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Lau JKC, Esuon F, Berden G, Oomens J, Hopkinson AC, Ryzhov V, Siu KWM. Generation, Characterization, and Dissociation of Radical Cations Derived from Prolyl-glycyl-glycine. J Phys Chem B 2021; 125:6121-6129. [PMID: 34097420 DOI: 10.1021/acs.jpcb.1c01732] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Radical cations of an aliphatic tripeptide prolyl-glycyl-glycine (PGG•+) and its sequence ions [a3 + H]•+ and [b2 - H]•+ have been generated by collision-induced dissociation of the [Cu(Phen)(PGG)]•2+ complex, where Phen = 1,10-phenanthroline. Infrared multiple photon dissociation spectroscopy, ion-molecule reaction experiments, and theoretical calculations have been used to investigate the structures of these ions. The unpaired electron in these three radical cations is located at different α-carbons. The PGG•+ radical cation has a captodative structure with the radical at the α-carbon of the proline residue and the proton on the oxygen of the first amide group. This structure is at the global minimum on the potential energy surface (PES). By contrast, the [a3 + H]•+ and [b2 - H]•+ ions are not the lowest-energy structures on their respective PESs, and their radicals are formally located at the C-terminal and second α-carbons, respectively. Density functional theory calculations on the structures of the ternary copper(II) complex ion suggest that the charge-solvated isomer of the metal complex is the precursor ion that dissociates to give the PGG•+ radical cation. The isomer of the complex in which PGG is bound as a zwitterion dissociates to give the [a3 + H]•+ and [b2 - H]•+ ions.
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Affiliation(s)
- Justin Kai-Chi Lau
- Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada.,Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, Ontario N9B 3P4, Canada
| | - Francis Esuon
- Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States
| | - Giel Berden
- FELIX Laboratory, Institute for Molecules and Materials, Radboud University, Toernooiveld 7, Nijmegen 6525 ED, the Netherlands
| | - Jos Oomens
- FELIX Laboratory, Institute for Molecules and Materials, Radboud University, Toernooiveld 7, Nijmegen 6525 ED, the Netherlands
| | - Alan C Hopkinson
- Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada
| | - Victor Ryzhov
- Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States
| | - K W Michael Siu
- Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada.,Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, Ontario N9B 3P4, Canada
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Piatkivskyi A, Lau JKC, Berden G, Oomens J, Hopkinson AC, Siu KM, Ryzhov V. Hydrogen atom transfer in the radical cations of tryptophan-containing peptides AW and WA studied by mass spectrometry, infrared multiple-photon dissociation spectroscopy, and theoretical calculations. EUROPEAN JOURNAL OF MASS SPECTROMETRY (CHICHESTER, ENGLAND) 2019; 25:112-121. [PMID: 30282467 DOI: 10.1177/1469066718802547] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Two types of radical cations of tryptophan-the π-radical cation and the protonated tryptophan-N radical-have been studied in dipeptides AW and WA. The π-radical cation produced by removal of an electron during collision-induced dissociation of a ternary Cu(II) complex was only observed for the AW peptide. In the case of WA, only the ion corresponding to the loss of ammonia, [WA-NH3] •+, was observed from the copper complex. Both protonated tryptophan-N radicals were produced by N-nitrosylation of the neutral peptides followed by transfer to the gas phase via electrospray ionization and subsequent collision-induced dissociation. The regiospecifically formed N• species were characterized by infrared multiple-photon dissociation spectroscopy which revealed that the WA tryptophan-N• radical remains the nitrogen radical, while the AW nitrogen radical rearranges into the π-radical cation. These findings are supported by the density functional theory calculations that suggest a relatively high barrier for the radical rearrangement (N• to π) in WA (156.3 kJ mol-1) and a very low barrier in AW (6.1 kJ mol-1). The facile hydrogen atom migration in the AW system is also supported by the collision-induced dissociation of the tryptophan-N radical species that produces fragments characteristic of the tryptophan π-radical cation. Gas-phase ion-molecule reactions with n-propyl thiol have also been used to differentiate between the π-radical cations (react by hydrogen abstraction) and the tryptophan-N• species (unreactive) of AW.
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Affiliation(s)
- Andrii Piatkivskyi
- 1 Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL, USA
| | - Justin Kai-Chi Lau
- 2 Department of Chemistry and Centre for Research in Mass Spectrometry, York University, Ontario, Canada
- 3 Department of Chemistry and Biochemistry, University of Windsor, Ontario, Canada
| | - Giel Berden
- 4 Institute for Molecules and Materials, FELIX Laboratory Radboud University, ED Nijmegen, The Netherlands
| | - Jos Oomens
- 4 Institute for Molecules and Materials, FELIX Laboratory Radboud University, ED Nijmegen, The Netherlands
| | - Alan C Hopkinson
- 2 Department of Chemistry and Centre for Research in Mass Spectrometry, York University, Ontario, Canada
| | - Kw Michael Siu
- 2 Department of Chemistry and Centre for Research in Mass Spectrometry, York University, Ontario, Canada
- 3 Department of Chemistry and Biochemistry, University of Windsor, Ontario, Canada
| | - Victor Ryzhov
- 1 Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL, USA
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Asakawa D, Takahashi H, Iwamoto S, Tanaka K. Hydrogen atom attachment to histidine and tryptophan containing peptides in the gas phase. Phys Chem Chem Phys 2019; 21:11633-11641. [DOI: 10.1039/c9cp00083f] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
In this study, we focus on the gas-phase fragmentation induced by the attachment of hydrogen atoms to the histidine and tryptophan residue side-chains in the peptide that provides the fragment ions due to Cα–Cβ bond cleavage.
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Affiliation(s)
- Daiki Asakawa
- National Institute of Advanced Industrial Science and Technology (AIST)
- National Metrology Institute of Japan (NMIJ)
- Tsukuba
- Japan
| | - Hidenori Takahashi
- Koichi Tanaka Mass Spectrometry Research Laboratory
- Shimadzu Corporation
- Kyoto 604-8511
- Japan
| | - Shinichi Iwamoto
- Koichi Tanaka Mass Spectrometry Research Laboratory
- Shimadzu Corporation
- Kyoto 604-8511
- Japan
| | - Koichi Tanaka
- Koichi Tanaka Mass Spectrometry Research Laboratory
- Shimadzu Corporation
- Kyoto 604-8511
- Japan
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Mu X, Lau JKC, Lai CK, Siu KWM, Hopkinson AC, Chu IK. Isomerization versus dissociation of phenylalanylglycyltryptophan radical cations. Phys Chem Chem Phys 2017. [PMID: 28631796 DOI: 10.1039/c7cp02355c] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Four isomers of the radical cation of tripeptide phenylalanylglycyltryptophan, in which the initial location of the radical center is well defined, have been isolated and their collision-induced dissociation (CID) spectra examined. These ions, the π-centered [FGWπ˙]+, α-carbon- [FGα˙W]+, N-centered [FGWN˙]+ and ζ-carbon- [Fζ˙GW]+ radical cations, were generated via collision-induced dissociation (CID) of transition metal-ligand-peptide complexes, side chain fragmentation of a π-centered radical cation, homolytic cleavage of a labile nitrogen-nitrogen single bond, and laser induced dissociation of an iodinated peptide, respectively. The π-centered and tryptophan N-centered peptide radical cations produced almost identical CID spectra, despite the different locations of their initial radical sites, which indicated that interconversion between the π-centered and tryptophan N-centered radical cations is facile. By contrast, the α-carbon-glycyl radical [FGα˙W]+, and ζ-phenyl radical [Fζ˙GW]+, featured different dissociation product ions, suggesting that the interconversions among α-carbon, π-centered (or tryptophan N-centered) and ζ-carbon-radical cations have higher barriers than those to dissociation. Density functional theory calculations have been used to perform systematic mechanistic investigations on the interconversions between these isomers and to study selected fragmentation pathways for these isomeric peptide radical cations. The results showed that the energy barrier for interconversion between [FGWπ˙]+ and [FGWN˙]+ is only 31.1 kcal mol-1, much lower than the barriers to their dissociation (40.3 kcal mol-1). For the [FGWπ˙]+, [FGα˙W]+, and [Fζ˙GW]+, the barriers to interconversion are higher than those to dissociation, suggesting that interconversions among these isomers are not competitive with dissociations. The [z3 - H]˙+ ions isolated from [FGα˙W]+ and [Fζ˙GW]+ show distinctly different fragmentation patterns, indicating that the structures of these ions are different and this result is supported by the DFT calculations.
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Affiliation(s)
- Xiaoyan Mu
- Department of Chemistry, The University of Hong Kong, Hong Kong, China.
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Mu X, Song T, Siu CK, Chu IK. Tautomerization and Dissociation of Molecular Peptide Radical Cations. CHEM REC 2017. [DOI: 10.1002/tcr.201700013] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Affiliation(s)
- Xiaoyan Mu
- Department of Chemistry; University of Hong Kong; Pokfulam, Hong Kong SAR P. R. China
| | - Tao Song
- Department of Chemistry; University of Hong Kong; Pokfulam, Hong Kong SAR P. R. China
| | - Chi-Kit Siu
- Department of Biology and Chemistry; City University of Hong Kong; 83 Tat Chee Avenue Kowloon Tong, Hong Kong SAR P. R. China
| | - Ivan K. Chu
- Department of Chemistry; University of Hong Kong; Pokfulam, Hong Kong SAR P. R. China
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Mu X, Lau JKC, Lai CK, Siu KWM, Hopkinson AC, Chu IK. Nucleophilic substitution by amide nitrogen in the aromatic rings of [zn - H]˙⁺ ions; the structures of the [b₂ - H - 17]˙⁺ and [c1 - 17]⁺ ions. Phys Chem Chem Phys 2016; 18:11168-75. [PMID: 27048940 DOI: 10.1039/c6cp00405a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Peptide radical cations that contain an aromatic amino acid residue cleave to give [zn - H]˙⁺ ions with [b2 - H - 17]˙⁺ and [c1 - 17](+) ions, the dominant products in the dissociation of [zn - H]˙⁺, also present in lower abundance in the CID spectra. Isotopic labeling in the aromatic ring of [Yπ˙GG](+) establishes that in the formation of [b2 - H - 17]˙⁺ ions a hydrogen from the δ-position of the Y residue is lost, indicating that nucleophilic substitution on the aromatic ring has occurred. A preliminary DFT investigation of nine plausible structures for the [c1 - 17](+) ion derived from [Y(π)˙GG](+) shows that two structures resulting from attack on the aromatic ring by oxygen and nitrogen atoms from the peptide backbone have significantly better energies than other isomers. A detailed study of [Y(π)˙GG](+) using two density functionals, B3LYP and M06-2X, with a 6-31++G(d,p) basis set gives a higher barrier for attack on the aromatic ring of the [zn - H]˙⁺ ion by nitrogen than by the carbonyl oxygen. However, subsequent rearrangements involving proton transfers are much higher in energy for the oxygen-substituted isomer leading to the conclusion that the [c1 - 17](+) ions are the products of nucleophilic attack by nitrogen, protonated 2,7-dihydroxyquinoline ions. The [b2 - H - 17]˙⁺ ions are formed by loss of glycine from the same intermediates involved in the formation of the [c1 - 17](+) ions.
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Affiliation(s)
- Xiaoyan Mu
- Department of Chemistry, The University of Hong Kong, Hong Kong, China.
| | - Justin Kai-Chi Lau
- Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto ON, M3J 1P3, Canada. and Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada
| | - Cheuk-Kuen Lai
- Department of Chemistry, The University of Hong Kong, Hong Kong, China.
| | - K W Michael Siu
- Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto ON, M3J 1P3, Canada. and Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada
| | - Alan C Hopkinson
- Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto ON, M3J 1P3, Canada.
| | - Ivan K Chu
- Department of Chemistry, The University of Hong Kong, Hong Kong, China.
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Williams D, Lau JKC, Zhao J, Mädler S, Wang Y, Saminathan IS, Hopkinson AC, Siu KWM. Radical-induced, proton-transfer-driven fragmentations in [b(5)-H]˙(+) ions derived from pentaalanyl tryptophan. Phys Chem Chem Phys 2015; 17:10699-707. [PMID: 25811808 DOI: 10.1039/c5cp00178a] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
The collision-induced dissociation (CID) of [b5 - H]˙(+) ions containing four alanine residues and one tryptophan give identical spectra regardless of the initial location of the tryptophan indicating that, as proposed for b5(+) ions, sequence scrambling occurs prior to dissociation. Cleavage occurs predominantly at the peptide bonds and at the N-Cα bond of the alanine residue that is attached to the N-terminus of the tryptophan residue. The product of the latter pathway, an ion at m/z 240, is the base peak in all the mass spectra. With the exception of one minor channel giving a b3(+) ion, the product ions retain both the tryptophan residue and the radical. Experiments with one trideuterated alanine established the sequences of loss of alanine residues. Formation of identical products implies a common intermediate, a [b5 - H]˙(+) ion that has a 'linear' structure in which the tryptophan residue is present as an α-radical located in the oxazolone ring, structure Ie. Density functional theory calculations show this structure to be at the global minimum, 14.6 kcal mol(-1) below the macrocyclic structure, ion II. Loss of CO from the [b5 - H]˙(+) ions is inhibited by the presence of the radical centre in the oxazolone ring and migration of the proton from the oxazolone ring onto the peptide backbone induces cleavage of an N-Cα or peptide bond. Three calculated structures for the ion at m/z 240 all have an oxazolone ring. Two of these structures may be formed from Ie, depending upon which proton migrates onto the peptide chain prior to the dissociation. The barrier to interconversion between these two structures requires a 1,3-hydrogen atom shift and is high (51.0 kcal mol(-1)), but both can convert into a third isomer that readily loses CO2 (barrier 38.7 kcal mol(-1)). The lowest barrier to the loss of CO, the usual fragmentation path observed for protonated oxazolones, is 47.0 kcal mol(-1).
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Affiliation(s)
- Declan Williams
- Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada
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Laskin J. Surface-induced dissociation: a unique tool for studying energetics and kinetics of the gas-phase fragmentation of large ions. EUROPEAN JOURNAL OF MASS SPECTROMETRY (CHICHESTER, ENGLAND) 2015; 21:377-389. [PMID: 26307719 DOI: 10.1255/ejms.1358] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Surface-induced dissociation (SID) is a valuable tool for investigating the activation and dissociation of large ions in tandem mass spectrometry. This account summarizes key findings from studies of the energetics and mechanisms of complex ion dissociation in which SID experiments were combined with Rice-Ramsperger-Kassel-Marcus modeling of the experimental data. These studies used time- and collision-energy-resolved SID experiments and SID combined with resonant ejection of selected fragment ions on a specially designed Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Fast-ion activation by collision with a surface combined with the long and variable timescale of FT-ICR mass spectrometry is perfectly suited to studying the energetics and dynamics of complex ion dissociation in the gas phase. Modeling of time- and collision-energy-resolved SID enables the accurate determination of energy and entropy effects in the dissociation process. It has been demonstrated that entropy effects play an important role in determining the dissociation rates of both covalent and noncovalent bonds in large gaseous ions. SID studies have provided important insights on the competition between charge-directed and charge-remote fragmentation in even-electron peptide ions and the role of the charge and radical site on the energetics of the dissociation of odd-electron peptide ions. Furthermore, this work examined factors that affect the strength of noncovalent binding, as well as the competition between covalent and noncovalent bond cleavages and between proton and electron transfer in model systems. Finally, SID studies have been used to understand the factors affecting nucleation and growth of clusters in solution and in the gas phase.
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Affiliation(s)
- Julia Laskin
- Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA..
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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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Zhang X, Julian RR. Radical additions to aromatic residues in peptides facilitate unexpected side chain and backbone losses. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2014; 25:626-635. [PMID: 24488753 DOI: 10.1007/s13361-013-0810-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/06/2013] [Revised: 12/06/2013] [Accepted: 12/08/2013] [Indexed: 06/03/2023]
Abstract
Accurate identification of fragments in tandem mass spectrometry experiments is aided by knowledge of relevant fragmentation mechanisms. Herein, novel radical addition reactions that direct unexpected side-chain dissociations at tryptophan and tyrosine residues are reported. Various mechanisms that can account for the observed dissociation channels are investigated by experiment and theory. The propensity for radical addition at a particular site is found to be primarily under kinetic control, which is largely dictated by molecular structure. In certain peptides, intramolecular radical addition reactions are favored, which leads to the observation of numerous unexpected fragments. In one pathway, radical addition leads to migration of an aromatic side chain to another residue. Alternatively, radical addition followed by hydrogen atom loss leads to cyclization of the peptide and increased observation of internal sequence fragments. Radical addition reactions should be considered when assigning fragmentation spectra obtained from activation of hydrogen deficient peptides.
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Affiliation(s)
- Xing Zhang
- Department of Chemistry, University of California, Riverside, CA, 92521, USA
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Piatkivskyi A, Osburn S, Jaderberg K, Grzetic J, Steill JD, Oomens J, Zhao J, Lau JKC, Verkerk UH, Hopkinson AC, Siu KWM, Ryzhov V. Structure and reactivity of the distonic and aromatic radical cations of tryptophan. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2013; 24:513-523. [PMID: 23512424 DOI: 10.1007/s13361-013-0594-0] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/02/2012] [Revised: 01/24/2013] [Accepted: 01/30/2013] [Indexed: 06/01/2023]
Abstract
In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric forms of tryptophan radical cations-a distonic indolyl N-radical (H3N(+) - TrpN(•)) and a canonical aromatic π (Trp(•+)) radical cation. The distonic radical cation was generated by nitrosylating the indole nitrogen of tryptophan in solution followed by collision-induced dissociation (CID) of the resulting protonated N-nitroso tryptophan. The π-radical cation was produced via CID of the ternary [Cu(II)(terpy)(Trp)](•2+) complex. CID spectra of the two isomeric species were found to be very different, suggesting no interconversion between the isomers. In gas-phase ion-molecule reactions, the distonic radical cation was unreactive towards n-propylsulfide, whereas the π radical cation reacted by hydrogen atom abstraction. DFT calculations revealed that the distonic indolyl radical cation is about 82 kJ/mol higher in energy than the π radical cation of tryptophan. The low reactivity of the distonic nitrogen radical cation was explained by spin delocalization of the radical over the aromatic ring and the remote, localized charge (at the amino nitrogen). The lack of interconversion between the isomers under both trapping and CID conditions was explained by the high rearrangement barrier of ca.137 kJ/mol. Finally, the two isomers were characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy in the ~1000-1800 cm(-1) region. It was found that some of the main experimental IR features overlap between the two species, making their distinction by IRMPD spectroscopy in this region problematic. In addition, DFT theoretical calculations showed that the IR spectra are strongly conformation-dependent.
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Affiliation(s)
- Andrii Piatkivskyi
- Department of Chemistry and Biochemistry, and Center for Biochemical and Biophysical Studies, Northern Illinois University, DeKalb, IL 60115, USA
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Quan Q, Hao Q, Song T, Siu CK, Chu IK. Mechanistic investigation of phosphate ester bond cleavages of glycylphosphoserinyltryptophan radical cations under low-energy collision-induced dissociation. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2013; 24:554-562. [PMID: 23516067 DOI: 10.1007/s13361-013-0597-x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/06/2012] [Revised: 02/18/2013] [Accepted: 02/21/2013] [Indexed: 06/01/2023]
Abstract
Under the conditions of low-energy collision-induced dissociation (CID), the canonical glycylphosphoserinyltryptophan radical cation having its radical located on the side chain of the tryptophan residue ([G(p)SW](•+)) fragments differently from its tautomer with the radical initially generated on the α-carbon atom of the glycine residue ([G(•)(p)SW](+)). The dissociation of [G(•)(p)SW](+) is dominated by the neutral loss of H3PO4 (98 Da), with backbone cleavage forming the [b2 - H](•+)/y1(+) pair as the minor products. In contrast, for [G(p)SW](•+), competitive cleavages along the peptide backbone, such as the formation of [G(p)SW - CO2](•+) and the [c2 + 2H](+)/[z(1) - H](•+) pair, significantly suppress the loss of neutral H3PO4. In this study, we used density functional theory (DFT) to examine the mechanisms for the tautomerizations of [G(•)(p)SW](+) and [G(p)SW](•+) and their dissociation pathways. Our results suggest that the dissociation reactions of these two peptide radical cations are more efficient than their tautomerizations, as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. We also propose that the loss of H3PO4 from both of these two radical cationic tautomers is preferentially charge-driven, similar to the analogous dissociations of even-electron protonated peptides. The distonic radical cationic character of [G(•)(p)SW](+) results in its charge being more mobile, thereby favoring charge-driven loss of H3PO4; in contrast, radical-driven pathways are more competitive during the CID of [G(p)SW](•+).
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Affiliation(s)
- Quan Quan
- Department of Chemistry, University of Hong Kong, Hong Kong, China
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