FIGURE 4.

Acetylation stoichiometry is detected and quantified across a full range of values. Light and Δ5-acetic anhydride-labeled BSA protein digests were mixed at different ratios corresponding to 1–99% acetylation followed by MS analysis and quantitation. A, MS/MS spectrum for the acetylated BSA peptide, ALK(Ac)AWSVAR, is shown. (Product ions shown are charge 1⁺.) B–D, MS1 spectrum showing light and heavy forms of the peptide in A (charge 2⁺) corresponding to 10, 50, 90% acetylation, respectively, with a mass shift of ∼2.5 m/z. E, scatterplot comparing percent volume input (light/total) to the corrected measured stoichiometry of the BSA peptide, ALK(Ac)AWSVAR. Stoichiometry values were corrected due to the isotopic purity of ∼96%. F, linear regression analysis of all the acetyl peptides quantified shows the best fit line and high R² value.