Description General description Tetrazine-PEG5-NHS ester has an N-hydroxysuccinimide (NHS) ester that can be reacted with primary amines and tetrazine, which is reactive to trans-cyclooctenes. Hydrogen substituted tetrazines demonstrate exceptionally fast kinetics, generally at least 10-fold faster compared to methyl substituted tetrazines. The aqueous solubility of this reagent is substantially enhanced by a hydrophilic polyethylene glycol (PEG) spacer arm. Application Tetrazine-PEG5-NHS ester (Tz-PEG5-NHS) can be used in click chemistry applications such as the preparation of chitosan-polyacrylamide microspheres utilizing selective tetrazine-trans-cyclooctene (Tz-TCO) ligation and the construction of copper-64 radiolabeled antibody by the click reaction.