2,5-Dioxopyrrolidin-1-yl 4-chlorobenzoate is an active ester (NHS ester) of 4-chlorobenzoic acid, used primarily for the selective acylation of primary amines in peptides, proteins, and other biomolecules. It facilitates the formation of stable amide bonds under mild, aqueous conditions, enabling efficient labeling, modification, and conjugation without harsh reagents. This makes it valuable in bioconjugation applications, such as attaching fluorophores, drugs, or cross-linkers to antibodies and proteins for pharmaceutical development and biotech research. It is also employed in peptide synthesis for N-terminal protection or capping, and in the synthesis of bioactive conjugates.