Deactivation pathways of electronically excited states have been investigated in three protonated aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe). The protonated amino acids were generated by electrospray and excited with a 266 nm femtosecond laser, the subsequent decay of the excited states being monitored through fragmentation of the ions induced and/or enhanced by another femtosecond pulse at 800 nm. The excited state of TrpH+ decays in 380 fs and gives rise to two channels: hydrogen atom dissociation or internal conversion (IC). In TyrH, the decay is slowed down to 22.3 ps and the fragmentation efficiency of PheH+ is so low that the decay cannot be measured with the available laser. The variation of the excited state lifetime between TrpH+ and TyrH+ can be ascribed to energy differences between the dissociative pi sigma* state and the initially excited pi pi* state.

We studied the photoionization of 2-pyridone and its tautomer, 2-hydroxypyridine by means of VUV synchrotron radiation coupled to a velocity map imaging electron/ion coincidence spectrometer. The photoionization efficiency (PIE) spectrum is composed of steps. The state energies of the [2-pyridone](+) cation in the X[combining tilde] ground and A excited electronic states, as well as of the [2-hydroxypyridine](+) cation in the electronic ground state, are determined. The slow photoelectron spectra (SPES) are dominated by the 0(0)(0) transitions to the corresponding electronic states together with several weaker bands corresponding to the population of the pure or combination vibrational bands of the cations. These vibrationally-resolved spectra compare very well with state-of-the-art calculations. Close to the ionization thresholds, the photoionization of these molecules is found to be mainly dominated by a direct process whereas the indirect route (autoionization) may contribute at higher energies.

In recent experiments, the excited-state lifetimes of protonated aromatic amino acids (TrpH+ and TyrH+) have been recorded by means of pump-probe photodissociation technique. The lifetime of TyrH+ is much longer than that of TrpH+, which has been initially rationalized on the basis of a simple phenomenological model. Besides, specific photofragments including the formation of radical cation after hydrogen loss are observed for TrpH+ that are not found for TyrH+. The ab initio calculations reported here for TrpH+ and TyrH+ using a coupled-cluster method are meant to track the rich photochemistry of these protonated amino acids following UV excitation.

Fe + -( CO 2 ) n ion-molecule complexes are produced by laser vaporization in a pulsed-nozzle cluster source. These species are analyzed and mass-selected using a specially designed reflectron time-of-flight mass spectrometer. Infrared photodissociation of these complexes is investigated with an optical parametric oscillator/amplifier system using wavelengths near the CO2 asymmetric stretch vibration (2349 cm−1). Dissociation occurs by successive elimination of CO2 molecules. Tunable laser experiments obtain infrared resonance-enhanced photodissociation spectra for these complexes. Small complexes have CO2 asymmetric stretch resonances shifted to higher frequency than the free CO2 mode. The blueshift decreases initially with cluster size, but becomes nearly constant after the n=4 cluster. Argon-tagged complexes, e.g., Fe+-(CO2)n⋅Arm, photodissociate via the same CO2 resonances by elimination of argon. Except for the n=1 complex, bands for the tagged complexes occur at the same frequency as those for the corresponding CO2 complex without argon. Larger complexes exhibit additional resonances near the free CO2 asymmetric stretch indicating “surface” molecules not attached to the metal. Blueshifted resonances also persist in these complexes attributed to “core” ligands attached to the metal ion. In the largest clusters studied (n=9–14), additional resonances with an intermediate blueshift are measured associated with “caged” CO2 molecules not attached to the metal. These measurements demonstrate that infrared photodissociation spectroscopy has exciting potential to study clustering structures and dynamics around metal ions.

The gas-phase structures of protonated peptides are studied by means of resonant infrared multiphoton dissociation spectroscopy (R-IRMPD) performed with a free electron laser. The peptide structures and protonation sites are obtained through comparison between experimental IR spectra and their prediction from quantum chemistry calculations. Two different analyses are conducted. It is first supposed that only well-defined conformations, sufficiently populated according to a Boltzmann distribution, contribute to the observed spectra. On the contrary, DFT-based Car-Parrinello molecular dynamics simulations show that at 300 K protonated peptides no longer possess well-defined structures, but rather dynamically explore the set of conformations considered in the first conventional approach.

Finite temperature Car-Parrinello molecular dynamics simulations are performed for the protonated dialanine peptide in vacuo, in relation to infrared multiphoton dissociation experiments. The simulations emphasize the flexibility of the different torsional angles at room temperature and the dynamical exchange between different conformers which were previously identified as stable at 0 K. A proton transfer occurring spontaneously at the N-terminal side is also observed and characterized. The theoretical infrared absorption spectrum is computed from the dipole time correlation function, and, in contrast to traditional static electronic structure calculations, it accounts directly for anharmonic and finite temperature effects. The comparison to the experimental infrared multiphoton dissociation spectrum turns out very good in terms of both band positions and band shapes. It does help the identification of a predominant conformer and the attribution of the different bands. The synergy shown between the experimental and theoretical approaches opens the door to the study of the vibrational properties of complex and floppy biomolecules in the gas phase at finite temperature.

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Protonated dialanine cations have been isolated in a Fourier transform ion cyclotron resonance mass-spectrometer (FT-ICR-MS) and subjected to infrared multiphoton dissociation (IRMPD) at the free electron laser facility CLIO in Orsay (France). The spectral dependence of the IR induced fragmentation pattern in the mid-infrared region (800-2000 cm -1 ) is interpreted with the help of structure and vibrational spectrum calculations of the different protonated conformers. This comparison allows for the assignment of the proton on the terminal amino group, as the most favourable proton site, the neighbouring amide bond being in the trans conformation.2