Ribose and Deoxyribose Group Alter Excited-State Dynamics of 5-Azacytosine in Solution^†

5-Azacytosine (5-AC) is one of the best interesting noncanonical nucleobases due to its functionalization and structural imitation of natural bases. 5-AC can be used as the scaffold of two important chemotherapeutic medicines, 5-azacytidine and 2′-deoxy-5-azacytidine. Furthermore, increased sensitivity to UV leads to the photochemical effects of 5-AC also attracted attention. Yet, no study has been reported to explore the effect of glycosyl groups on the photophysical and photochemical properties of 5-AC, which can help to reveal the photostability of related actual clinic drugs. In this study, the excited-state dynamics of 5-azacytidine and 2′-deoxy-5-azacytidine are studied by femtosecond transient absorption and quantum-chemical calculations while revisiting that of 5-AC with a wider probe spectral range. It is shown that glycosyl substitution on the N1 position leads to ultrafast excited-state relaxation within several picoseconds in both nucleosides, which is distinct compared with the 17 ps lifetime seen in 5-AC. It is proposed that these changes are due to altering the energy level of the dark nπ* state. Moreover, our results suggest that it should be cautioned to simply replace sugar groups with methyl groups when doing a theoretical calculation study on nucleobases and their derivatives.