Elucidation of Fragmentation Mechanisms of Protonated Peptide Ions and Their Products:  A Case Study on Glycylglycylglycine Using Density Functional Theory and Threshold Collision-Induced Dissociation

Abstract

The fragmentation mechanisms of protonated triglycine and its first-generation dissociation products have been investigated using a combination of density functional theory calculations and threshold collision-induced dissociation experiments. The activation barrier measured for the fragmentation of protonated triglycine to the b(2) ion and glycine is in good agreement with a calculated barrier at the B3LYP/6-31++G(d,p) level of theory reported earlier [Rodriquez, C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The b(2) ion fragments to the a(2) ion via a transition state structure that is best described as acylium-like. Contrary to what is commonly assumed, the lowest energy structure of the a(2) ion is not an iminium ion, but a cyclic, protonated 4-imidazolidone. Furthermore, fragmentation of the b(2) to the a(1) ion proceeds not via a mechanism that results in HNCO and H(2)C=C=O as byproducts, as have been postulated, but via a transition state that contains an incipient a(1) ion and an incipient carbene. The fragmentation of a(2) to a(1) proceeds via a transition state structure that contains the a(1) ion, CO and an imine as incipient components.

Formation of a1 Ions Directly from Oxazolone b2 Ions: an Energy-Resolved and Computational Study

“…The fragmentation reactions of simple oxazolone b 2 ions have been studied in detail involving threshold CID and computational studies by Siu and co-workers [32,33] and by Armentrout and Clark [31]. These studies have established detailed pathways for the b 2 →a 2 and b 2 →a 1 fragmentation reactions.…”

“…However, it is less well-known that a 1 ions may arise, in some cases, directly by fragmentation of b 2 ions [10,15,35]. Again, the detailed pathway has been established from density functional theory and threshold collision-induced dissociation studies [31][32][33]. In the present work, we apply both energy-resolved CID studies and theory to a systematic study of the occurrence of the direct b 2 →a 1 reaction.…”

“…It is wellknown [10,15,31] that a major fragmentation channel of oxazolone b 2 ions is elimination of CO to form the a 2 ion. A detailed pathway for this fragmentation reaction has been established from density functional theory and threshold collision-induced dissociation studies [31][32][33]. It is wellknown [32][33][34] that a 2 ions fragment, in part, to form the a 1 iminium ion.…”

“…A detailed pathway for this fragmentation reaction has been established from density functional theory and threshold collision-induced dissociation studies [31][32][33]. It is wellknown [32][33][34] that a 2 ions fragment, in part, to form the a 1 iminium ion. However, it is less well-known that a 1 ions may arise, in some cases, directly by fragmentation of b 2 ions [10,15,35].…”

Intramolecular condensation reactions in protonated dipeptides: Carbon monoxide, water, and ammonia losses in competition

“…The other major fragments from b 2 can be rationalized by consecutive decompositions of a 2 , as depicted in Scheme 3. In the proposed mechanism, the carbonyl group of a 2 is eliminated as a CO molecule to yield [a 2 Ϫ CO] ϩ , a proton-bound complex of the imines belonging to the Xxx and Yyy residues [45]. Sequential dissociation of this complex ultimately generates the immonium ions of the Xxx residue (a 1 ) and Yyy residue (an "internal" immonium ion).…”