BCN-PEG4-acid (CAS No.: 1881221-47-1) is a highly active bioorthogonal click chemistry reagent. Its core structure consists of three components: the bicyclo[6.1.0]non-5-ene (BCN) group provides strain-promoted alkyne-azide cycloaddition (SPAAC) activity under copper-free conditions; the tetraethylene glycol (PEG4) segment enhances water solubility and reduces nonspecific adsorption; and the terminal carboxyl group (-COOH) can form stable amide bonds with primary amine groups after activation with EDC/NHS. The production process employs linear PEG chemical synthesis technology, achieving high-purity (≥95%) preparation through precise control of the coupling sites between BCN and the carboxyl group. This reagent is widely used in fields such as antibody-drug conjugate (ADC) development, PROTAC protein degrader construction, live cell labeling, and targeted nanocarrier modification. Its mild reaction conditions (physiological environment, 37°C) and modular design make it an ideal tool in biomolecular engineering.