Incorporation of selenomethionine into proteins through selenohomocysteine-mediated ligation

The chemical synthesis of moderate-sized proteins has benefited enormously from the development of chemoselective methods for the ligation of peptide fragments. In this regard, the reaction of peptide thioesters with peptides containing an N-terminal cysteine has proved particularly powerful. The thiol side chain of cysteine attacks the thioester and subsequent acyl transfer from sulfur to the amine yields a native peptide bond. Homocysteine, sulfur-containing auxiliary groups, and selenocysteine have also been exploited to mediate ligations to thioesters, thus broadening the scope of the method. Herein we show that selenohomocysteine can also facilitate efficient peptide ligation. Subsequent methylation of the resulting selenol affords selenomethionine at the ligation site, which is a useful probe for NMR spectroscopic and X-ray crystallographic investigations of biomolecules