# State-dependent fragmentation of protonated uracil and uridine

## TL;DR

This study investigates the energy-dependent fragmentation pathways of protonated uracil and uridine using tandem mass spectrometry combined with VUV photoexcitation, revealing electronic state correlations and stabilization effects.

## Contribution

It provides new insights into the electronic excitation and fragmentation mechanisms of nucleic acid components, linking observed fragments to specific electronic states through theoretical calculations.

## Key findings

- Multiple fragmentation maxima linked to electronic states.
- Uridine shows fewer fragmentation pathways, indicating stabilization.
- Electronic state correlations help understand nucleic acid fragmentation.

## Abstract

Tandem mass spectroscopy ($\textrm{MS}^2$) combined with single photon excitation in the VUV range (photon energy 4.5-9 eV) was performed on protonated uracil ($\textrm{UraH}^{+}$) and uridine ($\textrm{UrdH}^{+}$). The precursor ions with $m/z\;113$ and $m/z\;245$ respectively were produced by an Electrospray Ionization source (ESI) and accumulated inside a quadrupole ion trap mass spectrometer. After irradiation with tunable synchrotron radiation, product ion mass spectra were obtained. Fragment yields as a function of exciting energy show several maxima that can be attributed to the photo-excitation into different electronic states. For uracil, vertically excited states were calculated using the equation-of-motion coupled cluster approach (EOM-CCSD) and compared to the observed maxima. This allows to establish correlations between electronic states and resulting fragment masses and can thus help to disentangle the complex deexcitation and fragmentation pathways of nucleic acid building blocks above the first electronically excited state. Photofragmentation of the nucleoside uridine shows a significantly lower variety of fragments, indicating stabilization of the nucleobase by the attached sugar.