V. Lepère

Conformation of polyalanine and polyglycine dications in the gas phase: insight from ion mobility spectrometry and replica-exchange molecular dynamics.

The conformation of model [Arg(Ala)(4)X(Ala)(4)Lys+2H](2+) and [Arg(Gly)(4)X(Gly)(4)Lys+2H](2+) peptides has been systematically investigated as a function of the central amino acid X through a combined experimental and theoretical approach. Mass spectrometry-based ion mobility measurements have been performed together with conformational sampling using replica-exchange molecular dynamics to probe the influence of each amino acid on the stable peptide conformation. Satisfactory agreement is obtained between measured and calculated diffusion cross section distributions. The results confirm the propensity of alanine-based peptides to form alpha-helices in the gas phase, differences between peptides arising from the local arrangement of the central side chain with respect to the charged ends. More generally, we find that charge solvation plays a major role in secondary structure stabilization, especially in the case of glycine-based peptides. The rich variety of conformations exhibited by the latter is qualitatively captured by the simulations. This work illustrates the potentiality of such combined experimental/theoretical strategy to determine peptide secondary structures. The present polyalanine and polyglycine peptides also offer a series of benchmark systems for future conformation-resolved studies.

Comprehensive characterization of the photodissociation pathways of protonated tryptophan

The photofragmentation of protonated tryptophan has been investigated in a unique experimental setup, in which ion and neutral issued from the photofragmentation are detected in coincidence, in time and in position. From these data are extracted the kinetic energy, the number of neutral fragments associated with an ion, their masses, and the order of the fragmentation steps. Moreover, the fragmentation time scale ranging from tens of nanoseconds to milliseconds is obtained. From all these data, a comprehensive fragmentation mechanism is proposed.

Chiral recognition in cinchona alkaloid protonated dimers: mass spectrometry and UV photodissociation studies.

Chiral recognition in protonated cinchona alkaloid dimers has been studied in mass spectrometry experiments. The experimental setups involved a modified 7T FT-ICR (Fourier transform-ion cyclotron resonance) mass spectrometer (MS) and a modified Paul ion trap both equipped with an electrospray ionization source (ESI). The Paul ion trap has been coupled to a frequency-doubled dye laser. The fragmentation of protonated dimers made from cinchonidine (Cd) and the two pseudoenantiomers of quinine, namely, quinine (Qn) and quinidine (Qd), has been assessed by means of collision-induced dissociation (CID) as well as UV photodissociation (UVPD). Whereas CID fragmentation of the dimers only leads to the evaporation of the monomers, UVPD results in the additional loss of a neutral radical fragment corresponding to the quinuclidinyl radical. The effect of the excitation wavelength and of complexation with H(2)SO(4) has been studied to cast light on the reaction mechanism. Complexation with H(2)SO(4) modifies the photoreactivity of the dimers; only evaporation of the monomeric fragments, quinine, and cinchonidine is observed. Comparison between the mass spectra of the cinchona alkaloid (CdQnH(+)) or (CdQdH(+)) dimers resulting from the UVPD of (CdQnH(2)SO(4)H(+)) and that of bare (CdQnH(+)) helps propose a fragmentation mechanism, which is thought to involve fast proton transfer from the quinuclidine part of a molecular subunit to the quinoline ring. CID and UV fragmentation experiments show that the homochiral dimer is more strongly bound than the heterochiral adduct.

Conformational Analysis of Quinine and Its Pseudo Enantiomer Quinidine: A Combined Jet-Cooled Spectroscopy and Vibrational Circular Dichroism Study

Laser-desorbed quinine and quinidine have been studied in the gas phase by combining supersonic expansion with laser spectroscopy, namely, laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), and IR-UV double resonance experiments. Density funtional theory (DFT) calculations have been done in conjunction with the experimental work. The first electronic transition of quinine and quinidine is of π-π* nature, and the studied molecules weakly fluoresce in the gas phase, in contrast to what was observed in solution (Qin, W. W.; et al. J. Phys. Chem. C2009, 113, 11790). The two pseudo enantiomers quinine and quinidine show limited differences in the gas phase; their main conformation is of open type as it is in solution. However, vibrational circular dichroism (VCD) experiments in solution show that additional conformers exist in condensed phase for quinidine, which are not observed for quinine. This difference in behavior between the two pseudo enantiomers is discussed.

Statistical vs. non-statistical deactivation pathways in the UV photo-fragmentation of protonated tryptophan–leucine dipeptide

The excited state dynamics of protonated tryptophan-leucine ions WLH+, generated in an electrospray source, is investigated by photo-induced fragmentation in the gas phase, using femtosecond laser pulses. Two main features arise from the experiment. Firstly, the initially excited pipi* state decays very quickly with 2 time constants of 1 and 10 ps. Secondly, the transient signals recorded on different fragments are not the same which indicates two competing primary fragmentation processes. One involves a direct dissociation from the excited state that gives evidence for a non-statistical deactivation path. The other is attributed to a statistical decay following internal conversion to the ground electronic surface.

How do Pseudoenantiomers Structurally Differ in the Gas Phase? An IR/UV Spectroscopy Study of Jet‐Cooled Hydroquinine and Hydroquinidine

The gas-phase structures of the cinchona alkaloids, hydroquinine and its pseudoenantiomer hydroquinidine, are studied in a supersonic expansion by means of laser-induced fluorescence and IR/UV double-resonance spectroscopy. Vibrational spectroscopy combined with density functional calculations show that the conformational properties of the two pseudoenantiomers are identical. In both cases, they exist in two isoenergetic forms, with similar IR spectra. Both conformers are similar to the most stable cis-γ-open form of quinine; they differ from each other by the position of the ethyl substituent attached to the quinuclidine ring. Further differences between the two conformers are observed in the laser-induced fluorescence spectrum. The first electronic transition is characterized by time-dependent density functional theory and RI-cc2 calculations, and is of ππ* nature. The results described here emphasize the role of the ethyl substituent in the structural differences between pseudoenantiomers of cinchona alkaloids.

A zero dead-time, multihit, time and position sensitive detector based on micro-channel plates

We have developed a fast multihit position and time sensitive detector with zero dead-time for heavy particles in the keV energy range. This new type of detector makes use of a micro-channel plates (MCP) assembly and combines a detection based on delay line anode with a simultaneous particle imaging with a CCD-camera. The time pickup accuracy is enhanced by digitalizing the MCP biasing signal. This detector, operating at kHz repetition rate, allows a position resolution better than 100μm and a time resolution better than 100ps to be achieved.

Lifetime and yield of metastable Ar2+ ions

Ar2(+) ions produced in a cooled supersonic expansion by electron-impact ionization are accelerated at 2.5 keV and kept during few milliseconds inside a linear electrostatic trap. The lifetime of the metastable Ar2(+) ion is determined from the measurement of the rate of the argon atoms escaping the trap. The lifetime and the relative metastable populations are measured as a function of the pressure and temperature in the supersonic expansion, i.e., of the mean cluster size. Possible mechanisms responsible for the metastable formation are discussed.

Photodissociation dynamics of Ar2(+) and Ar3(+) excited by 527 nm photons.

The photofragmentation dynamics of Ar(2)(+) and Ar(3)(+) clusters has been investigated at a 527 nm wavelength (2.35 eV) using a setup that allows simultaneous detection of the ionic and neutral fragments in a coincidence experiment. Measurement of positions and times of flight enables in principle a complete description of the fragmentation dynamics. The photofragmentation dynamics of Ar(3)(+) clusters is similar to that of Ar(2)(+) with, in addition, the ejection of a third fragment that can be neutral or ionized via a resonant electron capture. This is attributed to the triangular geometry of the Ar(3)(+) ion.

Dissociative charge transfer and collision induced dissociation of Ar2+ and Ar3+ clusters in collisions with argon atoms at keV energies

The dynamics of dissociative charge transfer and collision induced dissociation of Ar(2) (+) and Ar(3) (+) clusters colliding with Ar atoms at 4.8 keV has been investigated using a novel multifragment detection scheme that maps the postcollision vectors of all particles simultaneously. Estimation of internal energies and measurement of pre- and postcollision vectors enables a full description of reaction dynamics. The prominence of electronic excitation in defining the dynamics of these collision systems is demonstrated. The dissociation dynamics of Ar(3) (+) clusters is distinctly different from that of Ar(2) (+). This is attributed to a combination of lower internal energies and predominantly triangular T-shape structure of the Ar(3) (+) ion.