Formation of ions of protonated peptides. An ab initio study

Béla Paizs, Zoltán Szlávik, György Lendvay, Károly Vékey, Sándor Suhai

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The mechanism of the formation of a2/+ ions from b2/+ ions occurring during fragmentation of protonated peptides is investigated using quantum chemical methods. The geometries of the stationary structures involved in two possible mechanisms, namely, a two-step mechanism via an open-chain acylium ion and a concerted pathway involving rupture of two covalent bonds of the cyclic isomer of the b2/+ ion, as well as the energetics of the reactions, were calculated at the MP2 and B3LYP levels, both combined with the 6- 31G(d,p) as well as the 6-31++G(d,p) basis sets for the simplest analog of the b2/+ ion. The energetically favored path is the direct expulsion of the CO molecule from the cyclic b2/+ ion. The ZPE-corrected barrier height for this reaction is 26.2 kcal mol-1 at the MP2/6-31G(d,p) level, while the highest barrier along the two step path is 31.4 kcal mol-1. The barrier height for the reverse reaction is 3.8 kcal mol-1, significantly smaller than the average kinetic energy release (KER) measured for larger b2/+ ions. The barrier height for the reverse reactions of the MeCO-NH-CHMeCO,+ NH2-iBuCH-CO-NH-CH2CO,+ and NH2-CH2-CONH-CH(i-Bu)CO+ b2/+ ions was found to be 11.3, 9.6, and 18.4 kcal mol-1, in reasonable agreement with the measured KER for these reactions, indicating that the simplest model compound has unique properties in this respect. Based on comparisons with G2- MP2 calculations, comments are made on the applicability of various levels of theory for the description of the reaction. Copyright (C) 2000 John Wiley and Sons, Ltd.

Original languageEnglish
Pages (from-to)746-755
Number of pages10
JournalRapid Communications in Mass Spectrometry
Issue number9
Publication statusPublished - Jan 1 2000

ASJC Scopus subject areas

  • Analytical Chemistry
  • Spectroscopy
  • Organic Chemistry

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