Fig. 4. Site selective peptide modification via quinone-mediated transamination reaction.

a Reaction conditions for COVID-19 spike protein fragment 21: (1) Transamination: 2 M Gn·HCl, Ox1 (10 mM, sat.), 200 mM Na₂HPO₄, pH 6.5, 6 h, 25 °C; (2) Oxime formation: EtONH₂·HCl (100 mM), pH 4.0, 3 h, 25 °C. b Reaction conditions for tetracosactide 22: (1) Transamination: Ox1 (10 mM, sat.), 200 mM Na₂HPO₄, pH 6.5, 3 h, 25 °C; (2) Oxime formation: EtONH₂·HCl (100 mM), pH 4.0, 3 h, 25 °C; c Reaction conditions for myoglobin 23a-b: (1) Transamination: Ox1 (10 mM, sat.), 200 mM Na₂HPO₄, pH 4.0, 5 h, 25 °C; (2) Oxime formation for 23a: EtONH₂·HCl (100 mM), pH 4.0, 3 h, 25 °C; oxime formation for 23b: extraction with EtOAc (2 ×1.0 mL), then biotinylated tag S5 (100 mM), pH 6.5, 16 h, 25 °C. d Reaction conditions for ubiquitin 24: (1) Transamination: 2 M Gn·HCl, Ox1 (10 mM, sat.), 200 mM Na₂HPO₄, pH 6.5, 3 h, 25 °C; (2) Oxime formation: EtONH₂·HCl (100 mM), pH 4.0, 3 h, 25 °C.