endo-BCN-PEG3-acid is a PEG derivative containing a BCN group with a terminal carboxylic acid. The terminal carboxylic acid can be reacted with primary amine groups in the presence of activators (e.g. EDC, or DCC) forming a stable amide bond. The BCN group can react with azide-tagged biomolecules. The hydrophilic PEG spacer increases solubility in aqueous media.