Selective photo labeling of tryptophan in <i>N</i> ‐acetyl‐L‐tryptophan ethyl ester and peptide <scp>WWCNDGR</scp> with a diaryl nitrenium ion

Abstract Diaryl nitrenium ions are short‐lived electrophiles that have been shown to react rapidly with various nucleic and amino acids. To determine whether these species could be used to covalently photomodify proteins or peptides, a diarylnitrenium ion precursor, N ‐(4,4′‐dibromodiphenylamino)‐2,4,6‐trimethylpyridinium ion, was photolyzed in the presence of tryptophan as well as a tryptophan‐containing peptide (WWCNDGR). It is shown that the resulting diarylnitrenium ion, N ‐(4,4′‐dibromodiphenyl)nitrenium ion, forms two covalent adducts with tryptophan, the first where the ortho‐ring carbon of the nitrenium ion couples to the C‐2 of the tryptophan and a second isomer where the indole N‐1 nitrogen of tryptophan couples to the ortho‐ring carbon of the nitrenium ion. When photolyzed in the presence of the peptide, tandem mass spectrometry demonstrates selective formation of adducts on the tryptophan residues. Laser flash photolysis experiments verify that trapping occurs via a nitrenium ion intermediate.