The promising results for lysozyme protein prompted us to attempt the reaction incorporating a biotinylated peptide (Ac-Cys(Acm)(O)-miniPEG2-Lys(Biotin)-NH2 ( )) into trastuzumab 15, a monoclonal antibody against HER2 antigen as a proof-of-concept trial (Figure ). The MgCl2 (30 mM)-mediated reaction of trastuzumab 15 (0.01 mM) with 14 (1.5 mM) in 5% H2O/BMPy•OTf containing 0.1% TFA for 24 h at 37 °C gave the corresponding labelled proteins 16. The SDS-PAGE analysis of the obtained protein sample followed by western-blotting with anti-biotin antibody-HRP conjugate showed that peptide label was incorporated on the heavy and light chains (Figures and). LC-MS/MS analyses of the protein showed that labeling was predominantly at the Trp110 residue in the heavy chain (Figure ). The HER2 antigen-recognition ability of the resulting trastuzumab-biotin conjugate 16 was confirmed by confocal microscopy using SK-BR-3 breast cancer cells (HER2 (+)) and MDA-MB231 cells (HER2 (-)). An immunofluorescence signal by an anti-human IgG-fluorescein dye conjugate was and). A signal by anti-biotin antibody was also detected (Figure ). Additionally, we conducted FACS analysis of the trastuzumab-biotin conjugate 16 using an antihuman-IgG antibody, which indicated that the affinity of the modified 16 remained unchanged. (Figure ). The ability to perform this reaction and obtain products that retain protein function highlights the potential utility for protein labeling of Trp-sulfenylation enabled by MgCl2-mediated activation of the S-Acm oxide unit.