State-Dependent Fragmentation of Protonated Uracil and Uridine.

Action spectroscopy using photon excitation in the VUV range (photon energy 4.5-9 eV) was performed on protonated uracil (UraH(+)) and uridine (UrdH(+)). The precursor ions with m/z 113 and m/z 245, respectively, were produced by an electrospray ionization source 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 excitation energy show several maxima that can be attributed to the photoexcitation into different electronic states. For uracil, vertically excited states were calculated using the equation-of-motion coupled cluster approach and compared to the observed maxima. This allows to establish correlations between electronic states and the resulting fragment masses and can thus help to disentangle the complex de-excitation and fragmentation pathways of nucleic acid building blocks. Photofragmentation of the nucleoside uridine shows a significantly lower variety of fragments, indicating stabilization of the nucleobase by the attached sugar.