[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 gl yco-peptide was analyzed with electrospray ionization in the negative ioniz ation mode. The sequence assignment of this highly post-translationally mod ified peptide was based on the mass analysis using a quadrupole ion trap to gether with information from both Edman and DNA sequencing. Because there w as little precedent for the loss of five protons from a ferric cationized p eptide, we utilized Fourier transform mass spectrometry accurate mass and t andem mass spectrometry analyses to verify the peptide ion composition. (J Am Soc Mass Spectrom 2000, 11, 83-87) (C) 2000 American Society for Mass Sp ectrometry.