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Solvent-Dependent Stabilization of a Charge Transfer State is the Key to Ultrafast Triplet State Formation in an Epigenetic DNA Nucleoside |
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| Authors: | Xueli Wang Prof Lara Martínez-Fernández Yuyuan Zhang Kun Zhang Prof Roberto Improta Prof Bern Kohler Prof Jianhua Xu Prof Jinquan Chen |
| Institution: | 1. State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200241 P. R. China
These authors contributed equally to this work.;2. Departamento de Química, Facultad de Ciencias and Institute for Advanced Research in Chemistry (IADCHEM), Universidad Autónoma de Madrid Campus de Excelencia UAM-CSIC Cantoblanco, 28049 Madrid, Spain These authors contributed equally to this work.;3. Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio, 43210 USA These authors contributed equally to this work.;4. State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200241 P. R. China;5. Istituto di Biostrutture e Bioimmagini CNR, Via Mezzocannone 16, 80134 Napoli, Italy;6. Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio, 43210 USA;7. State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai, 200241 P. R. China Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi, 030006 P. R. China |
| Abstract: | 2’-Deoxy-5-formylcytidine (5fdCyd), a naturally occurring nucleoside found in mammalian DNA and mitochondrial RNA, exhibits important epigenetic functionality in biological processes. Because it efficiently generates triplet excited states, it is an endogenous photosensitizer capable of damaging DNA, but the intersystem crossing (ISC) mechanism responsible for ultrafast triplet state generation is poorly understood. In this study, time-resolved mid-IR spectroscopy and quantum mechanical calculations reveal the distinct ultrafast ISC mechanisms of 5fdCyd in water versus acetonitrile. Our experiment indicates that in water, ISC to triplet states occurs within 1 ps after 285 nm excitation. PCM-TD-DFT computations suggest that this ultrafast ISC is mediated by a singlet state with significant cytosine-to-formyl charge-transfer (CT) character. In contrast, ISC in acetonitrile proceeds via a dark 1nπ* state with a lifetime of ∼3 ps. CT-induced ISC is not favored in acetonitrile because reaching the minimum of the gateway CT state is hampered by intramolecular hydrogen bonding, which enforces planarity between the aldehyde group and the aromatic group. Our study provides a comprehensive picture of the non-radiative decay of 5fdCyd in solution and new insights into the factors governing ISC in biomolecules. We propose that the intramolecular CT state observed here is a key to the excited-state dynamics of epigenetic nucleosides with modified exocyclic functional groups, paving the way to study their effects in DNA strands. |
| Keywords: | DNA photophysics and photochemistry charge transfer states epigenetic nucleoside intersystem crossing triplet excited states |