Excited States of Proton-bound DNA/RNA Base Homo-dimers: Pyrimidines

We are presenting the electronic photo fragment spectra of the protonated pyrimidine DNA bases homo-dimers. Only the thymine dimer exhibits a well structured vibrational progression, while protonated monomer shows broad vibrational bands. This shows that proton bonding can block some non radiative processes present in the monomer.Comment: We acknowledge the use of the computing facility cluster GMPCS of the LUMAT federation (FR LUMAT 2764

Tautomerism of cytosine probed by gas phase IR spectroscopy

The tautomerism of the gaseous protonated cytosine is studied using infrared multiple photon dissociation (IRMPD) spectroscopy of singly hydrated complexes of protonated cytosine in the 2700-3750 cm(-1) wavenumber range. The hydrated complexes are formed through argon-mediated collisions between bare electrosprayed cytosine and low-pressure water vapor. In the spectra, where X-H (X=C, O, and N) stretching vibrations are probed, evidence is found for the coexistence in the gas phase of hydrated complexes of two different cytosine tautomers. As the addition of a water molecule to either tautomer of protonated cytosine is energetically highly unlikely to induce interconversion, it is deduced that both C(2)=O and N(3) protonated tautomers of cytosine are formed under electrospray conditions. (C) 2009 Elsevier B.V. All rights reserved

Gas-phase titration of C7H9+ ion mixtures by FT-ICR mass spectrometry: Semiquantitative determination of ion populations generated by CI-induced protonation of C7H8 isomers and by EI-induced fragmentation of some monoterpenes

Mormann M, Salpin JY, Kuck D. Gas-phase titration of C7H9+ ion mixtures by FT-ICR mass spectrometry: Semiquantitative determination of ion populations generated by CI-induced protonation of C7H8 isomers and by EI-induced fragmentation of some monoterpenes. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY. 2006;249-250:340-352.The composition of various mixtures of isomeric gaseous C7H9+ ions, generated from four different C7H8 precursors by CI(methane) and CI(isobutane) and by EI-induced fragmentation of two monoterpenes, alpha-pinene (5) and limonene (6), in the external ion source of an FT-ICR mass spectrometer, has been detected by means of proton transfer reactions to selected bases ('' gas-phase titration '') within the ICR cell. Protonation of toluene (1) was found to give a uniform C7H9+ ion population containing exclusively the toluenium ions [1 + H](+) under both CI conditions. By contrast, protonation of 1,3,5-cycloheptatriene (2), 2,5-norbornadiene (3) and 6-mothylfulvene (4) was found to give mixtures of constitutionally isomeric C7H9+ ions, containing ions [1 + H](+) and protonated 6-methylfulvene, [4 + H](+), in all of the cases. Protonated norbornadiene, or its valence isomer, the 3-nortricyclyl ion [3'+H](+), was detected only in minor amounts by CI(CH4) of 2 and by CI of the neutral precursor, 3, itself. Protonation of 6-methylfulvene (4) by CI(CH4) gave rise to partial isomerization to ions [1 + H](+) only, but CI(i-C4H(10)) of 4 generated also protonated cycloheptatriene, [2 + H]+. Both of the C7H9+ ion mixtures formed by El-induced fragmentation of (X-pinene (5) and limonene (6) were found to consist mainly (> 85%) of tolueniurn ions, [1 + H](+), with only very small fractions (<= 10%) of dihydrotropylium ions, [2 + H](+), in contrast to previous reports in the literature. The gas-phase titration experiments of the C7H9+ mixtures was found to reflect the relative thermodynamic stabilities of the constitutional C7H9+ isomers and the exothermicity of the proton transfer step. The results are also discussed in view of the energy hypersurface of the isomeric C7H9+ ions, parts of which have been calculated by ab initio methods [MP2(full)/6-31I+G(3df,2p)// MP2(full)/6-31G(d)]. (c) 2005 Elsevier B.V. All rights reserved