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International audience; Dissociation chemistry of the diproline anion [Pro2-H]− is studied using chemical dynamics simulations coupled with quantum-chemical calculations and RRKM analysis. Pro2– is chosen due to its reduced size and the small number of sites where deprotonation can take place. The mechanisms leading to the two dominant collision-induced dissociation (CID) product ions are elucidated. Trajectories from a variety of isomers of [Pro2-H]− were followed in order to sample a larger range of possible reactivity. While different mechanisms yielding y1– product ions are proposed, there is only one mechanism yielding the b2– ion. This mechanism leads to formation of a b2– fragment with a diketopiperazine structure. The sole formation of a diketopiperazine b2 sequence ion is experimentally confirmed by infrared ion spectroscopy of the fragment anion. Furthermore, collisional and internal energy activation simulations are used in parallel to identify the different dynamical aspects of the observed reactivity.

The enthalpy and entropy changes for the formation of the 1:1 complexes of methanol with various gaseous protonated amino acids have been measured using pulsed-ionization high-pressure mass spectrometry. The enthalpy changes for formation of the clusters Gly(MeOH)H(+), Ala(MeOH)H(+), Val(MeOH)H(+), Leu(MeOH)H(+), Ile(MeOH)H(+), and Pro(MeOH)H(+) have been determined to be -92.0, -83.3, -82.4, -79.5, -78.7, and -73.6 kJ mol(-1), respectively. These values agree very well with the energetic values computed at the MP2(full)/6-311++G(2d,2p)//B3LYP/6-311+G(d,p) level of theory for the lowest energy adducts in each system. Both experimental observations and computational determinations of the potential energy surface for the glycine system suggest that a mixture of low-lying isomers may be present for each of the cluster systems examined. The primary structural motif for these clusters is the coordination of the methanol molecule to the ammonium group of the protonated amino acid via a strong ionic hydrogen bond. For the amino acids studied here, computational results reveal that one methanol molecule does not sufficiently stabilize any zwitterionic structure such that no appreciable extent of proton transfer from the amino acid to methanol was observed.

Ionic hydrogen-bonding interactions have been found in several clusters formed by 5-fluorocytosine (5-FC). The chloride and trimethylammonium cluster ions, along with the cationic (proton-bound) dimer have been characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy and electronic structure calculations performed at the B2PLYP/aug-cc-pVTZ//B3LYP/6-311+G(d,p) level of theory. IRMPD action spectra, in combination with calculated spectra and relative energetics, indicate that it is most probable that predominantly a single isomer exists in each experiment. For the 5-FC-trimethylammonium cluster specifically, the calculated spectrum of the lowest-energy isomer convincingly matches the experimental spectrum. Interestingly, the cationic dimer of 5-FC was found to have a single energetically relevant isomer (Cationic-IV) involving a tridentate ionic hydrogen-bonding interaction. The three sites of intermolecular ionic hydrogen bonds in this isomer interact very efficiently, leading to a significant calculated binding energy of 180 kJ/mol. The magnitude of the calculated binding energy for this species, in combination with the strong correlation between the simulated and IRMPD spectra, suggests that a tridentate-proton-bound dimer was observed predominantly in the experiments. Comparison of the calculated relative Gibbs free energies (298 K) for this species and several of the other isomers considered also supports the likelihood of the dominant protonated dimer existing as Cationic-IV.