DBCO-PEG4-Amine is a carboxyl-reactive building block with enhanced solubility in aqueous media used to derivatise carboxyl groups in the presence of activators (e.g. EDC, DCC) or activated esters (e.g. NHS esters) through a stable amide bond. DBCO reagents (ADIBO/DIBAC) are the most commonly used substrates for copper-free Click chemistry. DBCO compounds react with azide-containing compounds/biomolecules without the need for a Cu(I) catalyst to result in a stable triazole linkage. The hydrophilic PEG linker increases the solubility of the compound in water.