Azido-PEG4-CH2CO2H is a very popular click chemistry linker containing an azide group with a terminal carboxylic acid. The hydrophilic PEG spacer increases solubility in aqueous media. The azide group can react with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or HATU).
