1
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Gruber E, Teiwes R, Kjær C, Brøndsted Nielsen S, Andersen LH. Tuning fast excited-state decay by ligand attachment in isolated chlorophyll a. Phys Chem Chem Phys 2021; 24:149-155. [PMID: 34901981 DOI: 10.1039/d1cp04356k] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
Abstract
Excited-state dynamics plays a key role for light harvesting and energy transport in photosynthetic proteins but it is nontrivial to separate the intrinsic photophysics of the light-absorbers (chlorophylls) from interactions with the protein matrix. Here we study chlorophyll a (4-coordinate complex) and axially ligated chlorophyll a (5-coordinate complex) isolated in vacuo applying mass spectrometry to shed light on the intrinsic dynamics in the absence of nearby chlorophylls, carotenoids, amino acids, and water molecules. The 4-coordinate complexes are tagged by quaternary ammonium ions while the charge is provided by a formate ligand in the case of 5-coordinate complexes. Regardless of excitation to the Soret band or the Q band, a fast ps decay is observed, which is ascribed to the decay of the lowest excited singlet state either by intersystem crossing (ISC) to nearby triplet states or by excited-state relaxation on the excited-state potential-energy surface. The lifetime of the first excited state is 15 ps with Mg2+ at the chlorophyll center, but only 1.7 ps when formate is attached to Mg2+. When the Soret band is excited, an initial sup-ps relaxation is observed which is ascribed to fast internal conversion to the first excited state. With respect to ISC, two factors seem to play a role for the reduced lifetime of the formate-chlorophyll complex: (i) The Mg ion is pulled out of the porphyrin plane thus reducing the symmetry of the chromophore, and (ii) the first excited state (Q band) and T3 are tuned almost into resonance by the ligand, which increases the singlet-triplet mixing.
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Affiliation(s)
- Elisabeth Gruber
- Department of Physics and Astronomy, Aarhus University, Aarhus 8000C, Denmark.
| | - Ricky Teiwes
- Department of Physics and Astronomy, Aarhus University, Aarhus 8000C, Denmark.
| | - Christina Kjær
- Department of Physics and Astronomy, Aarhus University, Aarhus 8000C, Denmark.
| | | | - Lars H Andersen
- Department of Physics and Astronomy, Aarhus University, Aarhus 8000C, Denmark.
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2
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Sohn CH, Yin S, Peng I, Loo JA, Beauchamp JL. Investigation of the Mechanism of Electron Capture and Electron Transfer Dissociation of Peptides with a Covalently Attached Free Radical Hydrogen Atom Scavenger. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY 2015; 390:49-55. [PMID: 27275130 PMCID: PMC4892187 DOI: 10.1016/j.ijms.2015.07.007] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
The mechanisms of electron capture and electron transfer dissociation (ECD and ETD) are investigated by covalently attaching a free-radical hydrogen atom scavenger to a peptide. The 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO) radical was chosen as the scavenger due to its high hydrogen atom affinity (ca. 280 kJ/mol) and low electron affinity (ca. 0.45 ev), and was derivatized to the model peptide, FQXTEMPOEEQQQTEDELQDK. The XTEMPO residue represents a cysteinyl residue derivatized with an acetamido-TEMPO group. The acetamide group without TEMPO was also examined as a control. The gas phase proton affinity (882 kJ/mol) of TEMPO is similar to backbone amide carbonyls (889 kJ/mol), minimizing perturbation to internal solvation and sites of protonation of the derivatized peptides. Collision induced dissociation (CID) of the TEMPO tagged peptide dication generated stable odd-electron b and y type ions without indication of any TEMPO radical induced fragmentation initiated by hydrogen abstraction. The type and abundance of fragment ions observed in the CID spectra of the TEMPO and acetamide tagged peptides are very similar. However, ECD of the TEMPO labeled peptide dication yielded no backbone cleavage. We propose that a labile hydrogen atom in the charge reduced radical ions is scavenged by the TEMPO radical moiety, resulting in inhibition of N-Cα backbone cleavage processes. Supplemental activation after electron attachment (ETcaD) and CID of the charge-reduced precursor ion generated by electron transfer of the TEMPO tagged peptide dication produced a series of b + H (bH) and y + H (yH) ions along with some c ions having suppressed intensities, consistent with stable O-H bond formation at the TEMPO group. In summary, the results indicate that ECD and ETD backbone cleavage processes are inhibited by scavenging of a labile hydrogen atom by the localized TEMPO radical moiety. This observation supports the conjecture that ECD and ETD processes involve long-lived intermediates formed by electron capture/transfer in which a labile hydrogen atom is present and plays a key role with low energy processes leading to c and z ion formation. Ab initio and density functional calculations are performed to support our conclusion, which depends most importantly on the proton affinity, electron affinity and hydrogen atom affinity of the TEMPO moiety.
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Affiliation(s)
- Chang Ho Sohn
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
| | - Sheng Yin
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095
| | - Ivory Peng
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095
| | - Joseph A. Loo
- Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095
- Department of Biological Chemistry, David Geffen School of Medicine at UCLA, University of California, Los Angeles, California 90095
- UCLA/DOE Institute of Genomics and Proteomics, University of California, Los Angeles, California 90095
| | - J. L. Beauchamp
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
- To whom correspondence should be addressed:
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3
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Riffet V, Jacquemin D, Cauët E, Frison G. Benchmarking DFT and TD-DFT Functionals for the Ground and Excited States of Hydrogen-Rich Peptide Radicals. J Chem Theory Comput 2014; 10:3308-18. [DOI: 10.1021/ct5004912] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Vanessa Riffet
- Laboratoire
de Chimie Moléculaire, Département de Chimie, Ecole
polytechnique and CNRS, 91128 Palaiseau cedex, France
| | - Denis Jacquemin
- Laboratoire
CEISAM, UMR CNRS 6230, Université de Nantes, 2 rue de la
Houssinière - BP 92208, 44322 Nantes cedex 3, France
- Institut Universitaire
de France, 103 bd Saint-Michel, F-75005 Paris Cedex 05, France
| | - Emilie Cauët
- Service
de Chimie quantique et Photophysique, Université Libre de Bruxelles, CP160/09, 50 av. F.D. Roosevelt, 1050 Bruxelles, Belgium
| | - Gilles Frison
- Laboratoire
de Chimie Moléculaire, Département de Chimie, Ecole
polytechnique and CNRS, 91128 Palaiseau cedex, France
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4
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Marek A, Pepin R, Peng B, Laszlo KJ, Bush MF, Tureček F. Electron transfer dissociation of photolabeled peptides. Backbone cleavages compete with diazirine ring rearrangements. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2013; 24:1641-1653. [PMID: 23633016 DOI: 10.1007/s13361-013-0630-0] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/28/2012] [Revised: 03/25/2013] [Accepted: 03/25/2013] [Indexed: 06/02/2023]
Abstract
Gas-phase conformations and electron transfer dissociations of pentapeptide ions containing the photo-Leu residue (L*) were studied. Exhaustive conformational search including molecular dynamics force-field, semi-empirical, ab initio, and density functional theory calculations established that the photo-Leu residue did not alter the gas-phase conformations of (GL*GGK + 2H)(2+) and (GL*GGK-NH2 + H)(+) ions, which showed the same conformer energy ranking as the unmodified Leu-containing ions. This finding is significant in that it simplifies conformational analysis of photo-labeled peptide ions. Electron transfer dissociation mass spectra of (GL*GGK + 2H)(2+), (GL*GGK-NH2 + 2H)(2+),(GL*GGKK + 2H)(2+), (GL*GLK + 2H)(2+), and (GL*LGK + 2H)(2+) showed 16 %-21 % fragment ions originating by radical rearrangements and cleavages in the diazirine ring. These side-chain dissociations resulted in eliminations of N2H3, N2H4, [N2H5], and [NH4O] neutral fragments and were particularly abundant in long-lived charge-reduced cation-radicals. Deuterium labeling established that the neutral hydrazine molecules mainly contained two exchangeable and two nonexchangeable hydrogen atoms from the peptide and underwent further H/D exchange in an ion-molecule complex. Electron structure calculations on the charge-reduced ions indicated that the unpaired electron was delocalized between the diazirine and amide π* electronic systems in the low electronic states of the cation-radicals. The diazirine moiety in GL*GGK-NH2was calculated to have an intrinsic electron affinity of 1.5 eV, which was further increased by the Coulomb effect of the peptide positive charge. Mechanisms are proposed for the unusual elimination of hydrazine from the photo-labeled peptide ions.
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Affiliation(s)
- Aleš Marek
- Department of Chemistry, Bagley Hall, University of Washington, Seattle, WA, USA
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5
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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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6
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Zimnicka M, Chung TW, Moss CL, Tureček F. Perturbing Peptide Cation-Radical Electronic States by Thioxoamide Groups: Formation, Dissociations, and Energetics of Thioxopeptide Cation-Radicals. J Phys Chem A 2012; 117:1265-75. [DOI: 10.1021/jp305865q] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Magdalena Zimnicka
- Department of Chemistry, Bagley Hall, Box
351700, University of Washington, Seattle,
Washington 98195-1700,
United States
| | - Thomas W. Chung
- Department of Chemistry, Bagley Hall, Box
351700, University of Washington, Seattle,
Washington 98195-1700,
United States
| | - Christopher L. Moss
- Department of Chemistry, Bagley Hall, Box
351700, University of Washington, Seattle,
Washington 98195-1700,
United States
| | - František Tureček
- Department of Chemistry, Bagley Hall, Box
351700, University of Washington, Seattle,
Washington 98195-1700,
United States
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7
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Moss CL, Liang W, Li X, Tureček F. The early life of a peptide cation-radical. Ground and excited-state trajectories of electron-based peptide dissociations during the first 330 femtoseconds. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2012; 23:446-459. [PMID: 22187160 DOI: 10.1007/s13361-011-0283-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/07/2011] [Revised: 10/11/2011] [Accepted: 10/18/2011] [Indexed: 05/31/2023]
Abstract
We report a new approach to investigating the mechanisms of fast peptide cation-radical dissociations based on an analysis of time-resolved reaction progress by Ehrenfest dynamics, as applied to an Ala-Arg cation-radical model system. Calculations of stationary points on the ground electronic state that were carried out with effective CCSD(T)/6-311++G(3df,2p) could not explain the experimental branching ratios for loss of a hydrogen atom, ammonia, and N-C(α) bond dissociation in (AR + 2H)(+•). The Ehrenfest dynamics results indicate that the ground and low-lying excited electronic states of (AR + 2H)(+•) follow different reaction courses in the first 330 femtoseconds after electron attachment. The ground (X) state undergoes competing loss of N-terminal ammonia and isomerization to an aminoketyl radical intermediate that depend on the vibrational energy of the charge-reduced ion. The A and B excited states involve electron capture in the Arg guanidine and carboxyl groups and are non-reactive on the short time scale. The C state is dissociative and progresses to a fast loss of an H atom from the Arg guanidine group. Analogous results were obtained by using the B3LYP and CAM-B3LYP density functionals for the excited state dynamics and including the universal M06-2X functional for ground electronic state calculations. The results of this Ehrenfest dynamics study indicate that reaction pathway branching into the various dissociation channels occurs in the early stages of electron attachment and is primarily determined by the electronic states being accessed. This represents a new paradigm for the discussion of peptide dissociations in electron based methods of mass spectrometry.
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Affiliation(s)
- Christopher L Moss
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA, USA
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8
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Hossan ASM, Abou-Melha HM, Refat MS. In situ synthesis, photometric and spectroscopic studies of chelating system during the 1,4,7,10,13,16-hexaoxacyclooctadecane charge transfer reaction with different acceptors. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2011; 79:583-593. [PMID: 21531613 DOI: 10.1016/j.saa.2011.03.039] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/03/2010] [Revised: 03/15/2011] [Accepted: 03/16/2011] [Indexed: 05/30/2023]
Abstract
Electron donor acceptor complexes (EDA) of the 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) as a rich donor were spectrophotometrically discussed and synthesized in solid form according the interactions with different nine of usual π-acceptors like 2,3,5,6-tetrachlorocyclohexa-2,5-diene-1,4-dione (p-chloranil; p-CHL), tetrachloro-1,2-benzoquinone (o-chloranil; o-CHL), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoquinodimethane (TCNQ), 2,6-dichloroquinone-4-chloroimide (DCQ), 2,6-dibromoquinone-4-chloroimide (DBQ), 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid; CLA), N-bromosuccinimide (NBS), 2,4,6-trinitrophenol (picric acid; PA). Spectroscopic and physical data such as formation constant (K(CT)), molar extinction coefficient (ɛ(CT)), standard free energy (ΔG°), oscillator strength (f), transition dipole moment (μ), resonance energy (R(N)) and ionization potential (I(p)) were estimated in chloroform or methanol at 25°C. Based on the elemental analysis and photometric titrations the CT-complexes were formed indicated the formation of 1:1 charge-transfer complexes for the o-CHL, TCNQ, DCQ, DBQ and NBS acceptors but 1:3 ratio for p-CHL, DDQ, CLA and PA, respectively. The charge-transfer interactions were interpretative according to the formation of dative ion pairs [18C6(•+), A(•-)], where A is acceptor. All of the resulting charge transfer complexes were isolated in amorphous form and the complexes formations on IR and (1)H NMR spectra were discussed.
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Affiliation(s)
- Aisha S M Hossan
- Faculty of Education of Girls, Scientific Departments, King Khaled University, Saudi Arabia
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9
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Sargaeva NP, Lin C, O’Connor PB. Unusual fragmentation of β-linked peptides by ExD tandem mass spectrometry. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2011; 22:480-91. [PMID: 21472566 PMCID: PMC4361814 DOI: 10.1007/s13361-010-0049-9] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/02/2010] [Revised: 11/18/2010] [Accepted: 11/18/2010] [Indexed: 05/11/2023]
Abstract
Ion-electron reaction based fragmentation methods (ExD) in tandem mass spectrometry (MS), such as electron capture dissociation (ECD) and electron transfer dissociation (ETD) represent a powerful tool for biological analysis. ExD methods have been used to differentiate the presence of the isoaspartate (isoAsp) from the aspartate (Asp) in peptides and proteins. IsoAsp is a β(3)-type amino acid that has an additional methylene group in the backbone, forming a C(α)-C(β) bond within the polypeptide chain. Cleavage of this bond provides specific fragments that allow differentiation of the isomers. The presence of a C(α)-C(β) bond within the backbone is unique to β-amino acids, suggesting a similar application of ExD toward the analysis of peptides containing other β-type amino acids. In the current study, ECD and ETD analysis of several β-amino acid containing peptides was performed. It was found that N-C(β) and C(α)-C(β) bond cleavages were rare, providing few c and z• type fragments, which was attributed to the instability of the C(β) radical. Instead, the electron capture resulted primarily in the formation of a• and y fragments, representing an alternative fragmentation pathway, likely initiated by the electron capture at a backbone amide nitrogen protonation site within the β amino acid residues.
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Affiliation(s)
- Nadezda P. Sargaeva
- Mass Spectrometry Resource, Department of Biochemistry, Boston University School of Medicine, 670 Albany Street, R504, Boston, Massachusetts 02118
| | - Cheng Lin
- Mass Spectrometry Resource, Department of Biochemistry, Boston University School of Medicine, 670 Albany Street, R504, Boston, Massachusetts 02118
| | - Peter B. O’Connor
- Mass Spectrometry Resource, Department of Biochemistry, Boston University School of Medicine, 670 Albany Street, R504, Boston, Massachusetts 02118
- Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
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10
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Chen X, Chan WYK, Wong PS, Yeung HS, Chan TWD. Formation of peptide radical cations (m+·) in electron capture dissociation of peptides adducted with group IIB metal ions. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2011; 22:233-244. [PMID: 21472583 DOI: 10.1007/s13361-010-0035-2] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/03/2010] [Revised: 11/09/2010] [Accepted: 11/11/2010] [Indexed: 05/30/2023]
Abstract
Peptides adducted with different divalent Group IIB metal ions (Zn(2+), Cd(2+), and Hg(2+)) were found to give very different ECD mass spectra. ECD of Zn(2+) adducted peptides gave series of c-/z-type fragment ions with and without metal ions. ECD of Cd(2+) and Hg(2+) adducted model peptides gave mostly a-type fragment ions with M(+•) and fragment ions corresponding to losses of neutral side chain from M(+•). No detectable a-ions could be observed in ECD spectra of Zn(2+) adducted peptides. We rationalized the present findings by invoking both proton-electron recombination and metal-ion reduction processes. As previously postulated, divalent metal-ions adducted peptides could adopt several forms, including (a) [M + Cat](2+), (b) [(M + Cat - H) + H](2+), and (c) [(M + Cat - 2H) + 2H](2+). The relative population of these precursor ions depends largely on the acidity of the metal-ion peptide complexes. Peptides adducted with divalent metal-ions of small ionic radii (i.e., Zn(2+)) would form predominantly species (b) and (c); whereas peptides adducted with metal ions of larger ionic radii (i.e., Hg(2+)) would adopt predominantly species (a). Species (b) and (c) are believed to be essential for proton-electron recombination process to give c-/z-type fragments via the labile ketylamino radical intermediates. Species (c) is particularly important for the formation of non-metalated c-/z-type fragments. Without any mobile protons, species (a) are believed to undergo metal ion reduction and subsequently induce spontaneous electron transfer from the peptide moiety to the charge-reduced metal ions. Depending on the exothermicity of the electron transfer reaction, the peptide radical cations might be formed with substantial internal energy and might undergo further dissociation to give structural related fragment ions.
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Affiliation(s)
- Xiangfeng Chen
- Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, SAR, China
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11
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Chung TW, Moss CL, Zimnicka M, Johnson RS, Moritz RL, Tureček F. Electron-capture and -transfer dissociation of peptides tagged with tunable fixed-charge groups: structures and dissociation energetics. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2011; 22:13-30. [PMID: 21472540 DOI: 10.1007/s13361-010-0012-9] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/23/2010] [Revised: 09/26/2010] [Accepted: 10/11/2010] [Indexed: 05/30/2023]
Abstract
Pyridiniummethylcarbonyl moieties that were previously designed on the basis of electronic structure analysis are now utilized as fixed-charge tags with tunable electronic properties to be used for N-terminal peptide derivatization and sequencing by electron-transfer dissociation. Dipeptides AK and KA were derivatized at the peptide N-terminus with 4-dimethylaminopyridinium-N-acetyl (DMAP-ac) and pyridinium-N-acetyl (pyrid-ac) tags of increasing intrinsic recombination energies. Upon the capture of a free electron or electron transfer from fluoranthene anions, (DMAP-ac-AK+H)(2+), (DMAP-ac-KA+H)(2+), (pyrid-ac-AK+H)(2+) and (pyrid-ac-KA+H)(2+) ions, as well as underivatized (AK+2H)(2+), completely dissociated. The fixed-charge tags steered the dissociation upon electron transfer to form abundant backbone N-C(α) bond cleavages, whereas the underivatized peptide mainly underwent H-atom and side-chain losses. Precursor ion structures for the tagged peptides were analyzed by an exhaustive conformational search combined with B3LYP/6-31+G(d,p) geometry optimization and single-point energy calculations in order to select the global energy minima. Structures, relative energies, transition states, ion-molecule complexes, and dissociation products were identified for several charge-reduced species from the tagged peptides. The electronic properties of the charge tags and their interactions with the peptide moieties are discussed. Electrospray ionization and electron-transfer dissociation of larger peptides are illustrated with a DMAP-tagged pentapeptide.
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Affiliation(s)
- Thomas W Chung
- Department of Chemistry, University of Washington, Bagley Hall, Box 351700, Seattle, WA 98195-1700, USA
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Jensen CS, Wyer JA, Houmøller J, Hvelplund P, Nielsen SB. Electron-capture induced dissociation of doubly charged dipeptides: on the neutral losses and N–Cα bond cleavages. Phys Chem Chem Phys 2011; 13:18373-8. [DOI: 10.1039/c1cp21549c] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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13
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Tureček F. Electron predators are hydrogen atom traps. Effects of aryl groups on N-C(α) bond dissociations of peptide radicals. JOURNAL OF MASS SPECTROMETRY : JMS 2010; 45:1280-1290. [PMID: 20812369 DOI: 10.1002/jms.1807] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/01/2010] [Accepted: 08/05/2010] [Indexed: 05/29/2023]
Abstract
Effects of substituted aryl groups on dissociations of peptide aminoketyl radicals were studied computationally for model tetrapeptide intermediates GXD(•) G where X was a cysteine residue that was derivatized by S-(3-nitrobenzyl), S-(3-cyanobenzyl), S-(3,5-dicyanobenzyl), S-(2,3,4,5,6-pentafluorobenzyl), and S-benzyl groups. The aminoketyl radical was placed within the Asp amide group. Aminoketyl radicals having the S-(3-nitrobenzyl) group were found to undergo spontaneous and highly exothermic migration of the hydroxyl hydrogen atom onto the nitro group in conformers allowing interaction between these groups. Competing reaction channels were investigated for aminoketyl radicals having the S-(3-cyanobenzyl) and S-(3,5-dicyanobenzyl) groups, e.g. H-atom migration to the C and N atoms of the C≡N group, migration to the C-4 position of the phenyl ring, and dissociation of the radical-activated NC(α) bond between the Asp and Gly residues. RRKM kinetic analysis on the combined B3LYP and ROMP2/6-311++G(2d,p) potential energy surface indicated > 99% H-atom transfer to the C≡N group forming a stable iminyl intermediate. The NC(α) bond dissociation was negligible. In contrast, peptides with the S-(2,3,4,5,6-pentafluorobenzyl) and S-benzyl groups showed preferential NC(α) bond dissociation that outcompeted H-atom migration to the C-4 position and fluorine substituents in the phenyl ring. These computational results are used to suggest an alternative mechanism for the quenching effect on electron-based peptide backbone dissociations of benzyl groups with electron-withdrawing substitutents, as reported recently.
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Affiliation(s)
- František Tureček
- Department of Chemistry, Bagley Hall, Box 351700, University of Washington, Seattle, WA 98195-1700, USA.
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14
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Sun F, Zong W, Liu R, Wang M, Zhang P, Xu Q. The relative charge ratio between C and N atoms in amide bond acts as a key factor to determine peptide fragment efficiency in different charge states. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2010; 21:1857-1862. [PMID: 20688527 DOI: 10.1016/j.jasms.2010.06.022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/07/2010] [Revised: 06/24/2010] [Accepted: 06/30/2010] [Indexed: 05/29/2023]
Abstract
The influence of charge state on the peptide dissociation behavior in tandem mass spectrometry (MS/MS) is worthy of discussion. Comparative studies of singly- and doubly-protonated peptide molecules are performed to explore the effect and mechanism of charge state on peptide fragmentation. In view of the charge-directed cleavage of protonated peptides described in the mobile proton model, radiolytic oxidation was applied to change the charge distribution of peptides but retain the sequence. Experimental studies of collision energy-dependent fragmentation efficiencies coupled with quantum chemical calculations indicated that the cleavage of ARRA and its side-chain oxidation products with oxygen atoms added followed a trend that doubly-protonated peptides fragment more easily than singly-protonated forms, while the oxidation product with the guanidine group deleted showed the opposite trend. By analyzing the charge distribution around the amide bonds, we found that the relative charge ratios between C and N atoms (Q(C)/Q(N)) in the amide bonds provided a reasonable explanation for peptide fragmentation efficiencies. An increase of the Q(C)/Q(N) value of the amide bond means that a peptide fragments more easily, and vice versa. The results described in this paper provide an experimental and calculation strategy for predicting peptide fragmentation efficiency.
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Affiliation(s)
- Feng Sun
- Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, China-America CRC for Environment and Health, Jinan, PR China
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15
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16
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Jensen CS, Wyer JA, Nielsen SB. Electron capture induced dissociation of dipeptide dications: where does the charge go? Phys Chem Chem Phys 2010; 12:12961-3. [DOI: 10.1039/c0cp00592d] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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