Role of Hydration in Excited-State Proton Transfer in Adenine-Thymine Nucleobase Pairs

This study investigates the excited-state proton transfer (ESPT) mechanism in canonical adenine-thymine (A-T) nucleobase pairs under gas-phase and explicitly hydrated conditions. Using a combination of time-dependent density functional theory (TDDFT), static potential energy surface (PES) analyses, and nonadiabatic surface hopping dynamics, we reveal that hydration induces a mechanistic switch from charge-transfer-driven ESPT in the gas phase to solvent-assisted proton relay in aqueous environments. Explicit hydration environment modulates both the energetic landscape and the nature of electronic transitions, reducing charge-transfer character and stabilizing proton-transferred intermediates. Difference density plots and UV-vis spectra highlight excited-state antiaromaticity as a potential driving force for ESPT, which is further supported by computed aromaticity indices. Dynamical simulations demonstrate that excitation to higher singlet states (S2) enhances access to proton transfer channels, particularly in hydrated systems. Overall, our results offer a unified mechanistic framework for understanding how hydration, excited-state reactivity, and photophysical stability are intricately linked in DNA base pairs, advancing insight into photoprotection and mutation pathways under biologically relevant conditions.