NCO^–, a Key Fragment Upon Dissociative Electron Attachment and Electron Transfer to Pyrimidine Bases: Site Selectivity for a Slow Decay Process

Abstract

We report gas phase studies on NCO(-) fragment formation from the nucleobases thymine and uracil and their N-site methylated derivatives upon dissociative electron attachment (DEA) and through electron transfer in potassium collisions. For comparison, the NCO(-) production in metastable decay of the nucleobases after deprotonation in matrix assisted laser desorption/ionization (MALDI) is also reported. We show that the delayed fragmentation of the dehydrogenated closed-shell anion into NCO(-) upon DEA proceeds few microseconds after the electron attachment process, indicating a rather slow unimolecular decomposition. Utilizing partially methylated thymine, we demonstrate that the remarkable site selectivity of the initial hydrogen loss as a function of the electron energy is preserved in the prompt as well as the metastable NCO(-) formation in DEA. Site selectivity in the NCO(-) yield is also pronounced after deprotonation in MALDI, though distinctly different from that observed in DEA. This is discussed in terms of the different electronic states subjected to metastable decay in these experiments. In potassium collisions with 1- and 3-methylthymine and 1- and 3-methyluracil, the dominant fragment is the NCO(-) ion and the branching ratios as a function of the collision energy show evidence of extraordinary site-selectivity in the reactions yielding its formation.

Low-energy electron-induced decomposition of 5-trifluoromethanesulfonyl-uracil: A potential radiosensitizer

“…Under the present experimental conditions, the ion yield seems to be reduced which hinders the identification of resonances at higher electron energies. It is worth noting that the underlying mechanism regarding the formation of NCO − upon DEA to pyrimidine bases 58 and halouracils XU (X = Cl, Br) 4,12 has been already intensively investigated in recent years (see Ref. 58).…”

Resonant Formation of Strand Breaks in Sensitized Oligonucleotides Induced by Low‐Energy Electrons (0.5–9 eV)

“…In the energy regime around 7 eV, all DNA subunits give rise to various resonant fragmentation pathways, including the cleavage of multiple bonds . These reactions are mediated by core‐excited resonances, in which the electron attachment is accompanied by an electronic excitation.…”

Resonante Bildung von Strangbrüchen in sensibilisierten Oligonukleotiden induziert durch niederenergetische Elektronen (0.5–9.0 eV)

“…Im Energiebereich um 7 eV wiesen in früheren Untersuchungen alle DNA‐Untereinheiten eine Vielzahl von resonanten Fragmentierungswegen auf, die häufig das Aufbrechen multipler Bindungen beinhalten . Diese Reaktionen werden durch Core‐excited ‐Resonanzen vermittelt, in denen die Elektronenanlagerung mit einer elektronischen Anregung einhergeht.…”