Statistical versus non-statistical photo-fragmentation of protonated GWG tri-peptide induced by UV excitation

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Gas phase dynamics, conformational transitions and spectroscopy of charged saccharides: the oxocarbenium ion, protonated anhydrogalactose and protonated methyl galactopyranoside

International audienceProtonated intermediates are postulated to be involved in the rate determining step of many sugar reactions. This paper presents a study of protonated sugar species, isolated in the gas phase, using a combination of infrared multiple photon dissociation (IRMPD) spectroscopy, classical ab initio molecular dynamics (AIMD) and quantum mechanical vibrational self-consistent field (VSCF) calculations. It provides a likely identification of the reactive intermediate oxocarbenium ion structure in a D-galactosyl system as well as the saccharide pyrolysis product anhydrogalactose (that suggests oxocarbenium ion stabilization), along with the spectrum of the protonated parent species: methyl D-galactopyranoside-H +. Its vibrational fingerprint indicates intramolecular proton sharing. Classical AIMD simulations for galactosyl oxocarbenium ions, conducted in the temperature range ~300-350 K (using B3LYP potentials on-the-fly) reveal efficient transitions on the picosecond timescale. Multiple conformers are likely to exist under the experimental conditions and along with static VSCF calculations, they have facilitated the identification of the individual structural motifs of the galactosyl oxocarbenium ion and protonated anhydrogalactose ion conformers that contribute to the observed experimental spectra. These results demonstrate the power of experimental IRMPD spectroscopy combined with dynamics simulations and with computational spectroscopy at the anharmonic level to unravel conformer structures of protonated saccharides, and to provide information on their lifetimes

Probing the glycosidic linkage: Uv and ir ion-dip spectroscopy of a lactoside

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HIGH-RESOLUTION UV SPECTRA OF BENZENE ISOTOPOMERS AND DIMERS IN HELIUM NANODROPLETS

Author Institution: Department of Chemistry, Princeton UniversityWe present high-resolution ultraviolet spectra of various benzene isotopomers and their dimers in helium nanodroplets in the region of the first Herzberg-Teller allowed vibronic transition $^{1}B_{2u} \leftarrow {^{1}}A_{1g} 6^{1}_{0}$ (a.k.a. the $A^{0}_{0}$ transition) at $\sim 260$ nm. Spectra were recorded in beam depletion and laser-induced fluorescence excitation. Unlike for many larger aromatic molecules, the monomer spectra consist of a single ``zero phonon'' line, blueshifted by about $30 cm ^{-1}$ from the gas phase value. The rotational moments of inertia of $C_{6}H_{6}$ are found to be at least 6 or 7 times larger than in the gas phase. The dimers present the same vibronic fine structure (though modestly compressed) as previously observed in the gas phase. The fluorescence lifetime and quantum yield of $(C_{6}H_{6})$ are found to be equal to those of $C_{6}H_{6}$, implying substantial inhibition of excimer formation in the dimer in helium nanodroplets

Adding water to sugar: A spectroscopic and computational study of alpha- and beta-phenylxyloside in the gas phase

The gas phase structures of phenyl alpha- and beta-d-xylopyranoside (alpha- and beta-pXyl) and their mono-hydrates have been investigated using a combination of resonant two-photon ionization (R2PI), ultra-violet hole-burning and resonant infrared ion dip spectroscopy, coupled with density functional theory (DFT) and ab initio computation. The hole-burning experiments indicate the population of a single conformer only, in each of the two anomers. Their experimental and calculated infrared spectra are both consistent with a conformational assignment corresponding to the computed global minimum configuration. All three OH groups are oriented towards the oxygen atom (O1) on the anomeric carbon atom to form an all trans(ttt) counter-clockwise chain of hydrogen bonds. The mono-hydrates, alpha- and beta-pXyl(H(2)O) each populate two distinct structures in the molecular beam environment, with the water molecule inserted between OH4 and OH3 or between OH3 and OH2 in alpha-pXyl(H2O), and between OH2 and O1 in either of two alternative orientations, in beta-pXyl(H2O). In all of the mono-hydrated xyloside complexes, the water molecule inserts into the weakest link of the sugar molecules' hydrogen-bonded chain of hydroxy groups, creating a single extended chain, strengthened by co-operativity. The all-trans configuration of the xylose moiety is retained and the mono-hydrate structures correspond to those calculated to lie at the lowest relative energies