Used in bioconjugation and chemical biology for selective protein modification. The propargyl group enables copper-catalyzed azide-alkyne cycloaddition (CuAAC), allowing attachment of fluorescent tags, affinity probes, or other functional molecules to proteins. The lysine backbone with protected side chain facilitates incorporation into peptides via solid-phase synthesis. After deprotection, the molecule can be used in site-specific labeling of proteins or in activity-based protein profiling. The hydrochloride salt improves solubility and stability during handling and storage.