Dissociative multi-photon ionization of isolated uracil and uracil-adenine complexes

Recent multi-photon ionization (MPI) experiments on uracil revealed a fragment ion at m/z 84 that was proposed as a potential marker for ring opening in the electronically excited neutral molecule. The present MPI measurements on deuterated uracil identify the fragment as C3H4N2O+ (uracil+ less CO), a plausible dissociative ionization product from the theoretically predicted open-ring isomer. Equivalent measurements on thymine do not reveal an analogous CO loss channel, suggesting greater stability of the excited DNA base. MPI and electron impact ionization experiments have been carried out on uracil-adenine clusters in order to better understand the radiation response of uracil within RNA. Evidence for C3H4N2O+ production from multi-photon-ionized uracil-adenine clusters is tentatively attributed to a significant population of π-stacked configurations in the neutral beam

Threshold behavior in metastable dissociation of multi-photon ionized thymine and uracil

Microsecond-timescale HNCO loss has been observed from single-color multi-photon ionized pyrimidine nucleobases in the gas phase. Photon energy thresholds for the metastable channels have been measured at 5.55 ± 0.02 eV for thymine and 5.57 ± 0.02 eV for uracil. We argue that these results can be attributed to accessing the molecules’ S1 states with additional vibrational energy matching the threshold energy for HNCO loss from the radical cation. Combined with previous photoionization energies, this enables the S1 adiabatic energies to be deduced: 3.67 ± 0.07 eV for thymine and 3.77 ± 0.07 eV for uracil. These values are consistent with recent calculations

Multi-photon ionization and fragmentation of uracil: neutral excited-state ring opening and hydration effects

Multi-photon ionization (MPI) of the RNA base uracil has been studied in the wavelength range 220–270 nm, coinciding with excitation to the S2(ππ*) state. A fragment ion at m/z = 84 was produced by 2-photon absorption at wavelengths ≤232 nm and assigned to C3H4N2O+ following CO abstraction. This ion has not been observed in alternative dissociative ionization processes (notably electron impact) and its threshold is close to recent calculations of the minimum activation energy for a ring opening conical intersection to a σ(n-π)π* closed shell state. Moreover, the predicted ring opening transition leaves a CO group at one end of the isomer, apparently vulnerable to abstraction. An MPI mass spectrum of uracil-water clusters is presented for the first time and compared with an equivalent dry measurement. Hydration enhances certain fragment ion pathways (particularly C3H3NO+) but represses C3H4N2O+ production. This indicates that hydrogen bonding to water stabilizes uracil with respect to neutral excited-state ring opening

Multi-photon and electron impact ionisation studies of reactivity in adenine–water clusters

Multi-photon ionisation (MPI) and electron impact ionisation (EII) mass spectrometry experiments have been carried out to probe unimolecular and intermolecular reactivities in hydrated adenine clusters. The effects of clustering with water on fragment ion production from adenine have been studied for the first time. While the observation of NH4+ fragments indicated the dissociation of protonated adenine, the dominant hydration effects were enhanced C4H4N4+ production and the suppression of dissociative ionisation pathways with high activation energies. These observations can be attributed to energy removal from the excited adenine radical cation via cluster dissociation. Comparisons of MPI and EII measurements provided the first experimental evidence supporting hypoxanthine formation in adenine–water clusters via theoretically predicted barrierless deamination reactions in closed shell complexes

Cross sections for ionization and ionic fragmentation of pyrimidine molecules by electron collisions

The electron impact mass spectroscopy and the total ion collection measurements were used to investigate the ionization and ionic fragmentation of the pyrimidine, C4H4N2, molecules in the gas phase. The cation mass spectra were measured in the 10−85 amu range and the observed mass peaks assigned to corresponding ionic fragments. The most abundant cation in the mass spectra is the parent cation, C4H4N2+, at 80 amu. The appearance energies of the ionic fragments were determined and the possible fragmentation processes are discussed. The electron impact absolute total and partial ionization cross sections in pyrimidine were obtained over the energy range from the respective ionization thresholds up to 150 eV

UV-visible emission as a probe of core excitations applied to the furan and carbon dioxide molecules

7 pags, 4 figs- -- 14th International Conference on X-Ray Absorption Fine Structure (XAFS14), Camerino, Italy, July 26-31, 2009The core excitations of the furan and carbon dioxide molecules have been studied using dispersed UV-visible fluorescence spectroscopy. Balmer-α (Balmer-β) emission was measured at the O 1s and C 1s (O 1s) excitations of furan, while emission due to an excited state of the neutral oxygen atom was measured at the O 1s excitations of CO2. The excitation functions of the emission lines display both valence and Rydberg resonances, but the latter are more enhanced. © 2009 IOP Publishing Ltd.Peer reviewe