An automatic method for disulfide bond assignment using dimethyl labeling and computational screening of a(1) ions with customized software, RADAR, is developed. By utilization of the enhanced a(1) ions generated from labeled peptides, the N-terminal amino acids from disulfide-linked peptides can be determined. In this study, we applied this method for structural characterization of recombinant monoclonal antibodies, an important group of therapeutic proteins. In addition to a(1) ion screening and molecular weight match, new RADAR is capable of confirming the matched peptide pairs by further comparing the collision-induced dissociation (CID) fragment ions. With the N-terminal amino acid identities as a threshold, the identification of disulfide-linked peptide pairs can be achieved rapidly at a higher confidence level. Unlike most current approaches, prior knowledge of disulfide linkages or a high-end mass spectrometer is not required, and tedious work or deliberate interpretation can be avoided in this study. Our approach makes it possible to analyze unknown disulfide bonds of protein pharmaceuticals as well as their degraded forms without further protein separation. It can be used as a convenient quality examination tool during biopharmaceutical development and manufacturing processes.
