TCO-PEG12-TFP ester is a PEG derivative containing a TCO moiety and an PFP ester. The hydrophilic PEG spacer increases water-solubility and decreases steric hindrance during ligation. This reagent can be used to label antibodies, proteins and other primary amine-containing macromolecules with TCO moiety. The PFP ester is amine-reactive, stable compound. PFP esters are less susceptible toward hydrolysis compared to NHS esters.