[M + Fe - 5H]2- peptide ion composition verified by Fourier transform mass spectrometry accurate mass and tandem mass spectrometry analyses.

Previously, the unusual ion composition [M + Fe - 5H]2- had been proposed as the major species observed when a gamma-carboxy glutamate-containing glyco-peptide was analyzed with electrospray ionization in the negative ionization mode. The sequence assignment of this highly post-translationally modified peptide was based on the mass analysis using a quadrupole ion trap together with information from both Edman and DNA sequencing. Because there was little precedent for the loss of five protons from a ferric cationized peptide, we utilized Fourier transform mass spectrometry accurate mass and tandem mass spectrometry analyses to verify the peptide ion composition.