A Twist in Electronic Relaxation of Pyrimidine Nucleosides and Nucleotides: Impact of C5 Methylation on Nonadiabatic Transition and Photohydration Damage

We report a combined experimental and theoretical study of the formation and decay dynamics of a ground-state twisted intermediate (TI) involved in the ultrafast internal conversion of UV-excited pyrimidine nucleosides and nucleotides in aqueous solution. Infrared transient absorption spectroscopy identifies a TI featuring a strongly twisted C5=C6 double bond, and its quantum yield (ΦTI) and lifetime (τTI) are determined for structurally distinct nucleosides and nucleotides. Photo-hydration rates (khyd) measured under continuous 266 nm irradiation correlate directly with ΦTI × τTI, providing unambiguous evidence that the TI mediates the hydration reaction. Comparison of C5−H and C5−CH3 derivatives reveals pronounced reductions in both ΦTI and khyd uponmethylation. Quantum mechanics/molecular mechanics dynamical simulations show that TI formation requires sufficient nuclear momentum along the TI-forming coordinate at the conical intersection, whereas vibrational energy randomization induced by C5 methylation and solvent interactions diminishes this momentum and consequently ΦTI. The TI is neither diradical nor zwitterionic but instead contains an elongated, highly reactive C5=C6 double bond whose polarization and hydration reactivity are attenuated by C5 methylation. Consistently, the normalized reactivity index, khyd/(ΦTI × τTI), is substantially lower for C5-methylated compounds. A high activation barrier limits the photohydration quantum yield (∼0.01), and τTI is primarily governed by isomerization back to the planar ground-state structure.