TCO-PEG3-amine is is a simple PEG derivative containing a TCO moiety and a free amine. This reagent can be used to derivatize carboxyl groups or activated esters (e.g. NHS esters) with the TCO moiety through a stable amide bond in the presence of of activators (e.g. EDC, or DCC). Hydrophilic PEG spacer improves water solubility.