Ultrafast Dynamics in Methylated Cytosine and Its Derivatives

DNA methylation is a stable epigenetic mark that has an important role in gene regulation. DNA methylation in the form of 5 methylcytosine (5mC) can be actively reversed to unmodified cytosine (C) through TET dioxygenase-mediated oxidation of 5mC to 5 hydroxymethylcytosine (5hmC), 5 formylcytosine (5fC) and 5 carboxylcytosine (5caC). Thus, 5-methylcytosine 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. We used femtosecond time-resolved spectroscopy to comprehensively study the excited state dynamics of 5mC, 5hmC, 5fC and 5caC in solution. Two distinct nonradiative decay channels were directly observed in 5mC and 5hmC singlet states while long-lived triplet state was seen in 5fC but not in 5caC. Potential double proton transfer were also observed in 5hmC, 5fC and 5caC.