Figure 4.

ETD MS/MS spectrum of m/z 516.55⁴⁺ derived from the C. difficile orf3::erm mutant. Peptide fragmentation provides strong evidence for the sequence DILAAQNLTTGAVILNK. c-ions are labeled in orange and z-ions in purple. Following the known ETD mechanism, the cleavage of the N–Cα bond occurs producing C-type (N-terminal) and Z-type (C-terminal) fragments. The mass difference between two adjacent c or z ions provides the mass and identity of the amino acid residue and any substitution, which here identifies the glycan as being attached to threonine 38 (see the text).