Time-resolved photoelectron and photoion fragmentation spectroscopy study of 9-methyladenine and its hydrates: a contribution to the understanding of the ultrafast radiationless decay of excited DNA bases.

The excited state dynamics of the purine base 9-methyladenine (9Me-Ade) has been investigated by time- and energy-resolved photoelectron imaging spectroscopy and mass-selected ion spectroscopy, in both vacuum and water-cluster environments. The specific probe processes used, namely a careful monitoring of time-resolved photoelectron energy distributions and of photoion fragmentation, together with the excellent temporal resolution achieved, enable us to derive additional information on the nature of the excited states (pp*, np*, ps*, triplet) involved in the electronic relaxation of adenine. The two-step pathway we propose to account for the double exponential decay observed agrees well with recent theoretical calculations. The near-UV photophysics of 9Me-Ade is dominated by the direct excitation of the pp* (1Lb) state (lifetime of 100 fs), followed by internal conversion to the np* state (lifetime in the ps range) via conical intersection. No evidence for the involvement of a ps* or a triplet state was found. 9Me- Ade–(H2O)n clusters have been studied, focusing on the fragmentation of these species after the probe process. A careful analysis of the fragments allowed us to provide evidence for a double exponential decay profile for the hydrates. The very weak second component observed, however, led us to conclude that the photophysics were very different compared with the isolated base, assigned to a competition between (i) a direct one-step decay of the initially excited state (pp* La and/or Lb, stabilised by hydration) to the ground state and (ii) a modified two-step decay scheme, qualitatively comparable to that occurring in the isolated molecule

Influence of structural isomerism and fluorine atom substitution on the self-association of naphthoic acid

The self-association of small aromatic systems driven by π-π stacking and hydrophobic interactions is well known. Understanding the nature of these interactions is important if they are to be used to control association. Here, we present results of an NMR study into the self-association of two isomers of naphthoic acid along with an investigation into the role of a fluorine substituent on that self- association. We interpret the results in terms of a simple isodesmic model of self- association and show that the addition of the fluorine atom appears to increase the stability of the aggregates by an order of magnitude (e.g. 1-naphothic acid vs 4- fluoro-1-naphthoic acid Keq = 0.05 increases to 0.35 M-1), a result which is supported by computational studies in the literature on the role of substituent effects on interaction energy. The use of fluorinated isomers to probe the assembly is also presented, with differing trends in fluorine-19 chemical shifts observed depending on the isomer substitution pattern

Dependence of ion pair formation on the velocity of high rydberg state argon atoms in collisions with CCl

Ionisation of argon atoms in high Rydberg states (HR) by collisions with CCl4 and CCl3F has been investigated. Absolute cross sections for the dissociative electron attachment are reported, together with the identities of the negatively and positively charged species produced in coincidence in the collision. The dependence of the cross sections on the principal quantum number n and the HR argon velocity alows us to determine the limits of the theoretical "free electron" model and the role of the core Ar+ in the outcome of the reaction

Near-UV Resonant Two-Photon Ionization Spectroscopy of Gas Phase Guanine: Evidence for the Observation of Three Rare Tautomers

International audienceIn light of a recently published study on the IR spectroscopy of guanine in He droplets (Choi, M. Y.; Miller, R. E. J. Am. Chem. Soc. 2006, 128, 7320), the present letter proposes a new interpretation of the resonant two-photon ionization (R2PI) experiments on gas phase guanine, which is supported by quantum chemistry calculations. Whereas He droplet experiments detect the most stable forms, only one of these forms is observed (very marginally) in the R2PI spectrum, which is actually dominated by three less stable “rare” tautomers, whose stabilities lie in the 3-7 kcal/mol range. The absence of the most stable forms in the R2PI spectrum suggests that a tautomer-dependent ultrafast relaxation process takes place in the excited state of these stable tautomers. The present reinterpretation modifies qualitatively the picture of the excited state of guanine tautomers and should contribute to the understanding of the deactivation mechanisms taking place in the excited state of DNA bases

Paysage conformationnel de petites chaînes peptidiques : étude par spectroscopie laser de double résonance IR/UV

National audienceLaisser se replier de petites chaînes peptiques en phase gazeuse sous l'action de leurs propres interactions intramoléculaires est un moyen efficace de caractériser les régions du paysage conformationnel pertinentes pour leur dynamique. C'est ce que réalisent les physico-chimistes de la phase gazeuse en désorbant des petits peptides par laser, les portant ainsi à haute température dans un état désordonné « dénaturé », puis en les refroidissant dans une détente supersonique. Le repliement est alors analysé a posteriori avec toute la précision des spectroscopies laser les plus modernes (technique de double résonance IR/UV) permettant, en croisant les informations spectroscopiques électroniques et vibrationnelles, de remonter à la structure des minima locaux du paysage conformationnel balayé, notamment en ce qui concerne le réseau de liaisons hydrogène intramoléculaire. Ainsi des structures secondaires typiques comme les coudes β et les hélices 310 des protéines ont pu être isolées et caractérisées avec une précision bien meilleure qu'en phase condensée, permettant de mettre à jour les interactions en jeu dans le repliement

Etude de mécanismes d'ionisation

L'interaction des rayonnements α, β ou γ avec la matière conduit à l'ionisation et l'excitation des molécules du milieu. La photoionisation à l'aide du rayonnement synchrotron ou de lasers permet l'étude détaillée des mécanismes primaires de production et de dissociation d'ions. Des études, à l'aide de lasers, de spectroscopie, d'énergie des états et de cinétique de fragmentation des ions sont décrites pour le phénol et le phénétole. Le rayonnement synchrotron permet d'étudier l'excitation et l'ionisation en couche interne dans le domaine d'énergie entre 20 et 1 000 eV. Une prédissociation rapide de HBr et CH3Br excitées en couche 3d a été observée ainsi qu'une dissociation sélective de l'ion doublement chargé CH3Br++

Gas phase hydrogen-bonded complexes of aromatic molecules : photoionization and energetics

International audienceThe present review discusses the possibility of measuring the dissociation energy of gas phase complexes from their dissociative photoionization. A compilation of recent results on hydrogen-bonded complexes of aromatic molecules, with a polar solvent molecule (water, alcohol, NH 3 , HCl, etc.), playing the role of either proton donor or proton acceptor is presented. We show that laser experiments begin to provide a consistent set of energetic data that can be considered as benchmarks to assess quantum calculations as well as to parametrize the force field models used in biochemistry