Chemical modifications of specific amino acids on proteins have paved the way for improvement of their chemical or pharmacological properties, thereby promoting the development of protein-based drugs. Among all proteinogenic residues, tryptophan (Trp) is considered an ideal residue selectively modified due to its less abundant exposure feature. Thus, the methodology allowing for Trp-targeting modification has been in demand for novel protein biologics. Recently, our group developed an Trp-selective modification reaction using S-methoxybenzyl cysteine sulfoxide (Cys(MBzl)(O) 1) in methansulfonic acid (MSA)−trifluoroacetic acid (TFA)^ref.1, which proceeds via formation of S-chlorocysteine (S-Cl Cys 2) accompanying the leaving of MBzl cation to generate Cys-Trp linkage (4) through SEAr reaction. The formation of the MBzl cation requires highly acidic conditions (MSA/TFA system), which, however, causes protein denaturation.

In this context, we explored the protein-friendly Trp-modification protocol by a cysteine sulfoxide with the combination of the sulfoxides and reaction conditions varying. Consequently, we found that S-acetamidomethyl (Acm) cysteine sulfoxide (Cys(Acm)(O): 5) serves as a suitable sulfenylating agent working under mild acid conditions (0.1% TFA in AcOH) in the presence of MgCl₂. Additionally, the application of the developed protocol to proteins will be presented.

Figure 1. The overview of the developed reaction.