossing point. In the T1 state, an energy barrier of 7.9 kcal mol-1 will trap 2,4-DT for a while. In parallel, for 2-AT, the system first relaxes to the S1 minimum and then S1 → S0 IC or S1 → T1 → S0 ISCs take place to the S0 state by surmounting a large barrier of 15.3 kcal mol-1 or 11.9 kcal mol-1, respectively, which heavily suppress electronic transition to the S0 state. Different from 2,4-DT, upon photoexcitation in the Franck-Condon region, 2-AT can quickly evolve in an essentially barrierless manner to nearby S2/S1 conical intersection, where the S2 and T1 states can be populated. Once it hops to the S2 state, the system will overcome a relatively small barrier (6.6 kcal mol-1vs. 15.3 kcal mol-1) through IC to the S0 state. Similarly, an energy barrier of 11.9 kcal mol-1 heavily suppresses the T1 state transformation to the S0 state. The present work manifests that the amination/deamination of the triazine rings can affect some degree of different vertical and adiabatic excitation energies and nonradiative decay pathways in solution. It not only rationalizes excited-state decay dynamics of 2,4-DT and 2-AT in aqueous solution but could also provide insights into the understanding of the photophysics of aza-nucleobases.
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Affiliation(s)
- Xue-Ping Chang
- College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China.
| | - Geng Zhao
- College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China.
| | - Teng-Shuo Zhang
- College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
| | - Bin-Bin Xie
- Hangzhou Institute of Advanced Studies, Zhejiang Normal University, Hangzhou 311231, China
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Jin P, Wang X, Pan H, Chen J. One order of magnitude increase of triplet state lifetime observed in deprotonated form selenium substituted uracil. Phys Chem Chem Phys 2022; 24:875-882. [PMID: 34908064 DOI: 10.1039/d1cp04811b] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
Selenium nucleic acids possess unique properties and have been demonstrated to have a wide range of applications such as in DNA X-ray crystallography and novel medical therapies. However, as a heavy atom, selenium substitution may easily alter the photophysical properties of a nucleic acid by red-shifting the absorption spectra and introducing effective intersystem crossing to triplet excited states. In present work, the excited state dynamics of a naturally occurring selenium substituted uracil (2-selenuracil, 2SeU) is studied by using femtosecond transient absorption spectroscopy as well as quantum chemistry calculations. Ultrafast intersystem crossing to the lowest triplet state (T1) and effective non-radiative decay of this state to the ground state (S0) are demonstrated in the neutral form 2SeU. However, the triplet lifetime of the deprotonated form 2SeU is found to be almost one order of magnitude longer than that in the neutral one. Quantum chemistry calculations indicate that the short triplet lifetime in 2SeU is due to excited state population decay through a crossing point between T1 and S0. In the deprotonated form, shortening the N1-C2 bond length makes the structural distortion more difficult and brings a larger energy barrier on the pathway to the T1/S0 crossing point, resulting in one order of magnitude increase of the triplet state lifetime. Our study reveals one key factor to regulate the triplet lifetime of 2SeU and sets the stage to further investigate the photophysical and photochemical properties of 2SeU-containing DNA/RNA duplexes.
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Affiliation(s)
- Peipei Jin
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China.
| | - Xueli Wang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China.
| | - Haifeng Pan
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China.
| | - Jinquan Chen
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China. .,Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
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Xu T, Hu Z, Lv M, Zhou Z, Xu J, Sun Z, Sun H, Chen J. Hydrogen atom and water complex determine the excited state dynamics of 8-azaguanine. Chem Phys 2021. [DOI: 10.1016/j.chemphys.2021.111118] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
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Zhang K, Wang F, Jiang Y, Wang X, Pan H, Sun Z, Sun H, Xu J, Chen J. New Insights about the Photostability of DNA/RNA Bases: Triplet nπ* State Leads to Effective Intersystem Crossing in Pyrimidinones. J Phys Chem B 2021; 125:2042-2049. [PMID: 33600186 DOI: 10.1021/acs.jpcb.0c10611] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The high photostability of DNA/RNA nucleobases is attributed to the effective internal conversions of their bright 1ππ* states to the ground state through conical intersections. Intersystem crossing (ISC) from singlet to triplet excited states is a minor decay pathway in nucleobases and it is observed with ∼1-2% quantum yields (QYs) in pyrimidine bases. Presumably, ISC in pyrimidines takes place from the dark singlet 1nπ* state to the lowest triplet 3ππ* state. However, recent studies showed that ISC from the initial populated bright 1ππ* state to higher energy triplet 3nπ* states indeed occurs in the subpicosecond timescale. Such a mechanism is still poorly understood since direct observation of this pathway is challenging. Herein, excited state dynamics of three pyrimidinones, which share the same skeleton with pyrimidine bases, is investigated in different solvents. Compared to canonical pyrimidine bases, removing the oxygen atom at the C4 position revokes the low-lying dark 1nπ* state in pyrimidinones, resulting in direct ISC from the S1 (1ππ*) state to triplet T3 (3nπ*) state with much higher QYs. Meanwhile, hydrogen bonding between the carbonyl group in pyrimidinones and protic solvents can accelerate vibrational cooling of the hot S1 (1ππ*) state, leading to higher fluorescence QYs and smaller ISC rate constants. These results not only evidence the hypothesis of the direct 1ππ* → 3nπ* ISC mechanism, but also contribute to a better understanding of triplet formation in pyrimidines.
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Affiliation(s)
- Kun Zhang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Fufang Wang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Yanrong Jiang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Xueli Wang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Haifeng Pan
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Zhenrong Sun
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Haitao Sun
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Jianhua Xu
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China.,Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
| | - Jinquan Chen
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China.,Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
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Fu L, Wang Z, Liu Y, Wang X, Xu R, Liu W, Chen J, Xu J. Observation of triplet nπ* state in ultrafast intersystem crossing of 6-azathymine. J Photochem Photobiol A Chem 2020. [DOI: 10.1016/j.jphotochem.2020.112491] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
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