Radiationless deactivation pathways of excited gas phase nucleobases were investigated using mass-selected femtosecond resolved pump-probe resonant ionization. By comparison between nucleobases and methylated species, in which tautomerism cannot occur, we can access intrinsic mechanisms at a time resolution never reported so far (80 fs). At this time resolution, and using appropriate substitution, real nuclear motion corresponding to active vibrational modes along deactivation coordinates can actually be probed. We provide evidence for the existence of a two-step decay mechanism, following a 267 nm excitation of the nucleobases. The time resolution achieved together with a careful zero time-delay calibration between lasers allow us to show that the first step does correspond to intrinsic dynamics rather than to a laser cross correlation. For adenine and 9-methyladenine a first decay component of about 100 fs has been measured. This first step is radically increased to 200 fs when the amino group hydrogen atoms of adenine are substituted by methyl groups. Our results could be rationalized according to the effect of the highly localized nature of the excitation combined to the presence of efficient deactivation pathway along both pyrimidine ring and amino group out-of-plane vibrational modes. These nuclear motions play a key role in the vibronic coupling between the initially excited pipi* and the dark npi* states. This seems to be the common mechanism that opens up the earlier phase of the internal conversion pathway which then, in consideration of the rather fast relaxation times observed, would probably proceed via conical intersection between the npi* relay state and high vibrational levels of the ground state.

The conformational structure of short peptide chains in the gas phase is studied by laser spectroscopy of a series of protected dipeptides, Ac-Xxx-Phe-NH 2 , XxxϭGly, Ala, and Val. The combination of laser desorption with supersonic expansion enables us to vaporize the peptide molecules and cool them internally; IR/UV double resonance spectroscopy in comparison to density functional theory calculations on Ac-Gly-Phe-NH 2 permits us to identify and characterize the conformers populated in the supersonic expansion. Two main conformations, corresponding to secondary structures of proteins, are found to compete in the present experiments. One is composed of a doubly ␥-fold corresponding to the 2 7 ribbon structure. Topologically, this motif is very close to a ␤-strand backbone conformation. The second conformation observed is the ␤-turn, responsible for the chain reversal in proteins. It is characterized by a relatively weak hydrogen bond linking remote NH and CO groups of the molecule and leading to a ten-membered ring. The present gas phase experiment illustrates the intrinsic folding properties of the peptide chain and the robustness of the ␤-turn structure, even in the absence of a solvent. The ␤-turn population is found to vary significantly with the residues within the sequence; the Ac-Val-Phe-NH 2 peptide, with its two bulky side chains, exhibits the largest ␤-turn population. This suggests that the intrinsic stabilities of the 2 7 ribbon and the ␤-turn are very similar and that weakly polar interactions occurring between side chains can be a decisive factor capable of controlling the secondary structure.

We report the first gas-phase spectroscopic study of a three-residue model of a peptide chain, Ac-Phe-Gly-Gly-NH2 (Ac = acetyl), using the IR/UV double resonance technique. The existence of at least five different conformers under supersonic expansion conditions is established, most of them exhibiting rather strong intramolecular H-bonds. One of the most populated conformers, however, exhibits a different H-bonding network characterized by two weak H-bonds. Comparison of the amide A and I/II experimental data with density functional theory calculations carried out on a series of selected conformations enables us to assign this conformer to two successive beta-turns along the peptide chain, the two H-bonds being of C10 type, i.e., each of them closing a 10-atom ring in the molecule. The corresponding form is found to be more stable than the 310 helix secondary structure (not observed), presumably because of specific effects due to the glycine residues.

To cite this version:C. Canuel, Mohamed Elhanine, M. Mons, F. Piuzzi, B. Tardivel, et al.. 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.. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2006, 8, pp.3978-3987. <10.1039/b606437j>. AbstractThe excited state dynamics of the purine base 9-methyladenine (9-MeAde) has been investigated by time-and energy-resolved photoelectron imaging spectroscopy and massselected ion spectroscopy both in vacuum and water-cluster environment. 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 (ππ*, nπ*, πσ*, triplet) states involved in the electronic relaxation of adenine. The two-step pathway we propose to account for the double exponential decay observed agrees well with the recent theoretical calculations. The near UV photophysics of 9MeAde is dominated by the direct excitation of the ππ* ( 1 L b ) state (lifetime of 100 fs), followed by internal conversion, proceeding to the nπ* state (lifetime in the ps range) via conical intersection. No evidence for the involvement of a πσ* or a triplet state was found. 9MeAde-water clusters have been studied, focussing onto 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, leads to conclude to a very different photophysics as compared to the isolated base, assigned to a competition between i) a direct one-step decay of the initially excited state (ππ* L a and/or L b , stabilised by hydration) to the ground state and ii) a modified two-step decay scheme, qualitatively comparable to that occurring in the isolated molecule.