Sulfo DBCO-PEG4-Amine is a water-soluble reagent used to derivatize carboxyl-containing molecules or activated esters (e.g. NHS esters) with DBCO moiety through a stable amide bond. 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 provides a long and flexible connection.