A carboxyl-reactive building block with enhanced solubility in aqueous media. Often a reagent of choice for modification biopolymers, peptides, and magnetic beads through carboxyl groups in the presence of activators (e.g. EDC, or DCC) or activated esters (e.g. NHS esters). A hydrophilic PEG3 spacer arm provides a long and flexible connection that minimizes steric hindrance involved with ligation to complementary tetrazine-containing molecules.