Used in bioconjugation reactions to link polyethylene glycol (PEG) spacers to primary amines on proteins, peptides, or other amine-containing molecules. The NHS ester group reacts efficiently with amines under mild conditions, forming stable amide bonds. The PEG2 spacer increases solubility, reduces aggregation, and extends the circulation time of conjugated biomolecules. Commonly applied in the development of antibody-drug conjugates (ADCs), targeted therapeutics, and functionalized nanoparticles. The NHPI (N-hydroxyphthalimide) group may serve as a radical mediator in some catalytic systems, adding versatility in synthetic applications. Suitable for use in aqueous or mixed aqueous-organic solvents, making it ideal for bioconjugation workflows.