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Oxidation of carboxyamidomethyl cysteine may add complexity to protein identification

Protein identification by interrogation of databases requires a comprehensive compilation of modified amino acids forms. Here, we describe the chemical oxidation of carboxyamidomethyl cysteine to the sulfoxide and sulfone forms, species that may add more complexity to peptide analyses. They can be easily distinguished by tandem mass spectrometry (MS/MS) due to their characteristic pattern of side chain neutral eliminations either from the parent ion or ion series that generate dehydroalanine as detected by MS3. This finding was supported by the MSn spectra recorded for a peptide isolated from a mixture of tryptic peptides and for a derivatized/oxidized synthetic peptide with a different sequence. These modifications and their diagnostic neutral losses should be included in the list of chemical modifications and in algorithms designed for the automatic sequencing of peptides and database searching.