TSTU (O-(N-Succinimidyl)-N,N,N',N'-tetramethyluronium tetrafluoroborate) is a highly efficient coupling reagent widely used in peptide synthesis and bioconjugation for forming amide bonds between carboxylic acids and amines. It activates carboxyl groups to form N-hydroxysuccinimide (NHS) esters, which are particularly reactive toward nucleophilic attack by amines under mild, aqueous or organic conditions. Compared to other coupling reagents like EDC, DCC, or HATU, TSTU offers several advantages: it generates fewer side products, is water-compatible, and produces more stable active esters than EDC, reducing hydrolysis before reaction. It also avoids urea byproducts common with DCC and is less toxic and more user-friendly than some alternatives like HATU or HBTU, making it a favored reagent in biological and pharmaceutical applications.