TSTU

TSTU — O-(N-succinimidyl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate — is a uronium-type coupling reagent that converts carboxylic acids into N-succinimidyl (OSu/NHS) active esters, either for isolation or for in-situ coupling with amines. With a tertiary amine base, the carboxylate is converted to its succinimidyl ester, releasing tetramethylurea, and the active ester then acylates a primary amine to give the carboxamide. TSTU is the tetrafluoroborate, succinimide-based member of the tetramethyluronium reagent family, and the tetrafluoroborate analogue of the hexafluorophosphate reagent HSTU. TSTU is a white crystalline solid, soluble in acetonitrile, DMF and chlorinated solvents, moisture-sensitive, and stored cold.

Common Scientific Applications

• N-Succinimidyl (OSu/NHS) esters for amide and peptide bond formation: TSTU is used as a coupling reagent in solution-phase peptide synthesis; it is selective for amines over alcohols, and couples N-protected α-amino acids without significant epimerisation.
• Active esters for aqueous-media coupling: TSTU forms succinimidyl esters efficiently even in the presence of water, and converts dyes, fluorophores and PEG building blocks to OSu active esters for conjugation in aqueous or mixed aqueous-organic media.
• Bioconjugation, sensors and surfaces: the succinimidyl esters formed with TSTU label biomolecules through their amino groups, activate the carboxyl termini of alkanethiols for surface plasmon resonance (SPR) sensors, and link amino-functionalised atomic force microscopy (AFM) tips.
• Radiolabelled prosthetic groups: TSTU is used to prepare N-succinimidyl ester radiolabelling agents, such as N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB), for attaching radiolabels to peptides and proteins.

Further Reading

For guidance on selecting between carbodiimides, aminium/uronium salts, phosphonium salts and other reagent classes for amide and peptide bond formation, see NorrChemica's Lab Journal guide: Choosing a Coupling Reagent for Amide and Peptide Bond Formation.

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