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.