Acetylation reactions for mass spectrometry analysis were performed like those for the aforementioned acetyltransferase assays. However, to obtain sufficient signal for detection after subsequent processing, 75 nM p300 or 200 nM CBP enzyme was used as well as 50 μM nonradioactive acetyl-CoA and 1 μM nucleosome. Reactions were quenched with 4 volumes of trichloroacetic acid, mixtures washed with acetone and dried, and free lysines blocked by treatment with propionic anhydride followed by pH adjustment with ammonium hydroxide and incubation for 1 h at 51 °C. The modified proteins were digested in trypsin overnight at 37 °C before separation of peptides on a Waters Acquity UPLC system with a BEH C18 column and identification on a Thermo TSQ Quantum Access triple-quadrupole mass spectrometer. Peptide elution was monitored by selected reaction monitoring for H3 and H4 acetylated and propionylated peptides.17 (link) Six time points were taken for each reaction, and the data were plotted with respect to time to determine rates and standard deviations. Statistical significance was determined using unpaired two-tailed t tests and the Holm–Sidak method to correct for multiple comparisons (α = 5.000%).