Elimination of water from the carboxyl group of GlyGlyH+

Bülent Balta, Viktorya Aviyente*, Chava Lifshitz

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

46 Citations (Scopus)

Abstract

The elimination of water from the carboxyl group of protonated diglycine has been investigated by density functional theory calculations. The resulting structure is identical to the b2 ion formed in the mass spectrometric fragmentation of protonated peptides (therefore named "b 2" in this study). The most stable geometry of the fragment ion ("b2") is an O-protonated diketopiperazine. However, its formation is kinetically disfavored as it requires a free energy of 58.2 kcal/mol. The experimentally observed N-protonated oxazolone is 3.0 kcal/mol less stable. The lowest energy pathway for the formation of the "b 2" ion requires a free energy of 37.5 kcal/mol and involves the proton transfer from the amide oxygen of protonated diglycine to the hydroxyl oxygen. Fragmentation initiated by proton transfer from the terminal nitrogen has also a comparable free energy of activation (39.4 kcal/mol). Proton transfer initiating the fragmentation, from the highly basic terminal nitrogen or amide oxygen to the less basic hydroxyl oxygen is feasible at energies reached in usual mass spectrometric experiments. Amide N-protonated diglycine structures are precursors of mainly y1 ions rather than "b 2" ions. In the lowest energy fragmentation channels, proton transfer to the hydroxylic oxygen, bond breaking and formation of an oxazolone ring occur concertedly but asynchronously. Proton transfer to hydroxyl oxygen and cleavage of the corresponding C O bond take place at the early stages of the fragmentation step, while ring closure to form an oxazolone geometry occurs at the later stages of the transition. The experimentally observed low kinetic energy release is expected to be due to the existence of a strongly hydrogen bonded protonated oxazolone-water complex in the exit channel. Whereas the threshold energy for "b2" ion formation (37.1 kcal/mol) is lower than for the y1 ion (38.4 kcal/mol), the former requires a tight transition state with an activation entropy, ΔS = -1.2 cal/mol.K and the latter has a loose transition state with ΔS = +8.8 cal/mol.K. This leads to y1 being the major fragment ion over a wide energy range.

Original languageEnglish
Pages (from-to)1192-1203
Number of pages12
JournalJournal of the American Society for Mass Spectrometry
Volume14
Issue number10
DOIs
Publication statusPublished - 1 Oct 2003
Externally publishedYes

Funding

The authors thank the Boğaziçi Üniversitesi Arastirma Fonu for support through project 01HB502D. The Farkas Research Center is supported by the Minerva Gesellschaft für die Forschung GmbH, München, Germany.

FundersFunder number
Minerva Gesellschaft für die Forschung GmbH

    Fingerprint

    Dive into the research topics of 'Elimination of water from the carboxyl group of GlyGlyH+'. Together they form a unique fingerprint.

    Cite this