Theoretical Carbon K-Edge X-ray Absorption Signatures of Twisted Intermediates and Hydration Reactivity in UV-Excited Nucleosides

Ultraviolet excitation of pyrimidine nucleosides leads to the ultrafast formation of a ground-state twisted intermediate featuring a highly distorted C5═C6 double bond. Here, we present a theoretical study on the feasibility of carbon K-edge X-ray absorption spectroscopy (XAS) as a site-specific probe of this twisted intermediate. Time-dependent density functional theory (TDDFT) calculations predict that twisting induces pronounced, site-dependent red shifts in the carbon K-edge spectra, providing distinct signatures that are experimentally observable using time-resolved XAS. The twisted state mediates the subsequent photohydration reaction; however, the hydration reactivity and its suppression by C5 methylation have not yet been clarified in terms of electronic structure. Conceptual DFT analysis suggests that twisting enhances electrophilic reactivity at the C6 site, whereas C5 methylation redistributes the LUMO toward C5 and reduces the reactivity at C6.