The Hidden Routes of DNA Photostability: Charge and Proton Transfer in Excited Cytosine-Guanine Tetramers

DNA's extraordinary resistance to UV-induced damage─essential to the survival of genetic material since prebiotic times ─ stems from its ability to rapidly and efficiently dissipate electronic excitation energy through damage-free relaxation channels. Multiple decay pathways, at different time scales, have been identified. Yet, the detailed interplay of these competing decay pathways has remained elusive. Using nonadiabatic surface-hopping dynamics at the TD-CAM-B3LYP level, we investigate the excited-state behavior of DNA tetramers composed of stacked guanine-cytosine (GC)₂ dimers in alternating and nonalternating sequences in the gas phase. Following photoexcitation, both systems populate a G → C charge-transfer state, with interstrand proton transfer emerging as the dominant relaxation mechanism. Overall, the simulations reveal a complex network of coupled charge- and proton-transfer events, highlighting the diversity and subtlety of DNA's excited-state dynamics. These findings provide a mechanistic picture of how stacked bases in DNA efficiently funnel excitation energy back to the ground state.