Analysis of the modification site in a small molecule-modified peptide by ion trap/time-of-flight hybrid mass spectrometry.

Ion trap/time-of-flight hybrid mass spectrometers are powerful tools for the detailed structural analysis of modified peptides. We have analyzed Met-Lys-bradykinin modified with deoxycholate at the amino-terminus or the epsilon-amino group as model peptides. These two modified peptides produced fragment ions with the same nominal but different exact masses in tandem mass spectrometry with low-energy collision-induced dissociation. Accurate high-resolution analysis coupled with MS(3) allowed us to distinguish the deoxycholate modification sites in the modified peptides.