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Volume 195, 2016
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Unimolecular dissociation of peptides: statistical vs. non-statistical fragmentation mechanisms and time scales

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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−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.

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Article information


Submitted
10 May 2016
Accepted
23 Jun 2016
First published
23 Jun 2016

Faraday Discuss., 2016,195, 599-618
Article type
Paper

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

R. Spezia, A. Martin-Somer, V. Macaluso, Z. Homayoon, S. Pratihar and W. L. Hase, Faraday Discuss., 2016, 195, 599 DOI: 10.1039/C6FD00126B

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