Specific Affinity Enrichment of Electrochemically Cleaved Peptides Based on Cu(II)-Mediated Spirolactone Tagging

Specific digestion of proteins is an essential step for mass spectrometry-based proteomics, and the chemical labeling of the resulting peptides is often used for peptide enrichment or the introduction of desirable tags. Electrochemical oxidation yielding specific cleavage C-terminal to tyrosine (Tyr) and tryptophan (Trp) residues provides a potential alternative to enzymatic digestion and a possibility for further chemical labeling by introducing reactive spirolactone moieties. However, spirolactone-containing peptides suffer from low stability due to hydrolysis and intramolecular side reactions. We found that Cu(II) ions stabilize the spirolactone and prevent intramolecular side reactions during chemical labeling, allowing efficient chemical tagging with a reduced excess of labeling reagent without intramolecular side reactions. On the basis of this reaction, we developed an analytical procedure combining electrochemical digestion, Cu(II)-mediated spirolactone biotinylation, and enrichment by avidin affinity chromatography with mass spectrometry. The method was optimized with the tripeptide LWL and subsequently applied to chicken egg white lysozyme, in which one biotinylated electrochemistry (EC)-cleaved peptide was identified after affinity enrichment. This proof-of-principle shows that specific enrichment of electrochemically cleaved spirolactone-containing peptides can be used for protein identification and notably that inclusion of Cu(II) ions is essential for stabilizing spirolactones for subsequent biotinylation.