A, selective ion chromatograms from LC-ESI-MS analysis of OA-NO2-modified peptide 18 (LCQ Deca; Thermo Electron Corp.). The nitroalkylation of peptide 18 (native peptide (M + H)+ = 1229.6, room temperature 27.9 min) by OA-NO2 increased the retention time to 76 min, and the mass of the OA-NO2-modified peptide 18 was increased by 327 Da, equivalent to the neutral ion mass of OA-NO2, becoming m/z 1556.7. B, LC nanospray MS/MS spectrum of the nitroalkylated peptide 18 (LTQ; Thermo Electron Corp.). MS/MS spectrum of the doubly charged ion at m/z 844.34. Colors are annotated in the corresponding table. The yo and bo nomenclature indicates the corresponding y-H2O and b-H2O fragments, respectively. Inset, amino acid sequence of peptide 18 indicating major C- and N-terminal fragment ions detected by full-scan MS/MS. C, MALDI-TOF mass spectrum of the tryptic digest of OA-NO2-treated GAPDH (Voyager DE Pro, Applied Biosystem, Foster City, CA), focusing on nitroalkylated-peptide 18 ((M + H)+ = 1556.9). D, PSD MALDI-TOF-MS analysis of modified peptide 18 gives a main product ion at m/z 437.1, corresponding to the immonium ion of the histidine (H)-OA-NO2 adduct. E, structure and fragmentation pattern of the His-OA-NO2 adduct, showing the immonium adduct fragment (H-OA-NO2). Table list of MS/MS fragment ions m/z from peptide 18. Ions that are detected are highlighted in color (B). AA, amino acids.