Theoretical Studies on the Photophysics and Photochemistry of 5-Formylcytosine and 5-Carboxylcytosine: The Oxidative Products of Epigenetic Modification of Cytosine in DNA.

Cytosine methylation and demethylation play crucial roles in understanding the genomic DNA expression regulation. The epigenetic modification of cytosine and its continuous oxidative products are called the "new four bases of DNA" including 5-methylcytosine (5mC), 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC). However, compared to the abundant studies on the classical DNA bases, the photophysical and photochemical properties of those new bases have not yet aroused people's excessive attention. In this contribution, a systematic study on the nonradiative decay and photochemical pathways via excited states or conical intersections upon photoexcitation has been explored through high-level computational approaches such as the complete active space self-consistent field method, complete active space with second-order perturbation theory, and density functional theory. Pathways like the ring-distortion deactivation, hydrogen dissociation, hydrogen transfer, and also Norrish type I and II photochemical reactions have been investigated, and it was proposed that intersystem crossing from the S state to the T state is the most effective route for 5fC. For 5caC, ring-pucking and intramolecular isomerism are effective deactivation ways at both neutral and protonated forms. In the meantime, the influences of two important environmental factors, the solution and acidic environment (i.e., the protonated state), were also considered in this study. From the theoretical perspective, the initial properties of the photostability and photochemical reactivity for 5fC and 5caC have become a crucial aspect to facilitate a further comprehension of their potential role in gene regulation and transcription.