FIGURE 5.

Sequencing of H3K9me2-labeled intermediate. MS/MS spectrum of the [M+2H]²⁺ ion at 523.318 m/z, which was determined to be generated from the peptide _prK_me2STGGK_prAPR. Expected mono-isotopic b- (top row) and y-type (bottom row) ion fragment masses are displayed, with fragments that could be annotated as underlined. We found that the Lys-9 residue contained both a single unlabeled and ¹³CD₃-labeled methyl group. [M+2H-H₂O]²⁺ refers to the +2 peptide with loss of water. pr, propionyl amide group from chemical derivatization of histone proteins.