Fig. 3.
A, Base peak chromatogram of SPAER-enriched low-abundance nitropeptides #IINEPTAAAIAYΔGLDK#, #VSFLSALEEYΔTK# and #GDYΔDLNAVR (where # indicates dimethylation of the amino terminus and YΔ denotes conversion of 3-nitrotyrosine to the 4-formylbenzoylamido derivative) showing interference-free data-dependent LC-ESI-MS/MS analysis of the tagged species. B, Full-scan ESI mass spectrum at tR = 28.74 min of #GDYΔDLNAVR ([M+2H]+ at m/z 599.2788) is shown as an example for derivatized and tagged nitropeptide, C, CID-MS/MS product-ion spectrum recorded from m/z 599.2788 as precursor. Peaks marked with dots were non-peptidic impurities, whereas the peak with the § was singly charged species excluded from MS/MS analysis upon data-dependent acquisition. As a minor constituent in the peak also marked with the + sign, the fully dimethylated BSA tryptic peptide #LFTFHADIC@TLPDTEK# ([M+2H]+ at m/z 655.33; @: carbamidomethylated Cys) was identified as the only non-modified matrix species present.