Representative tandem mass spectrum of a nitrotyrosine containing peptide ([M+2H]2+ ion at m/z 705.49). Following exposure to peroxynitrite, a +45-Da mass increment of the peptide is detected and can be attributed to the presence of a nitro group on either tyrosine or tryptophan residues. The fragmented ions unambiguously indicate the addition of the nitro group to the tyrosine residue, as inferred from the presence of the ions b3, b4, b5, and b6 at m/z 381.2,493.8, 565.3, and 679.4, respectively, and the ions y6, y7, and y8 at m/z 731.4,845.4, and 916.4, respectively. Y^, nitrated tyrosine.