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 hydrophilic sulfonated spacer arm greatly improves water solubility of DBCO derivatized molecules, in many cases making it completely soluble in aqueous media. The PEG spacer arm provides a long and flexible connection.