Figure 2.

¹H–¹³C HSQC spectra of the reference peptides for the detection of Met(O), lysozyme, and H₂O₂-treated rituximab. (a) Overlay of the reference spectra of peptides Ac–Gly–Gly–Met(O)–Gly–Gly–NH₂ (red) and Ac–Gly–Gly–Met–Gly–Gly–NH₂ (blue) under denaturing conditions (7 M urea, pH 2.3). (b) Overlay of the spectra of the reference peptide Ac–Gly–Gly–Met(O)–Gly–Gly–NH₂ (red) and denatured lysozyme (blue), to identify unique chemical shifts of Met(O) that differ from the random-coil chemical shift correlations of the 20 natural amino acids. (c) ¹H–¹³C HSQC spectrum of treated (0.35% H₂O₂ for 30 min at RT) rituximab (512 × 512 complex points, 104 scans, a recycle delay of 3 s, total measurement time of 2 days and 14 h) under denaturing conditions (7 M urea-d₄ in D₂O) at pH 2.3.