Intramolecular Charge Transfer in 5‑Halogen Cytidines Revealed by Femtosecond Time-Resolved Spectroscopy

Ultraviolet radiation induced damage to DNA/RNA can lead to chemical modifications to the nucleosides, and understanding the excited states involved is the key to reveal the mechanism of those reactions. 5-Halogen cytidines are metabolized DNA/RNA nucleoside byproducts that exhibit very important biological functions in the process of nucleic acid methylation as well as DNA/RNA damage repairing. However, despite the accumulation of knowledge about their biological functions, the effects of halogen substitution on the excited states of canonical nucleoside have not received much attention. In this work, the excited-state dynamics of 5-fluorocytidine, 5-chlorocytidine, and 5-bromocytidine is investigated. Excitation at 295 nm results in a bifurcation event that leads to sub-picosecond decay to ground state and population of intramolecular charge transfer states which have several to tens of picosecond lifetimes. The results elucidate the general excited-state relaxation pathways in 5-halogen cytidines, and the intrinsic charge transfer state may affect the halogen bonding that stabilizes DNA and protein structures when 5-halogen cytidines are excited.