HATU (O-(7-Azabenzotriazol-1-yl)-N,N,N',N'-tetramethyluronium hexafluorophosphate) is a highly reactive peptide coupling reagent prized for its exceptional efficiency and low racemization, especially in difficult or hindered peptide bond formations. It activates carboxylic acids through the formation of an OAt (7-aza-1-hydroxybenzotriazole) ester intermediate, which is more reactive than the OBt esters formed by HBTU, leading to faster reactions and higher yields. HATU is especially advantageous in solid-phase peptide synthesis (SPPS) when dealing with sterically hindered amino acids or sequences prone to epimerization. Compared to other reagents like HBTU, DCC, or EDC, HATU offers better coupling efficiency, minimal side product formation, and excellent performance under mild conditions, making it a top choice for synthesizing high-purity peptides and complex biomolecules.