School of Chemistry - Research Publications
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ItemNo Preview AvailableGas-phase structure and reactivity of the keto tautomer of the deoxyguanosine radical cation(ROYAL SOC CHEMISTRY, 2015)Guanine radical cations are formed upon oxidation of DNA. Deoxyguanosine (dG) is used as a model, and the gas-phase infrared (IR) spectroscopic signature and gas-phase unimolecular and bimolecular chemistry of its radical cation, dG˙(+), A, which is formed via direct electrospray ionisation (ESI/MS) of a methanolic solution of Cu(NO3)2 and dG, are examined. Quantum chemistry calculations have been carried out on 28 isomers and comparisons between their calculated IR spectra and the experimentally-measured spectra suggest that A exists as the ground-state keto tautomer. Collision-induced dissociation (CID) of A proceeds via cleavage of the glycosidic bond, while its ion–molecule reactions with amine bases occur via a number of pathways including hydrogen-atom abstraction, proton transfer and adduct formation. A hidden channel, involving isomerisation of the radical cation via adduct formation, is revealed through the use of two stages of CID, with the final stage of CID showing the loss of CH2O as a major fragmentation pathway from the reformed radical cation, dG˙(+). Quantum chemistry calculations on the unimolecular and bimolecular reactivity are also consistent with A being present as a ground-state keto tautomer.
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ItemGas-phase fragmentation of deprotonated tryptophan and its clusters [Trpn-H]- induced by different activation methods(WILEY, 2015-08-15)RATIONALE: Non-covalent amino acid clusters are the subject of intense research in diverse areas including peptide bond formation studies or the determination of proton affinities or methylating abilities of amino acids. However, most of the research has focused on positive ions and little is known about anionic clusters. METHODS: Fragmentation reactions of deprotonated tryptophan (Trp), [Trp-H](-) and Trp singly deprotonated non-covalently bound clusters [Trp(n) -H](-), n = 2, 3, 4, were investigated using low-energy collision-induced dissociation (CID) with He atoms, high-energy CID with Na atoms, and electron-induced dissociation (EID) with 20-35 eV electrons. Fragmentation of the monomeric Trp anion, where all labile hydrogens were exchanged for deuterium [d(4) -Trp-D](-), was investigated using low-energy CID and EID, in order to shed light on the dissociation mechanisms. RESULTS: The main fragmentation channel for Trp cluster anions, [Trp(n) -H](-), n >1, is the loss of the neutral monomer. The fragmentation of the deprotonated Trp monomer induced by electrons resembles the fragmentation induced by high-energy collisions through electronic excitation of the parent. However, the excitation must precede in a different way, shown through only monomer loss from larger clusters, n >1, in case of EID, but intracluster chemistry in the case of high-energy CID. CONCLUSIONS: The anion of the indole ring C(8)H(6) N(-) has been identified in the product ion spectra of [Trp(n) -H](-) using all activation methods, thus providing a diagnostic marker ion. No evidence was found for formation of peptide bonds as a route to prebiotic peptides in the fragmentation reactions of these singly deprotonated Trp cluster ions.
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ItemDecomposition of nitroimidazole ions: experiment and theory(ROYAL SOC CHEMISTRY, 2015)Nitroimidazoles are important compounds with chemotherapeutic applications as antibacterial drugs or as radiosensitizers in radiotherapy. Despite their use in biological applications, little is known about the fundamental properties of these compounds. Understanding the ionization reactions of these compounds is crucial in evaluating the radiosensitization potential and in developing new and more effective drugs. Thus, the present study investigates the decomposition of negative and positive ions of 2-nitroimidazole and 4(5)-nitroimidazole using low- and high-energy Collision-Induced Dissociation (CID) and Electron-Induced Dissociation (EID) by two different mass spectrometry techniques and is supported by quantum chemistry calculations. EID of [M+H](+) leads to more extensive fragmentation than CID and involves many radical cleavages including loss of H˙ leading to the formation of the radical cation, M˙(+). The stability (metastable decay) and the fragmentation (high-energy CID) of the radical cation M˙(+) have been probed in a crossed-beam experiment involving primary electron ionization of the neutral nitroimidazole. Thus, fragments in the EID spectra of [M+H](+) that come from further dissociation of radical cation M˙(+) have been highlighted. The loss of NO˙ radical from M˙(+) is associated with a high Kinetic Energy Release (KER) of 0.98 eV. EID of [M-H](-) also leads to additional fragments compared to CID, however, with much lower cross section. Only EID of [M+H](+) leads to a slight difference in the decomposition of 2-nitroimidazole and 4(5)-nitroimidazole.