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.

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.

Characterization of neutral fragments issued from the photodissociation of protonated tryptophane.

New information on the photo-fragmentation of biomolecules is obtained from the detection of neutral and ionic fragments using a time and position resolved coincidence technique that reveals whether an ionic photofragment is associated with one or more neutral fragments. In the case of a sequential dissociation, both fragmentation channels are identified as well as their time ordering.

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.

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.

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.

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.