Toward a general mechanism of electron capture dissociation



The effects of positive charge on the properties of ammonium and amide radicals were investigated by ab initio and density functional theory calculations with the goal of elucidating the energetics of electron capture dissociation (ECD) of multiply charged peptide ions. The electronic properties of the amide group in N-methylacetamide (NMA) are greatly affected by the presence of a remote charge in the form of a point charge, methylammonium, or guanidinium cations. The common effect of the remote charge is an increase of the electron affinity of the amide group, resulting in exothermic electron capture. The N—Cα bond dissociation and transition state energies in charge-stabilized NMA anions are 20–50 kJ mol−1 greater than in the hydrogen atom adduct. The zwitterions formed by electron capture have proton affinities that were calculated as 1030–1350 kJ mol−1, and are sufficiently basic for the amide carbonyl to exothermically abstract a proton from the ammonium, guanidinium and imidazolium groups in protonated lysine, arginine, and histidine residues, respectively. A new mechanism is proposed for ECD of multiply charged peptide and protein cations in which the electron enters a charge-stabilized electronic state delocalized over the amide group, which is a superbase that abstracts a proton from a sterically proximate amino acid residue to form a labile aminoketyl radical that dissociates by N—Cα bond cleavage. This mechanism explains the low selectivity of N—Cα bond dissociations induced by electron capture, and is applicable to dissociations of peptide ions in which the charge carriers are metal ions or quaternary ammonium groups. The new amide superbase and the previously proposed mechanisms of ECD can be uniformly viewed as being triggered by intramolecular proton transfer in charge-reduced amide cation-radicals. In contrast, remote charge affects N—H bond dissociation in weakly bound ground electronic states of hypervalent ammonium radicals, as represented by methylammonium, CH3NH3·, but has a negligible effect on the N—H bond dissociation in the strongly bound excited electronic states. This refutes previous speculations that loss of “hot hydrogen” can occur from an excited state of an ammonium radical.


  1. 1.
    Biemann, K. Peptides and Proteins Overview and Strategy. Meth. Enzymol. (McCloskey, J. A., Ed.) 1990, 193, 351–360.Google Scholar
  2. 2.
    Busch, K. L.; Glish, G. L.; McLuckey, S. A. Mass Spectrometry/Mass Spectrometry: Techniques and Applications of Tandem Mass Spectrometry; VCH Publishers: New York, 1988, pp 248–265.Google Scholar
  3. 3.
    Woodin, R. L.; Bomse, D. S.; Beauchamp, J. L. Multiphoton Dissociation of Molecules with Low Power Continuous Wave Infrared Laser Radiation. J. Am. Chem. Soc. 1978, 100, 3248–3250.CrossRefGoogle Scholar
  4. 4.(a)
    Gatlin, C. L.; Rao, R. D.; Turecček, F.; Vaisar, T. Carboxylate and Amine Terminus Directed Fragmentations in Gaseous Dipeptide Complexes with Copper(II) and Diimine Ligands Formed by Electrospray. Anal. Chem 1996, 68, 263–270.CrossRefGoogle Scholar
  5. 4.(b)
    Vaisar, T.; Gatlin, C. L.; Rao, R. D.; Seymour, J. L.; Turecček, F. Sequence Information, Distinction, and Quantitation of C-Terminal Leucine and Isoleucine in Ternary Complexes of Tripeptides with Cu(II) and 2,2′-Bipyridine. J. Mass Spectrom 2001, 36, 306–316.CrossRefGoogle Scholar
  6. 5.(a)
    Chu, I. K.; Rodriguez, C. F.; Lau, T. C.; Hopkinson, A. C.; Siu, K. W. M. Molecular Radical Cations of Oligopeptides. J. Phys. Chem. B 2000, 104, 3393–3397.CrossRefGoogle Scholar
  7. 5.(b)
    Bagheri-Majdi, E.; Ke, Y.; Orlova, G.; Chu, I. K.; Hopkinson, A. C.; Siu, K. W. M. Copper-Mediated Peptide Radical Ions in the Gas Phase. J. Phys. Chem. B 2004, 108, 11170–11181.CrossRefGoogle Scholar
  8. 6.
    Rowe, B. R.; Mitchell, J. B.; Canosa, A, Eds.; In Dissociative Recombination. Theory, Experiment, and Applications; Plenum Press: New York, 1993.Google Scholar
  9. 7.
    Zubarev, R. A.; Kelleher, N. L.; McLafferty, F. W. Electron Capture Dissociation of Multiply Charged Protein Cations. A Nonergodic Process. J. Am. Chem. Soc. 1998, 120, 3265–3266.CrossRefGoogle Scholar
  10. 8.
    Kruger, N. A.; Zubarev, R. A.; Carpenter, B. K.; Kelleher, N. L.; Horn, D. M.; McLafferty, F. W. Electron-Capture Dissociation of Multiply Charged Peptide Cations. Int. J. Mass Spectrom 1999, 183, 1–5.CrossRefGoogle Scholar
  11. 8.(a)
    McLafferty, F. W.; Horn, D. M.; Breuker, K.; Ge, Y.; Lewis, M. A.; Cerda, B.; Zubarev, R. A.; Carpenter, B. K. Electron Capture Dissociation of Gaseous Multiply Charged Ions by Fourier-Transform Ion Cyclotron Resonance. J. Am. Soc. Mass Spectrom 2001, 12, 245–249.CrossRefGoogle Scholar
  12. 8.(b)
    Zubarev, R. A.; Haselmann, K. F.; Budnik, B.; Kjeldsen, F.; Jensen, F. Towards an Understanding of the Mechanism of Electron-Capture Dissociation: A Hhistorical Perspective and Modern Ideas. Eur. J. Mass Spectrom 2002, 8, 337–349.CrossRefGoogle Scholar
  13. 8.(c)
    Zubarev, R. A. Reactions of Polypeptide Ions in the Gas Phase. Mass Spectrom. Re 2003, 22, 57–77.CrossRefGoogle Scholar
  14. 9.(a)
    Cooper, H. J.; Hudgins, R. R.; Hakansson, K.; Marshall, A. G. Secondary Fragmentation of Linear Peptides in Electron Capture Dissociation. Int. J. Mass Spectrom 2003, 228, 723–728.CrossRefGoogle Scholar
  15. 9.(b)
    Kjeldsen, F.; Zubarev, R. Secondary Losses via γ-Lactam Formation in Hot Electron Capture Dissociation: A Missing Link to Complete de Novo Sequencing of Proteins? J. Am. Chem. Soc 2003, 125, 6628–6629.CrossRefGoogle Scholar
  16. 9.(c)
    Leymarie, N.; Costello, C. E.; O’Connor, P. B. Electron Capture Dissociation Initiates a Free Radical Reaction Cascade. J. Am. Chem. Soc 2003, 125, 8949–8958.CrossRefGoogle Scholar
  17. 10.
    Syrstad, E. A.; Turecček, F. Hydrogen Atom Adducts to the Amide Bond. Generation and Energetics of the Amino(Hydroxy)Methyl Radical in the Gas Phase. J. Phys. Chem. A 2001, 105, 11144–11155.CrossRefGoogle Scholar
  18. 11.
    Syrstad, E. A.; Stephens, D. D.; Turecček, F. Hydrogen Atom Adducts to the Amide Bond. Generation and Energetics of Amide Radicals in the Gas Phase. J. Phys. Chem. A 2003, 107, 115–126.CrossRefGoogle Scholar
  19. 12.
    Konishi, H.; Yokotake, Y.; Ishibashi, T. Theoretical Study on the Electron Capture Dissociation Correlated with Proton Transfer Processes. J. Mass Spectrom. Soc. Jpn 2002, 50, 222–225.Google Scholar
  20. 13.
    Turecček, F.; Syrstad, E. A. Mechanism and Energetics of Intramolecular Hydrogen Transfer Atom Transfer in Amide and Peptide Radicals and Cation-Radicals. J. Am. Chem. Soc. 2003, 125, 3353–3369.CrossRefGoogle Scholar
  21. 14.
    Turecček, F. N—Cα Bond Dissociation Energies and Kinetics in Amide and Peptide Radicals. Is the Dissociation a Non-Ergodic Process? J. Am. Chem. Soc. 2003, 125, 5954–5963.CrossRefGoogle Scholar
  22. 15.
    Turecček, F.; Syrstad, E. A.; Seymour, J. L.; Chen, X.; Yao, C. Peptide Cation-Radicals: A Computational Study of the Competition Between Peptide N—Cα Bond Cleavage and Loss of the Side Chain in the [GlyPhe-NH2 + 2H] Cation Radical. J. Mass Spectrom. 2003, 38, 1093–1104.CrossRefGoogle Scholar
  23. 16.
    Turecček, F. Stereochemical Interactions in Ammonium Dications, Hypervalent Diammonium Cation-Radicals, and Ammonium Radicals: A B3-MP2 Computational Study. Eur. J. Mass Spectrom 2003, 9, 267–277.CrossRefGoogle Scholar
  24. 17.
    Sawicka, A.; Skurski, P.; Hudgins, R. R.; Simons, J. Model Calculations Relevant to Disulifde Bond Cleavage via Electron Capture Influenced by Positively Charged Groups. J. Phys. Chem. B 2003, 107, 13505–13511.CrossRefGoogle Scholar
  25. 18.
    Hudgins, R.R., Hakansson, K., Quinn, J.P.; Hendrickson,C. L.; Marshall, A.G. Electron Capture Dissociation of Peptides and Proteins Does Not Require a Hydrogen Atom Mechanism; Proceedings of the 50th ASMS Conference on Mass Spectrometry and Allied Topics; Orlando, FL, June, 2002; A020420.Google Scholar
  26. 19.
    Mihalca, R.; Kleinnijenhuis, A. J.; McDonnell, L. A.; Heck, A. J. R.; Heeren, R. M. A; Electron Capture Dissociation at Low Temperatures Reveals Selective Dissociations. J. Am. Soc. Mass Spectrom. 2004, in press.Google Scholar
  27. 20.
    Breuker, K.; Oh, H.-B.; Lin, C.; Carpenter, B. K.; McLafferty, F. W. Nonergodic and Conformational Control of the Electron Capture Dissociation of Protein Cations. Proc. Natl. Acad. Sci. U.S.A 2004, 101, 14011–14016.CrossRefGoogle Scholar
  28. 21.
    Demirev, P. A. Generation of Hydrogen Radicals for Reactivity Studies in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Rapid. Commun. Mass Spectrom. 2000, 14, 777–781.CrossRefGoogle Scholar
  29. 22.
    Bakken, V.; Helgaker, T.; Uggerud, E. Mechanism of Electron Capture Dissociation of Protonated Peptides. Presented at the 18th International Mass Spectrometry Conference; Edinburgh, August, 2003; Poster ThL-001.Google Scholar
  30. 23.
    Zubarev, R. A.; Horn, D. M.; Fridriksson, E. K.; Kelleher, N. L.; Kruger, N. A.; Lewis, M. A.; Carpenter, B. K.; McLafferty, F. W. Electron Capture Dissociation for Structural Characterization of Multiply Charged Protein Cations. Anal. Chem. 2000, 72, 563–573.CrossRefGoogle Scholar
  31. 24.
    Bates, D. R. Dissociative Recombination. Phys. Rev. 1950, 78, 492–493.CrossRefGoogle Scholar
  32. 25.
    Ketvirtis, A. E.; Simons, J. Dissociative Recombination of H3O+. J. Phys. Chem. A 1999, 103, 6552–6563.CrossRefGoogle Scholar
  33. 26.
    Bardsley, J. N. Theory of Dissociative Recombination. J. Phys. B 1968, 1, 365–380.CrossRefGoogle Scholar
  34. 27.
    (a) Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Zakrzewski, V. G., Montgomery, J. A., Stratmann, R. E., Burant, J. C., Dapprich, S., Millam, J. M., Daniels, A. D., Kudin, K. N., Strain, M. C., Farkas, O., Tomasi, J., Barone, V., Cossi, M., Cammi, R., Mennucci, B., Pomelli, C., Adamo, C., Clifford, S., Ochterski, J., Petersson, G. A., Ayala, P. Y., Cui, Q., Morokuma, K., Malick, D. K., Rabuck, A. D., Raghavachari, K., Foresman, J. B., Cioslowski, J., Ortiz, J. V., Stefanov, B. B., Liu, G., Liashenko, A., Piskorz, P., Komaromi, I., Gomperts, R., Martin, R. L., Fox, D. J., Keith, T., Al-Laham, M. A., Peng, C. Y., Nanayakkara, A., Gonzalez, C., Challacombe, M., Gill, P. M. W., Johnson, B. G., Chen, W., Wong, M. W., Andres, J. L., Head-Gordon, M., Replogle, E. S., Pople, J. A. Gaussian 98, Revision A.6; Gaussian, Inc., Pittsburgh, PA; 1998.Google Scholar
  35. 27.(b)
    Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Montgomery, J. A., Vreven, T., Kudin, K. N., Burant, J. C., Millam, J. M., Iyengar, J., Tomasi, J., Barone, V., Mennucci, B., Cossi, M., Scalmani, G., Rega, N., Petersson, G. A., Nakatsuji, H., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Klene, M., Li, X., Knox, J. E., Hratchian, H. P., Cross, J. B., Adamo, C., Jaramillo, J., Gomperts, R., Strattman, R. E., Yazyev, O., Austin, A. J., Cammi, R., Pomelli, C., Ochterski, J. W., Ayala, P. Y., Morokuma, K., Voth, G. A., Salvador, P., Dannenberg, J. J., Zakrzewski, V. G., apprich, S., Daniels, A. D., Strain, M. C., Farkas, O., Malick, D. K., Rabuck, A. D., Raghavachari, K., Foresman, J. B., Ortiz, J. V., Cui, Q., Baboul, A. G., Clifford, S., Cioslowski, J., Stefanov, B. B., Liu, G., Liashenko, A., Piskorz, P., Komaromi, I., Martin, R. L., Fox, D. J., Keith, T., Al-Laham, M. A., Peng, C. Y., Nanayakkara, A., Challacombe, M., Gill, P. M. W., Johnson, B., Chen, W., Wong, M. W., Gonzalez, C., Pople, J. A. Gaussian 03, Revision B.05, Gaussian, Inc., Pittsburgh, PA; 2003.Google Scholar
  36. 28.(a)
    Becke, A. D. A New Mixing of Hartree-Fock and Local Density-Functional Theories. J. Chem. Phys 1993, 98, 1372–1377.CrossRefGoogle Scholar
  37. 28.(b)
    Becke, A. D. Density Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys 1993, 98, 5648–5652.CrossRefGoogle Scholar
  38. 29.
    Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural Population Analysis. J. Chem. Phys. 1985, 83, 735–746.CrossRefGoogle Scholar
  39. 30.
    Dunning, T. H. Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007–1023.CrossRefGoogle Scholar
  40. 31.
    Curtiss, L. A.; Raghavachari, K.; Pople, J. A. Gaussian-2 Theory Using Reduced Moller-Plesset Orders. J. Chem. Phys. 1993, 98, 1293–1298.CrossRefGoogle Scholar
  41. 32.
    Pople, J. A.; Head-Gordon, M.; Raghavachari, K. Quadratic Configuration Interaction. A General Technique for Determining Electron Correlation Energies. J. Chem. Phys. 1987, 87, 5968–5975.CrossRefGoogle Scholar
  42. 33.(a)
    Turecček, F. Proton Affinity of Dimethyl Sulfoxide and Relative Stabilities of C2H6OS Molecules and C2H7OS+ Ions: A Comparative G2(MP2) ab Initio and Density Functional Theory Study. J. Phys. Chem. A 1998, 102, 4703–4713.CrossRefGoogle Scholar
  43. 33.(b)
    Turecček, F.; Wolken, J. K. Dissociation Energies and Kinetics of Aminopyrimidinium Radicals by ab Initio and Density Functional Theory. J. Phys. Chem. A 1999, 103, 1905–1912.CrossRefGoogle Scholar
  44. 33.(c)
    Turecček, F.; Polášek, M.; Frank, A. J.; Sadílek, M. Transient Hydrogen Atom Adducts to Disulfides. Formation and Energetics. J. Am. Chem. Soc 2000, 122, 2361–2370.CrossRefGoogle Scholar
  45. 33.(d)
    Polášek, M.; Turecček, F. Hydrogen Atom Adducts to Nitrobenzene. Formation of the Phenylnitronic Radical in the Gas Phase and Energetics of Wheland Intermediates. J. Am. Chem. Soc 2000, 122, 9511–9524.CrossRefGoogle Scholar
  46. 34.
    Cčížek, J.; Paldus, J.; Šroubková, L. Cluster Expansion Analysis for Delocalized Systems. Int. J. Quantum Chem. 1969, 3, 149–167.CrossRefGoogle Scholar
  47. 35.
    Purvis, G. D., III; Bartlett, R. J. A Full Coupled-Cluster Singles and Doubles Model: The Inclusion of Disconnected Triples. J. Chem. Phys 1982, 76, 1910–1918.CrossRefGoogle Scholar
  48. 36.(a)
    Schlegel, H. B. Potential Energy Curves Using Unrestricted Moller-Plesset Perturbation Theory with Spin Annihilation. J. Chem. Phys 1986, 84, 4530–4534.CrossRefGoogle Scholar
  49. 36.(b)
    Mayer, I. Spin-Projected UHF Method. IV. Comparison of Potential Curves Given by Different One-Electron Methods. Adv. Quantum Chem 1980, 12, 189–262.CrossRefGoogle Scholar
  50. 37.
    Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. An Efficient Implementation of Time-Dependent Density Functional Theory for the Calculation of Excitation Energies of Large Molecules. J. Chem. Phys. 1998, 109, 8218–8224.CrossRefGoogle Scholar
  51. 38.
    Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Toward a Systematic Molecular Orbital Theory for Excited States. J. Phys. Chem. 1992, 96, 135–149.CrossRefGoogle Scholar
  52. 39.
    Gellene, G. I.; Cleary, D. A.; Porter, R. F. Stability of the Ammonium and Methylammonium Radicals from Neutralized Ion-Beam Spectroscopy. J. Chem. Phys. 1982, 77, 3471–3477.CrossRefGoogle Scholar
  53. 40.
    Nguyen, V. Q.; Sadílek, M.; Ferrier, J. G.; Frank, A. J.; Turecček, F. Metastable States of Dimethylammonium Radical. J. Phys. Chem. A 1997, 101, 3789–3799.CrossRefGoogle Scholar
  54. 41.
    Shaffer, S. A.; Turecček, F. Hydrogentrimethylammonium. A Marginally Stable Hypervalent Radical. J. Am. Chem. Soc. 1994, 116, 8647–8653.CrossRefGoogle Scholar
  55. 42.
    Shaffer, S. A.; Turecček, F. Hydrogen Bonding in Bifunctional Hypervalent Ammonium and Oxonium Radicals. J. Am. Soc. Mass Spectrom. 1995, 6, 1004–1018.CrossRefGoogle Scholar
  56. 43.
    Shaffer, S. A.; Sadílek, M.; Turecček, F. Hypervalent Ammonium Radicals. Effects of Alkyl Groups and Aromatic Substituents. J. Org. Chem. 1996, 61, 5234–5245.CrossRefGoogle Scholar
  57. 44.
    Wolken, J. K.; Nguyen, V. Q.; Turecček, F. Bond Dissociations in Hypervalent Ammonium Radicals Prepared by Collisional Neutralization of Protonated Six-Membered Nitrogen Heterocycles. J. Mass Spectrom. 1997, 32, 1162–1169.CrossRefGoogle Scholar
  58. 45.
    Shaffer, S. A.; Wolken, J. K.; Turecček, F. Neutralization-Reionization of Alkenylammonium Cations: An Experimental and ab Initio Study of Intramolecular N—H...C=C Interactions in Cations and Hypervalent Ammonium Radicals. J. Am. Soc. Mass Spectrom. 1997, 8, 1111–1123.CrossRefGoogle Scholar
  59. 46.
    Frøsig, L.; Turecček, F. Hypervalent Pyrrolidinium Radicals by Neutralization-Reionization Mass Spectrometry. Metastability and Radical Leaving Group Abilities. J. Am. Soc. Mass Spectrom. 1998, 9, 242–254.CrossRefGoogle Scholar
  60. 47.
    Boldyrev, A. I.; Simons, J. Theoretical Search for Large Rydberg MoleculesMethyl Derivatives of Ammonium Radical [NH3CH3, NH2(CH3)2, NH(CH3)3, and N(CH3)4]. J. Chem. Phys. 1992, 97, 6621–6627.CrossRefGoogle Scholar
  61. 48.
    Okai, N.; Takahata, A.; Fuke, K. Electronic Structure, Stability, and Formation Dynamics of Hypervalent Molecular ClustersCH3NH3(CH3NH2)n. Chem. Phys. Lett 2004, 386, 442–447.CrossRefGoogle Scholar
  62. 49.
    Yao, C.; Turecček, F. Hypervalent Ammonium Radicals. Competitive N—C and N—H Bond Dissociations in Methylammonium and Ethylammonium; unpublished.Google Scholar
  63. 50.
    Hendricks, J. H.; Lyapustina, S. A.; de Clercq, H. L.; Bowen, K. H. The Dipole Bound-to-Covalent Anion Transformation in Uracil. J. Chem. Phys 1998, 108, 8–11.CrossRefGoogle Scholar
  64. 51.
    Rao, D. N. R.; Symons, M. C. R.; Stephenson, J. M. Radiation- Induced Electron Capture by Proteins Containing Disulfide Linkages: An Electron Spin Resonance Study. J. Chem. Soc. Perkin Trans. 2 1983, 727–730.Google Scholar
  65. 52.
    Stryer, L. Biochemistry; W. H. Freeman and Company: New York, 1975; p 34.Google Scholar
  66. 53.
    Hunter, E. P.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of MoleculesAn Update. J. Phys. Chem. Ref. Data 1998, 27, 413–656.CrossRefGoogle Scholar
  67. 54.
    Harrison, A. G. The Gas-Phase Basicities and Proton Affinities of Amino Acids and Peptides. Mass. Spectrom. Rev. 1997, 16, 201–217.CrossRefGoogle Scholar
  68. 55.
    Turecček, F.; Reid, P. J. Metastable States of Dimethyloxonium, (CH3)2OH·. Int. J. Mass Spectrom. 2003, 222, 49–61.CrossRefGoogle Scholar
  69. 56.(a)
    Chan, T.-W. D.; Ip, W. H. H. Optimization of Experimental Parameters for Electron Capture Dissociation of Peptides in a Fourier Transform Mass Spectrometer. J. Am. Soc. Mass Spectrom 2002, 8, 1396–1406.CrossRefGoogle Scholar
  70. 56.(b)
    Polfer, N. C.; Haselmann, K. F.; Zubarev, R. A.; Langridge-Smith, P. R. R. Electron Capture Dissociation of Polypeptides Using a 3 Tesla Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Rapid Commun. Mass Spectrom 2002, 16, 936–943.CrossRefGoogle Scholar
  71. 56.(c)
    Iavarone, A. T.; Paech, K.; Williams, E. R. Effects of Charge State and Cationizing Agent on the Electron Capture Dissociation of a Peptide. Anal. Chem 2004, 76, 2231–2238.CrossRefGoogle Scholar
  72. 57.(a)
    Vaisar, T.; Gatlin, C. L.; Turecček, F. Oxidation of Peptide—Copper Complexes by Alkali Metal Cations in the Gas Phase. J. Am. Chem. Soc 1996, 118, 5314–5315.CrossRefGoogle Scholar
  73. 57.(b)
    Vaisar, T.; Gatlin, C. L.; Turecček, F. Metal-Ligand Redox Reactions in Gas-Phase Quaternary Peptide—Metal Complexes by Electrospray Ionization Mass Spectrometry. Int. J. Mass Spectrom. Ion Processes 1997, 162, 77–87.CrossRefGoogle Scholar
  74. 58.
    Cournoyer, J. J.; Pittman, J. L.; Ivleva, V. B.; Fallows, E.; Waskell, L.; Costello, C. E.; O’Connor, P. B. Differentiation of α-versus β-Aspartic Acid Residues in Peptides by Electron Capture Dissociation. Proceedings of the 52nd ASMS Conference on Mass Spectrometry and Allied Topics; Nashville, TN, May, 2004, A041958.Google Scholar
  75. 59.
    Turecček, F.; unpublished.Google Scholar

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Authors and Affiliations

  1. 1.Department of ChemistryUniversity of WashingtonSeattleUSA

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