TCO-PEG3-Amine is a carboxyl-reactive building block with enhanced solubility in aqueous media. It is often the reagent of choice for the modification of biopolymers, peptides, and magnetic beads through carboxyl groups in the presence of activators (e.g. EDC, DCC) or activated esters (e.g. NHS esters). The PEG3 spacer arm is hydrophilic and provides a long, flexible connection that minimises steric hindrance in reactions with complementary tetrazine-containing molecules.