Ultrafast coherent vibrational dynamics and structural rearrangement following photodetachment of the aqueous adenine anion

Understanding the elementary processes that follow the interaction of ionizing radiation with biomolecules is of fundamental importance in radiation chemistry and radiation biology. Nucleobases are a class of biomolecules that are of particular interest in these fields due to their significance as the building blocks of DNA and the role of nucleobase radicals in DNA damage. Here, femtosecond optical pump–probe spectroscopy is employed to study the ultrafast vibrational wave packet dynamics of the aqueous adenine radical (Ade[-H]•) induced by photodetachment. Strong-field photodetachment of the aqueous adenine anion (Ade[-H]−) by intense, few-cycle pump pulses drives the formation of the hydrated electron and aqueous Ade[-H]•. Absorption signatures of the hydrated electron and Ade[-H]• are observed in the transient absorption spectra, probed in the visible–near-infrared and ultraviolet range. The pronounced modulations present in the transient absorption spectra encode vibrational wave packet dynamics of aqueous Ade[-H]•. This vibrational wave packet motion is resolved into ten vibrational modes along which structural reorganization occurs upon photodetachment. Density functional theory calculations assign the observed vibrational modes and quantify the extent of structural rearrangement following photodetachment. This work provides insights into the ultrafast structural dynamics that accompany the ionization of biomolecules in an aqueous environment.