title: Deamidation reactions of protonated asparagine and glutamine investigated by ion spectroscopy.
abstract: BACKGROUND
  Deamidation of Asn and Gln residues is a primary route for spontaneous post-translational protein modification. Several structures have been proposed for the deamidation products of the protonated amino acids. Here we verify these structures by ion spectroscopy, as well as the structures of parallel and sequential fragmentation products.
  
  
  METHODS
  Infrared ion spectroscopy using the free electron laser FELIX has been applied to the reaction products from deamidation of protonated glutamine and asparagine in a tandem mass spectrometer. IR spectra were recorded over the 800-1900 cm(-1) spectral range by infrared multiple-photon dissociation (IRMPD) spectroscopy. Molecular structures of the fragment ions are derived from comparison of the experimental spectra with spectra predicted for different candidate structures by density functional theory (DFT) calculations.
  
  
  RESULTS
  [AsnH(+) -NH3](+) is found to possess a 3-aminosuccinic anhydride structure protonated on the amino group. The dissociation reaction involving loss of H2O and CO forms a linear immonium ion. For [GlnH(+)-NH3](+), the N-terminal nitrogen acts as the nucleophile leading to an oxo-proline product ion structure. For [GlnH(+)-NH3](+), a sequential loss of [CO + H2O] is found, leading to a pyrolidone-like structure. We also confirm by IR spectroscopy that dehydration of protonated aspartic acid (AspH(+)) and glutamic acid (GluH(+)) leads to identical structures as to those found for the loss of NH3 from AsnH(+) and GlnH(+).
  
  
  CONCLUSIONS
  The structure determined for AsnH(+) is in agreement with the suggested structure derived from measured and computed activation energies. IR ion spectra for the NH3 -loss product from GlnH(+) establish that a different reaction mechanism occurs for this species as compared to AsnH(+). For both amino acids, loss of NH3 occurs from the side chain.
authors.affiliation: Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED, Nijmegen, The Netherlands.
authors.forename: Lisanne J M
authors.initials: LJ
authors.lastname: Kempkes
authors.affiliation: Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED, Nijmegen, The Netherlands.
authors.forename: Jonathan K
authors.initials: JK
authors.lastname: Martens
authors.affiliation: Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED, Nijmegen, The Netherlands.
authors.forename: Josipa
authors.initials: J
authors.lastname: Grzetic
authors.affiliation: Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED, Nijmegen, The Netherlands.
authors.forename: Giel
authors.initials: G
authors.lastname: Berden
authors.affiliation: Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED, Nijmegen, The Netherlands.|Van't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands.
authors.forename: Jos
authors.initials: J
authors.lastname: Oomens
mesh_terms: D001216:Asparagine / Q000737:chemistry*; D005973:Glutamine / Q000737:chemistry*; D015394:Molecular Structure; D011522:Protons; D013055:Spectrophotometry, Infrared / Q000379:methods*
chemical_list: D011522:Protons; D005973:Glutamine; D001216:Asparagine