Used in bioconjugation for linking molecules in drug development and bioanalytical research. Its NHS ester group reacts efficiently with primary amines on proteins or peptides, forming stable amide bonds. The PEG4 spacer improves solubility and reduces aggregation, while the propargyl group enables click chemistry reactions, such as copper-catalyzed azide-alkyne cycloaddition. This dual functionality allows sequential conjugation strategies, commonly applied in creating antibody-drug conjugates (ADCs), fluorescent probes, or immobilizing biomolecules on surfaces. The hydrophilic PEG linker also helps maintain the biological activity of conjugated proteins by minimizing steric hindrance.