DBCO-PEG4-acid is an analog of DBCO-Acid with PEG linker and a DBCO group. The DBCO groups is commonly used for copper-free Click Chemistry reactions due to its strain promoted high energy. The hydrophilic PEG chain allows for increased water solubility. 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.