Representative mass spectra of Hb β-globin peptides that contain cysteine (βCys93) in the presence and absence of Hp. (A) Nonoxidized Hb peptide (G₈₃TFATLSELC₉₃ [CH₂CONH₂] DKLHVDPENFR₁₀₄), showing the 56.0 mass unit alkylated Cys93 consistent with the absence of oxidation (m/z = 2585.2 [M + H]). (B) Oxidized Hb showing a mass addition of 47 to Cys93 (G₈₃TFATLSELC₉₃ [O3] DKLHVDPENFR₁₀₄) consistent with the trioxidation product, cysteic acid (m/z = 2576.2 [M + H]). Both panels C and D demonstrate the absence of significant oxidation on the Cys93 tryptic peptide before or after oxidation, consistent with spectra (A). LC/MS/MS amino acid sequence data for all oxidized peptides are included in Figure S1.