Selective Photo Labeling of Tryptophan with N-acetyl-L-Tryptophan Ethyl Ester and Peptide WWCNDGR

Selective amino acid labeling in peptides and proteins is important for understanding protein activity. Previously, it has been shown that aryl nitrenium ions react faster with tryptophan than any other amino acid. We hypothesized that we could label tryptophan in peptides with a diaryl nitrenium ion. We labeled N-acetyl-L-tryptophan ethyl ester and isolated two major adducts with N-(4,4’-dibromodiphenyl)nitrenium ion. We further labeled the peptide WWCNDGR and show that only tryptophan labeling is in the tandem mass spectrometry analysis. Therefore, aryl nitrenium ions are a new way to selectively label tryptophan in peptides and should continue being developed for labeling tryptophan in proteins.