TCO-PEG4-TFP ester is an amine-reactive labeling reagent used for the modification of proteins, antibodies, and other amine-containing biopolymers. TFP is a reactive ester that demonstrates superior stability during hydrolysis in aqueous media, enhancing the efficiency and reproducibility of biopolymer labeling. The TFP ester of carboxylic acid reacts with primary amines at a rate equivalent to NHS esters, forming a covalent amide bond identical to that produced by the reaction of primary amines with NHS or sulfo-NHS esters.