Description Application 5-[4-(1,2,4,5-Tetrazin-3-yl)benzylamino]-5-oxopentanoic acid may be used in the synthesis of a PEG-tetrazine (PEG-Tz) macromer via acid-amine conjugation. This macromer can undergo click reaction with norbornene-functionalized peptides to form hydrogels that is useful for 3D cell culture. Acid functionalized tetrazine for inverse electron demand Diels-Alder cycloaddition reactions. The tetrazine will react with strained alkenes such as transcyclooctene, norbornene and cyclopropene to yield a stable covalent linkage. Tetrazines have proven useful in bioorthogonal reactions for many biological imaging and bioconjugation applications. Packaging 10, 25 mg in clear glass bottle