Figure 4.(A) Precursor mass of the peptide with CH2 dimethyl labeling (top left) and the precursor mass of the peptide with CD2 dimethyl labeling (top right). The mass difference is not a constant 4 Da but 80 Da, accounting for the difference between S and Y (76 Da), as indicated in the figure. (B) CID-MS2 of the precursor ion from Figure 4A, CH2 dimethyl labeling (top) and CD2 dimethyl labeling (bottom).