Sulfo DBCO-PEG4-TFP Ester is a water-soluble, amine-reactive labelling reagent that enables the simple and efficient incorporation of Sulfo DBCO moiety onto amine-containing molecules. The sulfonated spacer arm is hydrophilic and greatly improves the solubility of DBCO-derivatized molecules in water and, in many cases, making them also soluble in aqueous buffers. The PEG spacer arm is hydrophilic and enhances the efficiency of labelling and provides a long and flexible connection.