FIGURE 4.

MS2 fragment ion pattern of hydroxylated (P-P-G)₅ and (P-P-G)₁₀ peptides. Assignment of b (blue) and y (red) fragment ion series in the MS2 scans from [(P-P-G)₅ +OH]⁺² (A) and [(P-P-G)₁₀ + OH]⁺³ (B) precursor ions. Using a variation of the Ascore algorithm, the hydroxylation was localized with >99% confidence to the Y position in the 4th repeat for (P-P-G)₅ and to the X position in the 5th repeat for (P-P-G)₁₀. The precursor peptides were selected from the single-turnover experiments.