Lysozyme oxidation by singlet molecular oxygen: Peptide characterization using [ 18 O]‐labeling oxygen and nLC‐MS/MS

Singlet molecular oxygen ( 1 O 2 ) is generated in biological systems and reacts with different biomolecules. Proteins are a major target for 1 O 2 , and His, Tyr, Met, Cys, and Trp are oxidized at physiological pH. In the present study, the modification of lysozyme protein by 1 O 2 was investigated using mass spectrometry approaches. The experimental findings showed methionine, histidine, and tryptophan oxidation. The experiments were achieved using [ 18 O]‐labeled 1 O 2 released from thermolabile endoperoxides in association with nano‐scale liquid chromatography coupled to electrospray ionization mass spectrometry. The structural characterization by nLC‐MS/MS of the amino acids in the tryptic peptides of the proteins showed addition of [ 18 O]‐labeling atoms in different amino acids.