The reduction of oxidized methionine residues in peptide thioesters with NH4I-Me2S

Oxidized methionine residues in peptide thioesters can be reduced rapidly with NH4I to the corresponding sulfide by using Me2S as coreductant. Comparative reduction studies employing a 28-amino acid peptide thioester with an N-terminal methionine oxide as model system revealed the importance of the Me2S addition to avoid hydrolysis of the reactive thioester functionality. In addition, an NH4I-Me2S containing cleavage cocktail has been used for the global deprotection of various thioesters which revealed no hydrolysis or oxidative side products. These results demonstrate the general applicability of sulfoxides as protecting groups in advanced peptide synthesis techniques by facilitating the preparation and handling of methionine containing peptide thioesters for native chemical ligation (NCL).