Two-body dissociation of isoxazole following double photoionization - an experimental PEPIPICO and theoretical DFT and MP2 study

The dissociative double photoionization of isoxazole molecules has been investigated experimentally and theoretically. The experiment has been carried out in the 27.5-36 eV photon energy range using vacuum ultraviolet (VUV) synchrotron radiation excitation combined with ion time-of-flight (TOF) spectrometry and photoelectron-photoion-photoion coincidence (PEPIPICO) technique. Five well-resolved two-body dissociation channels have been identified in the isoxazole's coincidence maps, and their appearance energies have been determined. The coincidence yield curves of these dissociation channels have been obtained in the photon energy ranges from their appearance energies up to 36 eV. The double photoionization of isoxazole produces a C3H3NO2+ transient dication, which decomposes into fragments differing from previously reported photofragmentation products of isoxazole. We have found no evidence of pathways leading to the C3H2NO+, HCN+, C2H2O+, C3HN+, or C2H2+ fragments or their neutral counterparts that have been observed in previous neutral photodissociation and single photoionization studies. Instead, the dissociation of isoxazole after the ejection of two electrons is bond-selective and is governed by two reactions, HCO+ + H2CCN+ and H2CO+ + HCCN+, whose appearance energies are 28.6 (±0.3) and 29.4 (±0.3) eV, respectively. A third dissociation channel turns out to be a variant of the most intense channel (HCO+ + H2CCN+), where one of the fragment ions contains a heavy isotope. Two minor dissociation channels occurring at higher energies, CO+ + CH3CN+ and CN+ + H3CCO+, are also identified. The density functional and ab initio quantum chemical calculations have been performed to elucidate the dissociative charge-separating mechanisms and determine the energies of the observed photoproducts. The present work unravels hitherto unexplored photodissociation mechanisms of isoxazole and thus provides deeper insight into the photophysics of five-membered heterocyclic molecules containing two heteroatoms.

Hydrogen bonding and protonation effects in amino acids' anthraquinone derivatives - Spectroscopic and electrochemical studies

Six novel amino acid chromophores were synthesized and their spectroscopic, acid-base, and electrochemical properties are discussed in this work. In studied compounds, selected amino acid residues (l-Aspartic acid, l-Glutamic acid, l-Glutamine, l-Histidine, l-Lysine, l-Arginine) are attached to the 1-(piperazine) 9,10-anthraquinone skeleton via the amide bond between the carboxyl group of amino acid and nitrogen atom of the piperazine ring. All derivatives have been characterized using a variety of spectroscopic techniques (mass spectrometry, 1HNMR, UV-Vis, IR spectroscopy), acid-base (electrochemical and UV-Vis) titrations, and cyclic voltammetry methods. Basing on observed experimental effects, supported by quantum chemical simulations, the structure-properties links were established. They are indicative of the specific interactions within and/or in-between amino acid side groups, which are prone to form both, intra- and intermolecular hydrogen bonds as well as electrostatic interactions with the anthraquinone system.

Stability of the Parent Anion of the Potential Radiosensitizer 8-Bromoadenine Formed by Low-Energy (<3 eV) Electron Attachment

8-Bromoadenine (8BrA) is a potential DNA radiosensitizer for cancer radiation therapy due to its efficient interaction with low-energy electrons (LEEs). LEEs are a short-living species generated during the radiation damage of DNA by high-energy radiation as it is applied in cancer radiation therapy. Electron attachment to 8BrA in the gas phase results in a stable parent anion below 3 eV electron energy in addition to fragmentation products formed by resonant exocyclic bond cleavages. Density functional theory (DFT) calculations of the 8BrA- anion reveal an exotic bond between the bromine and the C8 atom with a bond length of 2.6 Å, where the majority of the charge is located on bromine and the spin is mainly located on the C8 atom. The detailed understanding of such long-lived anionic states of nucleobase analogues supports the rational development of new therapeutic agents, in which the enhancement of dissociative electron transfer to the DNA backbone is critical to induce DNA strand breaks in cancerous tissue.

Sensitizing DNA Towards Low-Energy Electrons with 2-Fluoroadenine

2-Fluoroadenine ((2F) A) is a therapeutic agent, which is suggested for application in cancer radiotherapy. The molecular mechanism of DNA radiation damage can be ascribed to a significant extent to the action of low-energy (<20 eV) electrons (LEEs), which damage DNA by dissociative electron attachment. LEE induced reactions in (2F) A are characterized both isolated in the gas phase and in the condensed phase when it is incorporated into DNA. Information about negative ion resonances and anion-mediated fragmentation reactions is combined with an absolute quantification of DNA strand breaks in (2F) A-containing oligonucleotides upon irradiation with LEEs. The incorporation of (2F) A into DNA results in an enhanced strand breakage. The strand-break cross sections are clearly energy dependent, whereas the strand-break enhancements by (2F) A at 5.5, 10, and 15 eV are very similar. Thus, (2F) A can be considered an effective radiosensitizer operative at a wide range of electron energies.

Hydrogen migration in formation of NH(A³Π) radicals via superexcited states in photodissociation of isoxazole molecules

Formation of the excited NH(A(3)Π) free radicals in the photodissociation of isoxazole (C3H3NO) molecules has been studied over the 14-22 eV energy range using photon-induced fluorescence spectroscopy. The NH(A(3)Π) is produced through excitation of the isoxazole molecules into higher-lying superexcited states. Observation of the NH radical, which is not a structural unit of the isoxazole molecule, corroborates the hydrogen atom (or proton) migration within the molecule prior to dissociation. The vertical excitation energies of the superexcited states were determined and the dissociation mechanisms of isoxazole are discussed. The density functional and ab initio quantum chemical calculations have been performed to study the mechanism of the NH formation.