DBCO-PEG5-acid is an analog of DBCO-Acid with PEG linker and a DBCO group. DBCO group is a highly reactive cycloalkyne which is commonly used for copper-free Click Chemistry reactions. The hydrophilic PEG chain allows for increased water solubility of compounds in aqueous media. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or HATU) to form a stable amide bond.