Sulfo DBCO-PEG4-Amine is a water-soluble carboxyl-reactive building block with extended PEG spacer arm. In the presence of of activators (e.g. EDC, or DCC), this reagent can be used to derivatize carboxyl groups or activated esters (e.g. The NHS ester) through a stable amide bond. The hydrophilic sulfonated spacer arm greatly improves water solubility of DBCO derivatized molecules and a provides a long and flexible connection. DBCO is commonly used for copper-free Click Chemistry reactions.