Photochemical Stability of 5-Methylcytidine Relative to Cytidine: Photophysical Insight for mRNA Therapeutic Applications

5-Methylcytidine (5mCyd) has recently been investigated with renewed interest for utilization in mRNA therapeutics. However, its photostability following exposure to electromagnetic radiation has been overlooked. This Letter compares the photostability and excited state dynamics of 5mCyd with those of the canonical RNA nucleoside, cytidine (Cyd) using steady-state and femtosecond transient absorption spectroscopy under physiologic conditions. 5mCyd is shown to have a 5-fold higher fluorescence yield and a 5-fold longer 1ππ* excited state decay lifetime. Importantly, however, the excited state population in 5mCyd decays primarily by internal conversion, with a photodegradation rate 3 times smaller than in Cyd. In Cyd, the population of a 1nπ* state with a lifetime of ca. 45 ps is implicated in the formation 6-hydroxycytidine and other photoproducts.