Figure 2

Mass spectra of PTM-containing histone peptides. Mass spectra were collected with an Orbitrap mass analyzer for doubly charged peptides from bulk histone H3, H3K9acK14ac, bulk histone H4, and H4K12acK16ac. Blue circles indicate the isotopically light peak while red circles indicate the isotopically heavy peak. The percent isotopically light is shown in parentheses and in vivo formaldehyde (FA) cross-linking percentages are listed.