Mapping the tandem mass spectrometric characteristics of citrulline-containing peptides.

We found that the citrulline effect is very pronounced and could be used as a complementary tool for the confirmation of modification sites in addition to losses of isocyanic acids from the protonated molecules or from fragment ions. Low collision energy applied to peptide ions having partially mobile protons reveals the site of modification by generating specific and intensive fragments of the sequence. On the other hand, fragmenting precursor ions with mobile protons usually allows full sequence coverage, although citrulline-specific fragments may exhibit lower intensities compared to other fragments.