Controlling NO Production Upon Valence Ionization of Nitroimidazoles.

Nitroimidazole exhibits a remarkable regioselective fragmentation subsequent to valence ionization, which is characterized by ejection of NO. As NO is also considered to be an effective radiosensitizer, we investigated its production efficiency as a function of isomeric composition (the site of the NO2 nitro group). We observe strong dependence in the 8.6-15 eV binding energy range, and moreover, that the production of NO can be effectively suppressed by methylation of nitroimidazole. This behavior can be understood by modification of the valence electronic structure with respect to the dissociation threshold, which gives rise to varying effective density of dissociative states. We find the NO yield to follow the efficiency of the nitroimidazole dervivatives as radiosensitizers, found in preclinical studies.

Site-selective bond scission of methylbenzoate following core excitation.

The chemical bond scission of methylbenzoate (C6H5CO2CH3) following core excitation at the C and O K edges was examined from partial ion yield measurements across these edges using synchrotron radiation. Site-specific scission of the C-O bonds was observed at both edges. Theoretical X-ray absorption spectra (XAS) were obtained using density functional theory. Peak assignments in the observed spectra were found to be consistent with the theory. From core-excited state dynamics calculations, an elongation of the C-O bond was predicted and provides an explanation of the observed partial ion yield enhancement of CH3+ and C6H5CO+ at the core-excited resonances at both edges.

Gas-phase study on uridine: Conformation and X-ray photofragmentation.

Fragmentation of RNA nucleoside uridine, induced by carbon 1s core ionization, has been studied. The measurements by combined electron and ion spectroscopy have been performed in gas phase utilizing synchrotron radiation. As uridine is a combination of d-ribose and uracil, which have been studied earlier with the same method, this study also considers the effect of chemical environment and the relevant functional groups. Furthermore, since in core ionization the initial core hole is always highly localized, charge migration prior to fragmentation has been studied here. This study also demonstrates the destructive nature of core ionization as in most cases the C 1s ionization of uridine leads to concerted explosions producing only small fragments with masses ≤43 amu. In addition to fragmentation patterns, we found out that upon evaporation the sugar part of the uridine molecule attains hexagonal form.

Fragmentation dynamics of doubly charged methionine in the gas phase.

The dependence of the fragmentation of doubly charged gas-phase methionine (C5H11NO2S) on the electronic-state character of the parent ion is studied experimentally by energy-resolved electron ion-ion coincidence spectroscopy. The parent dication electronic states are populated by Auger transitions following site-specific sulfur 2p core ionization. Two fragmentation channels are observed to be strongly dependent on the electronic states with vacancies in weakly bound molecular orbitals. All-electron calculations are applied to assign doubly charged final states of sulfur 2p core ionized methionine. In addition, the Car-Parrinello method is applied to model fragmentation dynamics of doubly charged methionine molecules with various initial temperatures to understand the typical characteristics of the molecular dissociation and partly to support the interpretation of experimental data.