Selective labeling of tyrosine residues in proteins: insights from PTAD labeling and tandem mass spectrometry analysis.

Designing reagents for protein labeling is crucial for investigating cellular events and developing new therapeutics. Historically, much effort has been focused on labeling lysine and arginine residues due to their abundance on the protein periphery. The chemo-selectivity of these reagents is a challenging yet crucial parameter for deciphering properties specifically associated with the targeted amino acid. Consequently, there is a growing demand for new conjugation reagents and workflows that facilitate selective binding to amino acids other than lysine, cysteine, and arginine. Tyrosine, an aromatic amino acid, occurs moderately on the protein periphery, with its phenolic ring often buried within the tertiary protein structure. This presents a challenging environment for tyrosine-specific protein bioconjugation efforts. The hydrophobic aromatic side chain of tyrosine is known to engage in π-stacking interactions, while the hydroxyl group of the phenyl ring can participate in hydrogen bonding and form tyrosyl radicals involved in electron transfer. 4-Phenyl-3-1,2,4-triazole-3,5(4)-dione (PTAD) has been previously investigated for its ability to bind to tyrosine. This work presents an extensive structural proteomics investigation of tyrosine labeling across samples of varying complexity, ranging from peptides and proteins to entire cell lysates. Mass spectrometry is utilized to study the behavior of tyrosine-labeled samples through tandem mass spectrometry experiments. We believe these studies will offer valuable insights into tyrosine bioconjugation with PTAD and demonstrate its potential as a covalent labeling reagent for chemical proteomics research.