3-Bromopropionic acid N-hydroxysuccinimide ester is a bifunctional activating agent primarily used in peptide synthesis and bioconjugation. The NHS ester moiety enables efficient and selective coupling to primary amines (e.g., lysine residues on proteins and antibodies) under mild, aqueous conditions, forming stable amide bonds and introducing a 3-bromopropionyl linker. The terminal bromo group provides reactivity toward nucleophiles such as thiols (e.g., cysteine residues), facilitating subsequent alkylation for crosslinking or conjugation. This reagent is particularly valuable for preparing labeled biomolecules, antibody-drug conjugates, diagnostic probes, and immobilized biosensors. Its water-soluble activated ester minimizes side reactions, enhancing efficiency in pharmaceutical research, targeted drug delivery, and molecular biology applications.