Fragmentation reactions of a2 ions derived from deprotonated dipeptides-A synergy between experiment and theory

Gregory A. Chass, Christopher N J Marai, Alex G. Harrison, I. Csizmadia

Research output: Contribution to journalArticle

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Abstract

The fragmentation reactions of a number of a2 ions ([M-H-CO2]-) derived from dipeptides have been studied by energy-resolved mass spectrometry, isotopic labeling, and MS3 experiments. The general reaction sequence H2N-CH(R1)-C(=O)-N(R2)-CHR3 --HN-CH(R1)-C(=O) -N(R2)-CH2R3 (1) -HN-CH(R1)-C(=O)-N(R2)-CH2 R3 → R3CH2-N(R2) - C=O- + HN=CHR1 (2) R3CH2 -N(R2)-C=O- →R3CH2 -N(R2)- + CO (3) leading eventually to a deprotonated amine, is shown to occur, a reaction sequence first proposed by Styles and O'Hair (Rapid Commun. Mass Spectrom. 1998, 12, 809) from a study of the a2 ion derived from glycylglycine. When an amidic hydrogen (R2 = H) is present, the initial proton-transfer reaction 1 is nonreversible. However, when there is no amidic hydrogen, as in the a2 ions derived from H-Ala-Pro-OH or H-Gly-Sar-OH, the initial proton-transfer reaction 1 becomes reversible, leading to the interchange of N-bonded and C-bonded hydrogens. Ab initio calculations at the MP2/6-31+G(d) level of the energies and interconversion pathways of anions derived by deprotonation of glycine N-methylamide show a barrier of 8 kcal mol-1 for reaction 1, with reaction 2 being 23.8 kcal mol-1 endothermic. When an amidic hydrogen (R2 = H) is present, the amine-deprotonated species formed in reaction 1 abstracts a proton from the amide nitrogen to form the amide-deprotonated species, the most stable species on the potential energy surface. The system effectively becomes trapped in this low-energy well and exits upon activation by reactions 2 and 3 as observed when glycine N-methylamide is deprotonated directly. When no amidic hydrogen is present, this low-energy state does not exist, and reaction 1 becomes reversible, leading to the interchange of N-bonded and C-bonded hydrogens. In these cases, a significant population of the original a2 ion is formed, which fragments by the reaction H2N-CH(R1)-C(=O) -N(R2)-CHR3 -→ H2N-CH(R1)-C=O- + R3 CH=NR2 (4).

Original languageEnglish
Pages (from-to)9695-9704
Number of pages10
JournalJournal of Physical Chemistry A
Volume106
Issue number42
DOIs
Publication statusPublished - Oct 24 2002

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Dipeptides
Hydrogen
fragmentation
Ions
ions
Proton transfer
methylidyne
Experiments
Interchanges
Amides
Glycine
Amines
hydrogen
Glycylglycine
Potential energy surfaces
Deprotonation
glycine
Carbon Monoxide
amides
protons

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Fragmentation reactions of a2 ions derived from deprotonated dipeptides-A synergy between experiment and theory. / Chass, Gregory A.; Marai, Christopher N J; Harrison, Alex G.; Csizmadia, I.

In: Journal of Physical Chemistry A, Vol. 106, No. 42, 24.10.2002, p. 9695-9704.

Research output: Contribution to journalArticle

Chass, Gregory A. ; Marai, Christopher N J ; Harrison, Alex G. ; Csizmadia, I. / Fragmentation reactions of a2 ions derived from deprotonated dipeptides-A synergy between experiment and theory. In: Journal of Physical Chemistry A. 2002 ; Vol. 106, No. 42. pp. 9695-9704.
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AU - Marai, Christopher N J

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N2 - The fragmentation reactions of a number of a2 ions ([M-H-CO2]-) derived from dipeptides have been studied by energy-resolved mass spectrometry, isotopic labeling, and MS3 experiments. The general reaction sequence H2N-CH(R1)-C(=O)-N(R2)-CHR3 - → -HN-CH(R1)-C(=O) -N(R2)-CH2R3 (1) -HN-CH(R1)-C(=O)-N(R2)-CH2 R3 → R3CH2-N(R2) - C=O- + HN=CHR1 (2) R3CH2 -N(R2)-C=O- →R3CH2 -N(R2)- + CO (3) leading eventually to a deprotonated amine, is shown to occur, a reaction sequence first proposed by Styles and O'Hair (Rapid Commun. Mass Spectrom. 1998, 12, 809) from a study of the a2 ion derived from glycylglycine. When an amidic hydrogen (R2 = H) is present, the initial proton-transfer reaction 1 is nonreversible. However, when there is no amidic hydrogen, as in the a2 ions derived from H-Ala-Pro-OH or H-Gly-Sar-OH, the initial proton-transfer reaction 1 becomes reversible, leading to the interchange of N-bonded and C-bonded hydrogens. Ab initio calculations at the MP2/6-31+G(d) level of the energies and interconversion pathways of anions derived by deprotonation of glycine N-methylamide show a barrier of 8 kcal mol-1 for reaction 1, with reaction 2 being 23.8 kcal mol-1 endothermic. When an amidic hydrogen (R2 = H) is present, the amine-deprotonated species formed in reaction 1 abstracts a proton from the amide nitrogen to form the amide-deprotonated species, the most stable species on the potential energy surface. The system effectively becomes trapped in this low-energy well and exits upon activation by reactions 2 and 3 as observed when glycine N-methylamide is deprotonated directly. When no amidic hydrogen is present, this low-energy state does not exist, and reaction 1 becomes reversible, leading to the interchange of N-bonded and C-bonded hydrogens. In these cases, a significant population of the original a2 ion is formed, which fragments by the reaction H2N-CH(R1)-C(=O) -N(R2)-CHR3 -→ H2N-CH(R1)-C=O- + R3 CH=NR2 (4).

AB - The fragmentation reactions of a number of a2 ions ([M-H-CO2]-) derived from dipeptides have been studied by energy-resolved mass spectrometry, isotopic labeling, and MS3 experiments. The general reaction sequence H2N-CH(R1)-C(=O)-N(R2)-CHR3 - → -HN-CH(R1)-C(=O) -N(R2)-CH2R3 (1) -HN-CH(R1)-C(=O)-N(R2)-CH2 R3 → R3CH2-N(R2) - C=O- + HN=CHR1 (2) R3CH2 -N(R2)-C=O- →R3CH2 -N(R2)- + CO (3) leading eventually to a deprotonated amine, is shown to occur, a reaction sequence first proposed by Styles and O'Hair (Rapid Commun. Mass Spectrom. 1998, 12, 809) from a study of the a2 ion derived from glycylglycine. When an amidic hydrogen (R2 = H) is present, the initial proton-transfer reaction 1 is nonreversible. However, when there is no amidic hydrogen, as in the a2 ions derived from H-Ala-Pro-OH or H-Gly-Sar-OH, the initial proton-transfer reaction 1 becomes reversible, leading to the interchange of N-bonded and C-bonded hydrogens. Ab initio calculations at the MP2/6-31+G(d) level of the energies and interconversion pathways of anions derived by deprotonation of glycine N-methylamide show a barrier of 8 kcal mol-1 for reaction 1, with reaction 2 being 23.8 kcal mol-1 endothermic. When an amidic hydrogen (R2 = H) is present, the amine-deprotonated species formed in reaction 1 abstracts a proton from the amide nitrogen to form the amide-deprotonated species, the most stable species on the potential energy surface. The system effectively becomes trapped in this low-energy well and exits upon activation by reactions 2 and 3 as observed when glycine N-methylamide is deprotonated directly. When no amidic hydrogen is present, this low-energy state does not exist, and reaction 1 becomes reversible, leading to the interchange of N-bonded and C-bonded hydrogens. In these cases, a significant population of the original a2 ion is formed, which fragments by the reaction H2N-CH(R1)-C(=O) -N(R2)-CHR3 -→ H2N-CH(R1)-C=O- + R3 CH=NR2 (4).

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