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.
|Number of pages||10|
|Journal||Rapid Communications in Mass Spectrometry|
|Publication status||Published - Jan 1 2000|
ASJC Scopus subject areas
- Analytical Chemistry
- Organic Chemistry