title: Novel chemical degradation pathways of proteins mediated by tryptophan oxidation: tryptophan side chain fragmentation.
abstract: OBJECTIVE
  This minireview focuses on novel degradation pathways of proteins in solution via intermediary tryptophan (Trp) radical cations, which are generated via photo-induced electron transfer to suitable acceptors such as disulfide bonds.
  
  
  METHODS
  Gas-phase mass spectrometry studies had indicated the potential for Trp radical cations to fragment via release of 3-methylene-3H-indol-1-ium from the side chain. HPLC-MS/MS analysis demonstrates that analogous fragmentation reactions occur during the exposure of peptides and proteins to light or accelerated stability testing.
  
  
  RESULTS
  The light exposure of selected peptides and monoclonal antibodies leads to the conversion of Trp to glycine (Gly) or glycine hydroperoxide (GlyOOH), where GlyOOH could be reduced to hydroxyglycine, which undergoes subsequent cleavage. Product formation is consistent with Cα-Cβfragmentation of intermediary Trp radical cations. For the peptide octreotide and specific glycoforms of IgG1 Fc domains, Trp side chain cleavage in aqueous solution is indicated by the formation of 3-methyleneindolenine (3-MEI), which adds to nucleophilic side chains, for example to Lys residues adjacent to the original Trp residues.
  
  
  CONCLUSIONS
  Trp side chain cleavage leads to novel reaction products on specific peptide and protein sequences, which may have consequences for potency and immunogenicity.
authors.affiliation: Department of Pharmaceutical Chemistry, The University of Kansas, Lawrence, KS, USA.
authors.forename: Christian
authors.initials: C
authors.lastname: Schöneich
mesh_terms: D000595:Amino Acid Sequence; D002851:Chromatography, High Pressure Liquid; D008401:Gas Chromatography-Mass Spectrometry; D010084:Oxidation-Reduction; D010455:Peptides / Q000737:chemistry*; D055550:Protein Stability; D011506:Proteins / Q000737:chemistry*; D053719:Tandem Mass Spectrometry; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D010455:Peptides; D011506:Proteins; D014364:Tryptophan
keywords: fragmentation; oxidation; photochemistry; radical cation; tryptophan