Analysis of peptides containing oxidized methionine and/or tryptophan by fast atom bombardment mass spectrometry.

Treatment of peptides containing methionine and/or tryptophan with dimethylsulfoxide/hydrochloric acid/acetic acid resulted in oxidation of these amino acids respectively to methionine sulfoxide and oxyindolalanine. This reaction was monitored by fast atom bombardment mass spectrometry using a dithiothreitol/dithioerythritol liquid matrix. Under these conditions, only methionine and tryptophan were oxidized. Comparison of mass spectra of a sample before and after oxidation should provide a rapid screening procedure for determination of these residues in peptides.