Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation

Reprinted (adapted) with permission from Journal of Physical Chemistry A Copyright (2015) American Chemical SocietyInternational audienceThe electronic spectroscopy and the electronic excited state properties of cold protonated phenylalanine and protonated tyrosine have been revisited on a large spectral domain and interpreted by comparison with ab initio calculations. The protonated species are stored in a cryogenically cooled Paul trap, maintained at ~ 10K, and the parent and all the photo-fragment ions are mass-analyzed in a time-of-flight mass spectrometer, which allows detecting the ionic species with an improved mass resolution compared to what is routinely achieved with a quadrupole mass spectrometer. These new results emphasize the competition around the band origin between two proton transfer reactions from the ammonium group toward either the aromatic chromophore or the carboxylic acid group. These reactions are initiated by the coupling of the locally excited ππ* state with higher charge transfer states, the positions and coupling of which depend on the conformation of the protonated molecules. Each of these reaction processes gives rise to specific fragmentation channels that supports the conformer selectivity observed in the photofragmentation spectra of protonated Tyrosine and Phenylalanine

Photofragmentation spectroscopy of cold protonated aromatic amines in the gas phase

The electronic spectroscopy of cold protonated aromatic amines, anilineH+ C6H5-NH3+, benzylamineH+ C6H5-CH2-NH3+ and phenethylamineH+ C6H5-(CH2)2-NH3+ has been investigated experimentally in a large spectral domain and is compared to that of their hydroxy- homologues (OH containing). In the low energy region, the electronic spectra are similar to their neutral analogues, which reveals that their first excited state is of * character. A second transition is observed from 0.5 to 1 eV above the origin band, which is assigned to the excitation of the * state. In these protonated amine molecules, there is a competition between different fragmentation channels, some of them being specific of the UV excitation and are not observed in low-energy collision induced dissociation experiment. Besides, a drastic change in the fragmentation branching ratio can be observed within a very short energy range that reveals the complex excited state dynamics and fragmentation processes in these species. The experimental observations can be rationalized with a simple qualitative model, the /* model(PCCP-2002), which predicts that the excited state dynamics is controlled by the crossing between the * excited state and a state repulsive along the XH (X being O or N) coordinate

Unusual behavior in the first excited state lifetime of catechol

International audienceWe are presenting vibrationally selective pump-probe measurements of the first electronic excited-state (pp*) lifetime of jet-cooled neutral catechol (1,2-dihydroxybenzene). The lifetime of the 0-0 transition is very short (7 ps) as rationalized by the small pp*/psigma* gap calculated. However the lifetimes implying higher out-of-plane vibrational levels are longer (~11 ps). This emphasizes the role of the out-of-plane vibration in the pp*/psigma* coupling not only in its nature but also in the number of quant

UV photodissociation spectroscopy of cryogenic cooled gas phase host-guest complex ions of crown ethers

International audienceThe best determination of the most stable protonation site in aromatic molecules relies nowadays on the IR spectroscopy and ab initio calculations. It appears that these methods are not necessarily unambiguous and cannot always be safely employed. We present in this paper an example showing that electronic spectroscopy of cold ions complemented with ab initio calculations gives clear results on the protonation site. In the example given on the aminophenol isomers (in ortho, meta and para positions), the protonation site is assigned from the electronic spectroscopy and in particular we show that for the meta isomer the proton is not on the amino group as observed for the other isomers. It shows also that the protonation site is not conserved in the electrospray evaporation–ionization process

Photo-fragmentation spectroscopy of benzylium and 1-phenylethyl cations

The electronic spectra of cold benzylium (C6H5-CH2+) and 1-phenylethyl (C6H5-CH-CH3+)cations have been recorded via photofragment spectroscopy. Benzylium and 1-phenylethyl cations produced from electrosprayed benzylamine and phenylethylamine solutions, respectively, were stored in a cryogenically cooled quadrupole ion trap and photodissociated by an OPO laser, scanned in parts of the UV and visible regions (600-225 nm). The electronic states and active vibrational modes of the benzylium and 1-phenylethyl cations as well as those of their tropylium or methyl tropylium isomers have been calculated with ab initio methods for comparison with the spectra observed. Sharp vibrational progressions are observed in the visible region while the absorption features are much broader in the UV. The visible spectrum of the benzylium cation is similar to that obtained in an argon tagging experiment [V. Dryza, N. Chalyavi, J.A. Sanelli, and E.J. Bieske, J. Chem. Phys. 137, 204304 (2012)], with an additional splitting assigned to Fermi resonances. The visible spectrum of the 1-phenylethyl cation also shows vibrational progressions. For both cations, the second electronic transition is observed in the UV, around 33 000 cm-1 (4.1 eV), and shows a broadened vibrational progression. In both cases the S2 optimized geometry is non planar. The third electronic transition observed around 40 000 cm-1 (5.0 eV) is even broader with no apparent vibrational structures, which is indicative of either a fast non-radiative process or a very large change in geometry between the excited and the ground states. The oscillator strengths calculated for tropylium and methyl tropylium are weak. Therefore, these isomeric structures are most likely not responsible for these absorption features. Finally, the fragmentation pattern changes in the second and third electronic states: C2H2 loss becomes predominant at higher excitation energies, for both cations

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

Communication: Identification of daughter ions through their electronic spectroscopy at low temperature

International audienceWe present experimental results on photofragmentation of cold fragments issued from the photofragmentation of a cold parent ion. The cooling of the daughter ion at a few K allows its characterization not only through its fragmentation pattern but also through its well resolved electronic spectroscopy. This method is demonstrated on the photofragment resulting from the Ca-Cb bond rupture of protonated tyrosine. The analysis of the daughter ion (m/z 108) photofragmentation spectrum seems to be in agreement with the mechanism implying a proton transfer to the phenyl ring as the first step of the fragmentation mechanism of protonated tyrosine

The Effect of Ag+ on the Excited State Properties of Gas Phase (Cytosine)2Ag+ Complex: Electronic Transition and Estimated Lifetime

International audienceRecently, DNA molecules have received great attention because of their potential applications in material science. One interesting example is the production of highly fluorescent and tunable DNA-Agn clusters with cytosine (C) rich DNA strands. Here, we report the UV photofragmentation spectra of gas phase Cytosine...Ag+...Cytosine (C2Ag+) and Cytosine...H+...Cytosine (C2H+) complexes together with theoretical calculations. In both cases the excitation energy does not differ significantly from that of isolated cytosine or protonated-cytosine indicating that the excitation takes place on the DNA base. However, the excited state lifetime of the C2H+ ( = 85 fs), estimated from the bandwidth of the spectrum, is at least two orders of magnitude shorter than that of the C2Ag+ ( > 5300 fs). The increased excited state lifetime upon silver complexation is quite unexpected and it clearly opens the question about what factors are controlling the non-radiative decay in pyrimidine DNA bases? This is an important result for the expanding field of metal-mediated base pairing, and may also be important to the photophysical properties of DNA-templated, fluorescent silver clusters

Excited states of protonated DNA/RNA bases

International audienceThe very fast relaxation of the excited states to the ground state in DNA/RNA bases is a necessary process to ensure the photostability of DNA and its rate is highly sensitive to the tautomeric form of the bases. Protonation of the bases plays a crucial role in many biochemical and mutagenic processes and it can result in alternative tautomeric structures, thus making important the knowledge of the properties of protonated DNA/RNA bases. We report here the photofragmentation spectra of the five protonated DNA/RNA bases. In most of the cases, the spectra exhibit well resolved vibrational structures, with broad bands associated with very short excited state lifetimes. The similarity between the electronic properties e.g. excitation energy and very short excited state lifetimes for the canonical tautomers of protonated and neutral DNA bases, suggests that the former could also play an important role in the photostability mechanism of DNA