Chemo- and Regioselective Ethynylation of Tryptophan-Containing Peptides and Proteins

Ethynylation of various tryptophan-containing peptides and a single model protein was achieved using Waser's reagent, 1-[(triisopropylsilyl) ethynyl]-1,2-benziodoxol-3(1H)-one (TIPS-EBX), under gold(I) catalysis. It was demonstrated by NMR that the ethynylation occured selectively at the C2-position of the indole ring of tryptophan. Further, MS/MS showed that the tryptophan residues could be modified selectively with ethynyl functionalities even when the tryptophan was present as a part of the protein. Finally, the terminal alkyne was used to label a model peptide with a fluorophore by means of copper-catalyzed click chemistry.