1
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Schwob L, Dörner S, Atak K, Schubert K, Timm M, Bülow C, Zamudio-Bayer V, von Issendorff B, Lau JT, Techert S, Bari S. Site-Selective Dissociation upon Sulfur L-Edge X-ray Absorption in a Gas-Phase Protonated Peptide. J Phys Chem Lett 2020; 11:1215-1221. [PMID: 31978303 DOI: 10.1021/acs.jpclett.0c00041] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Site-selective dissociation induced by core photoexcitation of biomolecules is of key importance for the understanding of radiation damage processes and dynamics and for its promising use as "chemical scissors" in various applications. However, identifying products of site-selective dissociation in large molecules is challenging at the carbon, nitrogen, and oxygen edges because of the high recurrence of these atoms and related chemical groups. In this paper, we present the observation of site-selective dissociation at the sulfur L-edge in the gas-phase peptide methionine enkephalin, which contains only a single sulfur atom. Near-edge X-ray absorption mass spectrometry has revealed that the resonant S 2p → σ*C-S excitation of the sulfur contained in the methionine side chain leads to site-selective dissociation, which is not the case after core ionization above the sulfur L-edge. The prospects of such results for the study of charge dynamics in biomolecular systems are discussed.
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Affiliation(s)
- Lucas Schwob
- Deutsches Elektronen-Synchrotron DESY , Notkestrasse 85 , 22607 Hamburg , Germany
| | - Simon Dörner
- Deutsches Elektronen-Synchrotron DESY , Notkestrasse 85 , 22607 Hamburg , Germany
| | - Kaan Atak
- Deutsches Elektronen-Synchrotron DESY , Notkestrasse 85 , 22607 Hamburg , Germany
| | - Kaja Schubert
- Deutsches Elektronen-Synchrotron DESY , Notkestrasse 85 , 22607 Hamburg , Germany
| | - Martin Timm
- Abteilung für Hochempfindliche Röntgenspektroskopie , Helmholtz Zentrum Berlin für Materialien und Energie , Albert-Einstein-Strasse 15 , 12489 Berlin , Germany
| | - Christine Bülow
- Abteilung für Hochempfindliche Röntgenspektroskopie , Helmholtz Zentrum Berlin für Materialien und Energie , Albert-Einstein-Strasse 15 , 12489 Berlin , Germany
| | - Vicente Zamudio-Bayer
- Abteilung für Hochempfindliche Röntgenspektroskopie , Helmholtz Zentrum Berlin für Materialien und Energie , Albert-Einstein-Strasse 15 , 12489 Berlin , Germany
| | - Bernd von Issendorff
- Physikalisches Institut , Universität Freiburg , Hermann-Herder-Straße 3 , 79104 Freiburg , Germany
| | - J Tobias Lau
- Abteilung für Hochempfindliche Röntgenspektroskopie , Helmholtz Zentrum Berlin für Materialien und Energie , Albert-Einstein-Strasse 15 , 12489 Berlin , Germany
- Physikalisches Institut , Universität Freiburg , Hermann-Herder-Straße 3 , 79104 Freiburg , Germany
| | - Simone Techert
- Deutsches Elektronen-Synchrotron DESY , Notkestrasse 85 , 22607 Hamburg , Germany
- Institute of X-ray Physics , University of Göttingen , Friedrich-Hund-Platz 1 , 37077 Göttingen , Germany
| | - Sadia Bari
- Deutsches Elektronen-Synchrotron DESY , Notkestrasse 85 , 22607 Hamburg , Germany
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2
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Kempkes LJ, Martens J, Berden G, Houthuijs KJ, Oomens J. Investigation of the position of the radical in z3-ions resulting from electron transfer dissociation using infrared ion spectroscopy. Faraday Discuss 2019; 217:434-452. [DOI: 10.1039/c8fd00202a] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The molecular structures of six open-shell z3-ions resulting from electron transfer dissociation mass spectrometry (ETD MS) were investigated using infrared ion spectroscopy in combination with density functional theory and molecular mechanics/molecular dynamics calculations.
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Affiliation(s)
| | - Jonathan Martens
- Radboud University
- FELIX Laboratory
- 6525 ED Nijmegen
- The Netherlands
| | - Giel Berden
- Radboud University
- FELIX Laboratory
- 6525 ED Nijmegen
- The Netherlands
| | - Kas J. Houthuijs
- Radboud University
- FELIX Laboratory
- 6525 ED Nijmegen
- The Netherlands
| | - Jos Oomens
- Radboud University
- FELIX Laboratory
- 6525 ED Nijmegen
- The Netherlands
- Van’t Hoff Institute for Molecular Sciences
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3
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Chen X, Wang Z, Wong YLE, Wu R, Zhang F, Chan TWD. Electron-ion reaction-based dissociation: A powerful ion activation method for the elucidation of natural product structures. MASS SPECTROMETRY REVIEWS 2018; 37:793-810. [PMID: 29603345 DOI: 10.1002/mas.21563] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/27/2017] [Accepted: 02/12/2018] [Indexed: 05/16/2023]
Abstract
The structural elucidation of natural products (NPs) remains a challenge due to their structurally diversities and unpredictable functionalities, motifs, and scaffolds. Tandem mass spectrometry (MS/MS) is an effective method that assists the full elucidation of complicated NP structures. Ion activation methods play a key role in determining the fragmentation pathways and the structural information obtained from MS/MS. Electron-ion reaction-based dissociation (ExD) methods, including electron capture dissociation (ECD), electron transfer dissociation (ETD), electron-induced dissociation (EID), and electron detachment dissociation (EDD), can induce the breakage of specific chemical bonds and the generation of distinct fragment ions. This review article provides an overview of the mechanisms, instrumentation, and typical applications related to ExD MS/MS in the structural elucidation of NPs, primarly including lipids, oligosaccharides, glycoconjugates, metabolites, and pharmaceutical drugs. This work aims to reveal the capacity and potential of ExD mass spectrometry in analyzing NPs and consequently helping the NP communities to utilize the modern capabilities of MS/MS in the discovery and evaluation of novel NPs.
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Affiliation(s)
- Xiangfeng Chen
- Key Laboratory for Applied Technology of Sophisticated Analytical Instruments, Shandong Analysis and Test Centre, Qilu University of Technology (Shandong Academy of Science), Shandong, P.R. China
- Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, P.R. China
| | - Ze Wang
- Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, P.R. China
| | - Y-L Elaine Wong
- Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, P.R. China
| | - Ri Wu
- Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, P.R. China
| | - Feng Zhang
- Chinese Academy of Inspection and Quarantine, Beijing, P. R. China
| | - T-W Dominic Chan
- Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, P.R. China
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4
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Imaoka N, Houferak C, Murphy MP, Nguyen HTH, Dang A, Tureček F. Spontaneous Isomerization of Peptide Cation Radicals Following Electron Transfer Dissociation Revealed by UV-Vis Photodissociation Action Spectroscopy. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2018; 29:1768-1780. [PMID: 29340957 DOI: 10.1007/s13361-017-1871-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/19/2017] [Revised: 12/06/2017] [Accepted: 12/14/2017] [Indexed: 06/07/2023]
Abstract
Peptide cation radicals of the z-type were produced by electron transfer dissociation (ETD) of peptide dications and studied by UV-Vis photodissociation (UVPD) action spectroscopy. Cation radicals containing the Asp (D), Asn (N), Glu (E), and Gln (Q) residues were found to spontaneously isomerize by hydrogen atom migrations upon ETD. Canonical N-terminal [z4 + H]+● fragment ion-radicals of the R-C●H-CONH- type, initially formed by N-Cα bond cleavage, were found to be minor components of the stable ion fraction. Vibronically broadened UV-Vis absorption spectra were calculated by time-dependent density functional theory for several [●DAAR + H]+ isomers and used to assign structures to the action spectra. The potential energy surface of [●DAAR + H]+ isomers was mapped by ab initio and density functional theory calculations that revealed multiple isomerization pathways by hydrogen atom migrations. The transition-state energies for the isomerizations were found to be lower than the dissociation thresholds, accounting for the isomerization in non-dissociating ions. The facile isomerization in [●XAAR + H]+ ions (X = D, N, E, and Q) was attributed to low-energy intermediates having the radical defect in the side chain that can promote hydrogen migration along backbone Cα positions. A similar side-chain mediated mechanism is suggested for the facile intermolecular hydrogen migration between the c- and [z + H]●-ETD fragments containing Asp, Asn, Glu, and Gln residues. Graphical Abstract ᅟ.
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Affiliation(s)
- Naruaki Imaoka
- Department of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan
| | - Camille Houferak
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA, 98195-1700, USA
| | - Megan P Murphy
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA, 98195-1700, USA
| | - Huong T H Nguyen
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA, 98195-1700, USA
| | - Andy Dang
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA, 98195-1700, USA
| | - František Tureček
- Department of Chemistry, University of Washington, Box 351700, Seattle, WA, 98195-1700, USA.
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5
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Galano A, Reiter RJ. Melatonin and its metabolites vs oxidative stress: From individual actions to collective protection. J Pineal Res 2018; 65:e12514. [PMID: 29888508 DOI: 10.1111/jpi.12514] [Citation(s) in RCA: 197] [Impact Index Per Article: 32.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/08/2018] [Accepted: 06/05/2018] [Indexed: 12/11/2022]
Abstract
Oxidative stress (OS) represents a threat to the chemical integrity of biomolecules including lipids, proteins, and DNA. The associated molecular damage frequently results in serious health issues, which justifies our concern about this phenomenon. In addition to enzymatic defense mechanisms, there are compounds (usually referred to as antioxidants) that offer chemical protection against oxidative events. Among them, melatonin and its metabolites constitute a particularly efficient chemical family. They offer protection against OS as individual chemical entities through a wide variety of mechanisms including electron transfer, hydrogen transfer, radical adduct formation, and metal chelation, and by repairing biological targets. In fact, many of them including melatonin can be classified as multipurpose antioxidants. However, what seems to be unique to the melatonin's family is their collective effects. Because the members of this family are metabolically related, most of them are expected to be present in living organisms wherever melatonin is produced. Therefore, the protection exerted by melatonin against OS may be viewed as a result of the combined antioxidant effects of the parent molecule and its metabolites. Melatonin's family is rather exceptional in this regard, offering versatile and collective antioxidant protection against OS. It certainly seems that melatonin is one of the best nature's defenses against oxidative damage.
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Affiliation(s)
- Annia Galano
- Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, México City, México
| | - Russel J Reiter
- Department of Cellular and Structural Biology, UT Health Science Center, San Antonio, TX, USA
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6
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Nagoshi K, Yamakoshi M, Sakamoto K, Takayama M. Specific Cα-C Bond Cleavage of β-Carbon-Centered Radical Peptides Produced by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2018; 29:1473-1483. [PMID: 29675742 DOI: 10.1007/s13361-018-1958-2] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/09/2018] [Revised: 03/14/2018] [Accepted: 03/29/2018] [Indexed: 06/08/2023]
Abstract
Radical-driven dissociation (RDD) of hydrogen-deficient peptide ions [M - H + H]·+ has been examined using matrix-assisted laser dissociation/ionization in-source decay mass spectrometry (MALDI-ISD MS) with the hydrogen-abstracting matrices 4-nitro-1-naphthol (4,1-NNL) and 5-nitrosalicylic acid (5-NSA). The preferential fragment ions observed in the ISD spectra include N-terminal [a] + ions and C-terminal [x]+, [y + 2]+, and [w]+ ions which imply that β-carbon (Cβ)-centered radical peptide ions [M - Hβ + H]·+ are predominantly produced in MALDI conditions. RDD reactions from the peptide ions [M - Hβ + H]·+ successfully explains the fact that both [a]+ and [x]+ ions arising from cleavage at the Cα-C bond of the backbone of Gly-Xxx residues are missing from the ISD spectra. Furthermore, the formation of [a]+ ions originating from the cleavage of Cα-C bond of deuterated Ala(d3)-Xxx residues indicates that the [a]+ ions are produced from the peptide ions [M - Hβ + H]·+ generated by deuteron-abstraction from Ala(d3) residues. It is suggested that from the standpoint of hydrogen abstraction via direct interactions between the nitro group of matrix and hydrogen of peptides, the generation of the peptide radical ions [M - Hβ + H]·+ is more favorable than that of the α-carbon (Cα)-centered radical ions [M - Hα + H]·+ and the amide nitrogen-centered radical ions [M - HN + H]·+, while ab initio calculations indicate that the formation of [M - Hα + H]·+ is energetically most favorable. Graphical Abstract ᅟ.
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Affiliation(s)
- Keishiro Nagoshi
- Mass Spectrometry Laboratory, Graduate School in Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, 236-0027, Japan
| | - Mariko Yamakoshi
- Mass Spectrometry Laboratory, Graduate School in Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, 236-0027, Japan
| | - Kenya Sakamoto
- Mass Spectrometry Laboratory, Graduate School in Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, 236-0027, Japan
| | - Mitsuo Takayama
- Mass Spectrometry Laboratory, Graduate School in Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama, 236-0027, Japan.
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7
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Nguyen HTH, Tureček F. Near-UV Photodissociation of Tryptic Peptide Cation Radicals. Scope and Effects of Amino Acid Residues and Radical Sites. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2017; 28:1333-1344. [PMID: 28155086 DOI: 10.1007/s13361-016-1586-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/23/2016] [Revised: 12/20/2016] [Accepted: 12/21/2016] [Indexed: 06/06/2023]
Abstract
Peptide cation-radical fragment ions of the z-type, [●AXAR+], [●AXAK+], and [●XAR+], where X = A, C, D, E, F, G, H, K, L, M, N, P, Y, and W, were generated by electron transfer dissociation of peptide dications and investigated by MS3-near-ultraviolet photodissociation (UVPD) at 355 nm. Laser-pulse dependence measurements indicated that the ion populations were homogeneous for most X residues except phenylalanine. UVPD resulted in dissociations of backbone CO─NH bonds that were accompanied by hydrogen atom transfer, producing fragment ions of the [yn]+ type. Compared with collision-induced dissociation, UVPD yielded less side-chain dissociations even for residues that are sensitive to radical-induced side-chain bond cleavages. The backbone dissociations are triggered by transitions to second (B) excited electronic states in the peptide ion R-CH●-CONH- chromophores that are resonant with the 355-nm photon energy. Electron promotion increases the polarity of the B excited states, R-CH+-C●(O-)NH-, and steers the reaction to proceed by transfer of protons from proximate acidic Cα and amide nitrogen positions. Graphical Abstract ᅟ.
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Affiliation(s)
- Huong T H Nguyen
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA
| | - František Tureček
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA.
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8
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Nguyen HTH, Andrikopoulos PC, Bím D, Rulíšek L, Dang A, Tureček F. Radical Reactions Affecting Polar Groups in Threonine Peptide Ions. J Phys Chem B 2017; 121:6557-6569. [DOI: 10.1021/acs.jpcb.7b04661] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- Huong T. H. Nguyen
- Department
of Chemistry, University of Washington, Bagley Hall, Box
351700, Seattle, Washington 98195-1700, United States
| | - Prokopis C. Andrikopoulos
- Institute
of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Flemingovo nám 2, 16610 Prague, Czech Republic
| | - Daniel Bím
- Institute
of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Flemingovo nám 2, 16610 Prague, Czech Republic
| | - Lubomír Rulíšek
- Institute
of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Flemingovo nám 2, 16610 Prague, Czech Republic
| | - Andy Dang
- Department
of Chemistry, University of Washington, Bagley Hall, Box
351700, Seattle, Washington 98195-1700, United States
| | - František Tureček
- Department
of Chemistry, University of Washington, Bagley Hall, Box
351700, Seattle, Washington 98195-1700, United States
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9
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Qi Y, Volmer DA. Electron-based fragmentation methods in mass spectrometry: An overview. MASS SPECTROMETRY REVIEWS 2017; 36:4-15. [PMID: 26445267 DOI: 10.1002/mas.21482] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/10/2015] [Revised: 07/03/2015] [Accepted: 07/03/2015] [Indexed: 05/21/2023]
Abstract
Tandem mass spectrometry (MS/MS) provides detailed information for structural characterization of biomolecules. The combination of electron capture dissociation (ECD) techniques with Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) often provides unique ion-electron reactions and fragmentation channels in MS/MS. ECD is often a complimentary, sometimes even a superior tool to conventional MS/MS techniques. This article is aimed at providing a short overview of ECD-based fragmentation techniques (ExD) and optimization of ECD experiments for FTICR mass analyzers. Most importantly, it is meant to pique the interest of potential users for this exciting research field. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:4-15, 2017.
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Affiliation(s)
- Yulin Qi
- Institute of Bioanalytical Chemistry, Saarland University, Campus B2.2, Saarbrücken, 66123, Germany
| | - Dietrich A Volmer
- Institute of Bioanalytical Chemistry, Saarland University, Campus B2.2, Saarbrücken, 66123, Germany
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10
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Schwob L, Lalande M, Egorov D, Rangama J, Hoekstra R, Vizcaino V, Schlathölter T, Poully JC. Radical-driven processes within a peptidic sequence of type I collagen upon single-photon ionisation in the gas phase. Phys Chem Chem Phys 2017; 19:22895-22904. [DOI: 10.1039/c7cp03376a] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Radical creation after single-photon ionisation of collagen peptides induces the loss of molecules from amino-acid residue side-chains.
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Affiliation(s)
- Lucas Schwob
- CIMAP
- UMP 6252 (CEA/CNRS/ENSICAEN/Université de Caen Normandie)
- Caen
- France
| | - Mathieu Lalande
- CIMAP
- UMP 6252 (CEA/CNRS/ENSICAEN/Université de Caen Normandie)
- Caen
- France
| | - Dmitrii Egorov
- Zernike Institute for Advanced Materials
- University of Groningen
- 9747AG Groningen
- The Netherlands
| | - Jimmy Rangama
- CIMAP
- UMP 6252 (CEA/CNRS/ENSICAEN/Université de Caen Normandie)
- Caen
- France
| | - Ronnie Hoekstra
- Zernike Institute for Advanced Materials
- University of Groningen
- 9747AG Groningen
- The Netherlands
| | - Violaine Vizcaino
- CIMAP
- UMP 6252 (CEA/CNRS/ENSICAEN/Université de Caen Normandie)
- Caen
- France
| | - Thomas Schlathölter
- Zernike Institute for Advanced Materials
- University of Groningen
- 9747AG Groningen
- The Netherlands
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11
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Pepin R, Layton ED, Liu Y, Afonso C, Tureček F. Where Does the Electron Go? Stable and Metastable Peptide Cation Radicals Formed by Electron Transfer. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2017; 28:164-181. [PMID: 27709510 DOI: 10.1007/s13361-016-1512-z] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/08/2016] [Revised: 09/12/2016] [Accepted: 09/15/2016] [Indexed: 06/06/2023]
Abstract
Electron transfer to doubly and triply charged heptapeptide ions containing polar residues Arg, Lys, and Asp in combination with nonpolar Gly, Ala, and Pro or Leu generates stable and metastable charge-reduced ions, (M + 2H)+●, in addition to standard electron-transfer dissociation (ETD) fragment ions. The metastable (M + 2H)+● ions spontaneously dissociate upon resonant ejection from the linear ion trap, giving irregularly shaped peaks with offset m/z values. The fractions of stable and metastable (M + 2H)+● ions and their mass shifts depend on the presence of Pro-4 and Leu-4 residues in the peptides, with the Pro-4 sequences giving larger fractions of the stable ions while showing smaller mass shifts for the metastables. Conversion of the Asp and C-terminal carboxyl groups to methyl esters further lowers the charge-reduced ion stability. Collisional activation and photodissociation at 355 nm of mass-selected (M + 2H)+● results in different dissociations that give sequence specific MS3 spectra. With a single exception of charge-reduced (LKGLADR + 2H)+●, the MS3 spectra do not produce ETD sequence fragments of the c and z type. Hence, these (M + 2H)+● ions are covalent radicals, not ion-molecule complexes, undergoing dramatically different dissociations in the ground and excited electronic states. The increased stability of the Pro-4 containing (M + 2H)+● ions is attributed to radicals formed by opening of the Pro ring and undergoing further stabilization by hydrogen atom migrations. UV-VIS photodissociation action spectroscopy and time-dependent density functional theory calculations are used in a case in point study of the stable (LKGPADR + 2H)+● ion produced by ETD. In contrast to singly-reduced peptide ions, doubly reduced (M + 3H)+ ions are stable only when formed from the Pro-4 precursors and show all characteristics of even electron ions regarding no photon absorption at 355 nm or ion-molecule reactions, and exhibiting proton driven collision induced dissociations. Graphical Abstract ᅟ.
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Affiliation(s)
- Robert Pepin
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA
| | - Erik D Layton
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA
| | - Yang Liu
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA
| | - Carlos Afonso
- INSA Rouen, UNIROUEN, CNRS, COBRA, Normandie University, 76000, Rouen, France
| | - František Tureček
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA.
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12
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Commodore JJ, Cassady CJ. The Effects of Trivalent Lanthanide Cationization on the Electron Transfer Dissociation of Acidic Fibrinopeptide B and its Analogs. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2016; 27:1499-509. [PMID: 27294379 PMCID: PMC4974135 DOI: 10.1007/s13361-016-1428-7] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2016] [Revised: 05/22/2016] [Accepted: 05/23/2016] [Indexed: 05/08/2023]
Abstract
Electrospray ionization (ESI) on mixtures of acidic fibrinopeptide B and two peptide analogs with trivalent lanthanide salts generates [M + Met + H](4+), [M + Met](3+), and [M + Met -H](2+), where M = peptide and Met = metal (except radioactive promethium). These ions undergo extensive and highly efficient electron transfer dissociation (ETD) to form metallated and non-metallated c- and z-ions. All metal adducted product ions contain at least two acidic sites, which suggest attachment of the lanthanide cation at the side chains of one or more acidic residues. The three peptides undergo similar fragmentation. ETD on [M + Met + H](4+) leads to cleavage at every residue; the presence of both a metal ion and an extra proton is very effective in promoting sequence-informative fragmentation. Backbone dissociation of [M + Met](3+) is also extensive, although cleavage does not always occur between adjacent glutamic acid residues. For [M + Met - H ](2+), a more limited range of product ions form. All lanthanide metal peptide complexes display similar fragmentation except for europium (Eu). ETD on [M + Eu - H](2+) and [M + Eu](3+) yields a limited amount of peptide backbone cleavage; however, [M + Eu + H](4+) dissociates extensively with cleavage at every residue. With the exception of the results for Eu(III), metallated peptide ion formation by ESI, ETD fragmentation efficiencies, and product ion formation are unaffected by the identity of the lanthanide cation. Adduction with trivalent lanthanide metal ions is a promising tool for sequence analysis of acidic peptides by ETD. Graphical Abstract ᅟ.
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Affiliation(s)
| | - Carolyn J Cassady
- Department of Chemistry, The University of Alabama, Tuscaloosa, AL, 35487, USA.
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13
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Martens J, Grzetic J, Berden G, Oomens J. Structural identification of electron transfer dissociation products in mass spectrometry using infrared ion spectroscopy. Nat Commun 2016; 7:11754. [PMID: 27277826 PMCID: PMC4906228 DOI: 10.1038/ncomms11754] [Citation(s) in RCA: 71] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2015] [Accepted: 04/27/2016] [Indexed: 12/04/2022] Open
Abstract
Tandem mass spectrometry occupies a principle place among modern analytical methods and drives many developments in the ‘omics' sciences. Electron attachment induced dissociation methods, as alternatives for collision-induced dissociation have profoundly influenced the field of proteomics, enabling among others the top-down sequencing of entire proteins and the analysis of post-translational modifications. The technique, however, produces more complex mass spectra and its radical-driven reaction mechanisms remain incompletely understood. Here we demonstrate the facile structural characterization of electron transfer dissociation generated peptide fragments by infrared ion spectroscopy using the tunable free-electron laser FELIX, aiding the elucidation of the underlying dissociation mechanisms. We apply this method to verify and revise previously proposed product ion structures for an often studied model tryptic peptide, [AlaAlaHisAlaArg+2H]2+. Comparing experiment with theory reveals that structures that would be assigned using only theoretical thermodynamic considerations often do not correspond to the experimentally sampled species. Mass spectrometry is a leading method used for sequencing peptides and proteins by fragmentation followed by analysis of the sequence fragments. Here, the authors use infrared spectroscopy to characterize the structures of peptide fragments formed during electron transfer dissociation.
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Affiliation(s)
- Jonathan Martens
- Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The Netherlands
| | - Josipa Grzetic
- Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The Netherlands
| | - Giel Berden
- Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The Netherlands
| | - Jos Oomens
- Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The Netherlands.,Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 908, 1098XH Amsterdam, The Netherlands
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14
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Shaffer CJ, Marek A, Nguyen HTH, Tureček F. Combining Near-UV Photodissociation with Electron Transfer. Reduction of the Diazirine Ring in a Photomethionine-Labeled Peptide Ion. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2015; 26:1367-1381. [PMID: 25904063 DOI: 10.1007/s13361-015-1139-5] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/26/2015] [Revised: 02/26/2015] [Accepted: 03/12/2015] [Indexed: 06/04/2023]
Abstract
Electron transfer dissociation of peptide ions with the diazirine-containing residue photomethionine (M*) results in side-chain dissociations by loss of C3H7N2 radicals in addition to standard backbone cleavages. The side-chain dissociations are particularly prominent upon activation of long-lived, charge-reduced, cation radicals (GM*GGR + 2H)(+•). Investigation of these cation radicals by near-UV photodissociation and collisional activation revealed different fragmentation products and mechanisms resulting from these ion activation modes. The dissociations observed for photomethionine were dramatically different from those previously reported for the lower homologue photoleucine; here, a difference by a single methylene group in the side chain had a large effect on the chemistries of the cation radicals upon ETD and further activation. ETD intermediates and products were probed by tandem 355-nm UV photodissociation-collision induced dissociation and found to contain chromophores that resulted from electron attachment to the diazirine ring. The nature of the newly formed chromophores and ion energetics and kinetics were investigated by electron structure calculations combining ab initio and density functional theory methods and Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The dramatic difference between the dissociations of L* and M* containing peptide cation radicals is explained by electronic effects that play a role in stabilizing critical reaction intermediates and steer the dissociations into kinetically favored reaction channels. In addition, a new alternating UVPD-ETD-UVPD MS(4) experiment is introduced and utilized for ion structure elucidation.
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Affiliation(s)
- Christopher J Shaffer
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, 98195-1700, USA
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15
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Nguyen HTH, Shaffer CJ, Ledvina AR, Coon JJ, Tureček F. Serine effects on collision-induced dissociation and photodissociation of peptide cation radicals of the z+• -type. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY 2015; 378:20-30. [PMID: 26005367 PMCID: PMC4437545 DOI: 10.1016/j.ijms.2014.06.028] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
The serine residue displays specific effects on the dissociations of peptide fragment cation-radicals of the z+• type which are produced by electron transfer dissociation. Energy-resolved collision-induced dissociation (ER-CID), time-resolved infrared multiphoton dissociation (TR-IRMPD), and single-photon UV photodissociation at 355 nm revealed several competitive dissociation pathways consisting of loss of OH radical, water, and backbone cleavages occurring at N-terminal and C-terminal positions relative to the serine residue. The activation modes using slow-heating and UV photon absorption resulted in different relative intensities of fragment ions. This indicated that the dissociations proceeded through several channels with different energy-dependent kinetics. The experimental data were interpreted with the help of electron structure calculations that provided fully optimized structures and relative energies for cis and trans amide isomers of the z4+• ions as well as isomerization, dissociation, and transition state energies. UV photon absorption by the z4+• ions was due to Cα-radical amide groups created by ETD that provided a new chromophore absorbing at 355 nm.
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Affiliation(s)
- Huong T H Nguyen
- Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, WA, 98195-1700
| | - Christopher J Shaffer
- Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, WA, 98195-1700
| | - Aaron R Ledvina
- Department of Chemistry, University of Wisconsin, Madison, WI, USA
| | - Joshua J Coon
- Department of Chemistry, University of Wisconsin, Madison, WI, USA ; Department of Biomolecular Chemistry, University of Wisconsin, Madison, WI, USA
| | - František Tureček
- Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, WA, 98195-1700
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16
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Ledvina AR, Coon JJ, Tureček F. Competitive Hydrogen Atom Migrations Accompanying Cascade Dissociations of Peptide Cation-Radicals of the z+• Type. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY 2015; 377:44-53. [PMID: 25844055 PMCID: PMC4380023 DOI: 10.1016/j.ijms.2014.02.015] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
We report a combined experimental and computational study of energy-resolved collision-induced dissociation (ER-CID) and time-resolved infrared multiphoton dissociation (TR-IRMPD) of z4 ions prepared by electron transfer dissociation of peptide (Ala-Ala-Asn-Ala-Arg + 2H)2+ ions. The z4 cation-radicals, •ANAR+, undergo competitive dissociations by backbone cleavage and loss of a CONH2 radical from the Asn side chain. The backbone cleavage proceeds by radical-assisted dissociation of the Asn Cα-CO bond, forming an x2 ion intermediate which rapidly dissociates by HNCO elimination to yield a stable z2 fragment ion, •AR+. The ER-CID and TR-IRMPD data were consistent with the consecutive nature of the backbone dissociation but showed different branching ratios for the two major fragmentations. The ER-CID data showed branching ratios 0.6-1.0 for the side-chain and backbone cleavages whereas the TR-IRMPD data showed an earlier onset for the latter dissociation. Computational analysis of the potential energy surface with density functional theory and ab initio calculations was carried out to provide structures and energies for the reactant ions as well as several intermediates, products, and transition states. Dissociation pathways for cis and trans amide conformers were distinguished and their energies were evaluated. The threshold dissociation energies for the backbone and side-chain dissociations were similar in accordance with the experimental ER-CID branching ratio. The TR-IRMPD data were interpreted by different absorbances of intermediates produced by hydrogen atom migrations along the dissociation pathways.
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Affiliation(s)
- Aaron R Ledvina
- Department of Chemistry, University of Wisconsin, Madison, WI, USA
| | - Joshua J Coon
- Department of Chemistry, University of Wisconsin, Madison, WI, USA ; Department of Biomolecular Chemistry, University of Wisconsin, Madison, WI, USA
| | - František Tureček
- Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, WA 98195-1700, USA
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17
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Bythell BJ. Cα hydrogen atom transfer in post-cleavage radical-cation complexes: short and steep versus long winding road. J Phys Chem A 2014; 118:10797-803. [PMID: 25329622 DOI: 10.1021/jp507865h] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
Recently, I explored structurally straightforward pathways to Cα hydrogen atom, H(•), transfer reactions in the radical cation complex following electron capture/transfer of a series of polyprotonated peptides (J. Phys. Chem. A 2013, 117, 1189-1196). Here, I extend my analysis to incorporate detailed rearrangement processes potentially occurring prior to H(•) transfer. This comprises intracomplex isomerization of the initial iminol-terminated (-C(OH)═NH) form of the cn' species to the energetically more favorable, amide-terminated form (-C(O)-NH2) prior to Cα H(•) abstraction by the zm(•) species. The data indicate that the previously published H(•) transfer barriers are more energetically demanding than those of this multistep alternative. The rate-determining step is typically the intracomplex iminol isomerization, consistent with the substantial energetic favorability of the amide form of the cn species. The barriers to H(•) transfer still rise steeply as a function of the charge state. In agreement with experiment, evidence for product separation without H(•) transfer at a higher charge state is also provided.
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Affiliation(s)
- Benjamin J Bythell
- Department of Chemistry and Biochemistry, University of Missouri-St. Louis , St. Louis, Missouri 63121, United States
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18
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Bogdanov B, Zhao X, Robinson DB, Ren J. Electron capture dissociation studies of the fragmentation patterns of doubly protonated and mixed protonated-sodiated peptoids. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2014; 25:1202-1216. [PMID: 24845348 DOI: 10.1007/s13361-014-0869-0] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/21/2013] [Revised: 02/09/2014] [Accepted: 02/14/2014] [Indexed: 06/03/2023]
Abstract
The fragmentation patterns of a group of doubly protonated ([P + 2H](2+)) and mixed protonated-sodiated ([P + H + Na](2+)) peptide-mimicking oligomers, known as peptoids, have been studied using electron capturing dissociation (ECD) tandem mass spectrometry techniques. For all the peptoids studied, the primary backbone fragmentation occurred at the N-Cα bonds. The N-terminal fragment ions, the C-ions (protonated) and the C'-ions (sodiated) were observed universally for all the peptoids regardless of the types of charge carrier. The C-terminal ions varied depending on the type of charge carrier. The doubly protonated peptoids with at least one basic residue located at a position other than the N-terminus fragmented by producing the Z(•)-series of ions. In addition, most doubly protonated peptoids also produced the Y-series of ions with notable abundances. The mixed protonated-sodiated peptoids fragmented by yielding the Z(•)'-series of ions in addition to the C'-series. Chelation between the sodium cation and the amide groups of the peptoid chain might be an important factor that could stabilize both the N-terminal and the C-terminal fragment ions. Regardless of the types of the charge carrier, one notable fragmentation for all the peptoids was the elimination of a benzylic radical from the odd-electron positive ions of the protonated peptoids ([P + 2H](•+)) and the sodiated peptoids ([P + H + Na](•+)). The study showed potential utility of using the ECD technique for sequencing of peptoid libraries generated by combinatorial chemistry.
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Affiliation(s)
- Bogdan Bogdanov
- Department of Chemistry, University of the Pacific, Stockton, CA, 95211, USA
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19
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Mädler S, Kai-Chi Lau J, Williams D, Wang Y, Saminathan IS, Zhao J, Siu KWM, Hopkinson AC. Fragmentation of Peptide Radical Cations Containing a Tyrosine or Tryptophan Residue: Structural Features That Favor Formation of [x(n–1) + H]•+ and [z(n–1) + H]•+ Ions. J Phys Chem B 2014; 118:6123-33. [DOI: 10.1021/jp5030087] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Affiliation(s)
- Stefanie Mädler
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
| | - Justin Kai-Chi Lau
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
- Department
of Chemistry, University of Windsor, 401 Sunset Avenue, Windsor, Ontario Canada N9B 3P4
| | - Declan Williams
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
| | - Yating Wang
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
| | - Irine S. Saminathan
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
| | - Junfang Zhao
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
| | - K. W. Michael Siu
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
- Department
of Chemistry, University of Windsor, 401 Sunset Avenue, Windsor, Ontario Canada N9B 3P4
| | - Alan C. Hopkinson
- Department
of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario Canada M3J 1P3
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20
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Mu X, Song T, Xu M, Lai CK, Siu CK, Laskin J, Chu IK. Discovery and mechanistic studies of facile N-terminal Cα-C bond cleavages in the dissociation of tyrosine-containing peptide radical cations. J Phys Chem B 2014; 118:4273-81. [PMID: 24678922 DOI: 10.1021/jp410525f] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Fascinating N-terminal Cα-C bond cleavages in a series of nonbasic tyrosine-containing peptide radical cations have been observed under low-energy collision-induced dissociation (CID), leading to the generation of rarely observed x-type radical fragments, with significant abundances. CID experiments of the radical cations of the alanyltyrosylglycine tripeptide and its analogues suggested that the N-terminal Cα-C bond cleavage, yielding its [x2 + H](•+) radical cation, does not involve an N-terminal α-carbon-centered radical. Theoretical examination of a prototypical radical cation of the alanyltyrosine dipeptide, using density functional theory calculations, suggested that direct N-terminal Cα-C bond cleavage could produce an ion-molecule complex formed between the incipient a1(+) and x1(•) fragments. Subsequent proton transfer from the iminium nitrogen atom in a1(+) to the acyl carbon atom in x1(•) results in the observable [x1 + H](•+). The barriers against this novel Cα-C bond cleavage and the competitive N-Cα bond cleavage, forming the complementary [c1 + 2H](+)/[z1 - H](•+) ion pair, are similar (ca. 16 kcal mol(-1)). Rice-Ramsperger-Kassel-Marcus modeling revealed that [x1 + H](•+) and [c1 + 2H](+) species are formed with comparable rates, in agreement with energy-resolved CID experiments for [AY](•+).
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Affiliation(s)
- Xiaoyan Mu
- Department of Chemistry, The University of Hong Kong , Hong Kong, China
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21
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Wodrich MD, Zhurov KO, Corminboeuf C, Tsybin YO. On the viability of heterolytic peptide N-C(α) bond cleavage in electron capture and transfer dissociation mass spectrometry. J Phys Chem B 2014; 118:2985-92. [PMID: 24559292 DOI: 10.1021/jp500512a] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
While frequently employed as an experimental technique, the mechanistic picture surrounding the gas-phase dissociation of peptides carrying multiple positive charges during electron capture and electron transfer dissociation tandem mass spectrometry remains incomplete. Despite this mechanistic uncertainty, most proposals agree that the peptide backbone N-Cα bond located to the C-terminal (right) side of an aminoketyl radical formed in a peptide backbone during the electron capture process is homolytically cleaved. Recently, we introduced the "enol" mechanism, which proposes that a backbone N-Cα bond located to the N-terminal (left) side of an aminoketyl radical is cleaved heterolytically. Here, we further validate this mechanism using replica-exchange molecular dynamics to create unbiased representative sets of low-energy conformers for several model tryptic peptide systems (H-Alax-Lys-OH(2+), x = 3-5). Transition state barrier enthalpies for the cleavage of N-Cα bonds proceeding via the homolytic (right-side) and heterolytic (left-side) pathways, determined by density functional computations, identify the preferred cleavage route for each conformer. These findings support our original hypothesis that heterolytic N-Cα cleavage can exist in a competitive balance with homolytic cleavages, independent of the relative energy of the precursor dication species. Smaller peptide systems see decreased heterolytic N-Cα cleavage probabilities, likely resulting from an insufficient hydrogen-bonding network needed to stabilize and ultimately annihilate the transition state zwitterion. This observation may explain the early dismissal of left-side cleavage pathways based on computational studies employing small model systems.
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Affiliation(s)
- Matthew D Wodrich
- Biomolecular Mass Spectrometry Laboratory, Ecole Polytechnique Fédérale de Lausanne , 1015 Lausanne, Switzerland
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22
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Affiliation(s)
- František Tureček
- Department of Chemistry, Bagley Hall, University of Washington , Seattle, Washington 98195-1700, United States
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23
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Bythell BJ. To Jump or Not To Jump? Cα Hydrogen Atom Transfer in Post-cleavage Radical-Cation Complexes. J Phys Chem A 2012; 117:1189-96. [DOI: 10.1021/jp305277v] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Benjamin J. Bythell
- Ion Cyclotron Resonance
Program, National High Magnetic Field Laboratory, Florida State University,
1800 East Paul Dirac Drive, Tallahassee, Florida 32310-4005, United
States
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24
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Ledvina AR, Chung TW, Hui R, Coon JJ, Tureček F. Cascade dissociations of peptide cation-radicals. Part 2. Infrared multiphoton dissociation and mechanistic studies of z-ions from pentapeptides. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2012; 23:1351-1363. [PMID: 22669762 PMCID: PMC3619414 DOI: 10.1007/s13361-012-0409-8] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/30/2012] [Revised: 05/07/2012] [Accepted: 05/08/2012] [Indexed: 05/28/2023]
Abstract
Dissociations of z(4) ions from pentapeptides AAXAR where X=H, Y, F, W, and V produce dominant z(2) ions that account for >50 % of the fragment ion intensity. The dissociation has been studied in detail by experiment and theory and found to involve several isomerization and bond-breaking steps. Isomerizations in z(4) ions proceed by amide trans→cis rotations followed by radical-induced transfer of a β-hydrogen atom from the side chain, forming stable C(β) radical intermediates. These undergo rate-determining cleavage of the C(α)-CO bond at the X residue followed by loss of the neutral AX fragment, forming x(2) intermediates. The latter were detected by energy-resolved resonant excitation collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments. The x(2) intermediates undergo facile loss of HNCO to form z(2) fragment ions, as also confirmed by energy-resolved CID and IRMPD MS(4) experiments. The loss of HNCO from the x(2) ion from AAHWR is kinetically hampered by the Trp residue that traps the OCNH radical group in a cyclic intermediate.
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Affiliation(s)
- Aaron R Ledvina
- Department of Chemistry, University of Wisconsin, Madison, WI 53706, USA
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