IDENTIFICATION OF OXIDIZED METHIONINE IN PEPTIDES

The positive- and negative-ion collision induced dissociation spectra of peptides containing methionine, methionine sulphoxide and methionin e sulphone have been studied. Characteristic fragmentations were ident ified and evaluated as possible indicators for the presence of oxidize d methionine residues in peptides. It was found that the elimination o f CH3SOH (-64 u) from [M+H](+) is unique for peptides that contain met hionine sulphoxide. Sequence ions containing the oxidized methionine u ndergo the same elimination, allowing unambiguous sequence determinati on. Methionine sulphone exhibits an analogous elimination of CH3SO2H ( -80 u) from the protonated molecule, but not from sequence ions.