FIGURE 4.

Assignment of an O-Man α-DG glycopeptide. From the full scan acquired at 52.38 min (a), a peak at 895.8 m/z was selected for fragmentation. b, resulting MS/MS of 895.8 m/z yielded the neutral loss of terminal SA residue, which was then followed by MS³ fragmentation indicating the neutral loss of a Gal-GalNAc residue followed by a reducing end Man. The combined glycan structure was determined to belong to a peptide with 971.5 m/z. From examining the MS/MS spectra (data not shown) and the neutral loss-triggered MS³ spectra (c), the site of post-translational modification to the serine within the peptide LETASPPTR is assigned.