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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 dark1nπ* 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.
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This content will become publicly available on March 4, 2026
The Photophysical Path to the Triplet State in Light‐Oxygen‐Voltage (LOV) Domains
Abstract Upon blue‐light absorption, LOV domains efficiently undergo intersystem crossing (ISC) to the triplet state. Several factors potentially contribute to this efficiency. One often proposed in the literature is the heavy atom effect of the nearby (and eventually adduct‐forming) cysteine. However, some LOV domain derivatives that lack the cysteine residue also undergo ISC efficiently. Using hybrid multireference quantum mechanical/molecular mechanical (QM / MM) models, we investigated the effect of the electrostatic environment in a prototypal LOV domain,Arabidopsis thalianaPhototropin 1 LOV2 (AtLOV2), compared to the effect of the dielectric of an aqueous solution. We find that the electrostatic environment of AtLOV2 is especially well tuned to stabilize a triplet state, which we posit is the state involved in the ISC step. Other low‐lying triplet states that have and character are ruled out on the basis of energetics and/or their orbital character. The mechanistic picture that emerges from the calculations is one that involves the ISC of photoexcited flavin to a triplet state followed by rapid internal conversion to a triplet state, which is the state detected spectroscopically. This insight into the ISC mechanism can provide guidelines for tuning flavin's photophysics through mutations that alter the protein electrostatic environment and potentially helps to explain why ISC (and subsequent flavin photochemistry) does not occur readily in many classes of flavin‐binding enzymes.
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- Award ID(s):
- 2047667
- PAR ID:
- 10639326
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 31
- Issue:
- 22
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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