Used in bioconjugation to link molecules. The NHS ester reacts with primary amines (e.g., lysine or N-terminus of proteins) to form stable amide bonds under mild conditions, while the alkyne enables click chemistry reactions (e.g., CuAAC) with azides for further functionalization. The PEG4 spacer enhances water solubility and reduces steric hindrance, improving reaction efficiency in biological environments. Commonly employed in labeling peptides, proteins, and antibodies with fluorescent dyes, biotin, or other detection tags. Ideal for sensitive biomolecules in drug discovery, diagnostic assays, and development of bioconjugates for research and therapeutic purposes.