UV Excitation of Uracil Results in the Formation of a Ground-State Intermediate in Less Than One Picosecond and Its Decay is Quenched by Nucleophilic Water Addition

Investigating the excited-state dynamics of RNA is essential for understanding potential damage to biological systems, developing new biotechnologies, and gaining fundamental insights into molecular behavior and the chemical evolution of life on Earth. In this study, we investigate the excited-state dynamics of uracil after excitation at 267 nm in both aqueous and deuterated phosphate buffered solutions. We employ broadband transient absorption with femtosecond time resolution, complemented by steady-state absorption spectroscopy and quantum-chemical calculations. The primary focus is on detecting and investigating the reactivity toward water addition of a recently predicted reaction intermediate, which may lead to the formation of 6-hydroxy-5,6-dihydrouracil photohydrates. We support the experimentally determined transient absorption spectrum of this intermediate with the simulated absorption spectrum obtained through quantum-chemical calculations, which accounted for both explicit and implicit solvent effects. This comparison confirms that the transient species can be identified as a ground-state half-chair twisted uracil intermediate. The intermediate is formed in less than 1 ps from the decay of the ¹ππ* excited state. Additionally, its decay is driven by the nucleophilic addition of a water molecule, which occurs with lifetimes of 22.7 ± 0.2 ps in aqueous solutions and 30.8 ± 0.3 ps in deuterated water solutions. All things considered, the results presented in this study and elsewhere demonstrate that this reaction intermediate is the precursor of the 6-hydroxy-5,6-dihydrouracil photohydrates.