2,5-Dioxopyrrolidin-1-yl 6-(((((9H-fluoren-9-yl)methoxy)carbonyl)amino)hexanoate, commonly known as Fmoc-6-Ahx-OSu, is a heterobifunctional crosslinker and a key reagent in peptide synthesis. Its structure features an N-hydroxysuccinimide (NHS) ester at one end and an Fmoc-protected aminohexanoic acid (Ahx) spacer at the other. The NHS ester is highly reactive towards primary amines (e.g., lysine side chains or N-termini of peptides/proteins), forming stable amide bonds. The primary application of this compound is in solid-phase peptide synthesis (SPPS) and bioconjugation. The Fmoc group serves as a temporary protecting group for the amine on the Ahx linker, which can be deprotected under mild basic conditions (e.g., piperidine) to reveal a free amine for further coupling. This makes it an essential building block for introducing a spacer (Ahx) between a peptide/protein and another molecule (e.g., a fluorophore, biotin, or solid support), thereby improving solubility, reducing steric hindrance, and enhancing biological activity in the final conjugate. It is also widely used in the preparation of peptide antigens, antibody-drug conjugates (ADC) research, and surface immobilization of biomolecules. As an active ester, it must be stored under anhydrous conditions and handled carefully due to its moisture sensitivity.