Peptides hystidyl

Tyrosyl and hystidyl peptide bonds, which are usually cleaved by other brominating agents such as NBS, are sensitive to oxidation also but are not cleaved by TBC. Tyrosine was converted by TBC to 3,5-dibromotyrosine, cysteine to cysteic acid, methionine to methionine sulfoxide and tryptophan most probably converted to a bromooxindole derivative. Optimal conditions for the cleavage of the tryptophanyl peptide bond were found to be 3 equiv. of TBC at pH 3 for 5-15 min at room temperature. The model peptide N-carbobenzoxy-tryptophanyl-glycine was cleaved in about 50% yield NBS also cleaved this peptide to the same extent. In lysozyme, selective cleavage of the expected tryptophanyl peptide bonds (yields 5-60%) was obtained. 

Tryptophan residues in lysozyme form charge transfer complexes with imidazole, the residues being the charge donors. Studies of intramolecular indole-imidazolium complexes have been carried out using natural and synthetic peptides containing tryptophanyl and hystidyl residues. 

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