Unimolecular dissociation of peptides: statistical vs. non-statistical fragmentation mechanisms and time scales

• Published in: Faraday discussions 2016-01, Vol.195, p.599-618
• Main Authors: Spezia, Riccardo, Martin-Somer, Ana, Macaluso, Veronica, Homayoon, Zahra, Pratihar, Subha, Hase, William L
• Format: Article
• Language: English
• Subjects: Activation; Activation energy; Behavior; Chemical Physics; Collision dynamics; Decay; Energy of dissociation; Fragmentation; Life Sciences; Physics; Simulation
• ISSN: 1359-6640; 1364-5498
• DOI: 10.1039/c6fd00126b

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 −1 in 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.