An Aryl Diazonium‐Based Enrichment Method for Deep Coverage of Monomethyllysine Proteomics

Abstract Lysine methylation is an important post‐translational modification (PTM) that regulates diverse cellular processes. Proteomic analysis is a robust method to study PTMs, but a lack of good enrichment tool limits current understanding of lysine methylation. In a previous study, we demonstrated that aryl diazonium containing 2,6‐dimethoxy substitutions can conjugate monomethyllysine‐modified (Kme1) peptides with high selectivity and that the reaction is reversible under acidic conditions, allowing the release of Kme1 peptides. Therefore, such a warhead has great potential for the enrichment of low‐abundance Kme1 peptides from biological samples. Here, we report the preparation of aryl diazonium‐functionalized resins as enrichment tools and their application for mass spectrometry‐based proteomic studies of Kme1 peptides. In this procedure, aniline with a PEG linker as a precursor is synthesized and then coupled to a hydrophilic solid phase. After preparation of tryptic peptides from cell samples, the aniline groups on the resin are converted to aryl diazonium for Kme1 peptide capture. After sufficient treatment and washing, the covalent linkage is broken under acidic conditions to release the original Kme1 peptides from the resin. Finally, the enriched samples are processed by mass spectrometry scanning and data analysis to identify ∼10,000 Kme1 sites in cell or mouse tissue samples. Herein, we demonstrate an efficient Kme1 peptides enrichment strategy for deep coverage of the Kme1 proteome in biological samples. © 2025 Wiley Periodicals LLC. Basic Protocol 1 : Preparation of aryl aniline‐functionalized Sepharose resin Basic Protocol 2 : Preparation of tryptic peptides from biological samples Basic Protocol 3 : Enrichment of Kme1 peptides from whole‐cell lysate tryptic peptides Basic Protocol 4 : Data acquisition and analysis for Kme1 proteomics