title: Unimolecular dissociation of peptides: statistical vs. non-statistical fragmentation mechanisms and time scales.
abstract: In the present work we have investigated mechanisms of gas phase unimolecular dissociation of a relatively simple dipeptide, the di-proline anion, by means of chemical dynamics simulations, using the PM3 semi-empirical Hamiltonian. In particular, we have considered two activation processes that are representative limits of what occurs in collision induced dissociation experiments: (i) thermal activation, corresponding to several low energy collisions, in which the system is prepared with a microcanonical distribution of energy; (ii) collisional activation where a single shock of hundreds of kcal mol-1(300 kcal mol-1in the present case) can transfer sufficient energy to allow dissociation. From these two activation processes we obtained different product abundances, and for one particular fragmentation pathway a clear mechanistic difference for the two activation processes. This mechanism corresponds to the leaving of an OH-group and subsequent formation of water by taking a proton from the remaining molecule. This last reaction is always observed in thermal activation while in collisional activation it is less favoured and the formation of OH-as a final product is observed. More importantly, we show that while in thermal activation unimolecular dissociation follows exponential decay, in collision activation the initial population decays with non-exponential behaviour. Finally, from the thermal activation simulations it was possible to obtain rate constants as a function of temperature that show Arrhenius behaviour. Thus activation energies have also been extracted from these simulations.
authors.affiliation: Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, CEA CNRS, Université Paris Saclay, 91025 Evry, France. riccardo.spezia@univ-evry.fr and LAMBE, Université d'Evry-Val-d'Essonne, 91025 Evry, France.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Spezia
authors.affiliation: Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, CEA CNRS, Université Paris Saclay, 91025 Evry, France. riccardo.spezia@univ-evry.fr and LAMBE, Université d'Evry-Val-d'Essonne, 91025 Evry, France.
authors.forename: Ana
authors.initials: A
authors.lastname: Martin-Somer
authors.affiliation: Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, CEA CNRS, Université Paris Saclay, 91025 Evry, France. riccardo.spezia@univ-evry.fr and LAMBE, Université d'Evry-Val-d'Essonne, 91025 Evry, France.
authors.forename: Veronica
authors.initials: V
authors.lastname: Macaluso
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, USA.
authors.forename: Zahra
authors.initials: Z
authors.lastname: Homayoon
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, USA.
authors.forename: Subha
authors.initials: S
authors.lastname: Pratihar
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, USA.
authors.forename: William L
authors.initials: WL
authors.lastname: Hase